Other Engineered Features

Transcription

This page is left blank intentionally.
Pre- and Post-closure Engineering
Optimisation for the LLWR 2011
ESC
Final Report
Alan Paulley
Michael Egan
QRS-1443O-1
Version 2
April 2011
Document History
Title:
Pre- and Post-closure Engineering Optimisation for the LLWR 2011
ESC
Subtitle:
Final Report
Client:
LLW Repository Ltd
Document Number:
QRS-1443O-1
Version Number:
Version 0.1 (Draft for Discussion)
Notes:
Produced to support discussions on final report structure, also noting
Date:
January 2010
current ‘state of the argument’
Prepared by:
Alan Paulley and Michael Egan
Reviewed by:
For LLWR project team review
Version Number:
Version 0.2 (Workshop Draft)
Notes:
Produced as a draft briefing note for assessment workshop
Prepared by:
Michael Egan and Alan Paulley
Reviewed by:
For LLWR project team review
Version Number:
Version 1 (Draft for LLWR Review)
Notes:
Draft complete report for LLWR ESC project team review
Prepared by:
Alan Paulley and Michael Egan
Reviewed by:
David Hodgkinson
Version Number:
Version 2
Notes:
Contractor Approved following LLWR ESC project team comments
Prepared by:
Michael Egan
Reviewed by:
Richard Cummings, David Tonks, Alan Paulley
Approved by:
Michael Egan
Quintessa Limited
Chadwick House
Birchwood Park
Warrington WA3 6AE
United Kingdom
Date:
Date:
Date:
February 2010
August 2010
April 2011
Tel: +44 (0) 1925 885950
Fax: +44 (0) 1925 830688
[email protected]
www.quintessa.org
www.quintessa-online.com
QRS-1443O-1, Version 2
Summary
Background
LLW Repository Limited is undertaking a programme of work that will result in the
publication of a new Environmental Safety Case by May 2011 (the 2011 ESC). The 2011
ESC will be submitted to the Environment Agency in support of a formal application
for re-authorisation of disposal operations. The 2011 ESC is based on a reference
operational programme that assumes five additional disposal vaults will be created in
the future (i.e. Vaults 10 to 14) and that the facility will reach volumetric capacity at c.
2080.
Demonstration of optimisation – ensuring that radiological risks to members of the
public are as low as reasonably achievable (ALARA) – is an essential element of the
2011 ESC. This involves developing a description of the rationale behind choices
regarding the design and management of the LLWR. A diverse range of optimisation
and risk management considerations need to be taken into account, requiring inputs
from a wide range of information sources, analyses and arguments. Relevant
considerations include:
remedial actions relating to past disposals;
waste acceptance, conditioning and emplacement;
pre- and post-closure engineering; and
operational and post-emplacement management control.
The specific aspect of optimisation examined in this report is the engineering design of
the LLWR, from the perspective of both pre- and post-closure environmental safety.
This includes consideration of options for those engineered components of the facility
that will be expected to act as passive controls to provide isolation and containment of
the disposed waste (temporary/final cap, vaults, cut-off wall, leachate drains, etc.), as
well as the relationship between the scheduling of such measures and active controls,
such as leachate management.
Scope
Key considerations relevant to framing the current study include:
The focus of the ESC, and therefore on options for the engineering design, is on the
role of LLWR as a disposal facility (i.e. capable of final closure, with no intent to
retrieve waste at a later time). Justification of that role, as part of NDA’s national
1
strategy for LLW management, is part of the wider context of the ESC but outside
the scope of this particular study.
Options for the future design and operation of the LLWR are restricted to ‘vault’-
type waste emplacement. There is no practical scope for developing either a mined
cavern or silo-type disposal operation at the LLWR site.
The baseline for the ESC is the assumed development of the LLWR within the
‘reference disposal area’, extending vault disposals contiguously to the south-east
from Vault 9 until they reach a line roughly defined by the extended southern
perimeter of the trenches (i.e. to Vault 14). The possible implications of variant
cases designed to have a larger physical capacity are to be explored within the 2011
ESC. Thus it is relevant to consider the extent to which specific engineering options
may offer (or constrain) flexibility in future use of the site.
Within these constraints, alternative engineering design options have been identified
taking into account the types of passive control that are relevant to the main threats to
isolation and containment of the wastes. These threats are those associated with:
Natural disruption of the facility – in particular as a result of coastal erosion
and/or inundation.
Disruption of the facility by future human actions – in particular those actions that
have the potential to cause disturbance of the wastes, as well as direct or indirect
exposures to contaminants within the wastes.
Release of contaminated gases, generated either in the form of gaseous radioactive
elements or as radio-labelled gases produced within the wastes.
Generation and release of contaminated leachate by water entering the facility and
contacting the wastes.
Optimisation Approach
Demonstration of optimisation is fundamental to the regulatory regime that applies to
disposals to LLWR. However, optimisation considerations are not simply a matter of
achieving compliance with regulatory expectations. They are also an integral element
of the LLWR’s operational and closure planning process, with direct influences on
Lifetime Plan development and future use of the disposal facility. Such factors are also
inevitably of interest to the regulation of spatial planning and operational safety for the
site. Hence, although the regulatory process surrounding re-authorisation establishes
the primary driver in optimisation as being the control of radiological environmental
risk, demonstration of ALARA requires a wider set of values and criteria to be taken
into account. Regulatory Guidance on Requirements for Authorisation (the GRA)
(§6.3.58) underlines that “Although reducing radiological risk is important, it should not be
2
given a weight out of proportion to other considerations” (Environment Agency et al., 2009).
For the purposes of this study:
The main emphasis in comparing engineering options is whether there is a clear
preference from the perspective of establishing confidence in the environmental safety
performance of LLWR. It is then necessary to determine whether that preference may
be materially affected by wider considerations.
In practice, this means that the choice between options that are strongly differentiated
in terms of confidence in environmental safety performance is likely to be challenged
only where there is an equally strong (and contrary) differentiation in terms of other
important factors. Alternatively, options that cannot readily be discriminated in terms
of providing confidence in environmental safety may be differentiated on the basis of
other considerations.
Broadly speaking, there are contributions to optimisation from both a broad strategic
perspective (i.e. the role played by engineering controls) and a more detailed design
perspective (how those controls are implemented). The assessment of options is
necessarily iterative, reflecting the range of options available and the fact that more
general choices are informed by more detailed understanding. Informing and
consulting with stakeholders has also provided relevant inputs to the design of the
options assessment and understanding of the outcomes.
Inputs to the optimisation work reported here included a range of sources of evidence.
Understanding of safety performance and other factors has been gained from previous
assessment studies, including performance updates in response to the regulatory
requirements following the 2002 ESC and work supporting the rationale for the current
Vault 9 design. In addition, new hydrogeological modelling studies (and supporting
expert elicitation work) have been undertaken to explore the implications of alternative
design concepts.
Role of Engineered Barriers
The potential threats to environmental safety performance are varied, and engineering
controls are not the only means available for optimising protection in the “design,
construction, operation, closure and post-closure management” of the LLWR (Requirement
R8 of the GRA). Other types of action, such as controls on inventory, wasteform or site
management, may in some cases be equally or more effective in limiting discharges
and ensuring that related risks of exposure are ALARA.
Nevertheless, the safety functions performed by engineered features are an integral
part of what needs to be examined and demonstrated in the ESC. These safety
functions can be summarised as shown below.
3
Engineered Barrier Safety Functions
To protect against disturbance so far as is practicable
To minimise water flow through the system for as long as is practicable
To control gases and leachate that may be produced within the facility
To direct releases that may occur so as to minimise their impact
Features addressed in the optimisation of engineering design were identified by
examining the types of passive control that can be provided by different barriers
against identified threats to isolation and containment. Wider implications of specific
engineering features, and the potential interaction between different features, are also
taken into account in determining a preferred approach.
Threat from Natural Disruption
The precise timescale over which disruption by erosion might occur is uncertain, but
projections based on current best understanding of the implications of global climate
change on sea level, coupled with analysis of coastal processes in the vicinity of LLWR,
suggest that it is almost inevitable within a period of a few thousand years, and might
begin considerably earlier. Because the primary mechanism of disruption is expected to
be erosion by under-cutting at the base of the sea cliff, engineered features of a nearsurface vault disposal system (including the possible alternative of grouting waste in
situ or otherwise achieving a monolithic vault design) are not expected to offer
significant protection or impediment to the erosion process.
Taken together with the comparatively slow rate of decline in the residual hazard
presented by the wastes at the time coastal erosion becomes a threat, it is therefore
difficult to argue a case for engineered barriers playing any significant role in hazard
reduction by delaying disruption.
A monolithic vault design might be more resistant to erosion than current waste
emplacement and backfilling arrangements. In practice, however, once erosion of the
facility has started, a more resistant design might actually give rise to greater risks by
inhibiting dispersion and therefore prolonging the time period over which inadvertent
exposures to undiluted wastes could occur.
Shoreline and coastal defences intended to protect the LLWR against erosion are not
strictly an element of the facility engineering itself and would in any case need to be
maintained over a considerable period of time in order to be effective in delaying
4
disruption. It would be difficult to justify any claims for the effectiveness of such
measures beyond the period associated with post-closure site control plans.
The ESC will address the implications of coastal erosion leading to gross disruption of
LLWR as a ‘termination event’ associated with the expected natural evolution of the
facility and its environment. Disruption by coastal erosion therefore effectively sets a
limit to the timescale over which it is appropriate to consider in detail the optimisation
of engineering controls. It might be argued that, given the nature of the hazards
involved, disruption of the facility is unlikely to be a major priority compared with
other hazardous sites and facilities vulnerable to the effects of sea level rise around the
UK. Nevertheless, the GRA indicates that, should the levels of exposure from a
reasonably foreseeable natural process (albeit on timescales greater than those
normally attributable to authorised control over the facility) prove significant by
comparison with the regulatory risk guidance level, it is questionable whether a
‘period beyond authorisation’ could be contemplated. Safety arguments therefore
ultimately centre on the overall acceptability of disposal itself, given the potential
radiological implications of disruption. Inventory control (should it be necessary),
rather than passive engineering control, is then the most practicable way of providing
assurance that unacceptable consequences do not occur.
Other Threats to Isolation and Containment
Taking threats other than natural disruption into account, the following main features,
functions and characteristics have been identified for evaluation in the optimisation of
pre- and post-closure engineering:
The Engineered Cap for the facility including consideration of:
o
its role as a barrier to the inflow of water into the trenches and vaults;
o
its role in minimising the likelihood of disturbance of the wastes by future
human actions and bio-intrusion;
o
its role in controlling gas release;
o
the timing of installation, including implications for the balance between active
and passive controls on leachate discharge;
o
the possible implications for other engineered features and waste emplacement.
Future Vault Walls and Bases, including consideration of:
o
their role in contributing to the control of leachate that may be produced within
the facility;
5
o
the response of alternative design specifications under normal and ‘cap failure’
conditions;
o
the comparative implications of alternative water management strategies for
projected leachate release from the facility over the time frame relevant to
engineering optimisation (i.e. prior to disruption by natural processes);
o
the potential need for the base level of future vaults to fall as they are
developed towards the southern part of the site.
Cut-off Walls and their potential role in:
o
minimising the lateral inflow of water to the trenches and vaults, including the
control of waters that are shed at the cap perimeter;
o
minimising the possibility of leachate release to the surrounding environment
via near-surface pathways, in the event of degradation of cap performance.
Passive leachate discharge control, including consideration of:
o
the possible role for drains in providing passive control over the routing of
leachate released from the facility;
o
the anticipated performance of such features under normal and ‘cap failure’
conditions.
In the analysis that follows, the options for each of these engineering features are
assessed in turn.
Final Cap
The engineered cap is a principal component of the overall engineering design for the
LLWR. It plays key roles in relation to the passive control of leachate and the
protection of disposed wastes from inadvertent disturbance by human actions. The
design of the cap therefore needs to be optimised to minimise infiltration, to the extent
that it is less than the drainage capacity of the underlying geology, thereby creating
unsaturated zones beneath the vaults and trenches, for as long as reasonably
practicable. It also needs to provide effective protection against intrusion by humans,
deep rooting plants and burrowing animals.
Key considerations in cap design include:
overall thickness and the depth and composition of individual component layers;
profile; and
6
the scheduling of installation in relation to waste emplacement and active measures
for the monitoring and control of leachate.
The component layers of final cap design for the LLWR have been optimised over
many design cycles, based on principles consistent with best international practice. The
current baseline design includes a composite geomembrane on a bentonite-enhanced
soil hydraulic barrier, overlain by an internal drainage layer, biointrusion barrier and
upper layers designed to provide substrate for plant rooting, moisture retention and
filtration. Expert elicitation based on this design forms the basis for parameters
determined as inputs to the ESC hydrogeological model calculations.
The overall thickness of the engineered cap in the baseline design is 3m. Taking into
account that there will be profiling material underneath the cap, this means that there
will be a minimum distance of 4m between the cap surface and the underlying wastes.
As such, the cap is considered to provide significant protection against inadvertent
disruption of both trench and vault wastes as a result of commonplace human actions.
A thicker cap might potentially provide greater protection against disturbance
associated with certain types of bulk excavation. However, no significant mitigation of
the likelihood of disturbance from major earthworks would be provided even if the
engineered cap was considerably thicker than the baseline design. Moreover, even a
cap of double the thickness or more would not necessarily offer significantly greater
protection against minor disturbances associated with actions such as exploratory
borehole drilling.
Controlled passive gas venting (as adopted on municipal landfill cap designs) is not
necessarily required if it can be demonstrated with confidence that the volumes of bulk
gas from waste corrosion and degradation do not represent a threat to cap integrity
and performance. The absence of an engineered gas vent would have the advantage of
ensuring that any release of radio-labelled gases generated within the facility would
not be focused at a specific location near highest point of the cap, with associated
implications for risk to potential exposure groups following the lifting of controls on
site use. However, a passive venting scheme installed as part of the cap design would
facilitate confirmation monitoring of landfill gas and radio-labelled gas production
during post-operational active control of the site. Final closure of the vent could then
be achieved prior to the release of the site from active management control. The
reference assumption in the site management plan set out in the 2011 ESC (i.e. whether
or not the cap vent should be closed) will be decided before submission, informed by
the latest assessments of gas generation and transport.
Taking due account of planning considerations and local community concerns implies
that the profile of the cap should ensure that the minimum elevation is achieved,
consistent with providing assurance of long-term performance and stability.
7
Engineering design principles require a minimum gradient of 1:25, to ensure that the
drainage function of the cap will be maintained over the long term under expected
rates of settlement. In addition, the engineering optimisation process examined both
the advantages and disadvantages of a ‘gull wing’ and single dome cap design. A
single dome requires a significantly greater volume of profiling material underneath
the engineered cap, but its geometry and lower susceptibility to erosion (i.e. avoiding
the existence of a ‘gully’ between the trenches and vaults that collects water from a
substantial fraction of the overall cap area) is a critical factor in underpinning
confidence in long-term resistance to infiltration. This gives a slightly higher peak
elevation to the cap (by less than 4m) than would be obtained for a gull-wing design,
but this is not expected to be significant in terms of visual impact, when considered
across the 400m width of the facility over which the cap will be constructed.
The engineered cap needs to be supported by a suitable profiling layer. This does not
necessarily have to consist solely of inert fill materials; indeed, there is an opportunity
to use this profiling volume as additional capacity for waste emplacement, for example
by adopting higher stacking of disposal containers away from the outside perimeter of
the vaults. Because such waste disposals would be within the profiling shape, they
would have no effect on the final overall elevation of the cap. Provided that higher
stacking within the vaults can be shown not have a detrimental impact on the
environmental safety case or other aspects of operational safety management, such a
practice would be consistent with making optimum use of LLWR as a national asset.
Vault Design
Although the cap design is intended to minimise infiltration so far as is reasonably
practicable, water cannot be excluded completely and the rate of infiltration is expected
to increase over the long term as cap performance degrades. Expert elicitation of
performance, based on the proposed design, coupled with the ESC hydrogeological
model, provides an indication of anticipated rates of water input and flow through the
system. The optimisation study considered two broad strategies for the passive
management of leachate within the vaults and their implications for vault design.
The preferred design strategy is to decrease the potential for near-surface release by
seeking to minimise water contact with the wastes and to avoid ‘bath-tubbing’ in the
vaults. The overall aim in vault design is therefore to increase confidence that water
can be drained at the base over time faster than it enters from above via the cap.
Hydrogeological modelling suggests that such a design strategy will also achieve a
more predictable outcome regarding the behaviour of leachate than the alternative
approach of designing for (short-term) leachate retention. It is also considerably easier
to construct than the alternative.
8
During active control over the site, the intention is to ensure that leachate can be
effectively collected and its release managed for as long as this is required. The vault
bases are therefore assigned a sufficiently low permeability to support effective
operational leachate management for as long as active controls are in place. Bunding is
provided by surrounding liner walls to a height of 1m above the base in order to
protect against uncontrolled overflow in extreme rainfall events prior to capping.
However, there is no strong driver to implement a higher specification base that would
provide increased confidence in minimising permeability significantly beyond the
operational period.
After the period of active control, under passive conditions and with no collection of
leachate from the vault sumps, saturation over the depth of the waste column is
precluded by ensuring that, if leachate is retained by the base, it overflows the 1m
walls on the east and west sides to a drainage layer beneath the vault base. The
drainage layer provides a high horizontal permeability to support effective dispersal of
drainage waters to the underlying geology. In addition (and particularly for Vault 8),
some drainage to the unsaturated zone beneath the vaults is anticipated to occur
through the base itself, under the expectation that its performance over time as a
hydrological barrier will be less effective than the cap.
As the development of future vaults (10 to 14) continues southwards, base levels
reduce to best suit the existing topography. This obviates the need to construct disposal
platforms above the local ground level. Consequently, the top of the base slab reduces
from 15.9m AOD (current Vault 9) to 11m AOD for Vault 14. The drainage layers
beneath individual vaults are laterally isolated to impede flows from following a
preferential path with topography and reducing base levels to a more focused
discharge point at the south of the facility.
The internal north and south walls of the vaults are set slightly higher than the east and
west walls to give preferential drainage pathways to the sides, but with ultimate
hydraulic continuity (to provide contingency in the event that free drainage does not
occur locally) along Vaults 9 to 14. Vault 8 would be connected into the Vault 9 system
on cessation of pumped leachate control.
Cut-off Wall
The optimisation study examined whether a cut-off wall is required, and the
implications of extending such a wall to different depths. The primary function of such
a cut-off wall is to prevent the lateral infiltration of water (including water shed by the
cap) into the vault and trench wastes around the edges of the LLWR. It also plays a
contingency role in providing reassurance against the possibility of near-surface
release close to the facility in the unlikely event of early cap failure (leading to possible
9
saturation of the waste column), or as a result of preferential pathways for leachate that
might arise from higher waste stacking. The cut-off wall will be keyed into the
perimeter of the cap, to preclude the possibility of over-topping in the event of
saturated conditions developing within the wastes.
Expert elicitation and hydrogeological modelling suggests that there is advantage to be
gained from installing a cut-off wall, but there are no significant benefits to be gained
from taking the wall to a greater depth than 2m below the underside of the composite
basal liner, or including a geomembrane within the wall. Such a design is consistent
with the existing cut-off wall, constructed in the 1990s along the northern and eastern
perimeter of the trenches. Subject to ongoing monitoring showing no grounds for
concern in the performance of this feature, it is expected that the existing cut-off wall
can be incorporated into the overall perimeter barrier.
Passive Discharge Controls
A vertical drain was incorporated between the trenches and vaults in earlier baseline
designs. Its safety function was to mitigate the potential for near-surface release
associated with bath-tubbing within vaults designed for leachate containment, as well
as to provide a contingency in the event of failure of the gull-wing cap design.
However, given the change to the preferred cap profile, as well as the strategy for
leachate management within future vaults, this function is longer required.
Hence there are no substantial vertical drains in the engineering design identified as an
outcome from the current optimisation study. However, engineered drainage
pathways are provided by the sub-base features associated with the future vault
design, with the aim of maintaining low saturation conditions within the disposed
wastes and the ready dispersal of drainage waters from the vaults to the underlying
geology. For as long as the cap provides a barrier to infiltration that is greater than the
natural drainage capacity of the geology beneath the facility, passive leachate release
via these pathways is expected to be to unsaturated ground.
Timing of Cap Installation
The optimisation study determined that progressive capping should be implemented
in ‘strips’ down the site, linked to the construction of new vaults. Staged installation of
the cap over the vaults is preferred in order to ensure that the capacity of the leachate
management system is not threatened by the requirement to manage storm waters
from extreme weather events as new vault slabs are constructed. Moreover, rather than
waiting until the completion of disposal operations, earlier capping also serves to
provide a protective cover to early vault disposals that may have been exposed to the
elements for a considerable period of time. Staged capping is more efficient in terms of
10
resource use, and provides for a longer period of monitoring of cap performance prior
to final closure and withdrawal of active control over the site.
For the purpose of the ESC, it is therefore assumed that final capping over Vault 8 and
an associated fraction of the northern end of the trenches will be undertaken in
conjunction with the preparation of Vault 10. Likewise, capping for Vault 9 and the
adjacent area of the trenches will be undertaken in conjunction with the preparation of
Vault 11 et seq. Excavations required prior to construction of the next vault slab would
be expected to provide a proportion of the profiling material for the corresponding
stage in capping.
This assumption incorporates flexibility to enable changes to be made, consistent with
the overall aim of achieving a single dome final cap. In particular:
The schedule assumed for the ESC anticipates that no replacement of the current
interim cap over the trenches (minimum design life of 30 years from 1990) will be
required prior to installation of each stage of the final cap. Nevertheless, earlier
capping over the trenches could be undertaken if it were required. This would be
achieved either by constructing a new interim cap, a ‘Stage 1’ cap, or completing
the final cap, over the trenches in such a way that it could subsequently be
incorporated into the final single dome as future vaults were constructed and
capped.
Subject to further monitoring and analysis, there remains limited evidence and
some uncertainty regarding the degree of settlement that remains to be expressed,
particularly within the trench wastes. This could arise as a result of the additional
loads imposed during final capping, as well as the ongoing effects of waste
degradation. These processes have the potential to cause differential settlements of
the cap, across the wastes themselves, as well as across the bay walls, trench walls
and the secant pile wall between the trenches and the vaults. Although the design
intention within the ESC is for staged installation of the full, final engineered cap in
strips starting from the northern end of the facility, it is possible that a simpler
‘Stage 1’ cap (incorporating a geomembrane and appropriate surface cover as
temporary protection, supported by all the relevant profiling material) could be
constructed initially if it were determined that settlement over the trenches
represented a significant concern. Such a cap would have broadly similar
hydrological performance to the final cap during the operational period. However,
it would enable any remedial actions in response to settlement to be taken more
readily prior to installation of the final engineering layers.
One important control on the rate of capping is the expected arising of profiling
materials. If necessary to support the staged installation of the final cap, required
profiling materials would need to be brought to the site to supplement those
11
generated by excavations of future vaults. Although not incorporated in the
proposed design and inventory for the ESC, it is conceivable that profiling material
above the trenches might include VLLW, such as very lightly contaminated
building rubble or soils. Depending on the rate of such arisings, it is conceivable
that capping of the trenches might therefore need to progress more rapidly than
scheduled in relation to the development of new disposal vaults. Subject to national
policy and appropriate regulatory and planning approval, such disposals could be
accommodated with an adjustment to the preferred schedule for capping.
Leachate Management and Release Monitoring
The baseline assumption for the ESC is that leachate will continue to be actively
managed while waste emplacement continues and for a period after final capping over
the facility has been completed. Within the vaults, this is achieved by pumping to
collect leachate from sumps installed in the vault bases. Gravity drains leading to the
southern perimeter of the facility provide for leachate collection from the trenches. The
leachate is routed to marine holding tanks for monitoring prior to consented discharge
via the site pipeline to the sea. After final capping, the maintenance of pumps used to
remove leachate collected within the vault sumps will be undertaken with the support
of engineered man-access penetrations in the western side of the cap.
The assumption of continued active management and monitoring is consistent with
established practice for landfill operations. However, based on current understanding
of cap performance and estimated likely rates of leachate generation (subject to the
outcome of final hydrogeological modelling), it is expected that the vaults and trenches
will be effectively de-watered soon after the completion of final waste emplacement
(effectively within a few decades), with very low ongoing rates of leachate generation.
Monitoring during the post-operational period would be continued with the objective
of confirming such outcomes, for as long as necessary in order to fulfil regulatory or
other stakeholder requirements regarding assurance of system performance.
Likewise, the monitoring of landfill gas (and possible radioactive gas) via the gas vent
in the final cap would be designed to confirm the expectation that gaseous releases –
both in terms of volume and contamination levels – will be small, and present an
acceptably low risk in terms of fire, explosion or radiological exposure. The decision as
to whether it was appropriate finally to seal the gas vent would be informed by
monitoring information and taken, in consultation with regulatory authorities and
other stakeholders, prior to withdrawing active management control over the site.
12
Contents
1
Introduction
14
2
Context and Objectives
16
2.1 Requirements
16
2.2 Optimisation Constraints and Baseline Design
19
2.3 Key Issues for Options Identification and Assessment
21
2.4 Level of Detail
22
2.5 Summary of Objectives
22
Approach
24
3.1 Philosophy
24
3.2 Process
24
3.3 Evidence
28
Outcomes
35
4.1 Overview
35
4.2 Engineering Controls on Performance
35
4.3 Engineered Features for Assessment
40
4.4 Assessment of Component Options
41
3
4
5
4.4.1 Overview
41
4.4.2 Final Engineered Cap
43
4.4.3 Future Vault Walls and Base Options
48
4.4.4 Cut-off Wall
54
4.4.5 Passive Leachate Discharge Control Options
58
Final Optimised Pre- and Post-closure Engineering Strategy
References
Appendix A : Engineering Optimisation Scoping
Participants and Briefing Paper
62
76
Workshop Agenda,
78
Appendix B : Optimisation Assessment Workshop Agenda and Participants
83
Appendix C : Optimisation Presentation Agenda and Participants
86
Appendix D : Collation of Component Option Proformas
87
Appendix E : Commentary on Possible Implications of Different Component
Options for Post-closure Impacts
113
Appendix F : Systematic Identification of Strategy and Component Options
Against Threats to Repository Performance
146
Appendix G : Scoping Workshop Engineering Presentation showing Overview
of Basic Elements of the Previous Baseline Design
150
13
1 Introduction
LLW Repository Limited is undertaking a programme of work that will result in the
publication of a new Environmental Safety Case by May 2011 (the 2011 ESC). The 2011
ESC will be submitted to the Environment Agency in support of a formal application
for re-authorisation of disposal operations. The 2011 ESC is based on a reference
operational programme that assumes five additional disposal vaults will be created in
the future (i.e. Vaults 10 to 14) and that the facility will reach volumetric capacity by
around 2080.
In responding to the previous 2002 Safety Cases submitted for LLWR, the Environment
Agency (EA) noted that those cases had failed to make a sufficiently adequate or robust
argument for continued disposals. A key element of this weakness was described as
being “insufficient consideration of optimisation and risk management, to demonstrate that
impacts will be as low as reasonably achievable (ALARA)” (Environment Agency, 2005).
The requirements specified in Schedule 9 of the ensuing Authorisation specifically
underlined the need to demonstrate optimisation in the 2011 ESC. More recently,
updated regulatory Guidance on Requirements for Authorisation (GRA) for Nearsurface Disposal Facilities on Land for Solid Radioactive Wastes (Environment Agency
et al., 2009) has further reinforced the importance of optimisation within the regulatory
control regime.
A systematic description of the rationale that underpins design, use and operational
management choices made by LLW Repository Ltd will therefore be an essential
element of the 2011 ESC. A diverse range of optimisation and risk management
considerations need to be taken into account, requiring inputs from a wide range of
information sources, analyses and arguments. Relevant considerations include:
remedial actions relating to past disposals;
waste acceptance, conditioning and emplacement;
pre- and post-closure engineering; and
operational and post-emplacement management control.
The specific aspect of optimisation examined in this report is the engineering design of
the LLWR, from the perspective of both pre- and post-closure environmental safety.
This includes consideration of options for those engineered components of the facility
that will be expected to act as passive controls to provide isolation and containment of
the disposed waste (temporary/final cap, vaults, cut-off wall, leachate drains, etc.), as
well as the relationship between the scheduling of such measures and active controls,
such as leachate management.
14
The report is structured as follows.
Section 2 describes the context and objectives for this work.
Section 3 gives details of the process followed, and describes the evidence base
collated to underpin the analysis.
Section 4 describes the outcomes of the assessment and integration process.
Section 5 summarises the final, optimised pre- and post-closure engineering
strategy.
Appendices to the report contain information relating to workshops held in support of
the engineering optimisation process, and report a range of underpinning analyses
developed as part of the evidence base for decisions that were made.
Appendix A provides the Agenda and participant lists for the scoping workshop.
Appendix B reproduces the Agenda and participant list for the main assessment
workshop.
Appendix C provides equivalent information for the workshop at which the final
outcomes of the study were presented.
Appendix D collates the ‘component option’ proformas that were generated to
inform discussions at the assessment workshop.
Appendix E provides a commentary on factors relevant to differentiating between
component options from the perspective of post-closure impacts on the
environment.
Appendix F summarises the detailed analyses that were undertaken to support the
identification of engineering component options.
Appendix G reproduces slides from the scoping workshop presentation outlining
major features of the pre-existing baseline design and main component options.
15
2 Context and Objectives
2.1 Requirements
Environment Agency Regulatory Requirements
disposals to the LLWR. The EA, in its review of the 2002 Safety Cases and the
subsequent decision on disposal authorisation, was critical of how those submissions
addressed optimisation considerations. One consequence of this was that a number of
the requirements specified in Schedule 9 of the ensuing Authorisation reflected the
need for the LLWR Site Licence Company to pay more attention to the demonstration
of optimisation in the 2011 ESC. Subsequently, the recently updated Guidance on
Requirements for Authorisation (GRA) for Near-surface Disposal Facilities on Land for
Solid Radioactive Wastes (Environment Agency et al., 2009) has further reinforced the
importance of the concept of optimisation in application of the RSA93 regulatory
regime.
The following statements from the GRA articulate regulatory expectations in relation
to the demonstration of optimisation in ESC submissions.
Under Section 7.2 (General Guidance: What should the environmental safety case
demonstrate?), paragraphs 7.2.2 and 7.2.3 highlight the role of the central ‘safety
strategy’ as the focus of the ESC and, by extension, overall optimisation of the
safety case. The guidance states:
“The environmental safety case should include an environmental safety strategy supported
by detailed arguments to demonstrate environmental safety. The environmental safety
strategy should present a top level description of the fundamental approach taken to
demonstrate the environmental safety of the disposal system. It should include a clear
outline of the key environmental safety arguments and say how the major lines of reasoning
and underpinning evidence support these arguments. The strategy should explain, for
example, how the chosen site, design for passive safety and multiple barriers each contribute
to environmental safety... The environmental safety case should demonstrate, using a
structure based on clear linkages, how the environmental safety strategy is supported by the
detailed arguments and how the arguments are supported by evidence, analysis and
assessment. Internal consistency within the environmental safety case needs to be
established and maintained.”
The importance of considering the implications of uncertainties in developing a
robust safety strategy is considered in under Section 7.3 (Additional considerations:
Managing uncertainties), paragraph 7.3.11:
16
“The developer/operator should provide explanations for interested parties of the
significance of uncertainties important to the environmental safety case, by presenting these
explanations in a way that people will understand. This material could form part of the
environmental safety strategy... The relevant environment agency will provide its own view
on the developer/operator's statements.”
Particular attention is given within the GRA to exploring the concept of
optimisation, both in terms of demonstrating that the safety strategy is appropriate
and that the proposed implementation approach is consistent with the principles of
ALARA. This is enshrined in ‘Principle 2’ of the GRA, which is titled “Optimisation
(as low as reasonably achievable)”. The description of what optimisation means in
practice includes the following (taken from paragraphs 4.4.3 to 4.4.5):
“Optimisation is a continuing, forward-looking and iterative process aimed at maximising
the margin of benefit over harm. It takes into account both technical and socio-economic
factors, and requires qualitative as well as quantitative judgements. It involves continually
questioning whether everything reasonable has been done to reduce risks… Optimisation
decisions balance the detriment or harm associated with the radiological risk, together with
other benefits and detriments (economic, human, societal, political, etc.) associated with
disposing of the radioactive waste, both at the time the decisions are taken and in the future,
and the resources available for protecting people and the environment... The result of
optimisation provides a radiological risk at a suitably low level, but not necessarily the
option with the lowest possible radiological risk.... Careful attention needs to be paid to
optimisation in a way that is proportionate to the radiological hazard. Where the
radiological hazard of the waste is low, only limited effort will be required to reach an
optimised radiological risk; conversely, for disposal facilities where the radiological hazard of
the waste is high, considerable effort will be required to reach an optimised radiological
risk... Optimisation needs to be embedded in the developer/operator’s organisational culture
so that it is regular practice to explore possible alternative options and to make the best
choice among them.”
