The TRADE Experiment: Status of the Project and Physics

PHYSOR 2004 -The Physics of Fuel Cycles and Advanced Nuclear Systems: Global Developments
Chicago, Illinois, April 25-29, 2004, on CD-ROM, American Nuclear Society, Lagrange Park, IL. (2004)
The TRADE Experiment: Status of the Project and Physics of the
Spallation Target
C. Rubbia1, P. Agostini1, M. Carta1, S. Monti1, M. Palomba1, F. Pisacane1,
C. Krakowiak2, M. Salvatores2, Y. Kadi*3, A. Herrera-Martinez3, L. Maciocco4
1
ENEA, Lungotevere Thaon di Revel, 00196 Rome, ITALY
2
CEA, CEN-Cadarache, 13108 Saint-Paul-Lez-Durance, FRANCE
3
CERN, 1211 Geneva 23, SWITZERLAND
4
AAA, 01630 Saint-Genis-Pouilly, FRANCE
The neutronic characteristics of the target-core system of the TRADE
facility have been established and optimized for a reference proton energy of
140 MeV. Similar simulations have been repeated for two successive
upgrades of the proton energy, 200 and 300 MeV, corresponding to different
performances and design requirements and different characteristics of the
proposed cyclotron and, as a consequence, of the proton beam. An extensive
comparison of the main physical parameters has been also carried out, in
order to evaluate advantages and disadvantages of different proton beam
energies in the design of the spallation target and to allow the optimal
engineering design of the whole TRADE facility.
KEYWORDS: TRADE facility, ADS, Spallation target physics,
Experimentation, External neutron sources, intermediate proton energy.
1. Introduction
The TRADE “TRiga Accelerator Driven Experiment”, to be performed in the existing
TRIGA reactor of the ENEA Casaccia Centre, has been proposed as a major project in the
way for validating the ADS concept. Actually, TRADE will be the first experiment in which
the three main components of an ADS – the accelerator, the spallation target and the subcritical blanket – will be coupled at a power level sufficient to appreciate feedback reactivity
effects. As such, the TRADE experiment represents the necessary intermediate step in the
development of hybrid transmutation systems, its expected outcomes being considered crucial
– in terms of proof of stable operability, dynamic behavior and licensing issues – for the
subsequent realization of an ADS Transmutation Demonstrator.
As already shown in previous papers [1,2], the experiments of relevance that can be
performed in TRADE concern:
∑ the dynamic system behavior of an ADS vs. the neutron importance of the external
source at different sub-criticality levels, thus providing important information on the
optimal sub-criticality level;
∑ Sub-criticality measurements at significant power;
∑ Correlation between reactor power and proton current;
∑ Reactivity control (neutron source importance method);
∑ Compensation of power effects of reactivity swing with control rods movements or
with proton current variation;
∑ Start-up and shutdown procedures, including suitable techniques and instrumentation.
*
Corresponding author, Tel. +41-22-767-9569, FAX +41-22-767-7555, E-mail: [email protected]
In this paper, we will present the status of the project, as well as results of recent
simulations and analyses concerning the physics of the spallation target for different values of
the proton energy, from 140 MeV up to 2-300 MeV.
2. Status of the Project
The TRADE experiment - aimed at a global demonstration of the ADS concept - is an
original idea of Carlo Rubbia developed through a feasibility study carried out by an ENEA
and CEA Working Group over 2001-beginning of 2002. These feasibility studies have been
followed by further conceptual design activities performed in 2002-2003 by an International
TRADE collaboration set up by ENEA (Italy), CEA (France), FZK (Germany), DOE (USA),
CIEMAT (Spain), CNRS (France), AAA (France), and ANSALDO (Italy).
The overall layout of the facility - selected after a quite comprehensive comparison among
others - is shown in Figure 1 [3]. It foresees the erection of a new bunker, close to the existing
TRIGA building, to house the accelerator and the test station for proton beam test and
adjustment. The proton beam is transferred from one building to the other via a section of the
transfer line that is particularly simple, since the cyclotron is at the same level of the top of the
reactor. The beam transport line is protected by a massive shielded tunnel which extends into
the TRIGA building up to the reactor top. Through the straight section of the transport line,
the beam is transferred to the final bending section composed of two magnetic dipoles and
three magnetic quadrupoles, which have the duty of directing the beam, with the correct size,
to the spallation target placed in the central thimble of the reactor.
