Why do mechanisms matter in radioactive waste management? Or: Know your enemy

Why do mechanisms matter in radioactive waste
management?
Or: Know your enemy
Francis Livens
Centre for Radiochemistry Research
University of Manchester
With thanks to:
Rick Parkman, Rebeca Alvarez, Laura Butchins, Emmanuelle Chardon, Jo Renshaw, Helen
Steele, Mike Wharton, Kath Morris, Jon Lloyd, David Vaughan, Steve Parry, Richard Pattrick,
John Charnock
Contents
What do we mean by mechanism?
Two examplesMicrocrystalline materials- iron oxides and sulfides
Biological transformations- bioreduction
Legacy wastesPu behaviour in storage ponds
Why Do We Need to Know Mechanisms?
We often need to extrapolate, e.g. scaling up to process plant or evaluating long
term wasteform performance.
Understanding physical and chemical mechanism allows realistic description and
gives greater confidence in these extrapolations
A Simple Example- Radioactivity Measurements
100
95
Count rate
C ount ra te
100
90
85
80
60
40
20
80
0
0
1
Time (days)
2
0
5
10
15
20
25
Time (days)
Count rate = 100 - 0.08 x Time
Count rate = (Count rate)t=0 e –k x Time
Decrease to 50% after 6.25 days
Decrease to 50% after 8 days
But the real world is complicated
Problems- low concentrations, complex mixtures, fine particle size,
heterogeneity, variability, biological activity
Optical
High Resolution Optical
1 mm
1 cm
SEM
100 µm
TEM
SEM image of aquatic sediment
100 nm
Concrete is heterogeneous
on scales from cm to nm
How can we study mechanism in these complex systems?
Use laboratory models to reduce variability, provide greater control, and
allow the use of higher concentrations of radioelements.
This allows use of a wider range of techniques and characterisation at the
molecular and near-molecular scale.
But, because we are working with simplified systems, we need to interpret
the results with caution
NpO2+
NpO22+
Microcrystalline Materials
The X-ray Absorption Spectroscopy Experiment
Use synchrotron X-ray sourcemonochromatic, intense (106 x X-ray tube)
Eject core electron from absorber atom
Outgoing photoelectron wave reflected back
from neighbouring atoms
Interference pattern contains information on
number, type, distance of backscatterers
Double crystal
monochromator
Sample
White beam
Monochromatic
I0
It
X-rays
If
Imon
XANES (X-ray
absorption near edge
spectroscopy)fingerprints
oxidation state
4
3
2
1
0
21.0
21.1
21.2
21.3
XANES Spectrum
4
3
2
15.0
1
0
20.5
10.0
5.0
20.7
20.9
21.1
21.3
21.5
21.7
21.9
22.1
0.0
-5.0
Raw Spectrum
-10.0
-15.0
0
EXAFS (extended X-ray
absorption fine
structure)- quantitative
modelling of
coordination
environment (n ± 20%;
r ± 0.02 Å)
2
4
6
8
10
12
14
16
Isolated EXAFS
80
60
40
20
0
0
2
4
6
8
Fourier Transform
10
Example- Uranyl Ion Reacting with Hydrous Fe Oxides
Proposed coordination environment of uranyl on the iron oxide surface
EXAFS spectroscopy gives:
Number
Type
Distance (Å)
2
O
1.79
3
O
2.35
2
O
2.52
1
Fe
3.42
From: Waite et al., Geochimica et Cosmochimica Acta 58, 5465-5478 (1994)
Why Iron Sulfides?
Important and widespread mineral phases in anaerobic conditions such
as aquatic sediments
Microcrystalline with high surface area and redox active surface
Originates from use of alternative electron acceptors in
bacterial metabolism
Fe(III)→ Fe(II)
SO42- → S2Fe2+ + S2- → FeS
Microbiology and Radioactivity (eds M J Keith-Roach and F R Livens), Elsevier Dec
2001
Removal of UO22+ and NpO2+ from Solution by FeS
Uranium uptake is almost
quantitative and is
independent of solution
concentration
120
100
UO22+
80
60
40
20
NpO2+
0
0
2
4
Initia l S o lution Con c (mM)
6
Neptunium uptake is
relatively low but
independent of solution
concentration
Reactions with mineral surfaces studied with XANES spectroscopy
Uranyl reacted with Fe phases
Neptunyl reacted with FeS
Surface reactions with FeS studied by EXAFS
Uranium
Aqueous
Increasing [U]
Neptunium
60
50
0.27 mM Np soln.