Fundamental to the GRA’s portrayal of optimisation is the recognition that it is equally
applicable to disposal strategy (i.e. the engineered components of the disposal system
and their functions in assuring safety) as it is to detailed aspects of engineering design
(layout, scheduling, dimensions and materials). Addressing regulatory expectations
within the 2011 ESC therefore requires an approach that embeds more detailed design
optimisation in a well-understood and justified environmental safety strategy.
Requirements of Other Stakeholders
Although the regulatory permitting process establishes the primary driver for
optimisation as being the control of radiological environmental risk, demonstration of
ALARA requires a wider set of values and criteria to be taken into account. The GRA
17
(§6.3.58) underlines that “Although reducing radiological risk is important, it should not be
given a weight out of proportion to other considerations” (Environment Agency et al., 2009).
This reflects the need for design and operational decisions to take into account the
wider interests of other stakeholders, including the local community, planning
authorities, other regulators and funding bodies (on behalf of public sector spending).
Stakeholders with an interest in LLWR’s engineering optimisation processes include:
the planning authority (i.e. Cumbria County Council) who will have the
responsibility
for
granting
planning
permission
for
disposal
and
implementation of associated engineering;
the Nuclear Decommissioning Authority (NDA),
local bodies who will be consulted by the planning authority (e.g. Copeland
Borough Council, Drigg and Carleton Parish Council) through the planning
process;
other bodies representative of stakeholders who might be impacted upon by
any decisions made by LLWR (e.g. the West Cumbria Sites Stakeholder Group,
including the LLW sub-committee); and
other regulators with a statutory interest in LLWR operations, including the
Nuclear Installations Inspectorate of the Health and Safety Executive.
Best-practice guidance resources for decision–support frameworks such as Best
Practicable Environmental Option (BPEO) and Best Available Techniques (BAT) (e.g.
EA and SEPA, 2004) further underline the principle that engagement is essential to
ensuring openness and transparency in strategic decision making processes associated
with waste management.
Direct Benefits to LLWR
In addition to addressing regulatory requirements and wider stakeholder interests, a
comprehensive optimisation process is also of strategic importance to the future
management of the LLWR. It provides information on the key benefits and trade-offs
associated with different design philosophies and implementation approaches,
supported by a clearly structured analysis of underpinning data and rationale. This is a
very useful tool in developing plans and programmes for facility development and
closure.
18
2.2 Optimisation Constraints and Baseline Design
Constraints
Key factors relevant to framing the current study include:
The focus of the ESC, and therefore on options for the engineering design, is on the
role of LLWR as a disposal facility. Justification of that role rests with the NDA’s
national strategy for the management of solid low-level waste (NDA, 2010), and is
therefore outside the scope of the study. However, that aim is that an authorisation
will be sought to emplace waste at LLWR with no intent to retrieve it at a later
time. Retrieval may be possible, but the objective for future management of LLWR
is founded on making a safety case for a disposal facility that will be capable of
final closure.
Options for the future design and operation of the LLWR are restricted to ‘vault’-
type waste emplacement. There is no practical scope for developing either a mined
cavern or silo-type disposal operation at the LLWR site.
Notwithstanding recent controversy regarding the work of the IPCC as well as the
uncertainties inherent in projections of climate change, current understanding of
global sea level rise and local coastal erosion (which underpins the ESC) is that the
LLWR is under threat of eventual disruption by erosion or inundation. This is
expected to occur within a period of a few hundred to a few thousand years. As
such, it sets a natural limit to the timescale over which it is appropriate to consider
the role of engineering controls on facility performance.
The baseline for engineering design is the assumed development of the LLWR
within the ‘reference disposal area’, extending vault disposals southwards until
they reach the line of the trenches (i.e. to Vault 14). The possible implications of
variant cases designed to have a larger physical capacity are to be explored in the
2011 ESC. Thus it is relevant to consider the extent to which specific engineering
options may offer (or constrain) flexibility in future use of the site. It is also relevant
to note that, because of the natural topography, there is very likely to be a
necessary fall in the base level of future vaults as they are developed towards the
southern part of the site.
Baseline Design
The presence of existing and in-construction disposal facilities at the LLWR provides
important context for consideration of the optimised design for future engineering
19
components. To summarise briefly, the baseline design at the start of the study was
based upon that submitted in support of the storage planning application for Vault 9
(Williams and Proctor, 2007). This followed the previous disposal design philosophy
represented in the 2002 PCSC (BNFL, 2002a, b). Both designs included the following
engineering components:
a concrete reinforced vault base, with suitable stepping / gradients to support
active leachate management, and with a waterproof liner(s)
a low-permeability engineered final cap;
passive leachate discharge control (vertical drains); and
a complete encircling, deep cut-off wall.
Although the Vault 8 and Vault 9 designs involved similar engineering components,
there were substantial differences in philosophy. For example, the 2002 PCSC (Vault 8)
design has a lower specification (single bentonite-enhanced soil (BES) layer) base than
for Vault 9. Also, as the design philosophy was focused on avoiding ‘bath-tubbing’ (i.e.
complete saturation of the waste column within the vaults) by providing vertical
drainage post-closure, it was envisaged that a large single vertical drain to the south of
the facility would be installed once disposal operations were complete, to direct any
infiltrating leachate to deeper systems. The Vault 9 design meanwhile included a
double composite membrane on BES layer base and walls to maximise water retention,
as part of a safety strategy intended to contain infiltrating water for as long as possible.
In this concept, should the vaults eventually (as expected) become fully saturated,
overtopping leachate would then be directed to a series of vertical borehole drains to
be implemented between the line of Vault 9 (and any future vaults) and the trenches.
It is important to note that the context for the development of the existing baseline
Vault 9 design was very different from that for the current study.
It was developed in lieu of a disposal authorisation, and thus was focused on
‘storage’ rather than disposal. Hence one of the key objectives that emerged from
the design process was that it should maximise leachate retention during what
could be an extended operational (storage) period.
The main stakeholder associated with the Vault 9 design process was the planning
authority (Cumbria County Council). Other stakeholders, such as the EA, the NII
and other local bodies were consulted, but the focus was on gaining planning
permission for a storage facility.
20
One of the main benefits of the chosen design, however, was its flexibility. In the
options assessment and associated engagement process (Williams and Proctor,
2007), it was recognised that LLWR would in future present a second planning
application for Vault 9, to convert its purpose from storage to disposal, provided
that the EA granted a disposal authorisation following its review of the 2011 ESC. It
was therefore acknowledged that the design could be adapted, were alternative
‘non bath-tubbing’ approaches to passive leachate discharge control identified as
the preferred long-term strategy.
The present Vault 9 baseline design (Williams and Proctor, 2007) therefore provides a
useful baseline for the purposes of optimisation study presented in this document.
However the difference in objectives from that and the present study requires that the
components needed and associated designs should be considered afresh.
2.3 Key Issues for Options Identification and
Assessment
disposals to LLWR. However, optimisation considerations are not simply a matter of
achieving compliance with regulatory expectations. They are also an integral element
of the LLWR’s operational and closure planning process, with direct influences on
Lifetime Plan development and future use of the disposal facility. Such factors are also
inevitably of interest to the regulation of spatial planning and operational safety for the
site. For the purposes of this study:
The main emphasis in comparing engineering options is whether there is a clear
preference from the perspective of establishing confidence in the environmental safety
performance of LLWR. It is then necessary to determine whether that preference may
be materially affected by wider considerations.
In practice, this means that the choice between options that are strongly differentiated
in terms of confidence in environmental safety performance is likely to be challenged
only where there is an equally strong (and contrary) differentiation in terms of other
important factors. Alternatively, options that cannot readily be discriminated in terms
of providing confidence in environmental safety may be differentiated on the basis of
other considerations.
This means that alternative engineering design options need to be identified that take
into account the types of passive control that are relevant to the main threats to
isolation and containment of the wastes. These threats are those associated with:
21
Natural disruption of the facility – in particular as a result of coastal erosion
and/or inundation.
Disruption of the facility by future human actions – in particular those actions that
have the potential to cause disturbance of the wastes, as well as direct or indirect
exposures to contaminants within the wastes.
elements or as radio-labelled gases produced within the wastes.
Generation and release of contaminated leachate by water entering the facility and
contacting the wastes.
A detailed discussion of these threats and their implications is set out in Section 4.2.
Other factors that could be relevant in discriminating between options relate to, for
example, any wider environmental impacts (e.g. discharges), health and safety risks
(e.g. materials transport), engineering practicability (e.g. constructability, flexibility),
and lifetime cost and affordability (see Section 3.3).
2.4 Level of Detail
The output from the optimisation process needs to be defined to a level of detail
sufficient to establish a coherent design strategy, and to underpin subsequent ESC
studies such as safety assessment calculations. The emphasis here is at higher levels in
the hierarchy of decision making. Subsequent to delivery of the ESC, further iterations
of detailed design optimisation will be required following completion of the present
process. For example, the present process sought to identify the need, location, vertical
and lateral extent of any proposed cut-off walls, including consideration of the
principles underpinning the material choices and likely construction methods.
However, detailed implementation decisions and exact specification of wall
thicknesses and depths at different points would then be a matter for further work
through the ESC and vault construction projects.
2.5 Summary of Objectives
The main objectives of the pre- and post-closure engineering optimisation process can
be summarised as follows.
Identification of a preferred future engineering design strategy for the LLWR,
including overall objectives and preferred design concepts for each component of
the engineering.
22
Consideration of pre- and post-closure environmental impacts that might be
associated with different engineering options, mapped against known threats to the
isolation and containment of wastes within the LLWR. Appropriate consideration
also needs to be given to a range of other factors relevant to practicability, in order
to demonstrate the design is consistent with the overall goal of ensuring that
radiological risks to the public associated with the facility are ALARA.
Integration of the technical assessment with an appropriate engagement strategy in
order to seek and consider stakeholder views.
Ensuring that all decisions are robust and underpinned by an appropriate evidence
base.
23
3 Approach
3.1 Philosophy
The development and implementation of an options assessment process for the
optimisation study was framed by the goal of consistency with the principles of BPEO
and BAT. This implied a hierarchical, evidence-based study involving stakeholder
engagement. The approach was designed to address the specific requirements of the
study, in particular recognising that many of the decisions to be made are coupled. For
example, the approach to passive leachate discharge control depends strongly upon the
design and likely future performance of the cap.
The following discussion summarises the approach that was followed. The focus is on
describing the mechanics of the process, enabling the outcomes to be combined and
summarised in a narrative style in Sections 4 and 5.
3.2 Process
The approach followed is illustrated in Figure 1. It involved the various stages set out
below.
Scoping
As a key preparatory element of the study, the scoping phase was designed to:
Identify the main aims, regulatory requirements, assumptions and other drivers
and framing arguments that are relevant to the optimisation process.
Develop and refine a proposed approach to options identification and assessment.
Identify the main safety functions required of the repository engineering strategy,
mapped against the main threats to long-term isolation and containment of hazards
associated with the wastes.
Plan options development and evidence collation activities to underpin the main
appraisal stages.
Undertake an initial identification of potential options in order to identify and
explore potential differentiating factors.
Engage with stakeholders to present and gain consensus on the identified
objectives and drivers, as well as on the overall approach to be followed.
24
Figure 1: Overview of Pre- and Post-closure Engineering Optimisation Process
Scoping
Identification of Design Strategy and
Individual Component Options
Identify Strategy Options
Identify Individual Component Options
Evidence Collation:
Review of Previous Design Iterations
Hydrogeological Modelling
Assessment of Post-closure Impacts for
Different Options
Development of Outline Designs
Further Screening of Component Options
by Strategy (if possible)
Main Component Options Assessment
Integration
Report Outcomes (This Document)
It was understood from the outset that much of the analysis undertaken would involve
discussion of detailed technical considerations. It was therefore recognised that the
process stages that would benefit most from stakeholder input would be this (scoping)
phase and subsequent assessment outcomes presentation and discussion stages. This
assumption was also presented to and tested with stakeholders.
This phase therefore involved a significant amount of preparatory analysis that
culminated in a ‘scoping workshop’ held on 17th December 2009. The agenda,
attendance list and a copy of the briefing paper provided for the workshop are
presented in Appendix A. Participants included representatives from the EA, NII,
CCC, NDA, the ESC Peer Review team, consulting organisations supporting the
LLWR, and LLWR, including ESC and other project teams.
Participants were invited to review the proposed process approach and the principles
underpinning it through a combination of presentation and feedback, and interactive
sessions. Discussions were focused on two main sessions. The first addressed the main
safety functions required of engineering components. The second considered in more
25
detail potential approaches to the assessment of individual components of the
engineering design and the issues relevant to identifying and assessing options.
Outcomes from these sessions helped to shape a common understanding of constraints
on the optimisation process, as well as capturing the key issues to be taken into
account, as summarised in Sections 2.2 and 2.3.
Representatives of Copeland Borough Council, Drigg and Carleton Parish Council and
the WCSSG LLW sub-group were consulted at specific separate meetings.
In general, feedback was positive. In particular, the principles of the proposed
approach were supported, as was the approach to engagement, although the need to
recognise the role and interests of CCC as (in effect) a key regulator for the site was
particularly emphasised. Where suggestions for improvements to the process were
made, these were noted and were integrated into the approaches for relevant process
steps. A number of other issues were raised; where relevant these have been addressed
in the detailed options assessment and integration processes reported in Section 4.
Other feedback that was more broadly relevant to higher-level strategic questions was
passed on to the LLWR ESC and other project teams for appropriate consideration.
Identification of Design Strategy and Individual Component Options
This phase involved considering the principal goals of the pre- and post-closure
engineering for the LLWR in order to identify candidate design strategies associated
with optimising safety performance. The preliminary steps of this analysis overlapped
with the discussions presented during the scoping phase; the main principles of the
analyses conducted were therefore explicitly discussed at the scoping workshop. The
aims were as follows:
To further develop an understanding of the safety functions required of the
engineering design strategy as a whole, and of individual components within that
design strategy.
To identify the ways in which barriers might be combined in order to satisfy those
required safety functions, and thereby to generate a set of design strategy options
based on a comprehensive set of individual component design options.
To conduct an initial assessment of the design strategy options, in order to see if
unsuitable strategies can be screened out or a preferred strategy identified using a
‘top-down’ approach, thereby reducing the number of individual component
options that need to be considered.
In the detailed analysis undertaken for this phase of the process, it proved not to be
possible to identify a clear preferred design strategy option directly from a top-down
perspective. This is because the alternative strategies identified could not be clearly
26
differentiated on the basis of post-closure performance without detailed analysis of the
design options for individual components (i.e. a decision could not be made without
the more detailed analyses planned for the next phase of work). The rationale for this,
including a description of the options identified, is incorporated into the discussion on
process outcomes presented in Section 4.4.3.
The outcomes of this phase of the process were the systematic identification of a range
of strategic and detailed component options that needed to be considered in the main
assessment process, the identification and development of a range of analyses to
support the evidence base to underpin subsequent judgements, and the development
and refinement of the process for the main stage of assessment (see also Appendix F).
Main Component Options Assessment
The previous phases of work provided a framework for the main options assessment
phase. This included:
A substantial amount of work to further develop understanding of the options for
different engineering components, and to develop the evidence base required to
assess their performance against the criteria of interest.
Development of a hierarchical options assessment process that recognised the
coupling required between decisions for different component options.
Application of this process via a two-day expert workshop to assess the options
and derive a proposed strategy.
The aim of the workshop was to undertake a structured assessment of the identified
component options, informed by the expertise of participants and the evidence sources
that had been collated in advance of the meeting, to derive the preferred overall preand post-closure engineering design strategy. The workshop was held on the 24th and
25th of February, 2010, at the Sellafield Centre and subsequently Greengarth, West
Cumbria. In line with the approach agreed during the scoping phase, expert
participants were drawn from LLWR staff and supporting organisations.
An agenda and participant list for the workshop is provided in Appendix B. The
briefing material provided in support of the workshop, and indeed utilised throughout
the later stages of the process, is summarised in Section 3.3. The present report, in
particular Section 4, provides the main record of the outcomes of the workshop,
although further details were added during the subsequent integration phase.
27
Integration
The purpose of the integration phase was to draw together the outcomes of the
assessment workshop into a formal proposal for the engineering design strategy. An
important part of this was engagement on the outcomes with stakeholders, as well as
ensuring buy-in from LLWR senior management.
Following internal LLWR meetings to agree the proposed strategy with LLWR senior
management, a stakeholder event was held at the Samuel Lindow Building, Westlakes
Science and Technology Park on Tuesday 29th April 2010. An agenda and participant
list for the meeting is provided in Appendix C.
In broad terms, participants were generally supportive of the strategy outlined, and
therefore the decision was taken to ‘freeze’ the design following the event, as a new
baseline to underpin the 2011 ESC. However, the need to demonstrate, as part of any
future planning application, the need for engineered components such as the cap and
for those features to be implemented to particular timescales, was specifically noted,
and a number of points were raised that require ongoing liaison with regulators.
Other Stakeholder Engagement Activities
The stakeholder engagement activities referred to above represent the major events at
which stakeholders were consulted. However, stakeholder engagement was carried
out continuously throughout the optimisation process. Engagement activities also
included:
Presentations to the WCSSG LLW sub-group.
Presentation to the Drigg and Carleton Parish Council.
Meetings with Copeland Borough Council.
Utilisation of regular liaison meetings with the EA, NII and CCC for briefing and
updating on progress.
3.3 Evidence
A fundamental component of the approach to the pre- and post-closure engineering
optimisation study was that all decisions should be evidence-based, and that the
evidence should cover (so far as practicable) a suitably comprehensive range of factors
relevant to differentiation between options. This section summarises the evidence
collation and production activities undertaken in support of the optimisation process.
28
The use of relevant evidence sources, including expert judgements on their
interpretation, to underpin assessments of engineering component options is
documented in Section 4.
The process outcomes resulted from the structured analysis undertaken by the experts
at the main assessment workshop and the final project integration meetings, leading to
consensus judgements that shape the preferred strategy. The workshop analysis
needed to be informed by a suitable range of more detailed analyses exploring the
relevant factors. The following subsections describe the evidence collation process
utilised to generate an appropriate evidence set.
Options Proformas
A series of supporting ‘options proformas’ was created to record the collated evidence
relating to components of the facility design. This took place in two stages.
First, a set of ‘strategic approach’ proformas was created. The aim behind creation of
these documents was to explore, at a high level, the philosophies behind alternative
engineering strategies, and to identify whether there was any clear rationale for
differentiating strategic options. For example, options analysed included those
associated with keeping the vault wastes unsaturated for as long as possible, or those
associated with allowing waste saturation (‘bath-tubbing’) within the system.
A structured analysis of the advantages and disadvantages of each strategic option was
presented, matched against a range of assessment criteria covering factors relevant to
determining practicability, under the following broad criteria groups:.
Safety (worker safety, public safety).
Impact on the environment (habitats/construction, authorised discharges,
operational environmental impact, post-closure Impacts, non-LLW waste volumes,
resource use).
Technical factors (ease of implementation, confidence in performance, timescales
for implementation, impact on operations, flexibility, impacts on existing
engineered features, capacity).
Community and socio-economic factors (impacts on local community, support for
local community).
Cost (total implementation cost, affordability).
The analyses presented were underpinned by a range of other resources, including:
29
the outcomes of detailed hydrogeological modelling studies;
detailed engineering drawings and associated practicability studies; and
discussion papers outlining the qualitative and quantitative arguments for and
against the likely environmental safety performance of different strategic options.
The latter set of papers included an analysis of whether, considering ‘realistic’ rather
than ‘conservative’ assumptions, it is possible to predict whether an unsaturated or a
saturated bath-tubbing design strategy would be likely to lead to lower environmental
impacts than the other (etc). They are not included here as the outcomes of the analysis
are summarised in the collated proformas.
As the structured evidence gathering and analysis process continued, it became
evident that no sufficiently convincing arguments were emerging at the strategic level
to indicate a direct preference for any one of the identified strategic options. The
options analysis therefore moved on to consider directly each of the key components of
the facility engineering design, to identify, compare and contrast different options for
implementation. In the absence of a driver to focus on a particular strategic approach,
the proformas considered all identified credible implementation alternatives.
The initial strategic approach proformas were therefore entirely superseded by a set of
‘component option’ proformas. Together these covered the following components of
the facility:
final cap;
future vaults base slab and walls;
cut-off wall;
passive post-closure leachate management approaches; and
other engineered features.
Section 4 describes how the options for individual components outlined in these
proformas were considered in a structured, hierarchical manner at the assessment
workshop to derive the preferred strategy on the basis of more detailed considerations.
As with the original ‘strategic approach’ proformas, a range of different potential
implementation options were presented for each component. Each option was then
compared against the same criteria as had been identified for the strategic analysis, in
order to identify any significant differentiating factors. The outcomes of supporting
studies (i.e. the evidence sources identified above) were utilised to explore potential
30
differentiating factors through qualitative, and in some cases high-level quantitative
analysis. In this respect it is relevant to note that the proformas and associated analyses
were intended to provide a starting point for discussions, rather than offering a fully
comprehensive analysis.
The completed proformas, summarising the initial analysis of component options
against criteria, are reproduced in Appendix D. A separate document was produced to
explore options in terms of their implications for post-closure radiological impacts,
taking into account potential releases via the groundwater, human intrusion, coastal
erosion and gaseous pathways. The aim was to support optimisation considerations
by allowing more opportunity to assess implications of different options in more detail
from a ‘realistic’ perspective, rather than the more cautious approach typically adopted
in performance assessment studies. The outcome of this provisional analysis (prior to
the main options assessment workshop) is reproduced in Appendix E.
Component Option Identification
In order to ensure that all relevant options were considered for each component of the
engineering design, a set of structured ‘top down’ and ‘bottom up’ studies were
undertaken to identify how key safety functions might be provided and thereby
mitigate threats to the isolation and containment of hazards within the facility. This
process was used to audit and update the component option list. Appendix F
reproduces a set of ‘mind map’ diagrams illustrating how both top-down and bottomup approaches were utilised to identify a full range of engineering component options
and associated strategies, to ensure that the analysis could be demonstrated to be
comprehensive.
Hydrogeological Modelling Outputs
In addition to providing evidence used in compiling the options proformas (discussed
above), a key direct input to expert judgements made throughout the optimisation
process was provided by the outcomes of detailed hydrogeological modelling studies1
(Hartley et al., 2009; 2011). These studies were designed to explore key considerations
relevant to environmental performance for alternative design options. The
understanding generated by analysis of the model results provided important inputs to
the deliberations undertaken in the optimisation study. For example, the
1
The analysis of engineering options described in this report was informed by preliminary
outputs from these modelling studies. Although some modelling details changed prior to
publication, the broad conclusions on which decisions were made remain the same.
31
hydrogeological model results were central to decisions regarding the required depth
of the cut-off wall (see Section 4.4.4).
Engineering Design Drawings and Related Considerations
A significant amount of engineering design and analysis was also undertaken in
support of the study. The analysis was used to provide evidence compiled within the
options proformas, articulating the principles underpinning the existing (Vault 9)
baseline design, and was also used directly during the options assessment workshop.
A large number of engineering drawings were utilised throughout the process; not all
of these are provided here, in the interests of brevity. As an illustration of the sorts of
information provided, however, Appendix G reproduces the contents of a presentation
on the baseline design and alternatives for the cap delivered at the scoping workshop.
Section 5 provides engineering drawings describing the outcomes of the whole process.
Evidence from Previous Design Development Iterations
As described in Section 2.2, the vault design that represented the baseline prior to this
study was derived through a process focused on long-term storage, rather than
disposal. Nevertheless, the detailed analyses, drawings, identified options etc that were
derived as part of that process provide a very helpful set of supporting references for
the present study. Similarly, the outcomes of assessments based on previous design
iterations were also reviewed, to ensure that all potentially relevant evidence sources
were utilised, and to provide an audit tool to help ensure that the analysis of
component options was sufficiently comprehensive. The proformas collated in
Appendix D illustrate how those various evidence sources were reviewed against the
updated context of the current study to support assessments of option performance.
A brief summary, in reverse chronological order, of some of the more important
studies of relevance is provided in Table 1.
32
Table 1: Summary of Previous Design Development Studies Undertaken Since 2000
Study
Reference(s)
Notes
2008 Performance Update for
the LLWR (Schedule 9,
Requirement 2)
Baker (2008), Baker et al.
(2008) and underpinning
references
Provided an update of the expected long-term environmental performance of the LLWR, incorporating
assumptions that the future vaults will all be constructed and closed according to the present Modular
Vaults Single Design (see below). Described an assessment of options for the reduction of future impacts
from the LLWR through a ‘toolkit’ approach, including the approach to stakeholder engagement. The
EA’s response highlighted a number of ongoing concerns that are discussed elsewhere in this report.
Outline Construction
Method Statement: Modular
Vaults – Cut-Off Wall,
Capping & Vertical Drains
Hillary (2008)
Considered options for the construction of various elements of the Vaults Single Option design. Note that
the ‘cut-off wall’ design was not considered to the same level of detail, as it is a ‘closure’ feature
specifically designed for a ‘disposal’ concept.
Vault Single Option selection
process
Williams and Proctor
(2007), Newham et al.
(2007), and underpinning
references e.g. Belton
(2007), Barclay (2007),
Gallagher (2007).
Provides a detailed analysis of the rationale for selection of the preferred Modular Vault design, which is
presently being implemented for Vault 9 and also represents the present baseline design for any future
vaults. It outlines a number of strategic options, their advantages and disadvantages. Long-term
environmental impacts were considered through qualitative judgements comparing different designs with
the outputs of the 2002 Post-closure Safety Case (PCSC). A range of information on cost and wider socioeconomic impacts was also developed to aid this process. The present option was chosen as it ‘better
satisfies the design principles, as defined in the applicable UK regulations and Best International Practice (i.e.
containment) and will therefore be more acceptable to the regulators and other stakeholders. It was also the clear
preference of the stakeholder elected representatives.’
Summary of optimisation
studies
Fowler et al. (2005)
Reviewed recent options studies for the vaults design and other facility risk management options in the
context of their implications for post-closure safety. Qualitative and semi-quantitative arguments against a
range of criteria including socio-economic factors suggested that approaches involving a good cap but a
porous base and/or enhanced use of vertical drains to avoid vault bath-tubbing may lead to reduced
environmental impacts.
2004 Modular Vaults design
option selection process
Kane et al. (2008), Sola
(2003).
Describes the previous (2004) iteration of options development and assessment for the vaults, undertaken
prior to the Agency’s authorisation decision based upon the 2002 Safety Cases. The preferred option was
broadly similar in concept to that taken forward by the 2007 Single Option studies.
33
Study
Reference(s)
Notes
Preliminary assessment of
LLWR management options
Edwards and Alexander
(2004)
Described the outcomes of two workshops considering options by which future predicted impacts from
the LLWR might be reduced, based upon consideration of the 2002 PCSC against an appropriate range of
technical and non-technical criteria. It was suggested that the 2002 PCSC design represents an ‘appropriate
solution for the optimum management of the long-term risks and uncertainties associated with the (LLWR) site.’
2002 PCSC and
underpinning studies
BNFL (2002a), BNFL
(2002b) and
underpinning
references
The 2002 PCSC presented a baseline design including multiple redundancy in drainage routes to minimise
the risk of the vaults bath-tubbing, and provided a detailed assessment of the potential performance of
that design. The design had been subject to some development since submission of the previous ‘status
report’ (BNFL, 2000).
34
4 Outcomes
4.1 Overview
This section presents the overall outcomes of the engineering optimisation process. It is
based on the conclusions of the main options assessment workshop, with some minor
additional modifications introduced through the subsequent integration processes. The
outcomes are presented as a narrative, starting with an overview of the key functions
identified as being required of the facility pre- and post-closure engineering, and
leading into a hierarchical discussion of the outcomes of the options assessment for
individual components against those functions. A summary of the combined optimised
strategy determined as a result of the process is provided in Section 5.
4.2 Engineering Controls on Performance
Given the priority attached to optimisation of environmental safety performance (albeit
proportionate to other factors), a logical approach to the identification of options is to
structure the analysis in terms of the types of engineering control that might be
established in respect of the identified main threats to isolation and containment of the
wastes. As noted in Section 2.3, the emphasis in engineering design (particularly in
relation to the long term) is on passive controls, although the overall management
strategy through time, including the scheduling of closure engineering with respect to
ongoing active leachate management, needs to be based on an appropriate
combination of active and passive measures.
The primary threats are those associated with:
Natural disruption of the facility, in particular as a result of coastal erosion and/or
inundation. In this respect it is recognised that:
o
Engineering features, such as perimeter drains around the final cap, can be built
into the design to minimise the likelihood of localised flooding as a result of
storm events. However, it is much more difficult to protect against threats from
encroaching coastline and rising sea levels.
o
Shoreline and coastal defences to protect against threats from erosion and/or
flooding would need to be maintained over a substantial period in order to be
effective in risk mitigation. It would be difficult to justify claims for the
effectiveness of such defences as part of site control plans for a safety case
extending several thousands of years into the future. Nevertheless, there are
35
likely to be wider plans for coastal management in Cumbria that will impact on
coastline evolution in the vicinity of LLWR.
o
Some form of resistant material could potentially be embedded in land
surrounding the LLWR. However, because the primary coastal erosion process
is likely to be one of under-cutting at the base of the sea cliff (potentially below
the base level of the vaults), such measures would have to be very deeply
embedded to be worth considering at all. Likewise, because of the way in
which disruption is expected to occur, grouting waste in situ or otherwise
achieving a monolithic vault design are not expected to offer significant
protection or impediment to the erosion process.
o
Taken together with the comparatively slow rate of decline in the residual
radiological hazard from long-lived radionuclides at the time coastal erosion
becomes a threat, it would be very difficult to establish confidence in the
effectiveness of realistically achievable physical barriers in providing significant
mitigation of risk through delaying disruption.
o
In practice, once erosion of the facility has started, potential radiological
impacts in the vicinity are likely to be smaller if such erosion takes place more
rapidly (thereby ensuring faster dispersion of any residual contamination),
rather than being prolonged by a more resistant design. Even so, it might be
argued that, almost irrespective of projections of potential impact, disruption of
LLWR is unlikely to be a major priority compared with other hazardous sites
and facilities vulnerable to the effects of sea level rise around the UK.
Nevertheless, the GRA indicates that, were the likely levels of exposure from a
reasonably foreseeable natural process (albeit on timescales greater than those
normally attributable to authorised control over the facility) to be significant by
comparison with the regulatory risk guidance level, it is questionable whether a
‘period beyond authorisation’ could be contemplated.
o
Given the difficulty of identifying engineering measures that can provide
effective passive control against threats from coastal erosion and inundation,
arguments in the ESC ultimately centre on the overall acceptability of disposal
itself, given the potential radiological implications of disruption. Inventory
control (should it be necessary) is then likely to be the most practicable way of
providing assurance that unacceptable consequences do not occur.
o
Hence, as noted previously, disruption of the facility by coastal erosion and/or
inundation effectively sets a limit to the timescale over which it is appropriate
to consider the optimisation of engineering controls.
36
Disruption of the facility by future human actions; in particular those actions that
have the potential to cause disturbance of, and direct or indirect exposures to,
contaminants within the wastes. Radiological impacts associated with disturbance
of the wastes by human actions will largely depend on the inventory of the wastes
and how they are emplaced in the repository, so the primary consideration in
engineering design is the potential for reducing the likelihood of exposure.
o
Measures to reduce the likelihood of disturbance by future human actions
include the use of barriers and other controls to prevent access to the site,
preservation of information relating to its presence and content in order to
support planning controls over use of the site, and the use of signage and
markers. In general, these are all part of the site management arrangements,
rather than being passive engineering controls. Nevertheless, they are
potentially significant in terms of minimising the possibility of disruption to the
facility over timescales relevant to some of the shorter-lived radionuclides
present in the waste.
o
The primary engineering control to reduce the likelihood of disruption is the
depth of waste beneath the ground surface. This is essentially related to the
thickness of the final cap over the facility, coupled with the depth below surface
at which disposal takes place. Markers, or marker layers, built into the cap
design may also be considered to provide a passive function in alerting
inadvertent intruders to the hazards within the facility. The overall aim of such
a barrier is to minimise the likelihood that disturbance by reasonably
foreseeable actions might give rise to radiologically significant impacts.
o
There are both potential advantages and disadvantages associated with the use
of markers. On the one hand they potentially provide an enduring, low
maintenance mechanism for maintaining local information regarding the site.