Fig.1 Reference layout of the TRADE facility (vertical section)
The studies performed so far by the TRADE International Collaboration have concerned:
∑ The neutronics of selected possible configurations, along with a neutronic benchmark
to define codes and tools to be used for the neutronic design and the interpretation of
the experimental measurements;
∑ The core and target thermal hydraulics, using both natural or forced convection,
including the target coupling to the reactor at power;
∑ The target performances and characteristics as well as the conceptual design of the
target, its cooling system and the definition of the tests needed for its qualification;
∑ A conceptual design of the Beam Transport Line;
∑ The shielding and activation aspects in order to gain insight on the dose issues.
∑ The safety and licensing aspects related to the plant modifications induced by the
TRADE experiment, including considerations on general safety criteria, possible
accident initiators and a preliminary hazards analysis;
∑ The overall experimental program to be performed in TRADE;
∑ The representativity of the foreseen experiments in terms of dynamic behaviour,
neutron spectrum, reactivity control, proton current/power relation, operation at startup and shut-down, external source importance measurements, etc..
∑ A preliminary cost and time schedule evaluation.
Furthermore, a preliminary experimentation in the TRIGA RC1 reactor was carried out in
fall 2002, to characterize the TRADE reference core; a new experimental phase - which will
include measurements in a mock-up of the TRADE core with Californium, DD and DT
external sources – is being performed over 2003-2004 (reported in another paper at this
conference).
As for the accelerator, several configurations were studied for a suitable proton energy
beam of 140, 200 and 300 MeV and with a beam current in the range 100-300 µA. Taking into
account the constraints related to the cost and the relatively short time scale for the
implementation of the TRADE experiment, a room temperature H- cyclotron is considered the
most affordable solution. Actually, even if the requested performances for TRADE (140 MeV,
2-300 µA) are rather challenging with respect to the ones of the existing machines, the
TRADE cyclotron can be regarded as an evolution of a typical H- machine for radioisotope
production (Ep around 30 MeV) or for hadron-therapy (Ep around 60 MeV).
As for international agreements to implement the TRADE experiment, a Memorandum of
Understanding (MOU) among “funding” partners - ENEA, CEA, DOE and FZK – was signed
in 2003 by ENEA, CEA and FZK. TRADE-PLUS – the part of the TRADE project devoted to
the design of the facility as well as to the experiments and their interpretation and
transposition to the future ADS Demonstrator - is also one of the major subprojects of the
Integrated Project EUROTRANS, which is being presented to the European Commission by
several European associations within the EURATOM 6th Framework Programme in the
Thematic Priority Area “Management of Radioactive Waste: Transmutation”.
3. Physics of the Spallation Target
The spallation target is the key component of any ADS concept. Even in the TRADE
facility, despite the relative small power of the proton beam (<40 kW), the development and
design of the target implies a detailed assessment of different aspects mutually interacting,
from the physics of spallation reaction - including neutron generation and distribution,
spallation products yields and damage rates – to technological issues, such as choice of the
most suitable material, power density distribution, heat removal, thermo-mechanics,
fabricability, etc. In particular, accurate and rigorous assessment of nuclear parameters under
different physical conditions is the prerequisite for an optimal design of the target and its
interaction with the (subcritical) TRIGA core.
This work aims at evaluating, by probabilistic transport codes (FLUKA and EA-MC), the
main neutronic and physical parameters such as yield and energy and angular distribution of
the spallation neutrons, proton and neutron flux around the target, energy deposition, radiation
damage, spallation product yields and radioactivity. The calculations have been performed for
different intermediate energies (140, 200 and 300 MeV) of the proton beam impinging on a
solid Tantalum target, allowing the evaluation of pros and cons of different solutions as well
as the most suitable cyclotron for the TRADE facility and its successive utilizations. The
performances and the impact on the target design of different shapes of the proton beam and
geometrical configurations of the spallation target have been also assessed.
The guidelines which have been followed for the target development together with the
main constraints and interfaces have been extensively discussed in ref. [4]. The geometry is
described below to provide justification of the choices in relation with the aforementioned
design elements.
3.1 Geometrical description of the target
The draft prototype described here corresponds to configuration #10 (Figure 2).
Fig.2 Target configuration #10
3.1.1 Inner geometry
The inner geometry (Figure 2) is characterized by three conical cavities having different
angles and total length equal to the active height of the TRIGA core. The cone tip (lowest
cone) is exposed to the highest power density for two reasons:
∑ the relevant proton current at the centre of the Gaussian distribution,
∑ the forward scattering of protons as a consequence of the conical angle steepness.