-1
(650 µ g g )
50
40
Amplitude
20
3
k χ(k) (arbitrary units)
Increasing [Np]
40
0.68 mM Np soln.
-1
(1 625 µ g g )
30
0.27 ppm Np soln.
-1
(650 µ g g )
30
0.68 ppm Np soln.
-1
(1 650 µ g g )
20
10
2.74 mM Np soln.
-1
(6 500 µ g g )
10
0
2.74 ppm Np soln.
-1
(6 500 µ g g )
0
-10
3
5
7
9
-1
k (Å )
11
13
0
2
4
6
r (Å)
8
10
EXAFS Fitting Parameters
Concentration
4500 ppm
24000 ppm
113000 ppm
Uranium
Concentration
Number,Type
Distance (Å)
2O
1.81
4O
2.40
2O
1.81
2O
2.14
4O
2.36
2O
1.83
1O
2.07
5O
2.31
Number,Type
Distance (Å)
4O
2.25
3S
2.63
4O
2.25
3S
2.61
6500 ppm
4O
2.26
Neptunium
3S
2.64
650 ppm
1625 ppm
Proposed coordination environment of
Np on FeS surface
L N Moyes et al., Environmental
Science and Technology, 34, 10621068 (2000); Environmental Science
and Technology, 36, 179-183 (2002)
Microbiological Transformations
Bacterial Redox Processes- Geobacter sulfurreducens
Known transformationsTcO4- (soluble)
TcO2.2H2O (insoluble)
UO22+ (soluble)
UO2.2H2O (insoluble)
So how does Np (NpO2+ is potentially mobile) behave?
Effect of G. sulfurreducens on NpO2+
0.14
0.12
Absorbance
% Remaining in Solution
0.1
100
0.08
0.06
0.04
0.02
0
800
90
900
1000
1100
Wavelength (nm)
With acetate
No e- donor
Dead cells
Control (no cells)
With hydrogen
80
70
0
100
Time after Inoculation (h)
200
1200
Chemical Transformations
•
Assumed mechanism: 2e- reduction U(VI) Æ U(IV)
•
Alternative mechanism: 1 e- reduction U(VI) Æ U(V)
– U disproportionates Æ U(VI) + U(IV)
– Cycle of U(VI) reduction until nearly all U(IV)
UO22+
2 eU4+
UO22+
eUO2+ Æ UO22+ +
U4+
Identification of U(V) by EXAFS Spectroscopy
100%
80%
60%
U(IV)
U(V)
40%
U(VI)
20%
0%
0
2
4
8
24
Hours
Abundance of Different U Species
U(VI)
U(V)
U(IV)
Plutonium in Ponds
Observations-
Discharges of Pu from ponds may be difficult to predict
These effluents need to be carefully managed
Useful Stuff and Important Questions
What is in the ponds?
Corroded Magnox sludge (CMS)
What happens to the
effluents?
Water
NaOH to bring pH to 11
Mg + 2 NaOH
NaOH + CO2
Mg(OH)2 + CO32-
Mg(OH)2 + 2 Na+
NaHCO3
MgCO3 + 2 OH-
Effluent pH 11
Sand bed
filter
Adjust to pH 7
with CO2 gas
Key QuestionsIs Pu in solution or particle associated?
What is the nature of the particles?
Zeolite ion
exchange
What is the nature of Pu solution species?
What are the effects of effluent treatments?
AlsoHow do we immobilise the Magnox sludges?
Discharge to
sea
ESEM Imaging of CMS Simulant
1600 x
3200 x
9400 x
Two morphologies
Wide distribution of particle sizes
XRD- brucite and artinite (MgCO3.Mg(OH)2.3H2O)
Solubility, filterability, interconversion?
What influences Pu solubility?
% of Pu(IV) retained on a 0.22 µm filter
Use filtration (0.22 µm) to define
“solubility”
Factorial experiment allows
quantitative assessment of
different controls on Pu
100
100
75
75
Analysis of variance
50
50
25
25
0
+ CMS
pH 11
- CMS
pH 7
+ CMS
pH 7
+ Si
pH 7
+ CMS
+Si
pH 7
Conclusions
We need to understand the mechanisms underlying waste behaviour.
There are complexities in defining the behaviour of the actinides,
arising both from the chemical complexity of the actinides and from
the complexity of heterogeneous waste systems.
Using modern spectroscopic and analytical techniques it is possible
to make progress.
We have a good deal of work still to do, especially for the transuranic
elements.
It would be very helpful to have robust, fundamental models (e.g
quantum) of these difficult systems