However, they also present potential scope of misinterpretation; if poorly
conceived, and assuming that other records are somehow lost, they run the risk
of generating interest in the site (e.g. for archaeological investigation), rather
than deterring disturbance. In any case, it can be anticipated that the cap will
itself readily be recognised as an unnatural feature in the landscape.
o
Features of the cap design also play a role in limiting the possibility of intrusion
caused by burrowing animals and deep-rooted plants.
elements (principally radon) or as radio-labelled gases produced within the wastes.
37
o
During operations, gas release from the trenches takes place via controlled
venting in the temporary cap. Potential exposures are limited by restricting
public access to the site; the areas in the vicinity of the vents are not routinely
occupied by members of the site workforce. A vent within the final cap would
enable controlled release and monitoring of gases to continue while public
access is denied until the cessation of security controls over the site.
o
The primary engineering controls on gaseous release pathways in the long term
are either to create a long diffusion path length for shorter-lived radioactive
gases (particularly Rn-222) or, so far as practicable, to absorb and/or disperse
releases of longer-lived gaseous radionuclides (specifically C-14). Both these
functions relate primarily to the design and properties of the repository cap. For
Rn-222, emplacement of Ra-226 bearing wastes at greater depth in the vault
waste stack can contribute to ensuring that radioactive decay occurs before the
radon is able to diffuse in significant concentrations to the underside of the cap.
o
In so far as gas generation and release from the waste may be promoted by
reactions with infiltrating water, engineering measures that are taken to reduce
water flows through the wastes (see below) will also play a role in mitigating
the threats associated with gas release. Even so, there may already be sufficient
moisture within the wasteform to promote the generation of C-14 labelled
gases. Other (non-engineering) measures to control C-14 gas generation within
and release from the waste include nature of the wasteform (e.g. treatment of
metals and organics), control of chemical conditions within the vaults (e.g. to
promote the chemical fixation carbon).
Generation and release of contaminated leachate.
o
Engineering controls have the potential to protect the wastes from disturbance
by infiltrating water (i.e. minimising volumes of water entering and passing
through the facility), to control the release of radionuclides from the facility (by
ensuring that contaminated leachate production is minimised and/or contained
as far as practicable), and to mitigate the consequences of residual leachate
release (by managing the pathways via which leachate is discharge to the wider
environment).
o
During operations, leachate from both the vaults and trenches is collected in
sumps and drainage lines, and controlled discharges made via holding tanks
and the sea line. These controls will cease at some stage, at or following the
completion of waste emplacement, as the disposal system moves from active to
passive control on leachate release. The interim cap currently provides an
engineering control on the volume of leachate produced within the trenches; as
38
the facility is extended and further vaults are added, additional engineering
measures to control leachate production are required to ensure that the capacity
of the leachate management system is not exceeded.
o
The function fulfilled by engineered features in controlling the volume of water
entering and passing through the wastes (both trenches and vaults) is
essentially one of acting as a barrier to water entry. The most important
component in this respect is the cap. However, there is also a need to ensure
that significant volumes of water do not enter the vaults and trenches as a result
of lateral inflows. In particular, measures may need to be taken (e.g. cut-off
walls and cap perimeter drains) to ensure that water shed by the cap does not
enter the facility around the periphery.
o
Even with an optimised approach to minimising water ingress, it remains
necessary to control any water that does enter the facility in order to ensure that
the possibility for leachate release is minimised. There are two basic strategies
for this, both of which rely on engineering design to ensure appropriate passive
control of leachate production. One is to encourage water that enters beneath
the cap to have minimum contact with the wastes, by providing preferential
flow pathways through the facility in order to minimise water saturation and
opportunity for contamination. The other (consistent with the Vault 9 baseline
design and standard approaches to landfill management) is to attempt to
capture the water within the vault and ensure that no leachate is released, for as
long as possible. The leachate management philosophy that applies to these
strategies is different; in both cases, however, the opportunity to optimise
engineering controls for passive leachate management is largely restricted to
the design of current and future vaults.
o
Finally, given that circumstances may arise (e.g. when barriers to inflow have
degraded) when it is no longer possible to avoid leachate release, it is possible
to conceive of engineering measures that seek to direct the flow of leachate to
those parts of the environment where the impact will be lowest. As a general
rule, this means avoiding discharges near to surface in the vicinity of the
facility, and instead seeking to direct leachate to depth where it can be diluted
in larger volumes of groundwater before eventual release the marine
environment. Potential passive controls on leachate release include vertical
drains and downstream cut-off walls.
39
4.3 Engineered Features for Assessment
Taking into account the analysis presented above, it is possible to identify the
following main features to be considered in the optimisation of pre- and post-closure
engineering.
The Final Engineered Cap for the facility, including consideration of:
o
its role as a barrier to the inflow of water into the trenches and vaults;
o
its role in minimising the likelihood of disturbance of the wastes by future
human actions and bio-intrusion;
o
its role in controlling gas release;
o
the timing of installation, including implications for the balance between active
and passive controls on leachate discharge;
o
its potential implications for waste stacking within the vaults, and other aspects
of waste emplacement and closure engineering.
Cut-off Walls and their potential role in:
o
minimising the lateral inflow of water to the trenches and vaults, including the
control of waters that are shed at the cap perimeter;
o
minimising the possibility of leachate release to the surrounding environment
via near-surface pathways, in the event of degraded cap performance;
o
determining implications for other engineered components.
Future Vault Walls and Bases, including consideration of:
o
their functional role in supporting the control of water that enters the facility;
o
the response of alternative design specifications under normal and ‘cap failure’
conditions;
o
the comparative implications of alternative water management strategies for
projected leachate release from the facility over the time frame relevant to
engineering optimisation (i.e. prior to disruption by natural processes);
o
the potential need for the base level of future vaults to fall as they are
developed towards the southern part of the site; and
40
o
implications for other engineered components.
Passive leachate discharge control, including consideration of:
o
the possible role of drains in providing passive control over the routing of
leachate released from the facility;
o
the role of such features under normal and ‘cap failure’ conditions;
o
implications for other engineered components.
Other engineered features, including the possible roles of:
o
a stepped gravity drainage system for operational collection of leachate within
the trenches;
o
alternative vault backfills and their potential implications in conjunction with
different leachate control strategies, including the possibility of creating vault
‘monoliths’ by backfilling with grout.
In each case, options can be examined through a process of logical analysis, based on a
structured examination of key questions relevant to design choice. As noted at the
outset, the emphasis in the first instance is on establishing whether there is a clear
preference between options from the perspective of providing confidence in the
environmental safety performance of LLWR. In addition, consideration is also required
as to whether such a preference may be materially affected by wider considerations.
Broadly speaking, there are contributions to optimisation from both a broad strategic
perspective (i.e. the role played by engineering controls) and a more detailed design
perspective (how those controls are implemented). Assessment of options is necessarily
iterative, reflecting the range of options available and the fact that more general choices
are informed by more detailed understanding. Informing and consulting with
stakeholders also provided relevant inputs to the design of the options assessment and
understanding of the outcomes.
4.4 Assessment of Component Options
4.4.1 Overview
Informed in part by initial surveys of options undertaken as part of early phases of
scoping and options identification work, the assessment approach agreed with
stakeholders at the scoping workshop and with experts at the main workshop was for
a hierarchical, logical analysis process. The concept for this analysis was to identify a
41
preferred option for each component, taking into account decisions made for, and
interactions with, other components. This is made possible by starting with what is
clearly the most critical of closure engineering components for passive post-closure
facility performance, due to its role in controlling infiltration – the final cap – and then
working through the other components in turn. Thus the decision for the preferred
option for the cut-off wall is framed by the role it plays in controlling water given the
preferred cap concept, and so on.
Section 3.3 describes how a range of proformas were developed in advance of the main
assessment workshop that explore all the potential options for each component that
could be considered to provide benefit against the functional requirements outlined in
Sections 4.2 and 4.3. The analyses provided the experts at the main workshop with an
indicative assessment of potential differentiators between options, considering postclosure environmental safety performance alongside a suitable range of other
attributes. The analysis undertaken at the main workshop used these proformas and
associated evidence sources – in particular, the outcomes of hydrogeological modelling
studies – to support a comparison of the main option alternatives. This approach was
pursued in preference to undertaking an exhaustive analysis of all the advantages and
disadvantages of every component option against every assessment attribute. This
allowed a narrative approach to be adopted that helped the identification of a clear and
coherent strategy, uncluttered by detailed, essentially extraneous considerations. This
approach was also essential to allow the experts at the workshop to reach a provisional
conclusion in less than two days.
To achieve this, a set of key factors relevant to the differentiation of options for each
component was distilled ahead of the workshop from the underpinning analyses.
These were then presented to workshop participants as a set of questions that required
to be answered. Of these questions, some were quite simple, and were simply intended
to provide part of the overall logical structure; others were more complex, demanding
detailed consideration of underlying understanding and evidence.
For each component analysis, described below, the following is therefore presented:
1. the list of questions identified for that component, as presented to workshop
participants;
2. a summary of the current baseline and possible broad strategies for that
component, again as presented to workshop participants; and
3. an overview of the key factors that were debated, and the associated rationale,
matched against key evidence sources drawn from that wider evidence base that is
described in Section 3.3.
42
The outcomes are summarised in an integrated statement of the combined optimised
strategy, presented in Section 5.
4.4.2 Final Engineered Cap
Key questions:
Are there any circumstances in which it might be considered closing the facility
without a cap?
What functional role is required of the cap from an environmental safety
perspective:
o
Prior to completion of waste emplacement operations?
o
Following completion of operations, and in the period after authorisation?
What constraints are there on cap design, given the importance of achieving these
functions?
Within such constraints, what viable alternatives exist to the current cap design
(consistent with achieving the defined objectives)?
o
What purposes would be fulfilled (e.g. flexibility to accommodate more waste,
easier staged installation)?
o
Can ‘spare’ space within the cap be safely exploited for waste disposal without
compromising other aspects of performance?
o
How do the alternatives compare, in terms of degree of confidence in achieving
the desired performance?
When and how should the final cap be installed?
o
What environmental safety drivers support ‘early’ installation of the cap (e.g.
reducing demand on operational leachate controls, opportunity to demonstrate
performance)?
o
What environmental safety drivers support later installation of the cap (e.g.
allowing for waste settlement, not requiring interfaces between stages of
installation, allowing for improvements in technology)?
o
What other drivers exist?
What options exist for the timing of cap installation, and what are their
implications?
o
Leave current interim trench cap in place, installing the final cap on completion
of all emplacement operations?
o
Early installation of final cap over trenches only?
o
Progressive development of final cap over vaults and trenches?
43
What are the potential failure modes for the cap and their implications for
performance?
Status prior to the assessment workshop (to guide discussion):
There is no current argument for a cap being absent from the final closure design, and
general consensus that it performs a central role in passive control over post-closure
isolation and containment. Its performance cannot be guaranteed indefinitely, but it is
expected to be capable of providing for unsaturated conditions within the greater part
of the trenches and vaults for a considerable period of time – potentially up to the time
at which natural erosion processes threaten the facility. There appears to be a strong
argument that the engineered cap design is already optimised, and that it alone will be
sufficient to perform most of the safety functions required for the post-closure
engineering so long as confidence can be placed in the elicited infiltration values and
underpinning reasoning. Very little water is expected to pass through the cap for
perhaps up to 1000 years, so the wastes will be unsaturated for a prolonged period of
time; moreover, the likelihood of cap failure is thought to be low. Changing, adding or
subtracting layers would probably not gain much – there is a detailed reason for each
layer in the current design.
At present a progressively implemented single-domed cap appears the present ‘front
runner’ in terms of overall design – the benefits of a double-domed cap are unclear,
particularly as it is believed that progressive implementation of a single-domed cap
should not present substantial difficulties. A single domed cap may also be less
vulnerable to erosion, and would allow operational flexibility for additional volumes
of LLW or VLLW disposal into the cap dome. A ‘Stage 1’ cap (consisting of the first few
layers) could be implemented early, in order to provide an interim hydraulic barrier,
with the remainder of the engineered features added later. This could help costprofiling as well as placing less demand on operational leachate management.
Key Points from the Assessment
It is clear that a substantial final engineered cap is required for the LLWR, as it
provides a number of important functions. The experts were clear that, for a variety
of reasons, not least principles from regulatory guidance, a dilute-and-disperse
type approach is not acceptable, and a cap is required that will minimise infiltration
into the facility for a significant period of time.
It was recognised that the various layers of the cap have already been subject to a
number of iterations of detailed design optimisation (e.g. Belton (2007), Williams
and Proctor (2007), Kane et al (2007), BNFL (2002b). A specific function has been
identified for each layer together with an appropriate approach identified to
44
providing that function. The approach implemented is consistent with established
best practice and experience from other sites, e.g. landfills. The result is a cap that,
the experts agreed, will provide a level of performance that is unlikely to be
significantly improved upon by further analysis. In particular, the baseline design
is expected to be highly impermeable for a considerable period of time.
An important further role of the cap is to help minimise the likelihood of
substantial disruption of the facility through any future human actions. An
engineered barrier thickness of 3m (coupled with at least 1m of inert profiling
material between the cap and the waste) has been proposed as sufficient to ensure
that commonplace intrusions (e.g. building foundations for a house) would not be
likely to lead to significant consequences from direct disturbance of the wastes. The
present cap design is consistent with this requirement.
It is less clear whether cap design can substantially help to mitigate the impacts of
C-14 release in gas. The incorporation of an enhanced soil and vegetation layer on
the cap surface might help to prevent the release of C-14 to atmosphere through
dissolution in cap run-off waters, but it is not clear whether this would in practice
give rise to a substantial reduction in impacts. Depending on drainage patterns, it
could even lead to higher doses. The experts did not identify a strong rationale for
a deviation from the current proposed approach, noting that if required detailed
optimisation studies for the surface layers could be undertaken prior to
implementation.
Participants agreed that there is no longer a strong rationale for a double-dome
(‘gull-wing’) design, as it was agreed that a single dome approach is fully
implementable, and could be achieved in a progressive fashion.
The visual impact of the different cap designs was noted. A single dome cap would
be slightly higher, but a double dome cap would have two distinguishable crests,
which would be likely to present a stronger visual impact, especially when the
difference in peak elevation is considered across the 400m width of the facility as a
whole. From the perspective of visual amenity, known to be of concern to local
stakeholders, an argument can therefore be made in favour of the single dome cap
despite its greater elevation. Participants recognised that CCC regard this as an
important matter of concern, and that mock-up models/illustrations will be needed
to help illustrate these arguments in suitable future forums involving CCC and the
stakeholders they represent.
It was noted that the space underneath the cap, given the proposed profile, could
be utilised to store additional LLW volumes if a higher stacking disposal approach
is implemented and authorised. Although this potential opportunity was noted,
45
participants agreed that this should not be considered as a driver in favour of a
particular design.
Overall, no significant advantages of the double dome were identified; indeed the
opposite was true, as it was considered that a double-domed cap may be more
prone to erosion along the obvious ‘weak point’ at the line of the gully between the
two domes. Thus a single dome option was selected.
There was significant discussion on the question of gas venting, considering
whether it is important to include gas vents in the design to manage bulk gas
generation during the operational phase, and whether they should be sealed at the
point of cessation of institutional control to reduce the likelihood of post-closure
impacts associated with radio-labelled gases. The absence of an engineered gas
vent (especially once controls on site access have been withdrawn) would have the
advantage of ensuring that any release of radio-labelled gases generated within the
facility would not be focused at a specific location near the highest point of the cap.
This is potentially more significant for Rn-222, where the dominant route for
potential exposure is release into enclosed spaces, than it is for C-14 (incorporation
via foodchains), since in the latter case exposures will be effectively ‘diluted’ over
the total area from which foodstuffs are assumed to be taken by the potential
exposure group.
Conversely, it was suggested that the presence of localised gas vents (covered by
gravel or some other substrate to deter vegetation growth) might even reduce
impacts associated with C-14, since the main exposure routes are associated with
exposure through ingestion of foodstuffs that might be grown/farmed on the cap
and venting C-14 direct to atmosphere could reduce concentrations in relevant
media. However, because of its relatively short half-life, the potential impacts from
Rn-222 are very sensitive to the time taken for the gas to travel through the cap. If
there is no gas venting, decay during the time taken for Rn-222 to diffuse through
the cap will lead to significantly reduced impacts.
Overall, no strong preference on the topic of gas vents was identified at the
assessment workshop. This was therefore considered further during the integration
phase. Based on qualitative judgements regarding the likely impacts involved and
other practicalities, it was agreed (subject to the final results from the 2011 ESC
assessment calculations) that gas vents should be included in the baseline design
for the final cap and that, for the purposes of the 2011 ESC, they will be assumed to
be sealed several decades after the cap has been completed, allowing for a period of
further monitoring prior to the end of active institutional control.
46
Estimates of cap performance for the single domed cap were discussed, informed
by assessments such as those undertaken as part of the engineering performance
elicitation process2 (Jackson et al., 2011). It was agreed that the multi-layer
approach planned for cap implementation would lead to a high level of confidence
that the cap will provide a high level of performance for a prolonged period. It is
possible that the cap could degrade faster than anticipated, and could be subject to
sudden failure if, for example, there is major unexpected differential settlement
within the facility. However, whilst it was agreed that these scenarios should be
considered in the optimisation process, they represent ‘what-ifs’ that are
comparatively unlikely to occur compared to as-designed cap performance.
The timing and approach to implementation of the cap was discussed in detail. In
general, a progressive rather than one-off approach to cap construction was
preferred. Deferring construction of the final cap to the time of cessation of
disposals would maximise flexibility through increasing opportunities to revisit
design and structure. However, it was judged that the principle of minimising
environmental impacts by reducing infiltration into the disposal facility as soon as
practicable (as well as affording additional physical protection to waste containers
within the vaults) is of greater importance. Staged capping also provides for a
longer period of performance monitoring prior to final closure and withdrawal of
active control over the site.
Further discussion on the subject of timing centred on whether the performance of
the interim cap could be guaranteed in coming years, in particular noting its 30
year design life, and whether there was the potential that the trench wastes might
degrade substantially over the next few years. It was suggested that, unless the
trench cap is replaced or made redundant before the end of its design life, releases
from the trenches may increase as performance degrades. This represents an
argument in favour of early final capping. However, a separate matter of concern
relates to the need to avoid differential settlement and the challenge that this might
cause to cap integrity. The vaults will also be subject to some settlement but, due to
the engineered nature of disposals to those facilities, this will occur over timescales
that are sufficiently long not to influence this particular judgement.
As a result of discussions at the workshop and further consideration during the
integration phase, it was agreed that implementation of the final cap over the
trenches should be scheduled to allow time to monitor waste levels and ensure that
2
The analysis of engineering options described in this report was informed by expert inputs
derived in the first round of elicitation activities for the 2011 ESC. Some details in judgments
regarding system properties changed prior to publication; however, the general
understanding supporting design optimisation decisions remains the same.
47
degradation is substantially complete, giving confidence that differential settlement
will not significantly affect cap performance. After this, progressive capping by
strips will be undertaken in order to reduce infiltration into each disposal facility
component as soon as practicable. The current baseline assumption for the ESC will
be that capping commences in 2021 unless challenged by monitoring results.
The rate of capping will be limited partially by timescales for completion of
disposals to different vaults, but also according to timescales for arising of the large
amount of profiling material that will be required.
Participants also discussed the opportunity to implement a ‘Stage 1’ cap –
essentially the profile material and the bottom few (hydraulic barrier) layers of the
cap. This would be sufficient to ensure an impermeable barrier during active site
management over the trenches while they ‘expressed’ any remaining settlement
(both from existing voidage and that generated by future degradation up until
completion of the cap). This remains an option should monitoring of the trenches
not reach a strong conclusion on the potential for future differential settlement.
4.4.3 Future Vault Walls and Base Options
Key questions:
To what extent do constraints associated with ‘constructability’ requirements
influence decisions regarding the depth of future vault bases?
Given such constraints, and the roles played by the cap/cut-off wall, what
functions (and corresponding specifications) are required of the vault walls and
bases?
o
To provide a stable platform for disposal operations?
o
To provide for water collection and drainage during disposal operations?
AND
o
To promote the fast draining of infiltrating water (and thereby minimise contact
time between water and the wastes?
OR
o
To retain infiltrating water and leachate within the vaults for as long as
possible?
Given the role of the cap and cut-off walls (if installed) in seeking to minimise
water contact between the water and the wastes after closure, is there a logically
preferred alternative for controlling water that does enter the vaults?
48
Given ‘best estimate’ understanding of the likely evolution of hydrogeochemical
conditions in the vaults, is it possible to identify significant differences between the
two water management strategies in terms of the likely timing and rate of
contaminant release from LLWR:
o
For the expected evolution of cap performance?
o
Under ‘cap failure’ scenarios?
Could either strategy be enhanced by changes to other aspects of design (e.g.
backfill material, spacing between waste stacks), and what would be the wider
implications of such changes?
To what extent do vault design considerations have implications for other design
features, such as:
o
Implications of falling base depth for cap design?
o
Implications of waste stacking height for passive water management within the
facility?
If confidence is placed in the long-term effectiveness of the cap, to the extent that
saturation is precluded for several hundreds of years whatever specification is adopted
for the vault walls and base, it is difficult to argue that either alternative presents a
clear advantage. Current PA models are simplified to the extent that they cannot draw
obvious distinctions – so the emphasis is as likely to be on speculative failure modes as
it is on central estimates of performance. Moreover, given the philosophy of moving
from ‘storage’ to ‘disposal’, there would appear to be a strong cost and practicability
argument for adopting a ‘Vault 8’ rather than ‘Vault 9’ standard design.
It was agreed that the most basic, fundamental requirement for the future vault
bases is that they should provide a stable loading platform for disposal operations,
and that they should be impermeable during operations, to facilitate operational
leachate management (pumping) approaches, and more broadly to demonstrate
containment during the operational phase.
The choice between vault base options therefore is fundamentally between a design
such as that used for Vault 9 (with a double composite liner system) that is
specified to minimise permeability for as long as possible, against a lower
specification design that might degrade faster (e.g. something equivalent to the
present Vault 8 base design, with a single BES composite liner). Given that
49
operational requirements are satisfied by either approach, post-closure impacts are
the key driver here.
At the workshop, there were detailed discussions on the likely impacts that might
arise from the two different types of base. These were informed by hydrogeological
modelling calculations (Hartley et al., 2009; 2011) and analyses of potential impacts
such as those articulated in Appendix E. The outcome of these discussions was as
follows.
o
The Vault 9 approach (which includes impermeable walls) is designed to
maximise containment of any infiltration and thus leachate for as long as
possible. In the Vault 9 design concept, should the vaults eventually become
fully saturated and overtop, then the leachate would be directed to vertical
passive leachate discharge control components to direct it away from the
surface to deeper systems. However, the hydrogeological modelling studies
showed that it is unclear, based on elicited hydraulic properties of the cap and
vault base, whether completely saturated conditions will be achieved or not.
This is because the base, while designed to be of very low permeability, will still
allow some water to pass through; moreover, the ‘final’ leakage rate (i.e. when
and if the vault becomes fully saturated) is expected to be higher than the initial
rate, even before any degradation occurs. As with the cap, performance of the
base is expected to degrade over time; however, the elicitation process revealed
a wide range of uncertainty in the expected leakage rate, partly linked to the
potential for weaknesses at vault base/wall joints. Hence there is a possibility
that the base leakage rate could exceed the infiltration rate at some stage, or for
a period of time, post closure.
o
The modelling did, however, show that with a lower specification base, it is
highly likely that vertical drainage will be sufficient to ensure unsaturated
conditions persist for a long period of time.
o
The experts at the workshop agreed that, intuitively, one might expect lower
releases of contaminants for an unsaturated (lower specification base) system
because the small amount of water within the system should lead to limited
leaching of contaminants from the wastes, since a smaller proportion of the
wastes will be in contact with water and release mechanisms will be slower.
Fundamentally, therefore, the concept of keeping the wastes as unsaturated as
possible appears consistent with the principle of maximising containment of
contaminants associated with the wastes. The alternative (‘Vault 9’) concept of
allowing the wastes to saturate followed by directing waters that overtop the
vaults to deeper systems is consistent with maximising containment of leachate
during the operational phase, but at face value would appear to allow an
50
enhanced level of contact of wastes with water, potentially leading to higher
fluxes of contaminants being released from the facility post-closure. Figure 2
illustrates these alternative concepts.
o
However, when the physics of the two different systems are analysed in more
detail, it becomes more difficult to differentiate performance on this basis (see
Appendix E). The following issues need to be considered:
-
For a saturated (bath-tubbing) system, while the wastes will be in constant
contact with water, ‘new’ water infiltrating through the cap is unlikely to
mix significantly with the bulk of the porewaters within the vaults, as there
is no driving mechanism that could lead to substantial mixing. Qualitative
and semi-quantitative estimates of the level of mixing of new water with
the body of water already in the vaults were made considering processes
such as diffusion, and also temperature- and pressure- driven circulation,
and it was considered that only the top metre or so of the vaults would be
subject to significant mixing. Thus, the water infiltrating from above would
only mix with a small proportion of the contaminated water in the
saturated vault before being discharged via the drain, and concentrations in
discharged leachate may be lower than intuitively expected.
-
For an unsaturated system, unless there are preferential pathways outside
the waste containers, all the infiltrating water could (in principle) percolate
down through the ISO containers and would therefore ‘see’ a significant
proportion of the wastes. If the contaminants are available for immediate
release to the infiltrating moisture, the end product could be release of
water volumes through the base that are smaller than the overtopping
volumes for the saturated equivalent, but with concentrations of
contaminants that may be higher. Illustrative calculations indicated that,
given these assumptions, the resulting post-closure fluxes of contaminant
to the environment associated with the alternative concepts could be
broadly similar in magnitude.
-
However, workshop participants agreed that although this might be true if
all contaminants were available for ‘instantaneous’ release, it is likely that
this is a gross simplification and that, in reality, complex water flow
pathways and contaminant release mechanisms will apply. Specifically,
releases could prove to be much slower for unsaturated conditions due to
the more limited contact of wastes with water. Nevertheless, given the
complexities involved, these intuitive judgements cannot be fully
supported through the existing evidence base.
51
-
This difficulty in establishing evidence for a clear differentiation between
saturated and unsaturated design concepts was originally recognised
during the consideration of ‘strategic options’, as discussed in Section 3.2.
Figure 2: Schematic Illustrating the Principles of Saturated and Unsaturated Design
Concepts. Blue arrows indicate ‘clean’ rainfall and infiltration waters. Purple arrows indicate
potentially contaminated leachate. Schematics indicate undegraded performance at closure. Note
that the final design differs from both of these concepts; schematics are for illustration only.
Example Saturated Waste Concept
Cap
Profiling
Material
Ground level
Upper
Groundwater
(Saturated or
Largely
Saturated Zone)
Cut-off Wall
Vault 9 and Future Vaults
Trenches
Lower
Groundwater
Vertical Drains
Rock Surface
Example Unsaturated Waste Concept
Cap
Profiling
Material
Ground level
Upper
Groundwater
(Saturated or
Largely
Saturated Zone)
Cut-off Wall
Trenches
Vault 9 and Future Vaults
Lower
Groundwater
Vertical Drains
Rock Surface
52
o
The above arguments reflect consideration of ‘central estimate’ cap and base
performance parameters. Consideration was given to what might happen if the
future evolution of the facility differs from these central values. In particular,
the system hydrogeology calculations allowed for the possibility of early
degradation of cap performance, or even localised or general cap failure, to be
explored. It was agreed that, for such alternative cap performance scenarios,
bath-tubbing concepts are substantially more likely to lead to discharges to
surface or upper ground water systems. The principle was also agreed that such
discharges are more likely to lead to impacts to receptors than discharges to
deeper water systems. Participants reflected on the mitigating argument that
accelerated cap degradation represents a deviation from the expected
behaviour, and should not therefore be considered a ‘central case’. However the
following principles from GRA were noted (§7.3.3 & §7.3.4):
“The environmental safety case should include an explanation of, and substantiation
for, the environmental safety functions provided by each part of the system… The
environmental safety case for the period after closure of a disposal facility should not
depend unduly on any single function… The developer/operator will need to explore the
contribution that each environmental safety function makes to the environmental safety
case… The developer/operator will also need to explore the circumstances where more
than one function is impaired.” (Environment Agency et al., 2009).
In this context, the reduced likelihood that an ‘unsaturated’ vault design
concept would lead to surface or near-surface leachate discharges in the event
of poorer than expected cap performance represents a clear advantage.
From a post-closure environmental safety perspective, therefore, there are clear
arguments in favour of an approach that involves a substantial cap to minimise
infiltration, with management of water that does infiltrate in a manner consistent
with minimising waste saturation for as long as possible. However, the need collect
water in order to support active leachate management (both during the operational
period and post-closure institutional control) means that it is not appropriate to
adopt (for example) options for the vault base and walls that are porous or link
directly to passive vertical drainage systems from the outset.
Considering the various design options available to satisfy both active leachate
management and subsequent post-closure passive drainage functions, the
following approach was selected.
o
A vault base comprising a concrete layer underpinned by a single BES
composite liner would be sufficient to support active leachate management;
given that unsaturated conditions post-closure are preferred, there is no strong
53
driver to implement a higher specification base that would minimise
permeability significantly beyond the operational period (as would the Vault 9
design).
o
The lower parts of the vault walls (to a suitable height above the base) also need
to be impermeable during disposal operations in order to avoid loss of control
over drainage in storm events prior to capping.
o
To ensure that bath-tubbing to the top of the vault disposal system is avoided, it
was agreed that the impermeable section of the vault walls only needs to be 1m
above the base. It is appropriate to divert any leachate overtopping this level to
deeper systems.
The depth of the vault bases was considered. However, to ensure vertical drainage
capacity underneath the system, the concept requires an unsaturated zone to be
established underneath the vault base (see also the discussions on passive leachate
discharge management in Section 4.4.5). Thus it is important that the bottom of the
vault bases is sufficiently elevated compared with the water table. Hence it was
considered that the future vault bases should be emplaced at a level consistent with
the existing constructions, with additional stepping to take account of the
topography and the approach to operational leachate management.
It was agreed that this approach is a natural progression from the current ‘Vault 9’
design, involving a fairly limited number of modifications that are necessary for
transition to a ‘disposal’ design.
4.4.4 Cut-off Wall
Key questions:
Given a final cap and the existing cut-off wall (installed in the early 1990s to a
specification broadly similar to that proposed in the baseline design for a new
encircling wall), what advantages could be provided by alternative cut-off wall
installations, in terms of:
o
Contributing to minimising possible release/discharge (both controlled and
uncontrolled) while the repository is under authorisation?
o
Contributing as a barrier to lateral inflow in the long-term?
o
Contributing to avoiding near-surface releases in the vicinity of the LLWR
under conditions of leachate release?
What length/depth of cut-off wall is necessary to provide the necessary degree of
confidence in relation to:
54
o
The role of a cut-off wall in providing an ‘upstream’ function?
o
The role of a cut-off wall in providing a ‘downstream’ function?
How do considerations regarding the timing of cap installation influence the
schedule and design of a cut-off wall?
There is evidence that the existing cut-off wall is currently providing the ‘downstream’
functions for which it was designed, although the data are not conclusive. Tritium
concentrations in the railway drain were reduced substantially post installation.
However, the interim cap was installed simultaneously and it is unclear whether it was
the action of one or both components that led to the decline in near-surface tritium
discharge. In addition, a change in measurement methods may have influenced the
results. There is also some uncertainty whether the clay layer beneath the existing cutoff wall is sufficient to provide the original design requirement – i.e. to ensure an
impermeable flow barrier to a depth of 7.4 to 9m below ground level.
The existing cut-off wall (which extends along some 60-70% of the outer perimeter of
the trenches) should provide an ‘upstream’ function in protecting lateral infiltration of
water shed by the cap into trench wastes adjacent to the cap edge. The potential levels
of protection likely to be provided by an extended wall are difficult to determine
accurately.
In the case of cap failure, the case for a cut-off wall can prove equivocal – whilst it
should help to divert leachate release to deeper groundwaters, it could also lead to
ponding of waters within its confines, which in turn has the potential to promote nearsurface discharges. In the absence of hydro-modelling evidence, the overall balance of
benefit vs. disadvantages for this scenario is currently not clear. However, options
involving no cut-off wall, or replacing the wall by a perimeter vertical drain system
(which might have reduced longevity, but is unlikely to cause ponding within the
confines of the facility) merit consideration at the workshop.