Moreover, the deposited power at the tip of the cavity is very sensitive to the reduction of
the sigma value of the gaussian beam, as shown in Figure 3 for 140 MeV proton energy. To
better distribute the power, it is necessary to work out a small diameter at the extremity of the
cone. The former shape of the cone tip presented a series of drilled cylindrical bores which are
now substituted by a smooth conical surface. This improvement was demonstrated to be
feasible by the spark erosion technique up to a diameter of 1.5 mm as shown in Figure 4.
A
m
*
3
m
c
(
/
w
y
t
i
s
n
e
d
Power density at cone tip vs. sigma
40
35
30
r
e
w 25
o
p
20
7
7.5
8
8.5
Sigma of Gaussian (mm)
Fig.3 Power density at cone tip
9
Fig.4 Spark erosion bores in Ta
The conical wall angle of the lower cone (“Dett.C” in Figure 2), having predetermined
the larger diameter as 6 mm, is identified by its length: the larger the length, the smaller is the
linear power; nevertheless a maximum limit of 80 mm can be identified for fabrication
reasons. The smooth conical surface strongly improves the power distribution of the lower
cone which is more uniformly heated up. It is evident that, by increasing the cone length, one
can decrease the power distribution at the tip, but this procedure implies an increase of linear
power at the intermediate cone which should be balanced to better distribute the power in the
whole spallation volume. Elastic thermo-mechanical calculations show that a best
compromise can be reached with a lower cone length of 60 mm [5].
The intermediate truncated cone (“Dett.B” in Figure 2) has an angle of 7.35° and
receives the largest amount of power (77% of the total). In this region the forward scattering
of protons mainly occurs, therefore, in order to widen its angle, a conical region having
steeper angles was located below its position.
The upper truncated cone (“Dett.A” in Figure 2) is very short (21 mm) and has the
function to directly connect the target to the beam transport line; the tails of the Gaussian
profile are here truncated. This cone has the largest angle to keep constant the axial position
of the beginning of the spallation process, even in presence of small radial errors. The
impinging power is relatively low as shown by the calculated axial distribution of power
(section 3.4.5).
3.1.2 Outer geometry
The typical range of 140 MeV protons, being stopped in Tantalum by electronic
interactions, can be approximately expressed as:
ER( E ) = E 1.75 5811
where ER(E) is the electronic range (cm) and E is the proton energy (MeV).
The approximated electronic range was 9.8 mm and a thickness of 15 mm was assumed in
the design as the best compromise to assure the radial heat removal without intolerable
thermal stresses. At the lowest cone the 15 mm thickness produces excessive thermal stress
and has to be reduced. This is due to the shape factor “ 1 ln (re ri )” which affects the
integration of Fourier Law in axial-symmetric geometries. In Figure 5 the comparison
between the shape factors, is reported for the lowest cone in case of constant thickness and
reduced thickness.
e
m
i
d
a
(
Lowest cone shape factor
r
o 0.6
t
c
a 0.53
f
e 0.45
p
a 0.38
h
s
0.3
0
15
30
45
lowest cone length (mm)
60
constant thickness
reduced thickness
Fig.5 Variation of the shape factor versus lowest con length
The upper part of the cavity presents a thickness smaller than 15 mm because of the outer
geometrical constraint: 63 mm maximum diameter.
3.2 Thermal performances of the target
In presence of the design mass flow-rate of water (2.24 kg/s), the maximum thermal flux
at the outer wall of the target is 135 W/cm2 (Figure 6) thus assuring a margin large enough to
prevent the occurrence of Critical Heat Flux. Moreover the maximum temperature is 80°C
(Figure 7), which is significantly lower than the TRIGA saturation temperature.
Fig.6 Wall thermal flux
Fig.7 Outer wall temperature
The thermal performances of the target configuration #10 under the reference operating
conditions are reported in Table 1.
Table 1 Operating conditions
The velocity and temperature fields are reported in Figure 8.
Fig.8 Velocity and Temperature fields for the target configuration #10
This target configuration presents a better temperature distribution which comes out of a
more balanced power density in the target material, as evidenced by the lower temperature
peak.
3.3 Mechanical resistance of the target
The elastic stress calculations reported in [5] show a relevant stress value, which is higher
than the yield limit of the material (100 Mpa at 300ºC). Since the thermal loads are secondary
loads, they progressively disappear as soon as plastic deformations take place; this working
condition is defined as elastic-plastic. In nuclear applications the elastic-plastic condition of
structural materials is generally accepted provided that some design rules are respected. The
acceptability rules are mainly connected with the alternate nature of the loads; in particular
two phenomena have to be studied in our case: progressive plastic deformation (or ratcheting)
and fatigue. Due to the operating temperature that is lower than 815°C, the creep effects can
be neglected for Tantalum.