In practice the preferred design for the cut-off wall was evaluated in tandem with
options for passive leachate discharge control (see Section 4.4.5). This is because the
context for both decisions is framed by the wider strategy for leachate management,
given preferred designs for the cap and future vault bases and walls. Hydrogeological
modelling (Hartley et al., 2009; 2011) and associated discussions on potential impacts
(Appendix E) were amongst the evidence sources considered.
55
Given that infiltration into the facility will be small, according to central estimates
of cap performance, the main role of the cut-off wall is to limit lateral inflows of
water into the trenches. Although cap perimeter drains will be installed, these may
not be 100% reliable and the magnitude of lateral inflows will therefore be affected
by runoff from the cap. The hydrogeological modelling analyses suggest that a cutoff wall influences the saturation state of a fairly fraction of the trench wastes
(perhaps between 2 and 10% of the total trench waste volume) towards the edge of
the facility. However, so long as the cap performs as designed, the water flux
through those perimeter wastes could be significant compared to flows through the
remainder of the trench wastes. Modelling outputs examined at the workshop
suggested that the water volume passing through wastes close to the cap perimeter
could be of a similar magnitude to the total flow through the cap at closure.
Participants argued that, given that the trenches constitute loose tipped waste with
no engineered protection other than the closure engineering, it is important that the
wastes are protected from lateral inflows as far as practicable, and taking all other
aspects of the assessment into consideration, recommended that this benefit alone
was a strong argument for a closure cut-off wall.
In addition, the discussions on the preferred strategy for passive leachate discharge
management (see Section 4.4.5) involved consideration of a below-vault drainage
blanket that would not otherwise be protected from lateral groundwater inflows. In
order to protect and maximise the drainage capacity of such a feature, and to
maximise the extent of the unsaturated zone in the surrounding geology, it is
therefore consistent to include a cut-off wall surrounding the vaults. As saturation
levels are high in land adjacent to the vaults, a cut-off wall would also provide
protection against horizontal inflows and potential bath-tubbing that could occur,
particularly if the cap was to fail.
Given that arguments could be identified for protection of both the vault and
trench systems, the preference expressed was for a cut-off wall that will completely
encircle the facility (as opposed to a part-length equivalent designed to protect a
smaller section of the disposal area).
The secondary function of a cut-off wall in diverting outflows from the facility to
deeper groundwater systems was discussed. It was concluded that this represents a
contingency mitigation role for the cut-off wall. Compared with protecting against
inflow, the downstream function is of less importance because, particularly while
cap performance persists, the vertical drainage capacity within the geology is such
that leachate should in any case preferentially drain vertically. However, were
early cap failure to occur, a cut-off wall would help prevent releases to surface or
upper groundwater due to increasing saturation levels in the vaults.
56
Some potential drawbacks of a cut-off wall were discussed; in particular, it was
noted that, if the cap fails significantly and the amount of infiltration is significantly
increased, the cut-off wall could reduce lateral flows out of the facility area,
potentially leading to comprehensive saturation across the whole of the area it
would encircle. In some extreme situations water might be diverted over, rather
than under, the feature. However, it was agreed that the risks relating to such
scenarios could be mitigated at least in part by enhancing the vertical drainage
capacity of the system (see below). In addition, the level of failure that would be
required for this to occur would be equivalent to complete catastrophic failure of
the cap. Given the nature of the optimised, layered cap design, this was considered
to be very unlikely, even over periods of 500+ years from installation.
It was considered whether the functions required of the cut-off wall could instead
be fulfilled by a deep vertical perimeter drain, to draw water away from the wastes
through a high permeability feature, as opposed to forcing it around an
impermeable feature. However, were the cap to fail or any other process be
sufficient to cause the water table to rise to the base of the vaults or higher, such
drains would cease to provide a function, whereas an impermeable feature would
continue to provide a barrier of some sort between the facility and surrounding
near-surface waters. In addition, the potential for clogging of drains was noted.
Participants therefore favoured implementation of an impermeable feature.
The remainder of the options assessment focused on the required depth of the cut-
off wall. The pre-existing baseline design was for a complete encircling cut-off wall
installed to a depth of c.25m. The hydrogeological modelling work performed for
the current study suggested, however, that a significantly more modest depth
would be sufficient to fulfil the design functions described above. Considering the
outcomes of those studies and other practicalities (e.g. the depths of the trenches
and the vault constructions and the associated under slab drainage feature (see
below), it was concluded that an appropriate depth would be to 2m below the
trench/vault bases.
In principle, there is no reason why the existing cut-off wall should not be included
as part of the final cut-off wall construction, as its depth and construction method
are consistent with that required to achieve the design functions identified for the
full closure component.
The cut-off wall needs to be keyed into the cap, to maximise protection against
infiltrating waters. Its circumference will be within that that of the cap perimeter,
so it will protect the wastes against lateral components of water flow rather than
being in contact with a larger proportion of the cap runoff waters.
57
It was considered that the cut-off wall installation should be undertaken alongside
the capping strategy; i.e. it will be implemented in sections immediately prior to the
installation of equivalent sections of the cap.
4.4.5 Passive Leachate Discharge Control Options
Key questions:
What role is played by the base-case vertical drain:
o
For the expected evolution of cap performance?
o
Under ‘cap failure’ scenarios?
Is such a role critical to demonstrating confidence in long-term environmental
safety performance? If so, is the capacity sufficient and how (if necessary) could
such capacity be increased?
How does the control function played by the vertical drain vary according to
different assumptions regarding the strategy for management of infiltrating water
(i.e. different vault wall and base specifications)?
Could a vertical drain realistically replace a cut-off wall as a perimeter feature
surrounding the facility? What benefits (if any) might be gained?
How do considerations of a ‘stepped’ vault base depth (as well as possible
increased waste stacking within Cap void space) influence judgments regarding the
optimum location of a vertical drain?
Is drain location an important consideration in terms of allowing flexibility for
future vault developments beyond the existing southern boundary of the trenches?
How do considerations regarding the timing of cap installation influence the
schedule and design of a vertical drain?
Hydrogeological calculations suggest there is a strong argument that (for ‘as expected’
cap performance) the vault and trench wastes will de-saturate and remain de-saturated
for up to 1000 years, whether vertical drains are installed or not. In the 2002 PCSC
design, extensive drains to the south of the facility were included in the post-closure
design as a contingency measure, with the aim that they should work in harness with
the cut-off wall in the event of cap failure. The hydrogeological runs undertaken to
date suggest that, for the Vault 9 baseline design, vertical drains can offer an important
drainage role in such an event, but that they will nevertheless not be sufficient to drain
58
the vaults completely, and certainly not the trenches, as the distances and hydraulic
properties of the materials involved mean there is not a sufficiently strong hydrological
connection to transport the volumes of water that would be introduced into the system.
Hence, some discharges to surface or near-surface systems may occur in the event of
cap failure, even with the current vertical drains installed. This suggests an argument
for either not installing any such drains, or for extending their use and location so that
a larger proportion of the facility can be effectively drained by them.
An understanding of the context for passive leachate discharge control is essential
in order to identify corresponding design functions. In particular, the role of such
features needs to be considered alongside that of the cap. It is the latter feature that
is fundamental to ensuring containment of waste contaminants for as long as
possible. Additional drainage capacity provided by passive leachate discharge
controls then fulfils a secondary role, which needs to be considered alongside vault
base and wall design and performance. These controls are required to manage any
leachate generated as a result of infiltration through the cap, and to provide of an
additional layer of protection against early cap failure / degradation scenarios.
The principle that surface and upper ground water systems should be protected
preferentially to lower systems, as discharges to upper systems are likely to lead to
comparatively higher impacts, was noted. The main function of passive leachate
discharged control components should therefore be to help direct any leachate that
does arise to deeper systems, for as long a period post-closure as practicable.
The existing baseline ‘Vault 9’ approach was designed to fulfil just such a function.
The design includes a row of wide, deep vertical borehole drains installed between
Vault 9, the future vaults and the trenches, to direct any overtopping liquor from
the vaults to deeper systems. The hydrogeological modelling work suggested that
the drains would be effective in this role so long as cap performance as a hydraulic
barrier remains good. If degradation of the cap occurs, however, then although this
feature would be able to offer an important addition to drainage capacity beneath
the facility, it wouldn’t be sufficient to drain the vaults completely. This arises
because the distances and hydraulic properties of the materials involved mean
there is not a sufficiently strong hydrological connection to transport the volumes
of water that would be introduced into the system. Hence, some discharges to
surface or near-surface systems may occur in the event of cap failure.
Participants agreed that the facility must be provided with an effective passive
leachate discharge management route as a contingency, in order to reduce reliance
59
on the cap as the primary engineered barrier. Discussions focused on the best way
to address this issue. Overall, the following design requirements apply:
o
The design should be consistent with the aim to keep the wastes as unsaturated
as possible, noting the vault base and wall designs outlined above.
o
The design must create sufficient passive drainage capacity and connectivity for
the whole of the vault system to mitigate against enhanced cap degradation
scenarios.
o
The design should not unduly concentrate leachate within the environment.
The final requirement was noted as another potential disadvantage of the existing
baseline approach. The wide borehole drains envisaged in this approach were
designed to provide redundancy along a line of obvious potential weakness in the
cap (i.e. the ‘gully’ between the two domes in the existing baseline cap design) but
would involve concentration of overtopping leachate in environmental media
associated with the line of the base of the drains. It was also considered that the
drains could clog, as they are of limited surface area given the cumulative volumes
of leachate they might be required to transmit over the post-closure time period
(although it was also noted that, if they did clog, their permeability would most
likely remain higher than the surrounding geology and they would continue to
provide a limited drainage function).
Given the decision to move to a single dome design, the benefits provided by the
Vault 9 approach to passive leachate discharge control (in terms of mitigating
specific cap failure modes) no longer apply, but the disadvantages remain.
Participants therefore considered that other approaches should be assessed.
Following consideration of the advantages and disadvantages of alternative
options that have the potential to fulfil the design requirements listed above, the
following approach emerged as a clear preference.
o
The preferred design is based upon the implementation of horizontally
extensive drainage blanket layers under each of the vault bases. These would be
connected to vertical stone drains constructed along the vault walls, interfacing
with vault waters via weired edges to manage overtopping leachate at the 1m
level.
o
The concept behind this approach is to connect each of the vaults with an
extensive drainage layer that will interface with the underlying geology over a
wide area to maximise vertical drainage and avoid unnecessary concentration
of leachate.
60
Sections of the drainage blanket corresponding with the locations of the different
vaults will not be connected in this concept, to avoid concentration of leachate at
the southern end of the facility, which might otherwise occur as a result of the
stepped nature of the vault bases.
Should the water table rise to the level of the drainage blanket, the function
provided by this component could be substantially reduced. However, this would
be the same for any other passive leachate discharge management approach.
This approach was considered to be easier to construct, to be more predictable in
terms of reliability and performance, to provide more drainage capacity and to be
of lower cost than the existing baseline design. No other options were identified
with this combination of advantages.
61
5 Final Optimised Pre- and Post-closure
Engineering Strategy
The results of the assessment and integration stages are combined below in a record of
the outcome of optimisation process, setting out LLW Repository Ltd’s preferred
engineering strategy. Design drawings are presented in order to illustrate the strategy.
Final Cap
The engineered cap is a principal component of the overall engineering design for the
LLWR. It plays key roles in relation to the passive control of leachate and the
protection of disposed wastes from inadvertent disturbance. From a functional
perspective, the design of the cap needs to be optimised to minimise infiltration, to the
extent that it is less than the drainage capacity of the underlying geology, thereby
creating unsaturated zones beneath the vaults and trenches, for as long as reasonably
practicable. It also needs to provide effective protection against intrusion by humans,
deep rooting plants and burrowing animals.
Key considerations in cap design include:
overall thickness and the depth and composition of individual component layers;
profile; and
the scheduling of installation in relation to waste emplacement and active measures
for the monitoring and control of leachate.
The component layers of final cap design for the LLWR have been optimised over
many design cycles, based on principles consistent with best international practice. The
current baseline design includes a composite geomembrane on a BES hydraulic barrier,
overlain by an internal drainage layer, biointrusion barrier and upper layers designed
to provide substrate for plant rooting, moisture retention and filtration. Expert
elicitation based on this design forms the basis for parameters determined as inputs to
the ESC hydrogeological model calculations.
The overall thickness of the engineered cap in the baseline design is 3m. Taking into
account that there will be profiling material underneath the cap, this means that there
will be a minimum distance of 4m between the cap surface and the underling wastes.
As such, the cap is considered to provide significant protection against inadvertent
disruption of both trench and vault wastes as a result of commonplace human actions.
A thicker cap might potentially provide greater protection against disturbance
associated with certain types of bulk excavation. However, no significant mitigation of
the likelihood of disturbance from major earthworks would be provided even if the
62
engineered cap was considerably thicker than the baseline design. Moreover, even a
cap of double the thickness or more would not necessarily offer significantly greater
protection against minor disturbances associated with actions such as exploratory
borehole drilling.
Controlled passive gas venting (as adopted on municipal landfill cap designs) is not
necessarily required if it can be demonstrated with confidence that the volumes of bulk
gas from waste corrosion and degradation do not represent a threat to cap integrity
and performance. The absence of an engineered gas vent would have the advantage of
ensuring that any release of radio-labelled gases generated within the facility would
not be focused at a specific location near highest point of the cap, with associated
implications for risk to potential exposure groups following the lifting of controls on
site use. However, a passive venting scheme installed as part of the cap design would
facilitate confirmation monitoring of landfill gas and radio-labelled gas production
during post-operational active control of the site. Final closure of the vent could then
be achieved prior to the release of the site from active management control. The
reference assumption in the site management plan set out in the 2011 ESC (i.e. whether
or not the cap vent should be closed) will be decided before submission, informed by
the latest assessments of gas generation and transport.
Taking due account of planning considerations and local community interests implies
that the profile of the cap should ensure that the minimum elevation is achieved,
consistent with providing assurance of long-term performance and stability.
Engineering design principles require a minimum gradient of 1:25, to ensure that the
drainage function of the cap will be maintained over the long term under expected
rates of settlement. In addition, the engineering optimisation process considered the
relative merits of a ‘gull wing’ and single dome cap designs, with the potential
advantages and disadvantages of both alternatives being taken into account. A single
dome requires a significantly greater volume of profiling material underneath the
engineered cap, but its geometry and lower susceptibility to erosion (i.e. avoiding the
existence of a ‘gully’ between the trenches and vaults that collects water from a
substantial fraction of the overall cap area) is a critical factor in underpinning
confidence in long-term resistance to infiltration. This gives a slightly higher peak
elevation to the cap (by less than 4m) than would be obtained for a gull-wing design,
but it is not expected to be significant in terms of visual impact, when considered
across the 400m width of the facility over which the cap will be constructed.
The engineered cap needs to be supported by a suitable profiling layer. This does not
necessarily have to consist solely of inert fill materials; indeed, there is an opportunity
to use this profiling volume as additional capacity for waste emplacement, for example
by adopting higher stacking of disposal containers away from the outside perimeter of
the vaults. Because such waste disposals would be within the profiling shape, they
63
would have no effect on the final overall elevation of the cap. Provided that higher
stacking within the vaults can be shown not have a detrimental impact on the
environmental safety case or other aspects of operational safety management, such a
practice would be consistent with making optimum use of LLWR as a national asset.
Vault Design
Although the cap design is intended to minimise infiltration so far as is reasonably
practicable, water cannot be excluded completely and the rate of infiltration is expected
to increase over the long term as cap performance degrades. Expert elicitation of
performance, based on the proposed design, coupled with the ESC hydrogeological
model, provides an indication of anticipated rates of water input and flow through the
system. The optimisation study considered two broad strategies for the passive
management of leachate within the vaults and their implications for vault design.
The preferred design strategy is to decrease the potential for near-surface release by
seeking to minimise water contact with the wastes and to avoid ‘bath-tubbing’ in the
vaults. The overall aim in vault design is therefore to increase confidence that water
can be drained at the base over time faster than it enters from above via the cap.
Hydrogeological modelling suggests that such a design strategy will also achieve a
more predictable outcome regarding the behaviour of leachate than the alternative
approach of designing for (short-term) leachate retention. It is also considerably easier
to construct than the alternative.
During active control over the site, the intention is to ensure that leachate can be
effectively collected and its release managed for as long as this is required. The base
slabs are therefore assigned a sufficiently low permeability to support effective
operational leachate management for as long as active controls are in place. Bunding is
provided by surrounding liner walls to a height of 1m above the base in order to
protect against uncontrolled overflow in extreme rainfall events prior to capping.
However, there is no strong driver to implement a higher specification base that would
provide increased confidence in minimising permeability significantly beyond the
operational period.
After the period of active control, under passive conditions and with no collection of
leachate from the vault sumps, saturation over the depth of the waste column is
precluded by ensuring that, if leachate is retained by the base, it overflows the 1m
walls on the east and west sides to a drainage layer beneath the vault base. The
drainage layer provides a high horizontal permeability to support effective dispersal of
drainage waters to the underlying geology. In addition (and particularly for Vault 8),
some drainage to the unsaturated zone beneath the vaults is anticipated to occur
64
through the base itself, under the expectation that its performance over time as a
hydrological barrier will be less effective than the cap.
As the development of future Vaults (10 to 14) continues southwards, base levels
reduce to best suit the existing topography. This obviates the need to construct disposal
platforms above the local ground level. Consequently, the top of the base slab reduces
from 15.9m AOD (current Vault 9) to 11m AOD for Vault 14. The drainage layers
beneath individual vaults are laterally isolated to impede flows from following a
preferential path with topography and reducing base levels to a more focused
discharge point at the south of the facility.
The internal north and south walls of the vaults are set slightly higher than the east and
west walls to give preferential drainage pathways to the sides, but with ultimate
hydraulic continuity (to provide contingency in the event that free drainage does not
occur locally) along Vaults 9 to 14. Vault 8 would be connected into the Vault 9 system
on cessation of pumped disposal.
Cut-off Wall
The optimisation study examined whether a cut-off wall is required, and the
implications of extending such a wall to different depths. The primary function of such
a cut-off wall is to prevent the lateral infiltration of water (including water shed by the
cap) into the vault and trench wastes around the edges of the repository. It also plays a
contingency role in providing reassurance against the possibility of near-surface
release close to the facility in the unlikely event of early cap failure (leading to possible
saturation of the waste column), or as a result of preferential pathways for leachate that
might arise from higher waste stacking. The cut-off wall will be keyed into the
perimeter of the cap, to preclude the possibility of over-topping in the event of
saturated conditions developing within the wastes.
Expert elicitation and hydrogeological modelling suggests that there is advantage to be
gained from installing a cut-off wall, but there are no significant benefits to be gained
from taking the wall to a greater depth than 2m below the underside of the composite
basal liner, or including a geomembrane within the wall. Such a design is consistent
with the existing cut-off wall, constructed in the 1990s along the northern and eastern
perimeter of the trenches. Subject to ongoing monitoring showing no grounds for
concern in the performance of this feature, it is expected that the existing cut-off wall
can be incorporated into the overall perimeter barrier.
Passive Discharge Controls
A vertical drain was incorporated between the trenches and vaults in earlier baseline
designs. Its safety function was to mitigate the potential for near-surface release
65
associated with bath-tubbing within vaults designed for leachate containment, as well
as to provide a contingency in the event of failure of the gull-wing cap design.
However, given the change to the preferred cap profile, as well as the strategy for
leachate management within future vaults, this function is longer required.
Hence there are no substantial vertical drains in the engineering design identified as an
outcome from the current optimisation study. However, engineered drainage
pathways are provided by the sub-base features associated with the future vault
design, with the aim of maintaining low saturation conditions within the disposed
wastes and the ready dispersal of drainage waters from the vaults to the underlying
geology. For as long as the cap provides a barrier to infiltration that is greater than the
natural drainage capacity of the geology beneath the facility, passive leachate release
via these pathways is expected to be to unsaturated ground.
Timing of Cap Installation
The optimisation study determined that progressive capping should be implemented
in ‘strips’ down the site, linked to the construction of new vaults. Staged installation of
the cap over the vaults is preferred in order to ensure that the capacity of the leachate
management system is not threatened by the requirement to manage storm waters
from extreme weather events as new vault slabs are constructed. Moreover, rather than
waiting until the completion of disposal operations, earlier capping also serves to
provide a protective cover to early vault disposals that may have been exposed to the
elements for a considerable period of time. Staged capping is more efficient in terms of
resource use, and provides for a longer period of monitoring of cap performance prior
to final closure and withdrawal of active control over the site.
For the purpose of the ESC, it is therefore assumed that final capping over Vault 8 and
an associated fraction of the northern end of the trenches will be undertaken in
conjunction with the preparation of Vault 10. Likewise, capping for Vault 9 and the
adjacent area of the trenches will be undertaken in conjunction with the preparation of
Vault 11 et seq. Excavations required prior to construction of the next vault slab would
be expected to provide a proportion of the profiling material for the corresponding
stage in capping.
This assumption incorporates flexibility to enable changes to be made, consistent with
the overall aim of achieving a single dome final cap. In particular:
The schedule assumed for the ESC anticipates that no replacement of the current
interim cap over the trenches (minimum design life of 30 years from 1990) will be
required prior to installation of each stage of the final cap. Nevertheless, earlier
capping over the trenches could be undertaken if it were required. This would be
achieved either by constructing a new interim cap, a ‘Stage 1’ cap, or completing
66
the final cap, over the trenches in such a way that it could subsequently be
incorporated into the final single dome as future vaults were constructed and
capped.
Subject to further monitoring and analysis, there remains limited evidence and
some uncertainty regarding the degree of settlement that remains to be expressed,
particularly within the trench wastes. This could arise as a result of the additional
loads imposed during final capping, as well as the ongoing effects of waste
degradation. These processes have the potential to cause differential settlements of
the cap, across the wastes themselves, as well as across the bay walls, trench walls
and the secant pile wall between the trenches and the vaults. Although the design
intention within the ESC is for staged installation of the full, final engineered cap in
strips starting from the northern end of the facility, it is possible that a simpler
‘Stage 1’ cap (incorporating a geomembrane and appropriate surface cover as
temporary protection, supported by all the relevant profiling material) could be
constructed initially if it were determined that settlement over the trenches
represented a significant concern. Such a cap would have broadly similar
hydrological performance to the final cap over the operational period. However, it
would enable any remedial actions in response to settlement to be taken more
readily prior to installation of the final engineering layers.
One important control on the rate of capping is the expected arising of profiling
materials. If necessary to support the staged installation of the final cap, required
profiling materials would need to be brought to the site to supplement those
generated by excavations of future vaults. Although not incorporated in the
proposed design and inventory for the ESC, it is conceivable that profiling material
above the trenches might include VLLW, such as very lightly contaminated
building rubble or soils. Depending on the rate of such arisings, it is conceivable
that capping of the trenches might therefore need to progress more rapidly than
scheduled in relation to the development of new disposal vaults. Subject to national
policy and appropriate regulatory and planning approval, such disposals could be
accommodated with an adjustment to the preferred schedule for capping.
Leachate Management and Release Monitoring
The baseline assumption for the ESC is that leachate will continue to be actively
managed while waste emplacement continues and for a period after final capping over
the facility has been completed. Within the vaults, this is achieved by pumping to
collect leachate from sumps installed in the vault bases. Gravity drains leading to the
southern perimeter of the facility provide for leachate collection from the trenches. The
leachate is routed to marine holding tanks for monitoring prior to consented discharge
via the site pipeline to the sea. After final capping, the maintenance of pumps used to
67
remove leachate collected within the vault sumps will be undertaken with the support
of engineered man-access penetrations in the western side of the cap.
The assumption of continued active management and monitoring is consistent with
established practice for landfill operations. However, based on current understanding
of cap performance and estimated likely rates of leachate generation (subject to the
outcome of final hydrogeological modelling), it is expected that the vaults and trenches
will be effectively de-watered soon after the completion of final waste emplacement
(effectively within a few decades), with very low ongoing rates of leachate generation.
Monitoring during the post-operational period would be continued with the objective
of confirming such outcomes, for as long as necessary in order to fulfil regulatory or
other stakeholder requirements regarding assurance of system performance.
Likewise, the monitoring of landfill gas (and possible radioactive gas) via the gas vent
in the final cap would be designed to confirm the expectation that gaseous releases –
both in terms of volume and contamination levels – will be small, and present an
acceptably low risk in terms of fire, explosion or radiological exposure. The decision as
to whether it was appropriate finally to seal the gas vent would be informed by
monitoring information and taken, in consultation with regulatory authorities and
other stakeholders, prior to withdrawing active management control over the site.
Component Design Drawings
The following pages provide component design drawings illustrating the design
concept. The drawings include:
Figure 3: Overall LLWR Site Plan and Example Cross-section
Figure 4: Overall LLWR Site Plan and Schematic Long Section
Figure 5: Components of the Final Cap
Figure 6: Vault and Cap Edge Details
Figure 7: Cap – Vent Details
Figure 8: Trench and Cap Edge Details
Figure 9: Potential HHISO Stacking Contours within the Cap Profile
68
Figure 3: Overall LLWR Site Plan and Example Cross-section
69
Figure 4: Overall LLWR Site Plan and Schematic Long Section
70
Figure 5: Components of the Final Cap
71
Figure 6: Vault and Cap Edge Details
72
Figure 7: Cap – Vent Details
73
Figure 8: Trench and Cap Edge Details
74
Figure 9: Potential HHISO Stacking Contours within the Cap Profile
75
References
Baker A J (2008). LLWR Lifetime Plan: Managing Existing Liabilities and Future Disposals at
the LLWR. LLW Repository Ltd Report 10001 LLWR LTP Volume 1, Issue 1, April 2008.
Baker A J, Collier G D, Penfold J S S and Wood A J (2008), LLWR Lifetime Plan:
Assessment of Options for Reducing Future Impacts from the LLWR, LLW Repository Ltd
Report 10002/LLWR/LTP Volume 2, Issue 1, April 2008.
Barclay A (2007). LLWR Modular Vaults Project: Leachate Management Strategy. British
Nuclear Group Project Services Report BNGPSL/LLWR/MV/2.0/19/1, June 2007.
Belton J (2007). LLWR Modular Vaults Project: Capping Justification Report, British
Nuclear Group Project Services Report BNGPS/LLWR/MV/2.3/062/1, June 2007.
BNFL (2000). Status Report on the Development of the 2002 Drigg Post-Closure Safety Case.
BNFL (2002a). Drigg Post-closure Safety Case: Overview Report. BNFL report, September
2002.
BNFL (2002b). Drigg Post-closure Safety Case: Engineering Design Report, BNFL report,
September 2002.
Edwards J and Alexander S (2004). Preliminary Assessment of Options for the Management
of the Impact of the Drigg Low Level Radioactive Waste Disposal Facility. BNFL Nuclear
Sciences and Technology Services Report (04)5092 Issue 1, May 2004.
Environment Agency (2005). The Environment Agency’s Assessment of BNFL’s 2002
Environmental Safety Cases for the Low-Level Radioactive Waste Repository at Drigg.
NWAT/Drigg/05/001, Version 1, June 2005.
Environment Agency and Scottish Environment Protection Agency (2004). Guidance for
the Environment Agencies’ Assessment of Best Practicable Environmental Option Studies at
Nuclear Sites.
Environment
Agency,
Northern
Ireland
Environment
Agency
and
Scottish
Environment Protection Agency (2009). Near-surface Disposal Facilities on Land for Solid
Radioactive Wastes: Guidance on Requirements for Authorisation. February 2009.
Fowler L, Towler G H and Willans SM (2005). A Summary of Recent Optimisation Studies
for the Drigg Low Level Waste Radioactive Waste Disposal Site. BNFL Nuclear Sciences and
Technology Services Report NSTS (04)5127 Issue 2, September 2005.
76
Gallagher E (2007). LLWR Modular Vaults Project: Liner Component Design Report. British
Nuclear Group Project Services Report BNGPS/LLWR/MV/2/016, June 2007.
Hartley L, Applegate D, Couch M, Jackson P, James M and Roberts D (2009).
Hydrogeological Modelling for LLWR 2011 ESC: Phase 1. Serco Report TAS/003632/002
Issue 2.1, November 2009.
Hartley L, Applegate D, Couch M, Jackson C P and James M (2011). Hydrogeological
Modelling for LLWR 2011 ESC Phase 2. Serco report SERCO/TAS/003632/005 Issue 5.0,
April 2011.
Hilary J F (2008). Outline Construction Method Statement: Modular Vaults – Cut-Off Wall,
Capping & Vertical Drains: Low Level Waste Repository, Drigg. VT Nuclear Services report
MS/102917/460005916/CSA/00023.
Jackson C P, Couch M, Yates H, Smith V, Kelly M and James M (2011). Elicitation of
Uncertainties for LLWR. Serco report SERCO/TAS/E003796/010 Issue 2.0, April 2011.
Kane B, Price A, Tann V, Painter A, Saddington G, Mitchell R and Clark M (2007).
LLWR Modular Vaults Project: Evaluation of Options (2004 Process). British Nuclear Group
Project Services Report BNGPS/LLWR/MV/2.3/061/1, May 2007.
NDA (2010). UK Strategy for the Management of Solid Low Level Radioactive Waste from the
Nuclear Industry: UK Nuclear Industry LLW Strategy. Nuclear Decommissioning
Authority, August 2010.
Newham P, Painter A, Saddington G, Price A and Clark M (2007). LLWR Modular
Vaults Project: Options Development Report. British Nuclear Group Project Services
Report BNGPS/LLWR/MV/2.3/050/1, May 2007.
Sola R (2003), Viable Technologies Report, BNG Report RP/0102917/PROJ/00029, Rev A,
January 2003.
Williams L and Proctor A (2007). Low Level Waste Repository (LLWR) Modular Vaults
Project: Single Option Selection Process. British Nuclear Group Project Services Report
BNGPS/LLWR/MV/1/003/1, May 2007.
77
Appendix A: Engineering Optimisation Scoping
Workshop Agenda, Participants and Briefing
Paper
The Engineering Optimisation Scoping Workshop was held at the ENERGUS centre,
Workington, West Cumbria, from 10am to 5pm on Thursday 17th December 2009.
Participants included the following.
Richard Evans (Cumbria County Council)
Andrew Fairhurst, Ian Barraclough (Environment Agency)
Ollie Okeke (Nuclear Installations Inspectorate)
Dave Wetherburn (Nuclear Decommissioning Authority)
David Bennett (Terresalus / ESC Peer Review team)
Graham Garrard (Halcrow)
Joe Small (National Nuclear Laboratory)
Peter Jackson (Serco Assurance)
Richard Cummings, Neil Shaw, Dave Tonks, Andy Baker, Paul Pointon, Scott
Anderson, Trevor Sumerling, John Shevelan, Amy Huntingdon, Simon Hunter,
George Reeves (LLWR)
Mike Egan, Alan Paulley (facilitation team, Quintessa).
The Outline Agenda for the day was:
•
Introduction
•
Adoption of ‘Safety Strategy’ as basis for examination of options
–
•
Preventing disturbance / preventing release / controlling release
Threats and potential engineering controls
–
Identification/review of supporting evidence on functions
–
Categorisation: Required / Contingency / Excluded
–
Which main variants exist?
•
Basis for more detailed analysis
•
Recap and next steps
More details of the approach taken at the workshop are provided in the Briefing Paper.
78
14th December 2009
Workshop Briefing Paper
Environmental Safety Case
Engineering Optimisation Scoping Workshop
Thursday 17th December 2009
10.00 – 17.00, ENERGUS, Workington
Workshop Purpose
Examination of design and management alternatives as part of the demonstration of
optimisation is an essential element in developing the Environmental Safety Case for
LLWR. As well as satisfying regulatory requirements, a comprehensive optimisation
process is also important in guiding decision making on plans and programmes for
facility development and closure.
Optimisation covers a range of questions relating to the design, use and management of
the LLWR, including consideration of potential remedial actions relating to past disposal,
future waste conditioning and emplacement, and post-closure management control. The
specific focus of this workshop is on optimisation of pre- and post-closure engineering
design. The aim is to consult with key stakeholders and experts on the scope of, and
possible constraints on, the optimisation process, and to examine some of the key factors,
principles and evidence relevant to the evaluation and comparison of options.
The workshop will be facilitated by Michael Egan and Alan Paulley from Quintessa, who
have been contracted by LLW Repository Ltd to support this project as a contribution to
development of the 2011 ESC.
Overall Approach
Demonstration of optimisation requires an evaluation of alternative approaches to
achieving defined environmental safety goals, with the objective of ensuring that Best
Available Techniques are used to minimise releases to the environment and that impacts
are As Low As Reasonable Achievable. This process can be broadly divided into two
main elements:
•
Development and justification of an overall safety strategy for the control of
hazards relating to the disposal facility. In a long-term safety case for waste
disposal, the accent is on passive safety controls, with engineering optimisation
being directed towards the control measures provided by constructed elements of the
disposal system. A defined safety strategy therefore identifies the main functions
that are assigned to the different engineering control measures to be installed.
79
•
Optimisation of design and implementation of specific control measures for the
favoured strategy. Some understanding of how a specific design might be optimised
is of course relevant to the evaluation of overall strategies. In general, however, the
principle is that the design of engineered barriers and associated controls should be
optimised according to their required function and overall performance objectives
for the system. This involves consideration of factors such as detailed layout,
materials, quantities and timescales for implementation.
In both cases, the evaluation of options needs to be informed by available evidence and
understanding of how different engineering controls might work in the context of the site
and the disposed waste inventory.
Workshop Scope and Content
The focus of the workshop will be on the first of the above two elements. Specifically, we
will seek to develop and work through the main steps of a logical examination of the
choices involved in establishing the strategy for pre- and post-closure engineering design.
This will draw out the control functions required of different components of the pre- and
post-closure engineering in providing isolation and containment of the waste, and
examine alternative ways in which these functions might be provided.
Design features under consideration will include:
•
•
•
•
•
•
•
Cap options
Cut-off wall
Vertical drains
Base slab and wall designs for future vaults
Backfill and general waste emplacement options
Leachate management (pre- and post-closure)
Surface pathway modification
So far as possible, the aim of the workshop will be to define the control functions that
such features can provide, their consistency with regulatory principles, and the range of
design alternatives that need to be considered in more detailed optimisation.
We do not anticipate carrying out a full-scale Multi-Attribute Decision Analysis exercise
at this workshop. However, we will be seeking to identify those factors that are the most
important differentiators between strategic options. Where choices are strongly guided by
key principles, the aim will be to clarify these. Where suitable evidence (e.g. from
modelling studies) is available to inform understanding of the implications of different
choices, we will aim to use that. So far as possible, we will seek to narrow down the
options (e.g. whether particular functions or features must be present in the final design)
to an agreed set of feasible alternatives.
Relevant pointers towards more detailed optimisation considerations will be considered,
as appropriate. If it is determined that specific detailed analyses will be required in taking
the work further, we will ensure that is recorded.
80
Sources of Evidence
As may be expected for any complex options assessment process, the evidence for and
against performance of alternatives measured against different objectives may present
conflicts and trade-offs. For example, some options may prove beneficial from the
perspective of long-term groundwater pathway impact, but may be less preferred in
relation to potential gaseous releases. Similarly, there may be trade-offs to be considered
between near-term and long-term environmental performance. It is important that the
evidence base provides an appropriate level of detail at each stage of the process to help
understand the relevant issues in differentiating between alternatives.
To support this it is important that the evidence base is as ‘realistic’ as practicable.
Conservative assumptions may serve to skew estimates of comparative option
performance, particularly if they act to obscure the benefits of one or more of the
approaches being compared. The context for this optimisation study is therefore different
from that which usually applies to performance assessment calculations developed for
compliance purposes. At the same time, it may not be possible to identify realistic
assumptions that can be substantiated, and to attempt to do so may introduce significant
extra levels of uncertainty. A balance therefore needs to be struck when constructing the
evidence base.
Identified relevant sources of evidence include:
•
•
•
•
•
•
•
Ongoing hydrogeological modelling studies in preparation for the 2011 ESC
Ongoing development of approaches for impacts assessment in support for the
2011 ESC (groundwater, gas, human intrusion, coastal evolution and erosion
processes)
Assessments of vault waste-form innovations, and other near-field studies
2008 Performance Update for the LLWR (Schedule 9, Requirement 2)
Vault 9 Single Option selection process (2007)
2004 Modular Vaults design option selection process
2002 PCSC and underpinning studies
Process
This scoping workshop will follow an order of analysis similar to the following:
1. Is there agreement regarding the fundamental requirements of the LLWR pre- and
post-closure engineering, and the threats that must be addressed?
2. To meet these aims, what role is required of the LLWR cap and any cut-off walls in
isolating the wastes from disturbance or access by water?
3. Given the outcomes of point (2), what are the requirements for control and
containment of contaminants within the waste system itself? What are the main
strategies that could be considered (e.g. in relation to managing precipitation that
may enter the facility)? What are the key issues that need to be addressed in making
choices?
81
4. What ‘downstream’ functions may be required to manage the potential release of
contaminants from the facility? Is it appropriate to use engineering to ‘route’
contaminant release along particular preferred pathways and, if so, what should
determine the suitability of alternative options?
Following the workshop, the outcome will be documented as a logically derived
account of alternative engineering strategies and agreed principles for comparing
options.
82
Appendix B: Optimisation Assessment
Workshop Agenda and Participants
The Optimisation Assessment Workshop was held at the Sellafield Centre (Day 1) and
Greengarth, Cumbria (Day 2) on the 24th and 25th February 2010. Participants included
the following:
Richard Cummings, Amy Huntingdon, Andy Baker, Neil Shaw, Dave Tonks,
George Reeves (part-time) (LLWR)
Matt Couch, Lee Hartley (Serco Assurance)
Mike Thorne (first day only) (Mike Thorne Associates)
Joe Small (NNL)
The Original Agenda for the two days as set out below. In the event, the discussions
were inevitably more fluid. It was also noted ahead of the meeting the final step, or
even the last two or three steps, could be undertaken at the subsequent ‘integration’
meeting. Outcomes of the workshop reported in Section 4 of the main report.
Day 1 (Weds 24th Feb)
9.00
Welcome and introductions
9.15
Review of programme status and workshop objectives
9.40
Review framing assumptions
10.15
Final cap assessment:
o
Review baseline design
o
Review options for cap layers (in terms of potential benefits of deviations from
current design)
o
Review options for cap geometries
o
Review implications of timing issues
o
Assess cap layer options against criteria
o
Assess cap geometry options against criteria
83
10.45
Coffee
11.00
Continue cap assessments
12.30
Lunch
13.00
Continue cap assessments
14.45
Coffee
15.00
Vault base and Wall assessment
o
Review Vault 8 and Vault 9 designs as useful baseline options for comparison
o
Review other options
o
Assess options against criteria, noting outcomes of the cap assessment
17.30
Close
Day 2 (Thurs 25th Feb)
8.30
o
Cut-off walls assessment
Review current baseline, and 2002 PCSC designs as useful baseline options for
comparison
o
o
Assess options against criteria for ‘as expected’ cap performance scenario
o
Assess options against criteria for ‘early failure’ scenarios
o
Consider overall outcomes, noting following sessions, and final review and
integration session
10.15
Coffee
10.30
Continue cut-off walls assessment
11.00
Passive leachate management (vertical drains) assessment
o
Review current baseline, and 2002 PCSC designs as useful baseline options for
comparison
o
o
Assess options against criteria for ‘as expected’ cap performance scenario
o
Assess options against criteria for ‘early failure’ scenarios
o
Consider overall outcomes, noting following sessions, and final review and
integration session
12.30
Lunch
84
13.00
Continue vertical drains assessment
13.30
Operational leachate management and monitoring approaches
o
Review current approach
o
Identify whether any of the component options described so far would
significantly compromise approach, noting any adjustments that could be made
o
Agree conclusions, in particular noting any constraints on engineering strategy
selection
14.15
Backfill assessment
o
Review existing backfill decision, options and rationale
o
Review whether there are any drivers to change (e.g. impermeable grout)
o
Assess any alternatives
o
Recommend way forward
15.15
Coffee
15.30
Facility armouring / strengthening / local hardening
o
Review options
o
Assess options
o
Recommend way forward
16.30
o
Review outcomes
Agree relative importance of ‘as expected’ and ‘cap failure’ option performance
assessments
o
Describe preferred strategy
o
Agree next steps
o
Note issues remaining to be resolved (detailed design issues, and where further
assessment is necessary, e.g. through main assessment iteration)
o
Agree future approach/constraints re. addressing ongoing ‘sensitivity’ and
‘flexibility’ issues (e.g. more Vaults, higher stacking, stepped Vault bases)
17.15
Close
85
Appendix C: Optimisation Presentation Agenda
and Participants
The Engineering Optimisation Presentation Workshop was held at the Samuel Lindow
Building, Westlakes Science and Technology Park, on Thursday 29th April.
Participants included the following.
Richard Evans (Cumbria County Council)
Andrew Fairhurst, Ian Barraclough, Paul Robinson (Environment Agency)
Mike Calloway, Dave Wetherburn, Brenda Breen (Nuclear Decommissioning
Authority)
Steve Jones (Westlakes Scientific Consulting / ESC Peer Review team)
Lee Hartley (Serco Assurance)
Dick Raaz, Richard Cummings, Andy Baker, Paul Pointon, John Shevelan,
Trevor Sumerling, George Reeves, Dave Tonks, Neil Shaw, Nigel Lister, Amy
Huntington (LLWR)
Roger Coates (LLWR PBO Board)
The Agenda for the day was:
10.30: Welcome, objectives, agenda and introductions – Richard Cummings, LLWR.
11.00: Optimisation approach – Mike Egan, Quintessa.
11.30: Proposals for future vault and closure design and leachate management – RC.
12.30: Lunch
13.15: Discussion – ME.
14.45: Conclusions and actions – RC.
86
Appendix D: Collation of Component Option
Proformas
87
Engineering Component Option Description
Proforma
Final Capping Options
Version 1. Author: Alan Paulley.
Reviewed by: Dave Tonks, Mike Egan. Date: February 2010.
Component Options Summary
Main Role of Component In Strategy/Strategies
A final cap for the LLWR is required to provide a range of safety-related functions, such as the following.
The cap should provide an impermeable cover /barrier in order to minimise rainwater infiltration for as
long as possible. In doing so, it will help control the saturation state of the wastes, and indeed the
geological features directly underlying the facility, for as long as possible. In the absence of a cap
designed to minimise infiltration, the closure philosophy would need to be a ‘non-containment’
approach.
The cap will also help protect against inadvertent human intrusion into the wastes. It has been argued
that particular thicknesses of cap may help provide protection against specific potential inadvertent
intrusion mechanisms.
The cap may also include aspects designed to help manage potential impacts that might be associated
with emissions of bulk and radiolabelled gases.
A supporting design aim is to ensure it is robust against processes such as settlement, erosion and
biointrusion.
Timing issues are also relevant, as it may be considered that existing disposals to the trenches and Vault
8 require early protection (see discussions on progressive capping, and interim and ‘stage 1’ caps
below).
The final cap has been the subject of substantial amounts of study over the last two decades and the present
‘baseline’ design is considered to be consistent with best practice and experience from comparable
repositories and also other analogous sites and industries. The baseline design is well established and has
an underpinning logic. The role of the present assessment, therefore, can be seen as primarily testing
whether the baseline design is still appropriate, given the overall closure strategy identified.
Current plans (Williams and Proctor, 2007; Belton, 2007) involve sequential final capping once Vault 9
construction has been completed and any remediation or retrievals of the trenches have been undertaken
(this is the subject of an ongoing parallel study). For reasons of materials management, Vault 8 will be
capped in association with Vault 10 construction (Vault 9, Vault 10 etc will then be sequentially capped as
disposals are completed). The Vault caps will be ‘keyed in’ to the trench cap to produce, in effect, one large
cap structure. This modular design will lead to a ‘gull wing’ structure with a dome over the trenches and
one or more domes over the vaults. This may provide advantages in terms of spreading capping operations
over time – meaning that requirements for on-site infrastructure, volumes of material with time etc will be
reduced. In addition, the trenches (currently covered by an interim cap) will be protected by the final cap at
an appropriate stage.
88
A feature of the cap design and installation plan is a ‘Stage 1’ phase whereby profiling and initial
geomembrane barrier layers are installed, allowing the wastes to be protected from infiltration prior to
subsequent installation of the remainder. This allows flexibility regarding the timing of final construction
stages without adding cost.
The final cap will be approximately 3 metres thick in terms of the barrier protection, but including profiling
materials the height of the dome at its apex over the trenches will be several metres higher than this. Aside
from the backfill, the cap will contain (listed from bottom to top) a gas collection layer, a BES and
geomembrane hydraulic and gas barrier, sand and gravel drainage filter layers, a bio-intrusion cobble layer,
a sand and gravel filter layer, a silt and clay subsoil moisture retention layer, a surface soil layer, and
vegetation. The bottom gas collection layer consists of 500mm granular material and is provided with a
linear vent and gas rise arrangement for each arm of the gull-wing to disperse gases and control release. It is
intended that the vents will be sealed at site closure. The cap design is intended to provide a long diffusion
pathway for Rn, sufficient to ensure significant radioactive decay in-transit and thereby to minimise impacts
once the vents are sealed. Credit is also taken in the radon transport and dose calculations for diffusion
through and decay within the wastes, containers and backfill.
The baseline design is considered to be consistent with the requirements of landfill best practice and
regulations which provide a useful basis for comparison in the absence of prescriptive regulatory criteria for
near-surface radioactive waste disposal facility cap designs.
The proposed cap section is shown in the Figure below.
It is estimated that at closure the cap will be over 99.9% effective in preventing infiltration from rainfall. The
cap will degrade with time but is still expected to substantially reduce infiltration for timescales of a
thousand years and beyond.
According to present plans, the cap would be keyed in to a cut-off wall (which would be installed before the
cap) and run-off would be shed beyond the cut-off wall perimeter to surface drainage.
Belton (2007) also notes that, although the present baseline design restricts the extent of the cap to within
the site boundary, there may be an opportunity to extend the cap beyond the existing boundary in order to
provide a reduced gradient at the edge. This is noted for information, but is too detailed a question to
consider in detail through the current options assessment process; the issue is unlikely to impact
substantially on costs or post-closure impacts.
A key issue for this study is to confirm (or suitably modify) plans for modular capping. Previous iterations
of cap design have included the same ‘layering’ principle and basic approach (e.g. the 3.0m thick design
assumed in BNFL, 2002), but have considered a single domed cap implemented at site closure. The use of
the single domed cap might involve advantages not previously considered (e.g. maximising the volume of
space within the cap for LLW higher stacking and/or VLLW infill disposal). However, whereas the single
dome design was originally developed for late emplacement only, recent design work indicates that it
should be possible to progressively implement a single domed cap, even if a phased approach is taken
whereby the Stage 1 layers are implemented for some aspects of the facility (e.g. the trenches and Vault 8) in
advance of completion of the final layers.
Timing of cap emplacement is an important consideration in strategic management of the facility, and has
implications for operational leachate management as well as overall design. Although there may be design
advantages associated with implementing a single dome at site closure, this would mean that existing
facilities such as the trenches and Vault 8 would need to be managed for a substantial period of time prior
to final capping. This might necessitate a programme of monitoring and possible upgrading for the existing
interim trench cap, to help minimise the saturation state of the trench wastes prior to final capping, and
could even entail installation of a new interim cap over the trenches and/or Vault 8. However, as noted
above, the timely installation of the Stage 1 cap may address this issue.
Other important 'non-technical' issues that may influence the final decision on timing include:
NDA spend / affordability profiles.
Visual impact and local nuisance from construction, both for the final cap and of any interim caps.
89
Practical issues such as guaranteeing the availability of a suitably skilled workforce.
Other key optimisation issues relate to the potential environmental impacts that might be associated with
gas and human intrusion exposure pathways.
It would be useful to consider whether higher gradient/dome elevation is advantageous for both
designs.
It would be useful to consider whether a thicker cap could give additional benefits, for example by
providing additional protection against human intrusion; this could be particularly relevant if higher
stacking of waste is pursued.
It may be possible to include other design features to enhance C-14 / Rn-222 containment. This relates
to use of a cap that has been optimised to involve measures (e.g. a geomembrane) to ensure a sufficient
Rn diffusion time to maximise decay prior to potential exposure. Measures to protect against C-14
exposure, as its half-life is too long for increases in diffusion times to have an impact, include adding
chemical buffers to ‘trap’ C-14 through carbonation, or enhancing dissolution (e.g. through an enhanced
soil layer that will be saturated with rainfall, dissolving C-14 and transporting via runoff).
Consideration has been given in the past to adopting an ‘enhanced dispersion’ model to allow gases to
escape the facility post-closure with no restrictions, avoiding potential localised releases and associated
enhanced concentrations. An obvious way of doing this is to retain the gas vents within the cap postclosure; at present, the baseline is to seal them on final completion of all engineering. This might be
coupled with installation of an enhanced gas blanket in the cap. Both could be considered as variants
that could be applied to any of the main capping options.
Note that a ‘no cap’ (or, biointrusion-only cap) option would apply to a ‘Permeable or No Cap with
Controlled Dispersion’ type strategy and so should be considered for completeness.
Backfilling the cap with concrete as part of a strategy aiming to turn the vaults into a single monolith is
another important strategic option, to be treated separately.
The issues discussed above have been considered in developing the lists of options and exploring the
available evidence for performance against criteria of interest, as presented below.
Options and Variants:
Cap Baseline Option: Present Design (Progressively Implemented 3.1m Thick Two-dome cap)
Variants relating to the time of emplacement, and opportunities to implement different dome designs:
Timing Variant 1: Two Dome Design Implemented at Site Closure, with maintained trench / Vault 8 interim
or ‘Stage 1’ cap or upgraded trench interim cap.
Timing Variant 2: Progressively Implemented Single Domed Cap.
Timing Variant 3: Single Domed Cap Implemented at Site Closure, with maintained or upgraded trench /
Vault 8 interim or ‘Stage 1’ cap.
Design variants that could apply to all the above variants:
Design Variant 1: Additional Infiltration Barriers (geomembranes, other layers) to further reduce infiltration
Design Variant 2: Additional Material to Protect Against Intrusion (Thicker Cap)
Design Variant 3: Enhanced C-14/Rn-222 Containment Measures
Design Variant 4: Retain Gas Vents Post-closure, Enhanced Gas Blanket in Cap
Design Variant 5: Reduced number of layers (to reduce cost)
More detailed design variants e.g. minor changes to layer design are beyond the scope of the current study.
Alternative Baseline Option: Biointrusion Only Cap
Interactions with Other Components, and Other Optimisation Issues
90
Assuming such a feature is required, the design of any cut-off wall must be consistent with, and ideally
keyed into, the cap. The aim is that they should together to minimise water infiltration; also, by shedding
run-off outside the cut-off wall, the cap might establish a hydrological gradient across the cut-off wall
‘towards’ the area within the cut-off wall, thus supporting containment of potentially contaminated waters.
The current modular design assumes that Vertical Drains would be installed after the cap has been
installed, punching through and then repairing the cap. Other designs may allow for easier Vertical Drain
installation, assuming such features are required.
Caps with a large volume underneath the dome may allow for additional LLW/VLLW disposal in the void.
Key Issues Associated With Component Option Performance3
Criteria Group 1: Safety
(Worker Safety, Public Safety).
Health and safety aspects associated with implementation of the Cap include the following:
Conventional health and safety hazards associated with construction of substantial, complex structures
with multiple engineered layers.
Hazards associated with the transport of the materials required for construction of the cap and the fill to
go underneath it. Material volumes may differ substantially across options. For example, Williams and
Proctor (2007) suggests that the difference in volume of fill alone required between a domed and a gullwing cap may be as large as a factor of two (~1 million m3 for a domed cap, 500,000 m3 for a gull-wing
cap). Volumes of the engineered cap would also be around 1 million m3 in either case.
Radiological exposure, noting that workers are the most likely group to be exposed, but also that the
wastes are LLW, and this work should not involve direct disturbance or handling of wastes.
In summary, all the above hazards should in principle be readily manageable as they fall within the
boundaries of normal working conditions. Most of the material will be transported by rail (as for Vault 9
construction), and much of the infill material may be produced through vault excavations and thus will not
require transport.
Main Evidence Sources: Vaults Single Option documentation (Williams and Proctor, 2007). 2002 PCSC
(BNFL, 2002 and underpinning references).
Criteria Group 2: Impact on the Environment
(Habitats/Construction, Authorised Discharges, Operational Environmental Impact,
Post-closure Impacts, Non-LLW Waste Volumes, Resource Use)
Impacts on habitats should be limited. However this depends upon the footprint of the option chosen, and
the approach to construction. Care will need to be taken during construction not to disturb flora and fauna,
in particular associated with the adjacent SSSI; none is anticipated in current plans. The course of the
existing Drigg stream may be diverted during vault construction but that it not specifically a cap-related
issue. There may be dust etc blown on to the SSSI during construction but this is not likely to cause
significant impacts, and any required mitigation measures do not differentiate between engineered cap
3
Note that this is a summary of available evidence for options performance, provided as
input to the options assessment, to provide a starting point for discussion. The agreed
outcome of the options assessment may differ from the discussion presented here. Key
issues are presented ‘by exception’.
91
options.
Implications for authorised (operational gaseous and liquid) discharges should be minimal for all options.
There will be limited differences in operational environmental impact, except that environmental impacts
associated with the disposal of leachate from the trenches may be reduced by early implementation of a
final cap, unless an effective interim cap can be implemented prior to later final completion.
Post-closure impacts are discussed in the accompanying paper.
Non-LLW waste volumes will be generated across the suite of options being considered. However, it is
unlikely that waste production associated with cap implementation will be a significant differentiator
between the various options.
Resource use will principally include the construction material required for the cap and its infill, and the
energy required for associated transportation and construction operations. This could vary significantly
across options. However for variants on the 3m thick engineered cap, where existing materials are used for
infill, and the infill is generated either on site through vault excavations or transported by rail, the resource
use implications may not be strong differentiators. A thicker (e.g. 5m) cap would have a proportionate
impact on resource use, though would have limited effect on use of infill materials.
Main Evidence Sources: Vaults Single Option documentation (Williams and Proctor, 2007; Belton, 2007).
2002 PCSC (BNFL, 2002 and underpinning references), Schedule 9 Requirement 2 submission (Baker, 2008)
Criteria Group 3: Technical Factors
(Ease of Implementation, Confidence in Performance, Timescales for
Implementation, Impact on Operations, Flexibility, Impacts on Existing Engineered
Features, Capacity)
Ease of implementation – no clear case can be made that one option would be ‘easier’ to build than
another. All options involving a substantial cap would have their difficulties but are considered to be within
the capabilities of standard engineering practice. Progressive implementation might, however, add a further
layer of complexity in construction regarding assurance of performance for the final structure. Similar
arguments apply to Timescales for Implementation and for Impacts on Existing Engineered Features.
Confidence in performance – Normal practice is to direct cap waters to perimeter drainage; an extra risk to
confidence in performance may therefore be introduced by the gull-wing design, where the gully between
cap modules attracts a significant proportion of run-off and could be considered a potential weak point.
However, such risks can be mitigated by good design and implementation practice, and (potentially) by
locating ‘vertical drain’ features underneath the area of potential weakness. It would be difficult to argue
that confidence in the performance of a thinner cap, regarding it capability to prevent infiltration, would be
sufficient. By contrast, existing studies have not identified any potential major benefits in performance that
would arise from including extra layers, or extra thicknesses of existing layers. Confidence in the 3m cap
design arises from each component having an identifiable function; these have been the subject of
significant study regarding design and implementation, individually and as a system, and in long-term
performance evaluation.
Impacts on operations - Construction of a modular cap would involve passive engineering protection being
added to different disposal facility elements as soon as possible. Additional active operational management
(such as pumping and monitoring) might be required for a single cap implemented at site closure.
However, these considerations would not be expected to have a significant impact on disposal operations.
Flexibility - Delaying final capping would maximise opportunities for remediation or other activities. Later
capping may also allow enhance flexibility for plans regarding management of the facility to be modified in
response to improvements in knowledge and understanding, e.g. regarding threats from climate change,
sea-level rise and coastal erosion. Later implementation of the final cap also allows any further trench waste
settlement to occur and for design and construction approaches to be adapted as required. It is also
conceivable that cap design (e.g. geomembrane material) approaches will continue to advance over time.
92
Flexibility is also relevant to comparing single vs. double domed cap designs. A single domed cap would
maximise some aspects of flexibility in site operation, requiring additional infill volume that could offer
additional disposal capacity for higher stacked LLW and/or VLLW infill. It would also allow more
flexibility regarding cap gradient, which might further influence future capacity calculations (see below).
Overall decisions on waste capacity are beyond the scope of the current optimisation study, but the
flexibility to allow such schemes in the future remains a valid consideration for the present assessment.
Capacity – A single domed cap would allow additional vertical stacking in the vaults compared to a gullwing design. The additional LLW capacity could be of the order of 200,000 m3. Steeper cap gradients (up to
10% compared to 4%) may enable additional capacity of 1 million m3 compared to the present gull-wing
design. A range of designs and different capacities are of course available, but the single dome approach
potentially offers the greater flexibility (as noted above).
Main Evidence Sources: Engineering Report (in preparation), Williams and Proctor (2007) etc.
Criteria Group 4: Community and Socio-economic Factors
(Impacts on Local Community, Support for Local Community)
Construction of the cap would be a significant engineering activity compared with the day-to-day operation
of the LLWR. It may not be hugely significant in regional terms, but local requirements for man-power and
the delivery of (potentially) million(s) of cubic metres of material to construct the cap might involve a
handful of additional full-time staff and a team of contractors at the site for a number of years. This would
have some economic impact on the local community.
Detriments might be associated with local nuisance, and in particular the operation of earth-moving
equipment on site and transportation of materials, although it is expected that the majority of transports
would be by rail.
In addition there will be an impact on landscape and visual amenity for local residents, through the creation
of an engineered structure reaching an elevation of 34 m AOD (single dome) or 30m AOD m (gull-wing).
Previous studies (e.g. BNFL, 2002) have indicated that a new campaign of tree planting around the site
perimeter would be undertaken to help ‘screen’ the cap. Both types of cap could be landscaped, but it is
known there is some local sensitivity to permanent changes in land elevation.
2002 PCSC (BNFL, 2002 and underpinning references).
Criteria Group 5: Cost
(Total Implementation Cost, Affordability)
The overall cost of cap construction is estimated at £53.9M and would be similar in magnitude for caps of
similar thicknesses, whether one or two domes, implemented in a modular fashion or otherwise. Thicker
caps would be of a higher cost, in line with additional material volumes and/or requirements for additional
layers.
The cap is a high cost component of the overall disposal facility. There appears to be little opportunity for
further saving in costs and use of resources, given that most comes from fairly low unit costs spread over
very large areas. Reducing the thickness of the main infiltration barrier component from 600mm to 500mm
would save approaching £3m. (All costs are estimated as current, quoted as totals, including construction,
design, supervision and support costs).
Main Evidence Sources: Cap - Input Estimate 080809 R1 Vault & Trench Cap est.
93
Figure 1: Cap Section
94
Proforma
Cut-off Wall Options
Version 1. Author: Alan Paulley
Reviewed by: Dave Tonks, Mike Egan. Date: February 2010.
Implementation of a cut-off wall has been part of LLWR’s closure design since development of the 2002
PCSC, and remains an integral part of current plans. In evaluating the function that might be performed by a
cut-off wall, it is instructive to consider how it relates to other components of the ‘baseline’ closure design. It
is also relevant to note that the Vaults ‘Single Option’ process focused on gaining planning permission for
operation of Vault 9 as a storage facility, and did not significantly advance the engineering concept for the
cut-off wall. As a general rule, therefore, available evidence is based on assumptions made in the 2002 PCSC
(BNFL, 2002), updated to take into account the outcomes of the Vaults Single Option designs (Williams and
Proctor, 2007) and the assumptions made in the Schedule 9 Item 2 response (Baker, 2008).
The principal design functions of any closure cut-off wall are likely to be as follows:
(1) to help minimise / control lateral inflow of groundwater to the near-field for as long as possible,
supporting the cap in maintaining a low level of saturation within the trench and vault wastes (and in the
geological structures immediately beneath them), by diverting the horizontal-flow component of any
‘incoming’ groundwaters to deeper systems below the facility; and / or
(2) to help minimise / control lateral outflow of groundwater from the near-field to the near surface
environment for as long as possible, by diverting any waters that might contain concentrations of
contaminants originating from the trenches and the vaults to deeper systems, thereby reducing the potential
for release to the environment in the vicinity of the facility and enhancing the likelihood that any resulting
contamination plumes will discharge to sea rather than to land.
Design and implementation decisions associated with the cut-off wall need to be integrated with
consideration of the capping strategy.
The 2002 PCSC assumed that the wall will be “constructed to a depth below the base of the Upper groundwater
system. The cut-off wall will also be constructed within the plan extent of the capping system in order to ensure that cap
drainage is discharged to the outside of the wall”. The design was expected to be around 80cm wide and 20 to 25
metres in depth. Where the cut-off wall interfaces with the bedrock, the baseline design assumes that the rock
will be grouted to provide a 5m wide grout curtain of the same depth. In this design the cut-off wall will
therefore completely encircle the disposal facility. The proposed cut-off wall design (Carpenter and Proctor,
2007) specifies the use of bentonite slurry and/or cement.
The overall options assessment process for the cut-off wall needs to consider a number of issues, including
the following.
Determining whether the presence of a cut-off wall is indeed required as part of the overall combined
closure strategy (i.e. whether the benefits of the cut-off wall are not disproportionately small compared to
the implementation cost).
95
Determining if the existing cut-off wall is likely to be sufficient to provide the functions required.
The cut-off wall was constructed between 1989 and 1995 and is 470 m long and 1.1 m wide,
constructed in two sections between the north-east corner of Vault 8 and the southern end of
Trench 7. The design intention was to limit lateral migration of trench leachate to the railway
cutting, to control groundwater input to the trenches, and to reduce leachate production in
general by reducing horizontal ingress / egress. The design was for the wall to extend from 7.4 to
9 m below ground level. However, the ‘as built’ drawings suggest that it was keyed in to the clay
layer at around 5 m below ground level.
The existing cut-off wall provides protection to the north and east sides of the trenches (i.e.
around 60-70% of the length of their perimeter); it follows a line similar to that which has been
proposed for any fully encircling cut-off wall.
If a new/additional cut-off wall is required, it is necessary to determine whether it is necessary for it to be
constructed so that it completely encircles the facility in order to achieve its design functions. The
following variants can therefore be considered:
A part-length wall designed primarily to divert ‘incoming’ waters (particularly those shed by the
cap) to deeper systems, in order to keep the wastes as dry as possible;
A part-length cut-off wall designed to divert 'outgoing' leachate from the wastes to deeper
systems;
A complete encircling cut-off wall but with no grouting where it interfaces with bedrock.
Variations in cut-off wall depth – e.g. can a depth of 5-10m (that is, a cut-off wall implemented to
the same depth as the base of the trenches) deliver much the same overall benefit as one
constructed to a depth of 20-25m?
Determining whether or not there is a clear preference in terms of the material used, i.e. bentonite slurry
or cement (although this may be better considered in terms of design performance requirements, rather
than as a component of the current optimisation study).
It is unlikely that cut-off walls or any other passive water management measure would be emplaced for the
Vault Monolith type strategies as the whole aim of that strategy is to divert water away from the wastes.
Moreover, the absence of a cap (were a Permeable or No Cap with Controlled Dispersion strategy to be
adopted) would probably also imply that a cut-off wall should also be absent.
Cut-off Wall Baseline Option: As for existing ‘Schedule 9 Item 2’ design (see above), wall constructed from
cement/bentonite slurry
'Length' Variants
Length Variant 1: Protection against 'incoming' waters only (wall emplaced only where hydrogeological
understanding suggests there may be significant lateral inflows)
Length Variant 2: Protection against 'outgoing' waters only (wall emplaced only where hydrogeological
understanding suggests there may be significant lateral outflows)
Length Variant 3: No grouting of bedrock
'Depth' Variants
Depth Variant 1: Reduced depth, wall design reduced to a maximum of 10m AOD
Depth Variant 2: Reduced depth, wall design reduced to a depth to the base of the vaults (i.e. to provide a
minimum level of protection of the trenches against incoming lateral flows)
No New Cut-off Wall Variant:
No change to current arrangements (no new cut-off wall; retain existing structure)
96
Other engineered components of the disposal facility will also contribute to the twin aims of minimising
water saturation and diverting leachate to deeper systems. For example, the cut-off wall design must be
consistent with, and keyed into, the cap. Both components may be considered to act together to minimise
water infiltration; moreover, because the cap acts to shed precipitation to areas outside the cut-off wall, this
could potentially establish a hydrological gradient across the cut-off wall ‘towards’ the area it encloses, thus
supporting containment of potentially contaminated leachate.