Calculations of plastic strain on the target configuration #10 when exposed to 40 kW
beam power, by 140 MeV protons, are reported in Figures 9a (upper target) and 9b (lower
target).
Fig.9a Plastic strain of the upper target
Fig.9b Plastic strain of the lower target
The corresponding total strains are reported in Figures 10a and 10b.
Fig.10a Total strain of the upper target
Fig.10b Total strain of the lower target
The maximum strain (1.45 %) is located in the stepped region while a lower local
maximum (1.12 %) corresponds with the region of maximum temperature. The actually
preferred solution relies on a smooth lower surface (configuration #10), which will reduce the
highly stressed value of the stepped region [5]; nevertheless some rough fatigue
considerations can be drawn. In Figure 11 a series of calculated plots of the behaviour of
Tantalum and Tungsten at room temperature under strain controlled fatigue is reported [5]; the
plots are obtained after application of the Manson-Coffin numerical procedure to literature
results obtained by load controlled tests. It seems that Tantalum can survive a total strain
range of 1.4 % after 1000 cycles. The results have to be confirmed by more direct experiments
at operational temperature and are presently considered only a reference indication. In order to
enhance the target duration a lower level of proton beam power must be envisaged.
)
%
(
Fatigue Strain of W and Ta
10
n
i
a
r
t
s
l
a
t
o
t
1
0.1
3
1 .10
4
1 .10
5
cycles to rupture
1 .10
1 .10
6
Tungsten annealed at 1480°C, tested at room temperature
Tantalum in plate annealed at 1400°C,tested at room temperature
Tantalum in wire annealed at 1400°C, tested at room temperature
Fig.11 Calculated fatigue strain of Ta and W at room temperature
3.4 Nuclear performance of the target
The main neutron physic parameters of the target such as neutron yield and energy
spectra, power deposition, material damages and spallation product distributions are evaluated
by probabilistic transport codes (FLUKA/EA-MC and MCNPX).
The impact of different target parameters (material choice, geometry of the proton beam,
energy of the protons) has been studied extensively [6,10]. While the neutron yield and
spectra are mainly related to the nuclear behavior of the system, energy deposition is directly
related to the thermo-mechanics of the target and its cooling capabilities, which determine its
lifetime in the core.
3.4.1 Neutron production
The main goal of the spallation target is the neutron production. Calculations performed
for 140 MeV protons distributed according to a Gaussian profile (as discussed in section
3.1.1) on thick (configuration #1 and #10) and thin (configuration #4) geometries of the
Tantalum target show that the neutron yield (Table 2) is not affected significantly by the target
geometry since the protons are almost at the end of their range when they leave the target.
Even in the “thin” geometry, the material thickness (3 mm in the radial direction) is sufficient
to allow the whole spallation reactions to take place inside the target’s material.
Table 2 Neutron production process
Target type
Thick target (config. #1)
Neutron yield
0.80 n/p
Thin target 3 mm thick (config. #4)
0.79 n/p
Thick target (config. #10)
0.75 n/p
The neutron flux distribution for configuration #10 is reported in Figure 12. Since the
region of maximum production lies below the median plane of the core, it is necessary, in
order to achieve a better efficiency in the use of the source neutrons, to axially shift, by about
4 cm, the target body in the upward direction.
Fig.12 Neutron flux distribution in (n/cm2/s) per kW of beam
The neutron spectra shown in Figure 13 are the result of a complete simulation performed
with the target described above, inserted in the central channel of the TRIGA core where the
fuel pins have been removed (no contribution by the core thermal neutrons). In reality the
contribution by the core thermal neutrons is not negligible and will result in additional
activation of the spallation target. For high energies, it is possible to distinguish the wellknown peak at 1 MeV related to evaporation phase of the spallation process. A second broader
peak is clearly distinguishable at higher energies (above a few tens of MeV), which is related
to the INC phase (Intra-Nuclear Cascade) of the spallation process.
Fig.13 Neutron flux spectra at several locations inside the spallation target vessel in (n/cm2/s)
per kW of beam. The average energy of the high-energy neutrons (>20MeV) is also reported
Overall, the integrated flux of neutrons escaping the target (i.e. entering the core) is
reduced by a factor 10 as a result of the successive attenuation in the target body but also in
the thick shroud surrounding it. For neutrons above a few 100 keV’s, the attenuation reaches
almost a factor 100.