It needs to be established whether both the cut-off wall and the vertical drains are necessary to manage
contaminated leachate sufficiently to significantly reduce potential future impacts.
Health and safety aspects associated with implementation of cut-off wall include the following:
Conventional health and safety hazards associated with excavation and cut-off wall material disposition.
However, the excavation of a deep wall and infill using a significant volume of material implies that such
risks may be higher than for other, more routine construction operations. Nevertheless such should be
manageable – there is adequate experience of working to this depth - given good practice in construction
(CDM).
Hazards associated with the transport of excavated and cut-off wall construction materials (these include
the conventional risks associated with handling bulk quantities of such material, and also any impacts
from radiological or other contamination in material that is excavated). A cut-off wall that was 0.8m
wide, 10m deep, and 2800m in circumference would require excavation and emplacement of 20,000 m3 of
material (around 800-1400 lorry loads).
Transport risks associated with the off-site movement of construction materials, noting that most off-site
will be by rail and that excavated materials will be used on-site as cap profiling.
Hazards associated with the potential disturbance of existing wastes and structures. Any cut-off wall is
likely to be located around the perimeter of the facility within the footprint of the cap, so the likelihood of
disturbance is small. Adoption of appropriate operational approaches, taking into account that the
wastes are LLW, should ensure that radiological exposures to workers (or to the public) are not a matter
for concern.
Main Evidence Sources: Vaults Single Option documentation (Williams and Proctor, 2007; Carpenter, 2007).
Impacts on habitats should be limited. Care will need to be taken when handling machinery etc so as not to
4
97
disturb flora and fauna, in particular associated with the SSSI.
Implications for authorised discharges should be minimal. There will be limited differences in operational
environmental impact, except that environmental impacts associated with any ongoing leachate release from
the trenches may be reduced – see related arguments for ‘post-closure impacts’.
Post-closure impacts are discussed in the accompanying paper (Paulley, 2010).
Non-LLW waste volumes will be generated across the suite of options being considered. However excavated
material is planned to be used as infill for the cap. Some of this material may be contaminated (potentially as
VLLW) as a result of historic leachate releases from the trenches.
Resource use will principally include the construction material required for the cut-off wall, and the energy
use associated with excavation, transportation and construction operations. Requirements will scale
according to the extent and depth of the cut-off wall to be installed.
2002 PCSC (BNFL, 2002 and underpinning references), Schedule 9 Requirement 2 submission (Baker, 2008).
Features, Capacity)
Ease of Implementation. Construction methods would be consistent with established best practice, and
therefore within the boundary of well-understood approaches. Shorter walls and (particularly) shallower
depths will tend to be easier to implement. Down to about 15m is relatively routine. Greater depths are
increasingly challenging and less common. Beyond about 25m is rare, and represents a demanding exercise;
experience is limited to few specialist companies.
Construction will require an appropriate level of expertise, which rests with a limited number of international
specialist contractors having UK bases.
Timescales for Implementation. These should not be significant for any of the options, although it is likely to
scale with length / depth.
Confidence in Performance. There should not be a problem with achieving the design specification for any
of the options. See engineering elicitation report for more details. Emplacement schemes allow for
uncertainties in long term degradation.
Impact on Operations, Flexibility, Impacts on Existing Engineered Features, Capacity. It is expected that
the cut-off wall would be constructed immediately prior to final capping. There should not be any significant
impacts on operations or existing features, and no implications for capacity. However, as a perimeter feature,
its location cannot be finalised until overall plans for future development and capacity of LLWR are finalised.
Main Evidence Sources: 073 Cut Off Wall Report 07/06/2007
Issue P2. Nguyen M, Gallagher E and Thompson R P.
RP/102917/4600005916/CS&A/00023
40166mds0031 Cut-off wall. Construction Method Statement. Carpenter J, AJ Hooper, Apr 2007
Whilst the installation of the cut-off wall would be a significant exercise compared with the day-to-day
operation of the LLWR, this activity would not (of itself) require a long-term site presence, or provide any
other significant level of support to, or impact upon, the local community in an economic sense. The main
factor is likely to be associated with nuisance from noise etc. during excavation, and from the transport of
98
materials. Impacts will vary according to the length and depth of the wall.
There would be no export of materials, assuming a cap is constructed for which infill is required. All major
imports of materials to the site are expected to be via rail.
The cost of the current baseline design option is estimated at £37.2M, based on an approximate length of
2,800m length, 0.8m width and depth to -5m AOD. (Except for about 170m length to the NE of the site where
rockhead is above -5m AOD, where wall is sealed 2m into rock, which is grouted to -5m AOD).
This is a high cost component. There could be considerable saving in costs and use of resources for a shorter
or less deep wall, particularly as the technologies required will be simpler and since there is greater industrial
experience in working to lesser depths.
(All costs are given as current estimates, quoted as totals, including construction, design, supervision and
support costs).
Main Evidence Sources: Cut Off Walls - Input Estimate 080809 R3
99
Proforma
Future Vault Base Slab, Liner and Walls
Options
Reviewed by: Dave Tonks, Mike Egan. Date: January 2010.
The base slab, liner and walls serve to provide a number of functions.
The existing Vault 8 base slab and liner provides a suitable surface for storage and disposal operations,
and is intended to serve as an impermeable barrier for the duration of the operational period (as a
minimum). See below for more details.
The Vault 9 base slab and liner system currently being installed is more complex than the Vault 8
design. It is also designed to provide the loading /operations surface, but with an enhanced layering
(double composite lining) system designed to minimise permeability for as long as possible. Current
plans are for this very low permeability base to be combined with impermeable vault walls to maximise
containment of any infiltrating waters and associated leachate. Post-closure, this may eventually to lead
to complete saturation of the vault, in which case any released leachate would be directed to the vertical
drains. However, it is recognised that these plans allow flexibility and could be modified, for example
by implementing a lower or otherwise permeable east vault wall, or by not having such a wall at all,
which would tend to direct all infiltrating waters directly to the vertical drains, and thereby minimising
water contact with the wastes.
The current baseline plan for the Future Vaults is to continue with the existing Vault 9 philosophy. This
however is just one option to be considered in the present process. If it is decided that the optimal
strategy is, rather, to minimise water contact with wastes (i.e. to avoid saturation of the vaults for as
long as possible), a design more akin to the Vault 8 design (i.e. still essentially impermeable but with a
lower specification) may be considered appropriate, with infiltrating water drained from the base of the
vault.
More details of the existing vault base slab designs follow.
For Vault 9, the base slab has a robust double composite liner system including 2 sets of HDPE membranes
on bentonite enhanced sand (5% bentonite), with an intervening ‘leak detection and collection’ layers, all
underlying a reinforced concrete base slab. The base slopes to a drainage and collection system at the west
of the vault. The combined performance of these components is to provide hydraulic performance
equivalent to not less than ‘five meters of clay having permeability 10-9 m/s’. As installed the base is likely
to be more effective than this. The base concrete has a design life of 50 years and is installed for primarily
operational reasons, to support disposal activities and to provide suitable drainage surface. It will, however,
also provide an additional highly impermeable barrier for many years, but this function is expected to
reduce with time and due allowance has been made in assessments.
100
It is possible that future vaults could be constructed with descending / stepping base levels, falling
gradually to a depth of 11m AOD for Vault 14.
The Vault 8 and trench operational leachate drainage systems will be modified to accept leachate from the
Vault 9 culvert drain. During operational management, leachate release will be controlled through the
marine holding tank; after closure / end of Active Institutional Control (unless vertical drains are installed
to collect water directly from the base) infiltrating water exceeding the drainage capacity of the base will be
contained until the base slab degrades, after which partial waste saturation can still be expected.
A secant pile wall provides support along the eastern side of the vaults against the trenches area to enable
construction, but offers no long term support or impermeable barrier function. For Vault 9 vertical walls,
consisting of twin reinforced concrete walls with 1m thickness of BES infill, will be installed around all sides
of the vault. A modification to the design (more consistent with the Vault 8 design philosophy) could
involve not installing the ‘eastern’ vault wall, or adopting a permeable design, to encourage outflow to
vertical drains from the vault base, and thereby maintain unsaturated conditions for as long as possible.
The internal walls could be formed just 1m high, suitable for containment during operations, but thereafter
allowing leachate to flow southwards, coupled with a drop in base level of the vaults, to collection until end
of active institutional control and thereafter to vertical drains if necessary.
The Vault 8 design also includes a reinforced concrete base that is 250-300mm thick on a stone drainage
layer laid down on clay, or bentonite-enriched soil (BES) where the clay is less than 1m thick. The basal
drainage leads to a drainage culvert. The initial permeability of the slab is estimated to be equivalent to
several metres of 10-12 ms-1, due to the qualities of the concrete and the membrane. However after a hundred
years it is estimated that the barrier will have degraded to a permeability of around 2.5 metres of 10-9 ms-1
material.
Designs equivalent to the Vault 9 (‘baseline’) and Vault 8 (‘reduced permeability’) base slabs are appropriate
options for consideration in the design of the future vaults. The choice of base specification will depend on
the overall engineering strategy to be followed.
Other options include implementation of a ‘minimum standard base’ that provides the minimum
specification required to act as a sufficiently stable surface to enable disposal operations (i.e. a mechanically
robust concrete slab). Exploring this option helps to establish the relative benefits of more ‘hydrologically
robust’ alternatives. In addition, such an option would be particularly relevant to consider as part of the
‘Permeable or No Cap with Controlled Dispersion’ or ‘Vault Monolith’ type strategies.
There are also possible design options involving a ‘deeper vault base´. In such a case, the future vaults
would be constructed with the base being laid upon a deeper excavated surface. This has the potential to
provide the following benefits:
Reductions in base depth by a few metres could realise additional disposal volume without requiring a
further increase to the elevation of the final cap.
Alternatively, a reduction in base depth by a few metres but with no increase in the height of waste
stacking could potentially lead to reduced likelihood of near-surface contaminated water discharge.
If the base were installed deep enough, it is possible that future disposals to the facility could be
undertaken in such a way that the wastes would not be disrupted by coastal erosion / sea-level rise.
Vault Slab and Walls Baseline Option: Future Vaults Installed to Current Vault 9 Design (NB the possibility
of achieving the same specification with a single geomembrane liner is also relevant)
Base Specification Variants
Base Specification Variant 1: Reduced Specification Base for Future Vaults (Equivalent standard to Vault 8;
no geomembrane liner)
Base Specification Variant 2: Concrete Loading Base only (no additional long-term performance measures;
similar standard to ‘Dounreay’ design)
Base Depth Variants
101
Base Depth Variant 1: Lower Vault Bases compared to current plans (enhanced capacity or reduced
likelihood of near-surface discharge)
Base Depth Variant 2: Much Lower Vault Bases Compared to current plans (top of wastes below future sealevel)
Wall Variants
Wall Variant: No or Reduced Height Wall at East of Vault 9 (non bath-tubbing strategy, connecting leachate
directly to Vertical Drains).
Variants Noted to Test Sensitivities
Sensitivity Variant: Stepped Future Vault Bases (i.e. successively lower elevations for Vaults 10 to 14)
The base and wall design should be complementary. In the above option descriptions, the vault walls do not
vary significantly in design, except where discharge of infiltrating waters at the vault base is required. It is
assumed that their design is ‘standard’ and according to detailed optimisation work in previous studies (e.g.
Williams and Proctor, 2007, and underpinning references).
It seems unlikely that a containment strategy based on keeping the wastes unsaturated for as long as
possible (by promoting the passive drainage of leachate from the base of the facility) would be consistent
with a closure design that incorporated a ‘Vault 9’ standard base, since the lower permeability measures
associated with such a design would be redundant.
Health and safety aspects associated with implementation of the future Vault Base Slabs, Liners and Walls
include the following:
Standard health and safety hazards associated with similar types of construction activity.
Hazards associated with the movement on site of excavated and construction materials (these include
the conventional risks associated with handling bulk quantities of such material, and also any impacts
from radiological or other contamination in what has been excavated). The works will involve around
0.5 million m3 of excavated material which is planned to be retained for use in cap profiling.
Transport risks associated with off-site movement of construction materials, noting that most off-site
transport will be by rail.
Hazards associated with the potential disturbance of existing wastes and structures. Care will needed to
be taken to avoid waste disturbance; however, given appropriate operational approaches, and taking
into account that the wastes are LLW, radiological exposures to workers (or to the public) should not be
a matter for concern. Vault 8 and Vault 9 construction experience supports this.
All the above hazards should in principle be readily manageable for all options as they fall within the
boundaries of normal working conditions.
Main Evidence Sources: Vaults Single Option documentation (Williams and Proctor, 2007). 2002 PCSC
5
102
(BNFL, 2002 and underpinning references).
Impacts on habitats should be limited. However this depends upon the footprint of the option chosen, and
the construction approach. Care will need to be taken during construction not to disturb flora and fauna, in
particular associated with the adjacent SSSI; no significant disturbance is anticipated in current plans. The
existing course of the Drigg stream will be diverted during vault construction, as it has already been in the
past.
Implications for authorised discharges should be minimal for all options.
There will be limited differences in terms of operational environmental impact, provided that the trench
wastes and associated contamination are not unduly disturbed.
Post-closure impacts are discussed in the accompanying paper.
Non-LLW waste volumes will be generated across the suite of options being considered. However, it is
unlikely that waste production associated with vault construction will be a significant differentiator
between the various options.
Resource use will principally include the construction material required for the vaults, and the energy use
associated with transport and construction operations. In particular the volumes of material required for the
lining process could be large.
Features, Capacity)
Ease of implementation, Timescales for Implementation – standard engineering practice. Vault 9
experience indicates that more complex, multiple component bases are more difficult and time consuming
to construct to assured levels of quality and performance, but the difficulties are not insurmountable.
Experience from both Vault 8 and Vault 9 will help to inform future construction approaches.
Confidence in performance – the approach and quality measures taken in construction will assure
performance for whatever design is followed. Layers will be tested, validated and rectified if necessary.
Experience suggests that performance after emplacement is likely to be rather better than the design
specification, as discussed at the engineering performance elicitation meeting.
Impact on operations – all options discussed will provide an appropriate loading platform for future
disposals, and would be consistent with appropriate operational drainage approaches.
Flexibility – schemes consistent with the Vault 9 design can be converted from ‘storage’ to ‘disposal’ type
approaches; this flexibility (degradation in hydraulic containment performance can be more readily
engineered after construction than can improvement) was recognised in the original decision to adopt the
Vault 9 design when applying for planning consent. However, such a factor may be considered less
important in making the case for disposal authorisation via the ESC.
Impacts on Existing Engineered Features – existing disposal and drainage arrangement will need to be
considered during detailed design and implementation, but such factors do not present clear
103
differentiations between base/wall design options.
Capacity – deeper vault base options may be consistent with maximising disposal capacities, unless the
overall aim of adopting deeper base was to reduce the overall elevation of the final cap.
Whilst the installation of the future vault base slabs and walls would be a reasonably significant exercise
compared to the day-to-day operation of the LLWR, this activity does not (of itself) require a long-term site
presence, or provide any other significant level of support to, or impact upon, the local community in an
economic sense.
It is planned that excavated material will be re-used on site as infill, and all major material transports to the
site will be via rail. The main factor is therefore associated with nuisance from noise etc. during on-site
construction activities. This should be limited and manageable, as shown by experience in Vault 9
construction. It is unlikely that this would be a significant differentiator between options.
The cost of the Future Vaults base slab and walls construction is estimated at £110m. (Based on V9 being
£20m). Considerable savings might be possible, for example through:
Eliminating the second composite layer of BES 500mm and Geomembrane, leakage detection layers).
This could save around £10M.
Eliminating the internal BES wall and associated complex 2nd concrete wall (consistent with a non bathtubbing design). This could also save around £10M.
Deeper vaults, i.e. following the reducing ground profiles to about 11m AOD (top of slab) by Vault 14, tends
to optimise the geometric fit to the site topography. This can create around 0.22M m3 additional capacity
(existing scheme around 0.75M m3) (c 30%) for some £5-10M extra cost.
Future vaults are a high cost component of the engineered system. There appears to be little opportunity for
further saving in costs and use of resources, noting that most comes from fairly low unit costs spread over
large areas.
(All costs are given as current estimates, quoted as totals, including construction, design, supervision and
support costs.) .
104
Proforma
Post-closure Passive Leachate Management
System Options
Reviewed by: Dave Tonks, Mike Egan. Date: January 2010.
Post-closure leachate management systems can provide a passive design feature that can help to mitigate
the consequences of degradation and failure of other engineered barriers. Specifically, approaches such as
vertical drains may be implemented to provide the following functionality:
To permit excess water that may accumulate within the vaults system to be released from the
facility in a controlled manner that may limit environmental impacts from contaminated leachate.
Without a suitable drainage strategy for the facility, in which such features may play an important
part, it is possible that any ’bath-tubbing‘ waters in the vaults could eventually overspill into the
near-surface waters and soils surrounding the facility, thereby giving rise to enhanced
environmental impacts. Thus vertical drains provide a supplementary role to containment
measures, helping to minimise site impacts as those containment measures degrade over time.
To provide ‘redundancy’ in case of cap failure. The original rationale for the vertical drain(s)
(BNFL, 2000; BNFL, 2002) was that, if the cap were to fail significantly on a fairly short timescale,
it would be necessary to divert and drain excess water to deeper systems, thereby avoiding the
potential for leachate discharges to near-surface systems associated with ponding within the
confines of an encircling cut-off wall.
Previous reviews (e.g. Paulley, 2008) established the general principle that all near-surface repositories
feature some suitable means for the passive management of leachate from the disposal system. In
particular, vertical drains have been implemented for a small number of international facilities to provide
a mechanism for managing water in facilities that are otherwise designed to maximise containment over a
defined operational period. In the majority of these cases, however, the vertical drains connect to
underground collection and storage tanks rather than to deep hydrogeological features.
The current LLWR Modular Vaults Single Option design includes the assumption that two series of
contiguous 2 – 3.2 metre diameter drains will be constructed between Trench 3 and the line of Vault 9 and
the future modular vaults. This location has been identified as the most likely point of cap failure in the
future (given the current cap modular design) as a cap drainage gully follows that line.
The primary aim of the drains, within the present design option, is to control the fate of any leachate that
collects within the vaults following cap degradation, by ensuring that it is directed to the regional
groundwater system rather than having the potential to contaminate surface or near-surface water bodies.
The total cross-sectional area of the drains, in the present design, would be around 3000 m2, and the drains
are expected to be around 20-30 metres in vertical length. A key issue is whether the drains will remain of
105
adequate capacity (i.e. they will not collapse or become obstructed) for sufficient time to provide a
valuable contingency function should that be required. It is also possible that, even when clogged, the
drains will still provide a suitable higher conductivity preferential flow pathway that may provide longerterm benefits.
Other engineering components may also be emplaced to help manage leachate and other waters alone, or
in combination with the vertical drains; for example a cut-off wall would also help direct leachate to
deeper systems.
Other vertical drains options can also be envisaged:
It is relevant to consider whether the drains do indeed have the capability to perform a significant
passive control function, by evaluating a ‘no vertical drains’ option.
Although the current location of the vertical drains has a logic associated with it (i.e. being underneath
the most likely point of cap failure, assuming the gull-wing design), it is possible that the overall
function required of the drains could be equally well or better satisfied by other arrangements that
might offer additional advantages. For example, the main clogging mechanism is expected to be
related to interaction with leachate from the trenches, rather the than vaults. This suggests options
such as locating the vertical drains to the West side of the vault, or all around the entire facility. Such
options might also provide secondary contingency benefits in the event of the failure scenarios
described above.
The current design differs from the 2002 PCSC design in that it features an array of wide boreholes,
whereas the 2002 design considered a single rectangular area of vertical drainage, of significant
volume. For completeness is it appropriate to (briefly) revisit the logic for this change, noting the
different functional requirements behind the two designs.
The role of passive leachate management systems for strategy options associated with an intention to
maintain unsaturated conditions for as long as possible, is slightly different. Here, as well as providing the
functions described above, the strategy involves the management of all water that enters the vault system,
potentially also providing a level of protection for the trenches also, and not just overtopping waters from
the vaults after they become fully saturated. The drains would therefore need to be connected
hydrologically to the base of the vaults. Contaminant concentrations in waters discharged via the vertical
drains should therefore be lower than would eventually occur for the ‘Leachate Containment’ type
strategy. However, discharge via the drains (albeit at very low volume) would occur immediately,
whereas for the ‘Leachate Containment’ strategy, it will be delayed for an amount of time, as discharge
will not happen until the vaults are completely saturated.
Other engineered features may also assist in achieving the aims of a leachate management strategy; for
example, permeable vault bases may add to the total passive drainage capacity and any cut-off wall
would also assist in directing leachate to deeper waters.
The alternative vertical drain options identified above will also be relevant to a ‘Contaminant
Containment’ strategy, albeit providing slightly different functions. In addition, a further option suggests
itself for such an approach: additional drains could be emplaced that would either be either directly
connected to, or even placed within the future vault bases to maximise the overall drainage capacity and
enhance redundancy and to help minimise the potential impacts of any clogging that may occur.
Vertical drains may also provide a role within any option associated with the Permeable or No Cap with
Controlled Dispersion Strategy Option. It would be consistent with such a philosophy to maximise the
total vertical drainage through the system. The baseline option here would probably be as for the current
Single Option design plus additional boreholes surrounding the facility perimeter and within, or
connected to, future vault bases.
It is unlikely that vertical drains or any other passive leachate management measure would be adopted for
the Vault Monolith Strategy Option as the whole aim of that strategy would be to keep the wastes dry by
diverting waters away from the wastes.
Passive Leachate Management Baseline Option: Series of Wide Boreholes Vertical Drains Implemented
106
between Trench 3 and the modular vaults
Alternative Baseline Variant:
No Vertical Drains (with either reduced specification future vault base to otherwise enhance the overall
vertical drainage capacity, or vault base as for current design – see ‘vault base and walls’ proforma)
Vertical Drain Location Variants:
Location Variant 1: Vertical drains All Round Facility
Location Variant 2: Vertical drains the Other Side of the vaults
Location Variant 3: Vertical drains also In or Connected To Vault Bases and Drainage Systems
Alternative Design Variant:
2002 Design (one large deep rectangular drain).
The potential requirement for vertical drains is related to the function provided by a cut-off wall in
directing leachate to lower water systems (and vice versa). The location of the vertical drains will be
important in relation to other engineered components, particularly in terms of the likely rate at which they
become obstructed. ‘Contaminant Containment’ options (based on maintaining unsaturated conditions for
as long as possible) are likely to involve linking to elements of the existing leachate management system at
closure, rather than that system being entirely backfilled and closed.
Health and safety aspects associated with implementation of vertical drains include the following:
Conventional health and safety hazards associated with borehole drilling and completion;
Construction hazards associated with other forms of drain implementation, if adopting alternatives to
the current baseline;
Radiological exposure, noting that workers are the most likely group to be exposed, but that the wastes
are LLW, and this work should not involve direct disturbance or handling of wastes;
Hazards associated with the removal of excavated material (these include the conventional risks
associated with handling bulk quantities of such material, and also any impacts from radiological or
other contamination within the removed material).
6
It has been estimated that the amount of earth required to be excavated to implement the 2002
PCSC vertical drain design (BNFL, 2002b) would be in the region of 500,000 m3 (Carpenter,
2007).
The 2002 design assumed that the surface area of the vertical drains would need to be around
10% of the total vault area in order to cover all possible failure scenarios. Carpenter (2007)
argues that 1% would be more appropriate, and this provides the basis for the number of wide
boreholes in the current baseline design. This would require the removal (and, potentially,
107
disposal if contaminated) of around 45,000-50,000 m3 of material (2000 - 3000 lorry loads) and
the transport and installation of an equivalent volume of infill material.
Overall volumes would be proportionate to the number of boreholes implemented. For options
involving additional redundancy, in particular for the ‘all around the facility’ approach, the
volumes may scale by (as a rough estimate) factors of 2 to 3.
Transport risks associated with movement of excavated material, noting the potential requirements for
transportation noted above; and
Hazards associated with the potential disturbance of existing wastes and structures.
In summary, all the above hazards should in principle be readily manageable as they fall within the
boundaries of normal working conditions and engineering approaches. However, construction of a ‘2002
Design’ drain which would require a very large, single excavation and a difference in geometry and
construction method compared to the installation of wide boreholes. Transport of materials off site, if
required, would most likely be by rail.
Main Evidence Sources: Vaults Single Option documentation (Williams and Proctor, 2007; Carpenter,
2007). 2002 PCSC (BNFL, 2002 and underpinning references).
For the type and location of drains being considered, impacts on habitats should be limited. Similarly,
implications for authorised discharges should be reduced.
There will be limited differences between options in terms of operational environmental impact given that
issues such as waste generation are covered elsewhere. Consideration would need to be given to
interactions with the operational leachate management system to ensure that it is not compromised during
installation of passive drains, but that should be a manageable process. The drains will also influence the
local groundwater flow regime, including that associated with the trenches immediately upon
emplacement. However, if this occurs at closure and not before, the ‘operational’ impact should be limited.
Post-closure impacts are discussed in the accompanying paper (Paulley, 2010).
Non-LLW waste volumes will be generated across the suite of options being considered. Contaminated
material (probably VLLW) is likely to be encountered during the drain excavation processes (as a result of
past discharges of leachate from the trenches); such material may require treatment and re-disposal with
associated environmental impacts.
Resource use will principally include the infill material used for the drains, and the energy required for
associated transport and construction operations. These requirements will scale according to the number of
borehole drains emplaced.
2007). 2002 PCSC (BNFL, 2002 and underpinning references), Schedule 9 R2 submission (Baker, 2008).
Features, Capacity)
Ease of implementation, Timescales for Implementation – standard engineering practice, assuming wide
borehole drains. Implementation based on the ’2002 design’ may present greater challenges.
108
Impact on Operations, Flexibility, Impacts on Existing Engineered Features, Capacity: Installation would
take place at site closure so these should not be significant discriminators between options.
Confidence in performance. ‘Confidence in performance’ needs to be considered in terms of the likelihood
that the vertical drains will provide an effective free drainage route over the required timescales, that they
will provide for deep, rather than near-surface, discharge of vault waters over this period, and – as a
secondary measure of performance – that they will continue to provide a preferential groundwater flow
pathway even when degraded. In the main, such issues are largely addressed by the ‘post-closure impacts’
criterion noted above. However, the following additional points may be relevant.
The construction methods are well established and confidence can therefore be placed that they will
perform as designed at the time of emplacement.
Options involving drains in more than one location, and in particular those that include additional
drains to those in the baseline design, are likely to have a decreased risk of early clogging
degradation in performance across a significant proportion of the required drainage capacity.
It may be that locating drains elsewhere than the baseline location, to avoid the interception of
trench leachate, could extend the likely time period before clogging.
2007).
Whilst the installation of the vertical drains would be a reasonably significant exercise compared to the dayto-day operation of the LLWR, they will not (of themselves) require a long-term site presence, or provide
any other significant level of support to, or impact upon, the local community in an economic sense. The
main consideration is likely to be associated with nuisance during construction, and the transportation of
any materials through the local villages, in line with the volumes described above.
2007). 2002 PCSC (BNFL, 2002 and underpinning references).
The cost for the present baseline design is estimated at £11.9M, based on a total area of about 3000m2
(equivalent to 1% enclosed area), to a depth of -5m AOD. There could be considerable saving in costs and
use of resources for a smaller area (approximately proportionate) or less depth. (All costs are given as
current estimates, quoted as totals, including construction, design, supervision and support costs). Costs for
the 2002 design are likely to be far higher than for the borehole designs as the volumes for disposal and
imported stone would be great and the construction method would be particularly challenging. ( Quick
estimate > £20M plus cost of 500,000cu.m disposal on-site!)
Main Evidence Sources: Vert Drains - Input Estimate 080809
109
Proforma
Other Engineered Features
Version 1. Author: Alan Paulley, Dave Tonks.
Reviewed by: Mike Egan. Date: January 2010.
This proforma considers a range of further potentially important components of the engineered system
that may need to be addressed within the optimisation process, in addition to the ‘main’ engineered
components considered elsewhere. Such design components include:
1.
Alternative arrangements for operational leachate management approaches. It is important that the
approach to operational leachate management is consistent with, and not compromised by, the
approach to site closure. If certain designs for engineered barriers are followed, alternative leachate
management arrangements might be required.
For the vaults, drainage is / will be through granular fill and voidage between the HHISOs. Pipework
has been eliminated as unnecessary (and undesirable with respect to possible clogging issues).
Although deemed leachate, these waters will essentially comprise rainwater passing around the steel
HHISO containers for the operational phase, for up to perhaps 20 years before capping, and then
small infiltration quantities for a long time thereafter. By 100 years, there could be some limited iron
(rust) and alkali from degrading grout, but probably nothing of much consequence.
The vault bases are designed with a fall to manhole pumping chambers on the west side wall for ease
of access. The basal chambers have a low surround wall to prevent ingress of possible operational
fines from the vault base area. The voids and granular infilling between HHISOs will give ample
drainage within each vault and operational heads will be small. It is planned for leachate to continue
to be pumped from the manholes to a collector drain, then gravity fed to the marine holding tank
(MHT).
Stepped vaults base levels would allow gravity drainage southwards down to Vault 14. A 1m south
internal wall is proposed for each vault to allow suitable control during the operational stage. On
completion of each vault cell it can be designed to collect leachate weiring over from upstream. This
avoids potential concerns over leachate drainage systems clogging in the long term.
Vault 9 incorporates a leachate detection and collection layer beneath the upper basal geomembrane
and BES composite. Any leachate will be collected and monitored during operations and active
control. Dependent on observations from that monitoring, this is not proposed for future vaults.
On closure, the drainage would be closed off, and the LDC layer will saturate to be in equilibrium
with the materials above and below. Having high permeability, this layer will provide a source of
constant head to the lower geomembrane / BES layer, which thus will not significantly add to the
leachate control from the upper system.
For the trenches, leachate is gravity piped from the bases of the trenches to collection chambers and
110
thence pumped to the marine holding tank. However, the original drainage was not to modern
standards and arguably cannot be relied on. Work is in progress to retrofit new leachate drainage and
the leachate amounts and qualities will be re-assessed with monitoring over future years, once the
new drainage is working. Dependent on findings, it may be appropriate to ‘retro-fit’ additional
drainage in some trenches, at some locations, as might be done by horizontal boring techniques which
have been developing in recent years and becoming increasingly practical. However it will be difficult
to ever ‘guarantee’ that all of the trenches are effectively drained, due to heterogeneity of wastes and
the nature of material in the trench bases.
At present there are significant leachate heads in some trenches. These should be reduced with the
planned work. The hydrogeology studies indicate that the cap (even if functioning poorly after 20
years or so, to say 50mm / year infiltration) can create an unsaturated zone beneath the trenches. This
sets an important context for the performance requirements of the operational leachate management
systems.
Unless continued desaturation has an impact, degradation of the wastes in the trenches is expected to
continue to around current rates for the foreseeable future. Significant settlements will continue
accordingly. There will be further significant settlements from loading to form the cap.
In summary; note that BPM studies have been undertaken for operational leachate management and
monitoring undertaken elsewhere. It is anticipated that the operational leachate management
approach will not constrain choices for other components unless a significant deviation from the
current strategy is followed e.g. monolith-type approaches.
2.
Alternative vault backfills. The current baseline is to backfill any voids within the vaults with gravel
or other coarse granular material to facilitate drainage whilst minimising settlement. The ‘Void Filling
Report’ describes alternative backfills that could also provide a drainage function, and identified that
gravel (or equivalent) infill appears the ‘best’ option in this regard.
The main alternative is that the vaults could instead by backfilled with impermeable grout / concrete
in order to create a comparatively impermeable monolithic structure that would minimise water entry
into and release of leachate from the wastes. A drainage blanket may be required over the wastes to
ensure that waters entering above the facility are shed elsewhere. It might also be that the HHISO
containers in Vault 9 and the future vaults would need to be stacked with a sufficient gap to allow
grout to be placed between them. Potential problems that have been raised include the difficulty in
‘guaranteeing’ production of a meaningful monolith through this approach, as there could be cracks
and joints and voids which would propagate and potentially form meaningful pathways contrary to
the principal aims of such a backfill strategy.
3.
Facility armouring / strengthening / local hardening approaches to help mitigate disturbance by
coastal erosion and sea-level rise. Options include:
a.
Facility armouring, i.e. burying/emplacing boulders around the edge of the facility to present
a barrier to erosion processes.
b.