Nevertheless, the high-energy tail of the neutron distribution when entering the core inner
structures is still present and the high-energy neutrons (> 10 MeV), only slightly moderated,
still represent 4.5% of the spallation neutron population with an average energy of about 45
MeV. This fraction was of the order of 7.5% inside the spallation target with an average
energy of 51 MeV, as shown in Table 3.
Table 3 Neutron flux spectra
Energy groups
0. – 1. eV
1. eV – 1. keV
1. keV – 1. MeV
1. MeV – 10. MeV
> 10. MeV
Neutrons escaping
from vessel
(% of flux)
34.0
10.5
31.5
19.5
4.5
Neutrons escaping
from target
(% of flux)
0.6
0.8
51.9
39.7
7.0
These high-energy neutrons are very hard to shield, and contribute to a certain extent to
the radiation damage of the inner structure of the core but also to the ambient dose in the
TRIGA reactor building should they leak out. Moreover, they will most probably react with
the core coolant and produce N16 through (n,p) reactions on O16 which has a threshold at ~ 10
MeV and which cross section peaks at 0.15 barn compared to the 0.2 barn total cross section
of H.
This is further illustrated in Figure 14, which reports the spatial distribution of the highenergy component (> 20 MeV) of the neutron flux in the TRADE core.
Fig.14 Neutron flux distribution of the > 20 MeV in (n/cm2/s) per kW of beam
We estimate the flux of neutrons > 20 MeV reaching the inner most fuel elements to be of
the order of a few 109 n/cm2/s per kW of beam and up to 107 in the bottom part of the reactor
vessel. These neutrons tend to be forward peaked (> 130º), contrary to the low-energy
neutrons which are more or less centred along the core mid-plane (Figure 12). On the other
hand, any beam offset will produce an axial shift of the high-energy neutron distribution that
can be easily monitored by placing at several locations along the height of the target a series
of detectors sensitive to the recoils generated by the high-energy component of the neutron
flux (> 1 MeV).
These high-energy neutrons penetrate deep inside the reactor core and surrounding
biological shield (Figure 15) and very few leaks out into the reactor building, without
fortunately contributing to the ambient dose therein, as shown in Figure 16.
Fig.15 Radial distribution of the high-energy
component (> 20 MeV) of the neutron flux in
(n/cm2/s) per kW of beam
Fig.16 Ambient dose due to neutrons in
(mSv/h) per mA of beam
3.4.2 High-energy proton leakage
The release of protons out of the spallation target causes the production of unwanted
radionuclides in the cooling water, therefore it has to be taken into account to identify the
target acceptability and if possible minimized.
The primary proton flux distribution for the target configuration #10 is reported in Figure
17. The fraction of protons escaping the target vessel is almost negligible apart from the upper
part of the target (direct connection to the beam transport line) where the tails of the Gaussian
profile are truncated. In any case the majority of the protons escaping the spallation target are
either stopped in the cooling channel (riser) surrounding the target or inside the thick flow
guide, none reach the core internal structures and only a few are backscattered into the
vacuum beam pipe.
Fig.17 Primary proton flux distribution in (p/cm2/s) per kW of beam
Figure 18 reports the spectra of protons (both primary and secondary) escaping from the
target and from the target vessel. The proton flux above 10 MeV escaping from the target
vessel, has been reduced by almost three orders of magnitude. The high-energy tail, which is
still present, but at a much reduced scale, corresponds to those protons cut out from the tail of
the Gaussian profile at inlet. The low-energy proton flux results mostly from high-energy
neutron (n,Xp) reactions in oxygen and in the structural material, but also from low-energy
neutron elastic collisions on hydrogen, as shown in Figure 19. This is clearly illustrated by the
fact that the spatial distribution of the secondary protons emerging from the target coolant is
very similar to that of the high-energy neutrons, plotted in Figure 14.
Fig.18 Proton flux spectra at several locations
inside the spallation target vessel in (p/cm2/s)
per kW of beam
Fig.19 Primary and secondary proton flux
distribution in (p/cm2/s) per kW of beam
3.4.3 Residual products and activation
In the target cooling channels (down-comer and riser) as well as in the inner most ring of
the reactor (ring C), spallation and activation products (no contribution from the core
neutrons) are released in the water; their amount was calculated and reported in Figures 20a
and 20b.
Fig.20a Yield of residual products in water
vs. A in (nuclei/cm3/s) per kW of beam
Fig.20b Yield of residual products in water
vs. Z in (nuclei/cm3/s) per kW of beam
The volume of water contained in the target cooling channel (riser part) is approximately
338 cm3, therefore, according to the calculations, the maximum production of the most
problematic isotopes after one year of operation (2000 hours) is given in Table 4.
Table 4 Residual yield of the most troublesome isotopes in the target cooling channel in
(g/year) per kW of beam
Isotopes
H-3
Be-7
C-14
N-16
Riser
8.70 10-9
9.40 10-9
5.40 10-8
1.33 10-7
Down-comer
2.43 10-8
2.25 10-8
1.72 10-7
6.05 10-7
Burn-up calculations have been carried out to estimate the activation of the Tantalum
spallation target. As shown in Figure 21, it is worth noting that the activity of the spallation
target (expressed in Ci per year of irradiation, i.e. 2000 hours, per kW of beam) is dominated
by activation resulting from successive thermal neutron captures during the first year. After
one year the activity is dominated by the decay of the spallation products, mostly Hf, Lu and
Yb isotopes. At longer times (> 40 years) tritium is the only isotope of importance. Note that
tritium is the only troublesome volatile produced in the spallation target.
Fig.21 Evolution of the radioactivity of the Tantalum spallation target as a function of time in
(Ci/yr) per kW of beam
3.4.4 Radiation damage
The particle flux spectra generated by FLUKA [11] can be used to estimate the heating
and damage to structural materials by protons and neutrons with energy above and below 20
MeV. Indeed, in TRIGA one can consider separately the high-energy portion of the spectrum,
due to the primary proton shower, with its intensity proportional to the beam current, and a
lower energy region associated with the fission-multiplying medium, proportional to the
reactor power.
In practice, we have evaluated:
d (dpa) ·sEa Ò
=
◊ f ◊10 -21
2Ed
dt
h
[dpa/s]
Eq.(1)
where ·sEa Ò is the damage energy production cross section (barn-keV), Ed is the energy
required to displace an atom from its lattice position (eV), h = 0.8 is the collision efficiency
factor and f is the particle flux (cm-2.s-1).
As regards the damage induced by high-energy particles (> 20 MeV), we have used data
that provides values for proton- and neutron-induced displacement cross sections as calculated
using the default physics models in MCNPX [12] and illustrated in Figure 22.
Fig.22 Neutron and proton-induced damage-energy cross sections for Tantalum in (barn-keV)
Table 5 indicates the values of the integrated flux in the spallation target unit, due to highenergy particles (HE).
Table 5 Integrated flux per kW of beam power for different target configurations
HE proton Flux
(part/cm2.s)
Max
Ave
13
4.0x10
7.7x1012
4.7x1013 6.7x1012
2.9x1013 3.6x1012
1.8x1013 2.7x1012
Ta Target
110 MeV SOL-1
140 MeV SOL-7
140 MeV SOL-10 C4
140 MeV SOL-10 C200
HE neutron Flux
(part/cm2.s)
Max
Ave
10
8.4x10
1.3x1010
2.1x1011 1.7x1010
1.6x1011 3.1x1010
1.1x1011 2.5x1010
The gas production and the displacement rates (dpa/yr) obtained using Eq. (1) together
with the particle fluxes listed in Table 5 are reported in Table 6 for the same operating
conditions as those listed above, that is per kW of beam and for an expected duty factor
during the TRADE experiments of about 22% (2000 hours).
Table 6 Gas production and the displacement rates per kW of beam
Target
(Ta)
SOL-1(110MeV)
SOL-7
SOL-10 C4
SOL-10 C200
Average
Average
H3
Prot. Ener Neut. Ener Production
(MeV)
(MeV)
(appm/dpa)
60
90
90
90
46
50
51
51
1.45
0.40
2.01
0.99
He
Production
(appm/dpa)
58.1
25.0
165.8
54.8
HE proton
(dpa/yr)
Max
1.1
1.5
0.9
0.6
Ave
0.14
0.16
0.09
0.07
HE neutron
(dpa/yr)
Max
0.002
0.004
0.003
0.002
Ave
0.000
0.000
0.001
0.001
The particle damage are entirely dominated by the high-energy protons and localized on
the inner skin of the spallation target with a maximum situated at the tip of the inner cone. It is
further reduced (x 2) when the proton energy increases to 140 MeV.
3.4.5 Energy deposition
While the previous parameters were mainly related to the nuclear behaviour of the system,
energy deposition is directly related to both the thermo-mechanics of the target and its cooling
capabilities, which determine its lifetime in the core. The power density distribution, or the
distribution of the heat deposited, is a central factor in the thermal design of spallation targets.