Local hardening of the headland / estuary to promote future ‘passive’ local sediment
deposition (rather than erosion) in the area of the LLWR.
c.
Monolithic backfilling (i.e. using concrete as a backfill, as above) may help harden the
disposal system to help minimise the rate of erosion and exposure of wastes (see discussion
on backfill above).
Larger scale coastal defences are not considered in the current process as these are not consistent with a
diminishing reliance on active management and maintenance post closure. In order to have a substantial
impact, such features would need to be maintained over a prolonged timescale. This does not mean that
coastal defences are irrelevant to the future management strategy for the LLWR; rather that they cannot be
assumed to provide any benefit in terms of post-closure performance assessment for the ESC.
111
Present baseline option (operational leachate management arrangements as per Modular Vaults Single
Option design; gravel backfill; no facility armouring / strengthening / local hardening).
Operational leachate management alternatives:
None identified (see above).
Backfill variant:
Backfill Variant: Grout backfill.
Facility armouring / strengthening / local hardening variants:
Armouring / Strengthening / Hardening Variant 1: Facility Armouring
Armouring / Strengthening / Hardening Variant 2: Local Hardening of Headland / Estuary
Armouring / Strengthening / Hardening Variant 3: Monolithic Backfilling
The operational leachate management strategy followed must be consistent with the closure design and
the associated approach to construction.
Grout backfilling would have interactions with waste emplacement considerations, for example different
compositions would lead to different pH conditions within the system. It would also have implications for
the cap design. If backfilled, there would be no need for vertical drains.
The adoption of deeper vault bases as a potential option to mitigate the impacts of future inundation
and/or coastal erosion is covered in discussion of options for the vault Base and Walls.
Key Issues Associated With Component Option Performance
Features, Capacity)
Due to the nature of these options, a summary of evidence for and against performance for
each of the criteria groups has not been provided in this note. Rather, it is suggested that the
expert group involved in the relevant assessment workshop may be best placed to assess the
broad benefits and drawbacks of these options directly.
112
Appendix E: Commentary on Possible
Implications of Different Component Options
for Post-closure Impacts
113
Component Options – Post-closure Impacts
Commentary on Possible Implications of
Different Component Options for Post-closure
Impacts
Version 1. Author: Alan Paulley.
Checked: George Towler, Mike Egan. Date: February 2010.7
E.1
Introduction
The LLWR ESC pre- and post-closure engineering optimisation process is considering
the potential advantages and disadvantages of different options for each engineering
component. Workshops are being held to systematically evaluate and compare these
options in order to derive an overall optimised engineering strategy.
A set of notes has been developed to discuss factors related to each of the identified
component options, for review and further consideration within the project (Paulley,
2010a-e). The main focus of these notes is on issues relating to the practicability of
implementation for different alternatives, including safety during construction, general
environmental impacts, technical factors, community impacts and costs. However, they
do not discuss in any detail the implications of different component options for postclosure impacts. These issues are therefore explored in the present document.
The information presented here has been synthesised from a range of detailed
supporting studies undertaken for the ESC and from previous assessment iterations. It
also draws on notes of analyses on specific issues that have been produced in support
of this optimisation study.
7
Discussions at the assessment workshop held subsequent to the development of the notes
reproduced in this Appendix took this analysis into account alongside additional data from
updated hydrogeological calculations. The assessments of impact referred to in the main text
of this report include minor development of some of the arguments presented here.
114
E.2 Engineering Components Relevant to Minimising
Impacts
E.2.1 Release Pathways Associated with Potential Impacts
The pre- and post-closure engineering strategy needs to be consistent with the
principles of ALARA. The primary driver is therefore to identify strategy options that
can be optimised to minimise pre- and post-closure impacts to the environment. Such
impacts primarily relate to:
radioactive and non-radioactive contaminant concentrations in surface water and
upper groundwater systems as a result of disposals;
environmental impacts associated with contaminant releases to atmosphere (in
particular, associated with potential gaseous releases of C-14 and Rn-222);
threats associated with the potential for disturbance by human actions (e.g.
inadvertent intrusion into the wastes through future dwelling construction); and
potential impacts associated with site disruption as a result of natural processes
(e.g. coastal erosion and sea-level rise).
In addressing these threats, it is useful to consider in more detail the surface water and
groundwater
systems
that
represent
particularly
important
pathways
for
environmental impacts.
In the context of long-term environmental safety performance, where
dependence must be placed on passive environmental control, optimum
protection against impacts associated with releases to the environment involves
minimising contaminant fluxes from the disposal facility for as long as possible.
When it is no longer possible to avoid leachate release, the aim is to seek to direct
the flow of leachate to those parts of the environment where the impact will be
lowest. As a general rule, this means avoiding discharges near to surface in the
vicinity of the facility, and instead seeking to direct leachate to depth where it can
be diluted in larger volumes of groundwater before eventual delayed release the
marine environment.
If the vertical drainage capacity is not sufficient to deal with infiltration that
enters the facility, it is conceivable that uncontrolled overtopping of physical
barriers to leachate transport could occur. This, in turn, could lead to the direct
discharge of contaminated leachate to soils and waters around the perimeter of
the facility. Such a situation would be undesirable as it represents the shortest
115
conceivable pathway from source to receptor. An important element of
engineering design is therefore to mitigate the likelihood and magnitude of such
releases. However, it is unlikely that a cap alone would be considered sufficient
to provide the necessary level of confidence in protection against such
occurrences. A key consideration in optimisation is therefore the degree of
redundancy required to make the case that system performance does not rely
excessively on a single engineered component, and what other features should be
incorporated in order to provide a sufficient degree of defence in depth.
E.2.2 Overview of Relevant Components
Discussions at the scoping workshop, held at the Energus Centre on 17th December
2009, identified the following issues as being relevant to minimising contaminant
releases from the LLWR and controlling any releases that do occur.
Minimising water contact with the wastes is of primary importance, in particular
in terms of minimising groundwater pathway releases. Engineered features such
as the cap, and the nature of disposed waste forms and associated containers, are
relevant here.
The nature of the waste forms and containers is also relevant to minimising
release if water infiltration into the facility does occur. Optimisation of waste
forms is outside the scope of the present study; however, features and processes
related to waste form (e.g. congruent release, corrosion etc) are important factors
in differentiating between engineering options.
Engineered features such as base slabs, vertical drains and cut-off walls can be
relevant in controlling the nature and location of discharges, should there be
contamination of water passing into and through the facility.
There was general agreement at the scoping workshop that the following hierarchy of
engineering approaches are of relevance to these issues.
The most important feature of the LLWR engineering post-closure is likely to be
the final cap. Recent studies have suggested that the present cap design is likely
to reduce infiltration into the wastes substantially for a prolonged period of time.
The Vault 8 and Vault 9 base slabs will perform differently post-closure. Both
will be relatively impermeable at closure, but the low permeability of the Vault 9
slab is expected to persist for a longer period of time. The same arguments will
apply to whichever style of base slab is implemented for the future vaults.
However, even highly impermeable base slabs will allow a small amount of
116
water to pass through over time, as the permeabilities are small rather than zero,
and the areas of the base slabs are large compared with their depths. They will
therefore contribute to the overall passive vertical drainage capacity of the
system, and this contribution will increase with time through degradation. When
infiltration through the cap exceeds the vault drainage capacity (including any
contribution to water losses made by corrosion), the vault will tend to saturate
with water. Alternative future base slab design options (including options
without a geomembrane liner) are therefore being considered through the
current process.
Other engineered features are likely to be of secondary importance but may also
have a role in the final closure scheme, possibly as contingency measures. For
example, implementation of some form of cut-off wall, keyed in to the cap, might
help minimise lateral ingress of waters into the wastes. In addition, such a wall
could help to direct to deeper systems such leachate as may be released from the
wastes into upper groundwaters, thereby maximising dilution prior to final
discharge to the biosphere. It is possible that this contingency role would be of
particular relevance should the cap fail earlier than expected. The optimisation
process is considering whether these functions are of importance in terms of
minimising post-closure impacts, and what depth and length of cut-off wall
might be required to achieve them. Benefits are being explored through
comparison with those offered by the existing cut-off wall, installed adjacent to
the trenches in the early 1990s.
Vertical drains might also fulfil a similar contingency role, directing to deeper
groundwater systems any leachate that is produced, and/or possibly helping to
avoid the occurrence of bath-tubbing within the vaults (and thereby helping to
preclude high contaminant concentrations in near-surface leachate). Again, an
important aim of the vertical drains may be to provide reassurance that leachate
will be managed even if the cap fails. It is relevant to consider if these functions
are important, and what design and location of the drains might best provide the
functions if required.
Other aspects of the potential LLWR engineering design are important to
consider through the optimisation process (e.g. the approach to leachate
management and monitoring during the operational phase), but do not provide
functions relevant to long-term safety and so are not discussed in this note.
Subsequent sections of this note follow this general hierarchy in exploring the potential
implications of different component options for post-closure impacts.
117
E.3 Potential Implications for Post-closure Impacts
E.3.1 The Final Cap
Options and Variants
From the perspective of assessing capping options, the main issue for the optimisation
process is to test whether variations to the present design, which has been the subject
of detailed design optimisation studies over a number of years, could be modified to
yield additional significant benefits, for example in terms of cost savings or improved
protection for specific pathways (Paulley, 2010a).
The baseline option and key variants for the final cap (Paulley, 2010a) are reproduced
below.
Cap Baseline Option: Present Design (Progressively Implemented 3.1m Thick Two-dome cap)
Variants relating to the time of emplacement, and opportunities to implement different dome designs
Timing Variant 1: Two Dome Design Implemented at Site Closure, with maintained trench /
Vault 8 interim or ‘Stage 1’ cap or upgraded trench interim cap.
Timing Variant 2: Progressively Implemented Single Domed Cap.
Timing Variant 3: Single Domed Cap Implemented at Site Closure, with maintained or
upgraded trench / Vault 8 interim or ‘Stage 1’ cap
Design variants that could apply to all the above variants
Design Variant 1: Additional Infiltration Barriers (geomembranes, other layers) to further
reduce infiltration
Design Variant 2: Additional Material to Protect Against Intrusion (Thicker Cap)
Design Variant 3: Enhanced C-14/Rn-222 Containment Measures
Design Variant 4: Retain Gas Vents Post-closure, Enhanced Gas Blanket in Cap
Design Variant 5: Reduced number of layers (to reduce cost)
More detailed design variants e.g. minor changes to layer design are beyond the scope of the current
study
Alternative baseline option, for screening
Alternative Baseline Option: Biointrusion Only Cap
118
Implications of Cap Options for Groundwater Pathway
Releases
Participants at the elicitation meeting for hydrogeological modelling parameters
(Jackson et al., 2011) agreed that the baseline cap design is likely to be highly effective
in minimising infiltration for a substantial period of time (see also Thorne, 2008).
Central estimate values for infiltration elicited at the meeting were 1mm/yr ‘as built’,
10mm/yr at 2180 (assumed to represent 100 years after site closure), and 200 mm/yr at
3180.
Vaults
The implications of cap performance for evolution of the wastes are related to the
behaviour of other components of the engineered system, particularly in the case of the
vaults. If is assumed that there is zero leakage through the base of Vault 9 and the
future vaults (and that they are constructed and completed according to present
baseline plans), any of the above infiltration values would be sufficient to cause them
to become fully saturated over timescales of as little as tens of years. In reality,
however, even a base permeability as small as 10-10 s-1 will permit a small degree of
leakage; moreover, given the surface area of the vault bases, the total drainage capacity
may be significant relative to the expected infiltration levels. Indeed, if the cap
performs effectively, there is judged to be a significant possibility that the vault wastes
will initially de-saturate with time (i.e. water present in grout and elsewhere at closure
will be slowly lost) and that re-saturation will not occur. Recent hydrogeological
modelling work (Hartley et al., 2009; 2011) supports this interpretation, suggesting
that, even after 1000 years, there is a significant probability (estimated at c. 50%) that
much of the vault wastes will remain unsaturated.
However, the model outputs and the conclusions that can be drawn from them have
been shown to be highly sensitive to the way in which elicited parameter values for cap
infiltration and base permeabilities are combined. Estimates used in the base case
hydrogeological model runs are central values taken from the distributions elicited at
the model parameterisation elicitation workshop. These estimates have been found to
represent a condition whereby the model results are in ‘unstable equilibrium’, in so far
as small deviations from the chosen parameter values lead to large differences in
model outputs. In effect, the parameters correspond to a situation whereby infiltration
through the cap and drainage through the base at the time of closure is balanced; this is
logical given the presence of geomembranes in both structures. Small deviations from
the elicited parameter values disturb this balance, and lead to the model predicting
rapid saturation or desaturation of the vault system, with changes occurring over
model timescales as short as tens of years. The base case results show similar behaviour
119
over longer timescales, as the elicited parameters for the cap and base are changed to
represent degradation, altering the balance between infiltration and drainage such that
the model predicts a series of periods of saturated and unsaturated conditions between
closure and 1000 years.
Given such sensitivity in the modelling results it is not clear that the base case results
can be accurately described as representing the ‘likely’ evolution of the system. The
following points are relevant to consideration of the implications of this sensitivity.
It is reasonable to assume that there should be some correlation between the
parameter probability distributions when analysing their impacts. For example
degradation of the base is likely to be (at least in part) a function of the amount of
water passing through it, and therefore linked to performance of the cap.
However, it is very difficult to establish the extent of correlation across the
distributions elicited, and this was not a topic explicitly covered by the elicitation
process.
The elicited probability distributions encompass uncertainties associated with all
potential forms of degradation (and indeed other sources of uncertainty). The
implicit failure mechanisms ranged from slow degradation of concrete and liners
through to sudden failure / cracking. It is difficult to unpick the contributions
from different sources of uncertainty within the elicited distributions, and
therefore it is not easy to describe the potential importance of correlations across
the distributions elicited. Naturally this is less of an issue for a ‘conservative’
performance assessment (for which the elicitation process was designed) than it
is for an optimisation process that attempts to establish a more ‘realistic’ view.
The distributions elicited at the workshop (Jackson et al., 2011) were intended to
be ‘cautiously realistic’ as they were developed to support the main PA. For
example, the central estimates for the base permeability with time may therefore
reflect a faster degradation than would be appropriate to a real ‘best estimate’ of
performance.
It is also relevant to note that the Serco modelling work was focused on hydrogeology
and did not incorporate processes associated with the vault wasteform (such as metal
corrosion) that have the potential to absorb significant quantities of water and might
thereby reduce the likelihood of resaturation for a prolonged period (Paulley, 2009a).
Overall, therefore, it remains unclear whether the implementation of a ‘Vault 9’
standard base and high quality cap for future vaults would lead to persistent
unsaturated or saturated conditions, or a sequence of different saturation states.
120
However, as discussed in Section E.3.2, the predicted radiological impacts are likely to
be of the same order whichever state is established.
For any ‘lower standard’ (no liner) base design that might be implemented for the
future vaults (e.g. a base design equivalent to Vault 8, or simpler alternatives with no
bentonite liner) there appears to be a high probability, with or without absorption of
infiltrating water by metal corrosion, that unsaturated conditions will be established
and would persist for a significant period of time.
Trenches
The baseline cap design should also be effective in ensuring that the trenches become
desaturated. Given the drainage capacity of the geology underlying the trenches, it can
be anticipated that they will desaturate within a handful of years of an effective cap
being implemented. Indeed, the present interim cap may well already be performing
such a function, at least in part, although the available data (Hunter-Smith, 2009)
suggest that full desaturation of the trenches may not yet have been achieved.
The Serco modelling work indicates that, under central assumptions regarding cap
performance, the cut-off wall and vertical drains (see below) may play minor roles in
promoting unsaturated conditions underneath the repository footprint. Nevertheless, it
is the cap that is of principal importance as a barrier to water entry. There is, however,
a recognised low likelihood risk that the cap could fail. If failure were to take place,
with the current modular design, it is perhaps most likely to occur along the line of the
gully/valley between the two cap domes, which collects run-off from a substantial area
of the cap surface. In circumstances of cap failure, contingency measures to mitigate
the consequences of excess infiltration would come into play.
Previous studies have not identified any additional cap layers or design features that
are likely to provide further protection for the groundwater pathway. The modular
design was developed in order to facilitate early installation of the final cap over the
trenches (should that be preferred) as well as to keep the overall final height as low as
practicable. However, other than the potential for enhanced erosion along the line of
any gully between different modular cap components, the double and single dome
designs are considered to offer broadly the same level of protection against infiltration
and human intrusion once installed.
If the final cap is to be installed in one campaign at closure of the site, an appropriate
approach to interim capping of the trenches will need to be demonstrated in order to
provide effective control over releases while the site remains operational. It might also
121
be necessary to provide a basic interim cap for Vault 8 (and potentially for other vaults,
over time) in order to minimise the saturation state of those wastes and to reduce
demands on active leachate control systems. The ‘stage 1’ cap included as part of the
current design could possibly provide the required interim level of protection, if it can
be installed progressively.
Implications of Cap Options for Gas Pathway Releases
According to current understanding, the largest estimated impacts associated with the
gas pathway are from C-14 and, in particular, Rn-222 releases. The exposure
calculation associated with the maximum estimated doses is based on the assumed
release of contaminated gas into the basement of a house built on the cap.
Bulk gases are not considered a long-term threat to post-closure safety performance of
the facility, as they will not be produced in sufficient volumes to threaten the operation
of passive safety features (e.g. by disrupting engineered structures). However, they
have the potential to act as a transport vector for radiolabelled gases, and that is
partially why it is currently planned that the cap gas vents should be sealed at closure.
The other reason for sealing the gas vents is to maximise the diffusion pathlength for
Rn-222. Owing to its short half-life, the time taken for Rn-222 to diffuse through the
wastes and the cap is a significant factor in reducing the magnitude of release and
associated impacts. The incorporation of additional diffusion barriers within the cap,
such as extra geomembranes, would further reduce these impacts. However, estimated
impacts based on the current baseline design are currently projected to be beneath the
criteria for regulatory concern (Baker, 2008) and hence there is not considered to be a
strong driver for adding such additional complexity to the cap design.
C-14 labelled gas releases may be generated as a result of the degradation of organic
wastes, metal corrosion and other processes within the repository. The half-life of C-14
is sufficiently long that impacts will not be reduced by an extended diffusion path.
Issues relevant to the role of the cap in controlling long-term impacts include:
It is not a simple matter to estimate whether the release of C-14 labelled gas will
be reduced or enhanced by different saturation states within the wastes, as the
processes involved are complex and the wastes are heterogeneous.
Impacts could potentially be reduced by taking measures to enhance local
dilution (e.g. by keeping gas vents open post-closure to promote dispersion, or
including an enhanced gas blanket in the cap design). However, whilst such
measures might reduce C-14 concentrations above the cap, they would be
contrary to the aim of maximising the diffusion distance for Rn-222.
122
It has been suggested that inclusion of a thick layer of soil and vegetation on the
cap surface might maximise the potential for any C-14 release to interact with cap
surface waters and be dissolved. This would reduce gaseous impacts but could
enhance near-surface groundwater pathway related impacts.
Implications of Cap Options for Impacts Associated with
Inadvertent Human Intrusion
The detailed cap design is primarily intended to provide a stable long-term barrier to
the infiltration of water into the facility. However, the cap also plays an important role
in helping to mitigate risks associated with the threat of disruption by human activity
in the long term, when controls on site use may be less reliable.
The quantitative regulatory standard for assessing the significance of human intrusion
for a near surface disposal facility is expressed in terms of a ‘dose guidance level’
(Environment Agency et al., 2009). There is no absolute requirement for this standard
to be met, but it is intended to provide an indication of the range of circumstances in
which consideration needs to be given to the implementation of practical measures to
reduce the chance of exposures taking place.
It might be argued that a memory of the existence of the site may well persist for a
substantial proportion of the period between site closure and its anticipated eventual
natural disruption following sea-level rise and coastal erosion. Nevertheless, the GRA
suggests that it will be difficult to substantiate that human intrusion into a near-surface
disposal facility is unlikely to occur after the period of authorisation (Environment
Agency et al., 2009).
The recent performance assessment update for the LLWR (Baker, 2008) described a
range of potential intrusion scenarios. The most important scenario, taking into
account the type of disturbance that might arise from commonplace actions and
consequent radiological impact, was considered to be the construction of a house on
the cap. Such a development would disrupt the cap itself and potentially also the
wastes beneath.
Were disturbance of the wastes to occur, subsequent exposures might be significant
compared with the regulatory dose guidance level. However, it is argued that if the cap
were constructed such that it provides a 3m depth of ‘hard’ material beneath the
surface, the likelihood that a commonplace excavation (such as that associated with
house building) might result in intrusion into the wastes themselves would be
substantially reduced. Typical footings for a domestic house would be highly unlikely
to extend beneath the depth of the ‘intrusion barrier’, while deeper excavation would
reveal the hard-to-penetrate geomembrane Nevertheless, even if the wastes are not
123
disturbed, disruption of the cap as a result of intrusion could lead to the more rapid
migration of Rn-222 labelled gases from the wastes to the surface environment, or more
specifically into a building constructed on the cap. This relevant exposure scenario is
therefore the same as that considered for the gas pathway (Section 3.1.3).
Even with a 3m thick engineered cap to provide a barrier against disturbance, there
remains a possibility that other categories of commonplace action (i.e. not involving
bulk excavation), such as the drilling of investigative boreholes, could lead to direct
intrusion into the wastes. However, because they would not involve bulk excavation,
the consequences of such actions are judged to be less radiologically significant than
excavation for house construction. Alternatively, less likely scenarios, such as the
creation of foundations for a much larger building on the cap, might give rise to
considerably deeper excavation. In this case, it is argued that prior geotechnical
investigations would reveal the cap as being an unsuitable location for construction of
such a building.
Exposures associated with human intrusion scenarios could in principle be reduced if
the inventory were to be reduced, for example by accelerated leaching of contaminants
from the facility. This could occur, for example, if the emphasis in cap design were
placed solely on its function as a barrier to intrusion, rather than to infiltration. Clearly,
however, this would be contrary to the principle of containment and would most likely
lead to significantly higher doses via the groundwater pathway.
Implications of Cap Options for Impacts Associated with Site
Disruption through Coastal Erosion and Sea-level Rise
None of the different options for cap design is likely to mitigate significantly against
the potential for, and consequences of, eventual disruption of the facility as a result of
coastal erosion and sea-level rise. As with human intrusion, passive measures to
accelerate leaching and dispersion of the contents of the disposal facility could possibly
lead to a reduction in the remaining inventory at the time of disruption, but such an
approach would be contrary to the principle of containment, as well as working
against the protection measures required for the groundwater pathway. The viability
and implications of alternative measures such as coastal defences, local hardening and
other engineering approaches to facility protection are considered in Paulley (2010e).
124
Summary of Post-closure Impacts Relevant to Cap Options
The baseline cap design is mature and a range of existing studies suggest that it
will satisfactorily achieve its design criteria. There is considerable confidence that
the cap will reduce significantly infiltration into the wastes for a prolonged
period of time, leading to persistent unsaturated conditions within the trench
wastes and the geology underlying the facility, and a significant probability that
the future vault wastes will also be unsaturated. Use of lower-standard (unlined
(Vault 8) equivalent or simple concrete loading platform type) bases for the
future vaults would appear to be sufficient to ensure the vault wastes as a whole
would remain unsaturated for a prolonged period of time. This is consistent with
a strategy of minimising contact between the wastes and infiltrating water in
order to control releases from the facility via the groundwater pathway.
Previous studies have not identified any reasonably practicable additional cap
layers or design features that would provide significant additional protection for
the groundwater pathway.
The design of the cap is consistent with assertions regarding its long-term
performance. No additional reasonably practicable features have been identified
that could help further to reduce the likelihood of failure, except perhaps
avoiding the ‘gull-wing’ (two-dome) modular design and the associated gully
between modules that could potentially be a source of weakness for erosion.
The single and double domed cap designs will offer broadly the same level of
protection once completed. However, if the final cap is to be installed in a single
campaign at site closure, an appropriate approach to interim capping of the
trenches will be required. It may also be necessary to provide some form of
interim cap for Vault 8 (and other vaults), depending on the implications for
demand on active leachate control systems. The ‘stage 1’ cap included as part of
the current design could possibly provide the required interim level of
protection, if it can be installed progressively.
A further important role of the cap is to provide a long diffusion path-length for
Rn-222 gas. Additional layers (e.g. geomembranes) would give additional
protection; however, estimated impacts based on the current baseline design are
already projected to be beneath criteria for regulatory concern.
125
An important role of the cap is to help minimise the likelihood of substantial
disruption of the facility through any future human actions. An engineered
barrier thickness of 3m has been proposed as sufficient to ensure that
commonplace intrusions (e.g. building foundations for a house) would not be
likely to lead to significant consequences from direct disturbance of the wastes.
It is less clear whether cap design can substantially help to mitigate the impacts
of C-14 release in gas. The incorporation of an enhanced soil and vegetation layer
on the cap surface might help to trap C-14 through dissolution in cap run-off
waters, but it is not clear whether this would in practice give rise to a substantial
reduction in impacts, which are in any case (for C-14 release in gas) presently
estimated to be below criteria for regulatory concern.
An important role of the cap is to help minimise the likelihood of substantial
disruption of the facility through any future human actions. An engineered
barrier thickness of 3m has been proposed as sufficient to ensure that
commonplace intrusions (e.g. building foundations for a house) would not be
likely to lead to significant consequences from direct disturbance of the wastes.
E.3.2 Future Vault Base Slab and Walls
As described by Paulley (2010b), the main functions of the future vault base slabs and
walls are:
To provide a suitable engineered platform in support of waste disposal
operations; and
To support the isolation and containment of the disposed wastes pre- and postclosure.
As discussed at the Scoping Workshop (Egan, 2009), demonstrating containment does
not necessarily depend upon the adoption of a high-specification ‘bath-tubbing’
design, for the reasons discussed below.
With these aims in mind, the baseline option and important variants for the future
vaults base slabs and walls have been summarised by Paulley (2010b). They are
reproduced below.
126
Vault Slab and Walls Baseline Option: Future Vaults Installed to Current Vault 9 Design (NB
the possibility of achieving the same specification with a single geomembrane liner is also
relevant)
Base Specification Variants
Base Specification Variant 1: Reduced Specification Base for Future Vaults (equivalent standard
to Vault 8; no geomembrane liner)
Base Specification Variant 2: Concrete Loading Base only (no additional long-term performance
measures; similar standard to ‘Dounreay’ design)
Base Depth Variants
Base Depth Variant 1: Lower Vault Bases compared to current plans (enhanced capacity or
reduced likelihood of near-surface discharge)
Base Depth Variant 2: Much Lower Vault Bases Compared to current plans (top of wastes below
future sea-level)
Wall Variants
Wall Variant: Reduced Height or No Wall at East of Vault 9 (non bath-tubbing strategy,
connecting leachate directly to Vertical Drains).
Variants Noted to Test Sensitivities
Sensitivity Variant: Stepped Future Vault Bases (i.e. successively lower elevations for Vaults 10
to 14)
Implications of Vault Base Slab and Wall Options for
Groundwater Pathway Releases
Vault 9 vs. Lower Specification Bases
The double-liner approach implemented for Vault 9 was optimised for long-term
storage, but the design is also consistent with a closure philosophy that achieves
containment through retaining any infiltrating waters and resulting leachate within the
vaults system for as long as possible. This requires a highly impermeable base that
continues to perform post-closure.
By contrast, the (no geomembrane liner) Vault 8 approach was designed for a disposal
scheme that sought to promote containment by minimising contact between the
wasteform and infiltrating water (hence reducing the potential for generating
contaminated leachate). This was achieved by draining any infiltrating water at the
base of the vault, to avoid saturation and possible bath-tubbing. Such an approach still
requires that the base is impermeable during operations but it is less critical that this
performance persists after closure.
127
Both specifications are based on the assumption of a high performance cap, consistent
with the baseline design, that serves to minimise infiltration for several hundred years.
The issues involved in comparing the two options on a realistic basis as part of an
optimisation exercise are complex (Paulley, 2009a). However the qualitative arguments
developed by Paulley (2009a), supported by the hydrogeological model runs described
by Serco (Hartley et al., 2009; 2011), suggest that it is difficult to differentiate between
the two approaches from an impacts perspective on the basis of current evidence. The
arguments for this are as follows.
Existing assessments of post-closure safety performance for LLWR are based on a
conservative conceptual model that represents the vaults as being saturated and
well mixed, with contaminants instantaneously available for release from the
wastes. If these assumptions are used as a basis for comparing options, it can be
expected that the major difference between them will be that the Vault 9
specification design will lead to releases being delayed by the time it takes for the
vaults become fully saturated (as discussed in Paulley, 2009a and Towler, 2009a).
Logical arguments and hydrogeological modelling outputs (see Section 3.1)
support the view that the cap performance will ensure that infiltration is minimal
for a substantial period of time. Whichever base specification is adopted, the
vault wastes could well remain largely unsaturated (subject to the caveats noted
earlier for the ‘Vault 9’ base) for periods up to, and possibly beyond, 1000 years.
Moreover, even when the system is saturated, the porewaters within the vaults
will not be mobile or ‘well mixed’, nor will such waters mix efficiently with those
in the overlying drainage layer. In addition, the conditioning of the disposed
waste and the disposal containers themselves will serve to delay the release of
some radionuclides, such that a substantial proportion of the inventory is
unlikely to be ‘instantly available’ to infiltrating waters. These arguments imply
that contaminant releases in leachate and hence impacts will be low over such
timescales for both vault design options. From a purely hydrological perspective,
however, the main difference is that adoption of the Vault 8 (lower specification)
base would enhance the drainage of infiltrating water and hence increase the
likelihood of unsaturated conditions persisting within the vault wastes beyond
1000 years, when cap performance is expected to degrade.
Removal of the east wall of the vaults to enable direct release of any leachate to
adjacent vertical drains could in principle lead to additional reductions in the
saturation state of the vault wastes and underlying geology for the time period
over which the drains continue to provide a preferential pathway (see Section
E.3.4 for more details).
128
Other Vault Base Options
Given the arguments described above, it is logical to consider whether a ‘Vault 8’ style
base would provide any significant long-term performance advantages over a simple
concrete base with no geomembrane liner or BES layer. Indeed, from a post-closure
impacts perspective, there is a strong argument that any advantages would be
minimal, as implementation of either option would lead to persistent unsaturated
conditions within the vault wastes.
This option is essentially equivalent to the use of a base structure similar to that
planned for the Dounreay New LLW Facilities. Here a base consisting of 0.5m of 10-10
ms-1 hydraulic conductivity (as built) concrete is assumed for the purpose of PA. This is
higher than the elicited conductivities for the LLWR concrete in Vault 8 or Vault 9
which lead to an arguably more ‘realistic’ view of performance. It is appropriate
therefore to consider the latter conductivity values to be representative of the
performance that could be obtained from a ‘Dounreay-like’ reinforced concrete base
slab design.
Deeper Vault Bases
A decision to implement a ‘deeper future vault base’ option would be driven by the
intention either to mitigate the potential impacts of coastal erosion (Paulley, 2010b and
2010e) (provided that it were sufficiently deep to ensure that the wastes were below sea
level when erosion occurred), or to increase overall disposal capacity.
Both alternatives could have implications for groundwater releases. Wastes within
deep vaults (approaching a silo-type disposal concept) would very probably be located
within the regional groundwater system, but the potential for discharge of leachate
from the saturated wastes to near-surface systems would be reduced.
Implications of Vault Base Slab and Wall Options for Impacts
Associated with Inadvertent Human Intrusion
Deeper vaults could mean there would be a larger distance between the surface of the
disposed wastes and the surface of the cap, and hence less likelihood of disturbance by
commonplace human actions; however, the extent of benefit actually achieved would
depend on detailed cap design associated with such a system, as well as decisions on
waste stacking. Moreover, estimated exposures arising from U-238 series radionuclides
(which dominate estimated impacts for the human intrusion pathway) are highest in
relation to the trench inventory, so this pathway is unlikely to be a discriminating
factor for future vault base slab and wall designs.
129
Implications of Vault Base Slab and Wall Options for Gas
Pathway Releases
Similar arguments to those for human intrusion apply for the gas pathway. Moreover,
as noted previously (Section 3.1.3), it is not clear whether a more prolonged period of
unsaturated conditions (resulting from the increased drainage capacity associated with
a lower specification vault base) would lead to reduced or enhanced gaseous releases.