The largest value of the power density, which is equal to 175 W/cm3 per kW of beam of
140 MeV protons, is found at the tip of the conical cavity at the bottom of the target as shown
in Figures 23a and 23b.
Fig.23a Power deposition inside the target
in (W/cm3) per kW of beam at 140 MeV
Fig.23b Power deposition in the tip region
in (W/cm3) per kW of beam at 140 MeV
The distribution inside the target reveals a rather homogenous behaviour with an average
power density of ~ 4 W/cm3 per kW of beam. A discontinuity is observed in the aluminium
beam pipe (where the tail of the Gaussian profile is cut out) and in the upper truncated cone
region of the target where the cone has the largest angle to keep constant the axial position of
the spallation neutron source distribution, even in presence of small beam offsets. However,
the impinging power there is relatively low as shown by the calculated axial power
distribution reported in Figure 24.
Fig.24 Linear power distribution in (W/cm) per kW of beam for different target
configurations and beam profiles
3.5 Application of higher proton energies to the reference target configuration
The neutronic characteristics of the target-core system of the TRADE facility have been
optimized for a reference proton energy of 140 MeV. Similar simulations have been repeated
for two successive upgrades of the proton energy, 200 and 300 MeV, corresponding to
different performances and design requirements and different characteristics of the proposed
cyclotron and, as a consequence, of the proton beam. In this section, an extensive comparison
of the main physical parameters is carried out, in order to evaluate pros and cons of different
proton beam energies in the design of the spallation target and to allow the optimal
engineering design of the whole TRADE facility.
3.5.1 High-energy proton leakage
The fraction of protons escaping the target vessel increases with the proton beam energy.
On average between 109 and 1011 particles per kW of beam manage to reach the first fuelled
region. Their average energy is of the order of 30 MeV and reaches up to 65 MeV when the
beam proton energy is increased to 200 and 300 MeV respectively. They are in any case all
stopped in the core.
In Figures 25 and 26, the proton leakage from the spallation target unit is represented for
initial proton energies of 200 and 300 MeV respectively.
Fig.25 Primary proton leakage from the target unit in (p/cm2/s) per kW of beam at 200 MeV
Fig.26 Primary proton leakage from the target unit in (p/cm2/s) per kW of beam at 300 MeV
The induced radioactivity of the target cooling circuit increases when the proton energy is
increased since protons are escaping more readily out of the spallation target and are
undergoing more nuclear interactions in the water surrounding the target. Figure 27 reports
the production of the light isotopes in target cooling water. Figures 28a and 28b report the
activation of the target and its cooling circuit by the generated spallation products. Assuming
the water volume in the riser is approximately 338 cm3, one can calculate the water activation
by the protons. The latter reasoning is only applicable to protons up to 200 MeV, because at
higher energies the complete stopping of protons in the flow guide seems unfeasible by the
material thickness of the present target configuration.
Fig.27 Yield of residual products in the target cooling circuit in (nuclei/cm3/s) per kW of
beam at different incident proton energy
Fig.28a Activation of thespallation target
per kW of beam at different proton energies
Fig.28b Activation of the target coolant
per kW of beam at different proton energies
As far as the radiation damage of the target is concerned, the integrated fluxes of highenergy (HE) protons and neutrons have been obtained for different beam energies and
reported in Table 7.
Table 7 Integrated flux per kW of beam power for different beam energies
Ta Target
sol 10c200
140 MeV
200 MeV
300 MeV
HE proton Flux
(part/cm2.s)
Max
Ave
13
1.8x10
2.7x1012
1.3x1013 1.8x1012
8.3x1012 1.2x1012
HE neutron Flux
(part/cm2.s)
Max
Ave
11
1.1x10
2.5x1010
1.6x1011 4.3x1010
2.1x1011 6.3x1010
The gas production and the displacement rates (dpa/yr) obtained using Eq. (1) together
with the particle fluxes listed in Table 7 are reported in Table 8 for the same operating
conditions as those listed in section 3.4.4.
Table 8 Gas production and the displacement rates per kW of beam
Target
(Ta)
Average
Average
H3
Prot. Ener Neut. Ener Production
(MeV)
(MeV)
(appm/dpa)
He
Production
(appm/dpa)
HE proton
(dpa/yr)
HE neutron
(dpa/yr)
140 MeV
90
51
0.99
54.8
Max Ave Max Ave
0.6 0.07 0.002 0.001
200 MeV
300 MeV
115
155
65
88
2.92
6.93
130.
275.
0.5
0.4
0.05 0.003 0.001
0.04 0.005 0.002
The particle damage are entirely dominated by the high-energy protons and localized on
the inner skin of the spallation target with a maximum situated at the tip of the inner cone. It is
reduced with increasing proton energy, but on the other hand, gas production that is
responsible for the swelling of the target is drastically enhanced.
3.5.2 Neutron production
High-energy neutrons produced by evaporation reactions in the target are more difficult to
stop or to attenuate and travel a relatively long distance in the core. Their average energy is
about 45 MeV and increases up to 74 MeV, in the case of the most energetic beam. Their
spatial distribution is relatively forward peaked and has the tendency to escape through the
bottom part of the reactor vessel.
In fig.29 and 30 the neutron distribution close to the target and in the core are represented
for initial proton energies of 200 and 300 MeV respectively.
Fig.29 High-energy neutron flux distribution (n/cm2/s) per kW of beam at 200 MeV
Fig.30 High-energy neutron flux distribution (n/cm2/s) per kW of beam at 300 MeV
The presence of the high-energy neutrons in the core and their anisotropic distribution
result in a harder neutron spectrum and higher leakage component, which explains why the
neutron multiplication is reduced when the proton beam energy is raised (Table 9). This
mitigates the gain in neutron yield and therefore the corresponding reduction of the
accelerator intensity. It has to be observed that a good neutron distribution, in this case, will
require an upward axial shift of the target of about 20 cm.
Table 9 Variation of the main neutronic parameters with proton beam energy
140 MeV
170 MeV
200 MeV
240 MeV
300 MeV
Neutron Yield
(n/p)
0.80
1.20
1.65
2.34
3.56
Net Neutron
Multiplication
11.7
11.1
11.0
10.4
10.1
0.9143
0.9098
0.9093
0.9041
0.9007
Dk/k (pcm)
0.0
-500
-550
-1120
-1500
Scaling Factor
1.0
0.71
0.52
0.38
0.26
k
These high-energy neutrons penetrate deep inside the reactor core and surrounding
biological shield and contribute to the ambient dose inside the reactor building, as shown in
Figures 31a and 31b. We estimate the flux of neutrons > 20 MeV reaching the inner most fuel
elements to be between 109 and 1010 n/cm2/s per kW of beam and between 107 and 108 in the
bottom part of the reactor vessel for incident beams of 200 and 300 MeV respectively.
Fig.31a Ambient dose due to neutrons in
(mSv/h) per mA of beam of 200 MeV protons
Fig.31b Ambient dose due to neutrons in
(mSv/h) per mA of beam of 300 MeV protons
4. Conclusions
The thermo-mechanical issues (progressive plastic deformation and fatigue) becomes less
stringent at higher proton energies because the higher neutron yield allows a target power
reduction. The neutron multiplication is effective only if the target is axially re-positioned to
optimally match with the neutron flux of the TRIGA core. At higher proton energies, the flux
levels in the target are reduced together with dpa’s and power densities (easing therefore the
cooling capabilities of the target). We note however a considerable increase in the gas release
fraction at 300 MeV.
The issue of proton stopping becomes more critical and must be faced by a further
increase of the flow guide thickness. Since the forced convection of the target has to be kept,
to prevent flow disturbances in the TRIGA core, the radial space limitations have to be
accounted for. To electronically stop 200 MeV protons, a Ta target thickness of 18 mm. is
roughly required; this value is assured by the present target configuration. For higher energy
protons it is necessary to develop completely new design based for example on the use of
liquid metal. In any case the effects of partial proton stopping by water have to be studied and
the acceptability limits, in terms of safety, have to be established.
References
[1]
C. RUBBIA et al., “A Full Experimental Validation of the ADS Concept in a European
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[2] M. SALVATORES et. al., “The TRADE Experiment and Progress”, Global 2003, New
Orleans, Louisiana, November, 2003.
[3] C. RUBBIA et al., “TRADE Reference Design Report”, TRADE Technical report,
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[4] P. AGOSTINI et al., “Design Manual of the TRADE Spallation Target”, TRADE
Technical note, March 2004.
[5] C. KRAKOWIAK et al., “Thermo-Mechanical Analyses of the TRADE Spallation
Target”, TRADE Technical note, March 2004.
[6] F. PISACANE et al., “Evaluation of the Spallation Target Design Characteristics for the
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[10] N. BURGIO et al., “Upgrading the TRADE System: 300 MeV Proton Beam Energy on
Tantalum Target, Preliminary Results”, TRADE Technical note, PH1.ME.008.0 (2003).
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