Implications of Vault Base Slab and Wall Options for Impacts
Associated with Site Disruption through Coastal Erosion and
Sea-level Rise
The process of coastal erosion is expected eventually to lead to undercutting of the site
at a depth below the current base level (Baker, 2008). The vault bases would therefore
have to be very deep indeed to avoid disruption of the wastes they contain; indeed,
vaults constructed at an intermediate depth might be disrupted earlier than the
existing shallow vaults. Moreover, no significant additional protection against
inundation can be envisaged through developing future vaults with a deeper base.
Deep ‘silo’ designs have been excluded from the current optimisation work on the
basis of arguments from previous studies that the practical problems involved in
operating such a facility would be disproportionately large.
Summary of Impacts Discussions Relevant to Vault Base Slab
and Wall Options
It is not currently possible to undertake the realistic calculations necessary to
make comparative assessments of performance (in terms of leachate release) for
different approaches to base slab design and associated closure strategies.
Nevertheless, a range of arguments can be made that suggest the two approaches
(i.e. the Vault 8 and Vault 9 design specifications) are likely to lead to broadly
similar levels of post-closure environmental impact, and cannot therefore be
differentiated on this basis.
There are no strong arguments for differentiation between the main vault design
variants on the basis of potential threats associated with gas release, human
intrusion and coastal erosion/sea-level rise. Moreover, realistically practicable
variations to the depth of the future vault bases are unlikely to realise significant
benefits for protection against natural disruption.
130
E.3.3 Cut-off Wall
The main design functions that have been identified for the cut-off wall (Paulley,
2010c) are:
(1) to help minimise / control lateral inflow of groundwater to the near-field for as
long as possible, supporting the cap in maintaining a low level of saturation within the
trench and vault wastes (and in the geological structures immediately beneath them),
by diverting the horizontal-flow component of any ‘incoming’ groundwaters to deeper
systems below the facility; and/or
(2) to help minimise / control lateral outflow of groundwater from the near-field to
the near surface environment for as long as possible, diverting any waters that might
contain concentrations of contaminants originating from the trenches and the vaults to
deeper systems, thereby reducing the potential for release to the environment in the
vicinity of the facility and enhancing the likelihood that any resulting contamination
plumes will discharge to sea rather than to land.
With these functions in mind, the baseline option and key variants for the cut-off wall
have been summarised by Paulley (2010c). They are summarised below.
Cut-off Wall Baseline Option: As for existing ‘Schedule 9 Item 2’ design (keyed in to the cap,
emplaced all around the facility, around 80cm wide and 20 to 25 metres in depth, wall
constructed from cement / bentonite slurry)
‘Length’ Variants
Length Variant 1: Protection against 'incoming' waters only (wall emplaced only where
hydrogeological understanding suggests there may be significant lateral inflows)
Length Variant 2: Protection against 'outgoing' waters only (wall emplaced only where
hydrogeological understanding suggests there may be significant lateral outflows)
Length Variant 3: No grouting of bedrock
‘Depth’ Variants
Depth Variant 1: Reduced depth, wall design reduced to a maximum of 10m AOD Depth
Variant 2: Reduced depth, wall design reduced to a depth to the base of the vaults (i.e. to
provide a minimum level of protection of the trenches against incoming lateral flows)
No New Cut-off Wall Variant:
No change to current arrangements (no new cut-off wall; retain existing structure)
131
Implications of Cut-off Wall Options for Groundwater Pathway
Releases
Role of Cut-off Wall for the ‘As Expected’ Cap Performance Scenario
If the cap performs ‘as expected’, then it will be the most important engineered feature
contributing to minimising the saturation state of the wastes (Hartley et al., 2009; 2011).
An encircling cut-off wall might provide some additional protection against lateral
inflows, but this will very much be a secondary role.
In terms of exerting passive controls on the fate contaminated leachate that might be
released from the disposal system, hydrogeological analyses suggest that, for as long as
the cap operates effectively, the majority of contamination leaving the system will
migrate essentially vertically to the regional groundwater rather than laterally to
shallower systems, with or without a cut-off wall in place (Hartley et al., 2009; 2011).
This is due to the extensive drainage areas associated with the zones underneath the
LLWR, including the unsaturated zone maintained by the cap, as well as the influence
of topography and the gradients towards the centre of the facility that are likely to be
created by cap-run off. This again suggests that the role of a cut-off wall in minimising
potential impacts by directing outflows would be minimal, for as long as the cap
performs as expected.
The following considerations are relevant to assessing the importance of a cut-off wall
in terms of diverting lateral inflows away from the wastes (from Paulley, 2009b).
The vaults will be engineered in such a way as to provide effective isolation of the
wastes they contain from the impacts of any inflows. Although flows have been
observed into the Vault 8 under-slab drainage structure from outside the LLWR
engineered system during operations, the de-saturating effect of a cap is likely to
be sufficient to address this.
In so far as it might provide protection from inflows, the role of a cut-off wall is
therefore essentially restricted to the trenches.
The implications of increased saturation/flows within the trench wastes are best
assessed by considering the implications for specific radionuclides and waste
types.
o
Significant radionuclides in terms of impacts associated with the groundwater
pathway for trench wastes include C-14, Cl-36, Tc-99, I-129, Np-237, U-234 and
U-238 (Baker, 2008).
132
o
Uranium release from MgF2 crystals will be affected by reduced saturation
and/or flow, but the effect will be minimal as the dissolution rate is in any
case expected to be very slow.
o
U, Th, Tc, Np etc are solubility limited and so release from the near-field may
be delayed in unsaturated conditions.
o
The implications of different saturation regimes for cellulose degradation and
metal corrosion release mechanisms for C-14 are unclear, as these processes
will still operate in largely unsaturated conditions given sufficient residual
humidity.
o
It is possible that other radionuclides may be incorporated within other trench
waste forms and in practice would be subject to congruent release. However
the disposals are many and heterogeneous, and data are in any case not
available to support complex release models. For most contaminants,
therefore, instantaneous release on contact with water is likely to remain the
primary (albeit conservative) assumption for the purposes of long-term
performance assessment.
o
The trench wastes are heterogeneous and there may be preferential pathways
through them. However this is again difficult to substantiate and quantify.
o
Whilst there will be very little contaminant transport in essentially
unsaturated conditions, transport velocities do not scale with saturation.
Transport processes in partially saturated conditions can be faster than those
in a fully saturated environment.
For the purpose of comparing options, it is therefore considered reasonable to make
the following assumptions:
Very limited release of contaminants within and transport from the near-field will
occur for regions where the wastes are at a very low saturation state.
The release of most radionuclides within and from the trench near-field will
approximately scale with the fluxes of water passing through the relevant wastes.
The release of U, Th, Tc and Np from the near-field will be subject to further
retardation in unsaturated conditions due to the effect of solubility limitation. C-14
may also be affected by changes in the geochemistry, but the implications are
unclear.
Hartley et al. (2011) present a number of model analyses that examine the extent to
which the presence or absence of a cut-off wall might influence the saturation state of
133
the trench wastes for the central cap performance scenario. The analyses suggest that
the absence of a cut-off wall would influence the saturation state of only a fraction of
the trench wastes (perhaps 10 to 15% of the total trench waste volume). However,
flows through those wastes would be significant compared to flows through the
remainder of the trench wastes while cap performance persists, with model outputs
suggesting that the overall volume of sub-horizontal flow through the wastes close to
the cap perimeter could be of a similar magnitude to the total flow through the cap at
closure.
Nonetheless, because of variations in release rates for different nuclides, flow
properties in unsaturated conditions, and the subsequent dilution during transport
prior to exposure, it is expected that calculated impacts from the trenches with no cutoff wall (for the period for which cap performance persists) would not increase
substantially8. Indeed test calculations based on the Schedule 9 Requirement 2 GoldSim
PA model (with associated ‘conservative’ assumptions - in particular, no near-field
solubility controls) indicate that increased flow volumes through the parts of trenches
at the periphery could lead to enhanced dilution and thus to a (c.30%) reduction in peak
impacts to the shallow well water ingestion pathway, which is the exposure pathway
associated with the largest calculated risks. An increase in calculated impact (also by
c.30%) was estimated for the regional well water pathway. However, it was
subsequently noted that changes in concept for the 2011 ESC calculations may reverse
these outcomes.
If the simple assumption is made that the rate of release of ‘available’ contaminants
from the trench wastes will scale with water flow through them, it may be that the
effect of the cap alone will be sufficient to ensure that leachate release is substantially
reduced for a prolonged period. A cut-off wall could in principle provide an additional
contribution by helping to protect the wastes nearer to the edges of the trenches from
partial saturation and higher flows that might arise as a result of cap run-off, but the
implications for impacts are not likely to be large compared to the overall calculated
impact (see above), assuming that existing models are an accurate representation of the
geometries and processes involved.
Moreover, any benefits that might be gained in terms of restriction of inflow would
require the installation of a cut-off wall only down to the level of the base of the
8
This conclusion is based on the recognition that C-14, Cl-36, Tc-99 etc releases would
increase for the 10-15% of the trench wastes subject to higher flows, but releases would not
scale in a linear fashion due to waste-form, solubility and sorption effects. Moreover, doses
would not scale linearly to any change in release as transport from the trenches would be
vertical, and any contaminated plumes would then be subject to sorption and dilution in the
geosphere and biosphere.
134
trenches; any deeper construction (including any grout curtain) could be considered
largely superfluous.
It is also important to note that there is an existing cut-off wall, running between the
trenches and the railway drain. It is possible that this existing structure would be
sufficient to provide a substantial contribution towards the functions that would
otherwise be specified for a complete encircling cut-off wall. The benefits of a new
installation therefore need to be considered against the control functions provided by
the existing construction.
A summary of the intended and implemented design for the existing cut-off wall is
provided below.
The cut-off wall was constructed between 1989 and 1995 and is 470 m long and
1.1 m wide. It was constructed in two sections between the north-east corner of
Vault 8 and the southern end of Trench 7. The design intention was to limit lateral
migration of trench leachate to the railway cutting, to control groundwater input
to the trenches, and to reduce leachate production in general by reducing
horizontal inflows of water.
The existing cut-off wall therefore provides protection to the north and east sides
of the trenches (i.e. around 60-70% of the length of their perimeter, excluding the
common boundary with the vaults). It follows a line similar to that which has been
proposed for any fully encircling cut-off wall.
The construction method used for the installed wall is broadly consistent with the
baseline proposed design for any encircling cut-off wall, in terms of material and
liner systems.
The design was for the wall to extend from 7.4 to 9 m below ground level.
However, the ‘as built’ drawings suggest that it was keyed in to the clay layer at
around 5 m below ground level.
There is some evidence that the cut-off wall is currently providing the ‘downstream’
functions for which it was designed (Hunter-Smith, 2009), but the data is not
conclusive. Tritium concentrations in the railway drain were substantially reduced
post-installation. However, the interim trench cap was installed simultaneously and so
it is unclear whether the observed decrease was the result of one or both components.
In addition, a change in measurement methods (with different minimum detection
levels) may also have influenced the results.
Some concerns have been expressed that the function of the wall might be
compromised as it was not implemented to the design depth. One of the potential
135
issues is that excavations stopped wherever an apparently continuous clay layer was
encountered, and the base of the wall was then keyed in to that. The current geological
understanding suggests that such clay layers may be laterally and vertically
discontinuous. This means that there is uncertainty whether the clay layer underneath
the existing cut-off wall is sufficient to fulfil the original design requirement, i.e. to
ensure an impermeable flow barrier down to the depths noted above. A second issue is
that the rock head is high across some sections of the wall, and the wall was installed
only to the depth of the rock interface, and no other action was taken (i.e. the rock was
not grouted). A substantial section of the wall is therefore shallower than originally
intended.
To summarise, the existing cut-off wall is expected to play a significant role in fulfilling
the functions otherwise specified for a new encircling cut-off wall, but the levels of
protection actually be provided are difficult to ascertain with accuracy. These
performance uncertainties need to be considered when assessing the relative benefits
of any new feature.
Role of Cut-off Wall for Early Cap Failure Scenarios
A comprehensive safety analysis requires consideration to be given possibility and
likely implications of alternative scenarios for cap performance scenarios. Whilst the
outcomes of the expert elicitation process for cap performance (Jackson et al., 2011)
suggested that a high level of confidence can be placed in the cap fulfilling its required
control functions for a considerable period of time, there nevertheless remains the
possibility that its initial ‘as built’ performance will not be as specified, or that its
performance will degrade faster than anticipated.
Indeed, the original rationale for including an encircling cut-off wall in the design for
the 2002 PCSC (BNFL, 2002) was to provide redundancy in the engineering design
such that, if the cap were to fail, the cut-off wall would serve to divert leachate arising
within the facility to deeper groundwater systems. The possibility that, under such
conditions, the cut-off wall itself could actually cause ponding/bath-tubbing within its
confines was also recognised. Specifically, transient (seasonal or high rainfall) effects
on infiltration were recognised as potential causes of near-surface saturation and
leachate contamination, with the potential for temporary ponding within the cut-off
wall to give rise to direct discharge of contaminated waters direct to near-surface soils
and surface water systems adjacent to the facility. The vertical drain (see Section E.3.4)
was therefore incorporated in the design to provide an additional degree of protection
against such a scenario.
The implications of early cap failure are partially explored by Hartley et al. (2011). A
number of model runs have been undertaken with full or localised cap failure. The
136
outputs of the calculations suggest that the wastes will become saturated and that
discharge of leachate to upper groundwater can occur for these scenarios, even without
the transient effects on near surface saturation and flows being taken into account. The
model predicts full saturation within the confines of the cut-off wall, suggesting that it
does act to retain water within the facility. However the model results do not show
clearly whether the cut-off wall can provide an effective contribution to minimising
discharges to upper groundwater systems, because a significant proportion (perhaps
30–50%) of discharges represented in the model occur via the cap profiling material
(the remainder being diverted under the cut-off wall). This provides a fast lateral flow
pathway, thereby allowing contaminated waters to by-pass over the top of the cut-off
wall.
Summary of Implications for the Groundwater Pathway
In summary, the cap alone is likely to cause sufficient de-watering of trenches to
substantially reduce release rates of contaminants. According to the current water flow
model, and based on central estimates of cap performance, any cut-off wall may not
provide more than a minor supporting role through preventing the in-flow of run-off
water at the cap periphery, in particular given that the existing cut-off wall will provide
at least part of the required control function. If a new encircling cut-off wall was to be
installed, it might be that a relatively shallow installation (to the base of the trenches) is
all that would be required to fulfil such a purpose.
Any future cut-off wall (for preventing in-flow at the periphery of the facility) could
also provide additional ‘what-if’ protection against the consequences of significant
early cap failure. For such scenarios, it is possible deeper installations (e.g. to the top of
the regional groundwater – there does not appear to be a strong rationale for going any
deeper) could provide additional benefits in preventing leachate release to near-surface
hydrological systems. However, there remains a possibility that ponding/bath-tubbing
might occur within the confines of any cut-off wall following cap failure (that is, it may
not offer the protection intended for such ‘what-if’ events). The models presented by
Serco (Hartley et al., 2011) suggest that the influence of the vertical drains, at least
given the present design, would not be sufficient to mitigate against this effect.
Implications of Cut-off Wall Options for Impacts Associated
with Inadvertent Human Intrusion
The presence of otherwise of a cut-off wall will not have significant implications for
impacts related to the potential for inadvertent human intrusion.
137
Implications of Cut-off Wall Options for Gas Pathway Releases
Likewise, the presence of otherwise of a cut-off wall will not have significant
implications for impacts related to releases via the gas pathway.
Implications of Cut-off Wall Options for Impacts Associated
with Site Disruption through Coastal Erosion and Sea-level
Rise
The presence of otherwise of a cut-off wall will also not have significant implications
for impacts related to releases via coastal erosion and sea-level rise processes.
Summary of Impacts Discussions Relevant to Cut-off Wall
Options
For central estimates of projected cap performance, the role of any new encircling
cut-off wall will be limited. Leachate is likely to be routed to the regional
groundwater system, whether or not a cut-off wall is installed. Without a cut-off
wall, a small proportion of the trench wastes (close to the periphery of the cap)
may become partially saturated, but the associated impacts from releases via
groundwater are not judged significant, in particular considering the role that
might be provided by the existing cut-off wall. If required, a new cut-off wall
installed to the depth of the base of the trenches would be sufficient to mitigate
against any increase in impacts for this scenario.
The encircling cut-off wall was originally introduced into the LLWR closure design
to provide redundancy in controlling leachate discharge in the case of early cap
failure. The likelihood of major early failure of the cap was assessed to be very low
in the recent engineering elicitation meeting (Jackson et al., 2011).
Existing water flow models for the LLWR do not clearly show whether any new
cut-off wall would be effective in diverting to deeper systems any near-surface
leachate flows that might occur following cap failure. This is because the model
geometry allows preferential flows above the cut-off wall through the cap profiling
material. However the flows that are shown in the models do indicate that the cap
is effective in diverting flows in other situations.
138
The hydrogeological model does however suggest that ponding/bath-tubbing
may occur within the confines of an encircling cut-off wall following cap failure.
This potential was recognised in the 2002 PCSC, hence the introduction of the
vertical drain into the design, to provide further redundancy. Moreover, the water
flow model suggests that the vertical drain, according to the present baseline
design, will not be sufficient to drain the facility under such conditions, in
particular the trenches. This is not due to the capacity of the drains, but a
consequence of the geometry of the system. It may be that alternative drain
location designs (e.g. emplacement all around the facility, possibly with an overdrainage blanket in the base of the cap to promote flows to the drains, similar to
that discussed in the 2002 PCSC) could mitigate this, by providing additional
vertical drainage capacity to ensure sufficient hydrogeological interactions with
the vaults and the trenches. Indeed, it is possible that the installation of vertical
drains all around the facility would be a valid alternative to a cut-off wall,
providing a passive drainage route to protect against inputs from cap-run off and
a barrier to leachate transport in the upper water system. See Section E.3.4 for
further discussions on this topic.
Because of the potential for ponding/bath-tubbing within the confines of a cut-off
wall, an effective impermeable seal between the cap and the cut-off wall would be
required in order to avoid leachate flows in near-surface systems passing over the
top of the cut-off wall. It would also need to be demonstrated that the likelihood of
seal failure at the same time as more general or localised cap failure would be low.
E.3.4 Passive Post-closure Leachate Management (Vertical
Drains)
The main design functions that have been identified for the vertical drains (Paulley,
2010d) include:
To permit excess water that may accumulate within the vaults system to be
released from the facility in a controlled manner that may limit environmental
impacts from contaminated leachate. Without a suitable drainage strategy for the
facility, in which such features may provide an important part, it is possible that
any ‘bath-tubbing’ waters in the vaults could eventually overspill into the nearsurface waters and soils surrounding the facility, thereby giving rise to enhanced
139
environmental impacts. Thus vertical drains provide a supplementary role to
containment measures, helping to minimise site impacts as those containment
measures degrade over time.
To support a containment strategy in which the aim was to keep the wastes
unsaturated for as long as possible, by passively draining water from the base of
the vaults.
To provide ‘redundancy’ in case of cap failure. The original concept behind the
introduction of such features into the closure design (BNFL, 2000; BNFL, 2002) was
that, if the cap were to fail significantly on a fairly short timescale, it would be
necessary to divert and drain excess water to deeper systems, thereby avoiding the
potential for leachate discharges to near-surface systems associated with ponding
within the confines of an encircling cut-off wall. Provision of such redundancy
remains a potentially important function.
The baseline option and key variants for the vertical drains (Paulley, 2010d) are
reproduced below.
Passive Leachate Management Baseline Option: Series of Wide Boreholes Vertical Drains
Implemented between Trench 3 and the modular vaults
Alternative Baseline Variant:
No Vertical Drains (with either reduced specification future vault base to otherwise enhance
the overall vertical drainage capacity, or vault base as for current design – see 'Vault base and
walls' proforma)
Vertical Drain Location Variants:
Location Variant 1: Vertical Drains All Round Facility
Location Variant 2: Vertical Drains the Other Side of the Vaults
Location Variant 3: Vertical Drains also In or Connected To Vault Bases and Drainage Systems
Alternative Design Variant:
2002 Design (one large deep rectangular drain).
140
Implications of Vertical Drain Options for Groundwater
Pathway Releases9
A number of lines of reasoning are relevant to considering the potential advantages or
disadvantages of different options for the vertical drains (and passive post-closure
leachate management in general) (see e.g. Paulley, 2009a,b; Hartley et al., 2011).
Assuming that the cap performs ‘as expected’, it seems likely (as discussed above)
that the cap alone will be sufficient to ensure that the wastes, and indeed the
directly underlying geology, will remain partially or wholly unsaturated for a
significant period of time. The vaults are not expected to become fully saturated
(subject to the caveats and uncertainties previously discussed), whether or not
vertical drains are installed, until timescales of around or beyond 1000 years (i.e.
on timescales beyond which confidence can be placed in the vertical drains to
provide an effective preferential pathway).
Installing vertical drains that are hydrologically connected to the vault bases might
however have minor benefits in terms of further extending the timescales over
which the vault wastes are likely to remain partially or fully unsaturated as the cap
performance degrades.
For the central cap performance scenario the vertical drains will not offer any
substantial benefit to managing impacts from the trenches.
Should the assumptions outlined in Paulley (2009a,b) and the model predictions
described by Hartley et al. (2011) prove incorrect, such that the vaults become
saturated without requiring major cap failure to occur, the arguments presented
by Towler (2009a,b) suggest that the difference in impacts between ‘saturated’ and
‘unsaturated’ vault designs would be small (because of the limited mixing of
waters with the wastes that would occur prior to discharge in both cases).
Another potentially important design function for any vertical drains is to provide
redundancy measures to protect against the potential against cap failure. Here it is
important to consider the relationship of the drain to any cut-off wall that may be
present.
It appears reasonable to assume that any early localised or general cap failure
event is likely to lead to saturation of both the vault and trench disposal systems,
particularly if a cut-off wall is present, preventing lateral dispersion of waters.
9
Compliance or otherwise with regulations relating to UK implementation of the EC
Groundwater Directive are not discussed here or in subsequent subsections, which present a
purely ‘technical’ analysis of the potential advantages or disadvantages of different options.
141
Steady-state calculations for relevant scenarios undertaken by Serco (Hartley et al.,
2011) confirm the likelihood that, under such conditions, leachate discharge to
near-surface systems would be likely to occur, although the fate and extent of such
discharges remain unclear. Transient effects (i.e. seasonal and other high rainfall
occurrences) would seem likely to further increase the likelihood of such
discharges. Moreover the calculations undertaken included the presence of vertical
drains according to the present baseline designs. However:
o
The drains, as represented in the model, are not sufficient to divert all leachate
from the vault area to deeper systems at a sufficient rate to avoid near-surface
discharges, although model ‘particle pathline’ runs do show that a significant
proportion of vault wastes are likely to be drained by this route for as long as
the vertical drains persist in providing a preferential flow pathway. Geometry
(i.e. the distance between the edge of the vaults and the line of the drains) is a
key consideration here. It could be argued that further near-surface discharges
would occur if transient effects were to be included in the model, representing
the ability of the system to respond to variability in rainfall.
o
The vertical drains, as currently designed, are not able to offer significant
protection against trench saturation and potential discharges to surface or
near-surface systems, because of the geometries and conductivities involved.
Alternatives to the present baseline design for the vertical drain have the potential to
offer additional benefits. For example, emplacing vertical drains all around the facility
might offer sufficient drainage capacity to mitigate against cap failure events and
associated transient effects, at least for the vaults, and potentially also for the trenches.
It could be that such a ‘drain curtain’ would in fact be a more effective perimeter
barrier feature than a cut-off wall, as it would not promote bath-tubbing within its
confines under conditions of cap failure. Alternatively, cut-off wall and drainage
systems could both be installed as perimeter features, with or without the central linear
drains currently included in the design.
Implications of Vertical Drain Options for Impacts Associated
with Inadvertent Human Intrusion
The presence of otherwise of vertical drains will not have significant implications for
impacts related to the potential for inadvertent human intrusion.
Implications of Vertical Drain Options for Gas Pathway
Releases
142
Similarly the presence of otherwise of vertical drains will not have significant
implications for impacts related to releases via the gas pathway.
Implications of Vertical Drain Options for Impacts Associated
with Site Disruption through Coastal Erosion and Sea-level
Rise
The presence of otherwise of vertical drains will also not have significant implications
for impacts related to releases via coastal erosion and sea-level rise processes.
Summary of Impacts Discussions Relevant to Vertical Drain
Options
Assuming that the cap performs ‘as expected’, it seems likely that the cap alone
will be sufficient to ensure that the wastes, and indeed the directly underlying
geology, will remain partially or wholly unsaturated for a significant period of
time. The vaults are not expected to become fully saturated, whether or not
vertical drains are installed, until timescales of around or beyond 1000 years (i.e.
on timescales beyond which confidence can be placed in the vertical drains to
provide an effective preferential pathway).
Minor additional benefits may be obtained, in terms of prolonging the period for
which the vault wastes remain unsaturated, if vertical drains are directly
hydrologically connected to the vault bases.
Even if these assumptions prove incorrect and bath-tubbing within the vaults will
occur, mixing and other relevant arguments suggest that there is little to choose
between different options for the drains.
Should general or localised cap failure occur, calculations suggest that the trenches
and the vaults will saturate and discharges to near-surface pathways may occur,
even if transient effects are not taken into account, although the drains will
provide some benefits for vault leachate.
Other vertical drain geometries (e.g. installing them around the perimeter of the
facility as well as between the vaults and the trenches, possibly without the cut-off
wall) may offer additional benefits compared to the present baseline design in the
event of cap failure.
Based on the above arguments, the following issues need to be considered in
identifying a preferred option for the vertical drains (and indeed any cut-off wall):
143
To what extent is it necessary to demonstrate ‘redundancy’ in design as part of the
development of a robust ESC (noting GRA requirements that the safety case
should not rely excessively on one system component)?
Are the contingency benefits that might be obtained by one or more of the various
design options proportionate to the costs involved, and is there a particular option
that offers the best performance?
References 10
Baker A J (2008). LLWR Lifetime Plan: Managing Existing Liabilities and Future Disposals at
the LLWR. LLW Repository Ltd Report 10001 LLWR LTP Volume 1, Issue 1, April 2008.
BNFL (2000). Status Report on the Development of the 2002 Drigg Post-Closure Safety Case.
BNFL (2002a). Drigg Post-closure Safety Case: Overview Report. BNFL report, September
2002.
Hartley L, Applegate D, Couch M, Jackson C P and James M (2011). Hydrogeological
Modelling for LLWR 2011 ESC Phase 2. Serco report SERCO/TAS/003632/005 Issue 5.0,
April 2011.
Egan M J (2009). Workshop Briefing Paper: Environmental Safety Case Engineering
Optimisation Scoping Workshop.
Environment
Agency,
Northern
Ireland
Environment
Agency
and
Scottish
Environment Protection Agency (2009). Near-surface Disposal Facilities on Land for Solid
Radioactive Wastes: Guidance on Requirements for Authorisation. February 2009.
Hartley L, Applegate D, Couch M, Jackson P, James M and Roberts D (2009).
Hydrogeological Modelling for LLWR 2011 ESC: Phase 1. Serco Report TAS/003632/002
Issue 2.1, November 2009.
Hunter-Smith R (2009). 2009 Annual Performance Assessment of the Interim Cap and Cut-off
Wall at the LLWR Trenches. Schedule 9 Requirement 7 Trench Cap Report (09),
RP/103547/4510034188/PROJ/00019.
10
Several of the references quoted here refer to documents that were current at the time of
writing; many have since been largely superseded by the discussions and/or references
quoted in the main body of this document.
144
Jackson C P, Couch M, Yates H, Smith V, Kelly M and James M (2011). Elicitation of
Uncertainties for LLWR. Serco report SERCO/TAS/E003796/010 Issue 2.0, April 2011.
Paulley A (2009a). Estimating Impacts to Inform Option Comparisons – Groundwater
Pathway: In support of the LLWR Pre- and Post-closure Engineering Optimisation
Programme. Quintessa Technical Note QRS-1443O-TN4 (Draft for Discussion).
Paulley A (2009b). Potential Implications of Different Saturation Profiles Within the Trenches
for Release: Initial Thoughts to Inform Views on the Comparative Benefits of Specific LLWR
Engineering Component Options. Quintessa Technical Note QRS-1443O-TN5 (Draft for
Discussion).
Paulley A (2010a). Engineering Component Option Description Proforma: Final Capping
Options (see Appendix D).
Paulley A (2010b). Engineering Component Option Description Proforma: Future Vaults
Base Slab, Liner and Walls Options (see Appendix D) .
Paulley A (2010c). Engineering Component Option Description Proforma: Cut-off Wall
Options (see Appendix D).
Paulley A (2010d). Engineering Component Option Description Proforma: Post-closure
Passive Leachate Management System Options (see Appendix D).
Paulley A (2010e). Engineering Component Option Description Proforma: Other Engineered
Features (see Appendix D)
Thorne M C (2008). LLWR Lifetime Project: Estimates of Cap Infiltration and Erosion. Mike
Thorne and Associates Ltd. for Nexia Solutions, Report No. (08)9274, July 2008.
Towler G H (2009a). LLWR Optimisation: Consideration Comparison of Wet Waste and Dry
Waste Strategies. Quintessa Technical Note QRS-1443O-TN3 (Draft for Discussion).
Towler G H (2009b). LLWR Optimisation: Consideration of Near-Surface Pathways in
Engineering Optimisation. Quintessa Technical Note QRS-1443O-TN6 (Draft for
Discussion).
145
Appendix F: Systematic Identification of
Strategy and Component Options Against
Threats to Repository Performance
Section 3.3 of the main report text describes how, during the scoping and main options
assessment phase, systematic approaches to options identification were utilised. The
aims of this approach were twofold:
to ensure that evidence collation and analysis activities (e.g. the hydrogeological
modelling work) addresses a suitably comprehensive range of component options,
in order to inform the main options assessment stage; and
to provide an audit tool to build confidence that the analysis presented is
comprehensive, i.e. that no potentially important component options were missed.
The detailed analysis undertaken can be summarised by three ‘mind map’ diagrams
showing alternative approaches to identification of options. These are shown in
Figures F1, F2 and F3. They indicate, respectively:
an analysis of the different threats to facility performance against component
options that could help mitigate those threats;
an analysis of high-level potential broad engineering strategies against the detailed
component options that could apply; and
a summary of all the different component options identified during this phase of
the analysis.
The analysis as presented here is deliberately unconstrained. Discussions in the main
body of the report detail why certain options (e.g. monolith strategies, or strategies
involving a dilute and disperse (permeable/no cap) approach) were screened out from
further consideration.
The options identified here were further developed and refined during the main phase
of the optimisation assessment. For example, the ‘Vertical Drains Also in Future Vault
Bases’ option was developed to include the concept of a drainage layer underneath the
vault base to maximise the passive vertical drainage capacity.
146
Figure F1: Schematic of Pre- and Post-closure Threats to Facility Performance Mapped Against Strategies to Mitigate Those Threats
147
Figure F2: Schematic of Combined Strategy Options, Noting Key Variants Mapped to Threats to Facility Performance
148
Figure F3: Combined List of Engineering Component Options Identified During the Initial Options Identification Stage
149
Appendix G: Scoping Workshop Engineering
Presentation showing Overview of Basic
Elements of the Previous Baseline Design
LLWR Environmental Safety Case
Workshop December 2009
Engineering Features
Dave Tonks
LLWR
Engineering Optimisation
Technologies – generally established, proven…
–
–
–
–
–
but long term performance / design lives unproven
reasonable conservative approach
ways of reducing uncertainty & building confidence
monitor and manage. Continuous improvement
CQA – essential to validating
Records and Tracking.
Vault 9 Current Design & Construction
Future Vaults
Closure Engineering
150
Engineering Features
Overview of elements
–
–
–
–
Cap
Future Vaults
Cut-off wall
Vertical Drains
Site Photo c2006
151
Survey Plan 2009
3D Digital Models
152
Gull Wing Cap + Cut-off Wall
Gull Wing
Cap
Typical
Cross
Section
153
Gull Wing Cap
Western Perimeter Section
Gull Wing Cap
Central Section
154
Single Dome Cap
Vaults
Long
Section
155
Single Dome Cap
Single Dome Cap Western Perimeter
156
Single Dome Cap Central Area
Cap
157
Vault 9 Base and Western Wall
Vault 9 Base and Central Wall
158

Other Engineered Features

Transcription

Similar documents

Sharp Triangle Beads instruction

Note Using your system the receive TS440 pan

Ambassador of a charity for persons with

Bon Grace Cap - the Hooded Hare

the PDF

Lander - Wardcraft Homes

File

Cap Erquy, both nature and history

our PDF Brochure