1
Index
0. Objective ……………………………………………………………………………………3
1. Validation of simulation tool (Movak3D) .......................................................................... 3
1.1. Theoretical validation ................................................................................................. 3
1.1.1.
Cylindrical tube .................................................................................................. 3
1.1.2.
Cylindrical annulus ............................................................................................ 4
1.2. Experimental validation ............................................................................................. 5
2. Cryotrapping system for SPIRAL2 .................................................................................... 7
2.1. Function and efficiency of the cryotraps .................................................................... 7
2.2. Principle of the cryotraps ........................................................................................... 7
2.3. Activated charcoal behaviour ..................................................................................... 8
3. Source term of the contamination transfer ....................................................................... 10
3.1. Neutron production .................................................................................................. 10
3.2. Geometry configuration of the target ....................................................................... 10
3.3. Volatile gases in the uranium target ......................................................................... 11
4. Accommodation ............................................................................................................... 13
4.1. Accommodation interest .......................................................................................... 13
4.2. Accommodation models ........................................................................................... 14
5. Conclusion ........................................................................................................................ 18
6. References ........................................................................................................................ 19
Figures :
Figure 1 : Comparison of Movak3D calculation to analytical formula ..................................... 4
Figure 2 : Relative error between Levenson formula and MOVAK3D for the transmission
probability given for several Di/D0 ratios (from 0.1 to 0.8) ............................................... 5
Figure 3 : Experimental set-up in the vacuum laboratory at GANIL. The gas flow was
transmitted at 20 K. Results of the experiment compared to Movak3D calculation.......... 6
Figure 4 : The cryotraps as conceived by the SPIRAL2 team ................................................... 7
Figure 5 : Areas and groups of the cryotraps ............................................................................. 7
Figure 6 : Experimental set of activated charcoal under development in the Vacuum
Laboratory of GANIL ........................................................................................................ 8
Figure 7 : Nitrogen adsorption on metallic surface and on activated charcoal .......................... 9
Figure 8 : Adsorption of hydrogen and nitrogen on activated charcoal ..................................... 9
Figure 9 : Uranium carbide target in reality (left), and as modelled (right) ............................ 10
Figure 10 : Geometry configuration of the converter and the target as modelled.................... 11
Figure 11 : Beam line in the configuration of SPIRAL2 ........................................................ 13
Figure 12 Accommodation coefficient as a function of the ration of molecular masses of
respectively the gas and the wall ...................................................................................... 14
Figure 13 : Windows of the SCA code, developed by GANIL................................................ 15
Tables :
Table 1 : Gaseous elements in the target .................................................................................. 11
Table 2 : Activity of gases (Bq) in the target just after irradiation (0 hour) and after some
periods of cooling ............................................................................................................. 12
2
0. Objective
This report gives details about the contribution of GANIL to the SAFERIB program
concerning the subtask T-J10-3: Technical concept to confine volatile radioactivity.
This work deals with the radioactive contamination transfer in beam lines and the
technological solution to limit this migration, namely the solution called “cryotraps”.
The problem of the migration of the contamination is very constraining in e.g. the
SPIRAL2-project at GANIL, since a very high level of radioactivity will be produced. In this
project the fission process will be used to generate very intense beams of exotic nuclei. This
production scheme requires a very high fission rate. With 5 mA of 40-MeV deuterons on a
converter target followed by a high density uranium target (11 g.cm-3) the fission rate will be
about 5 1013 up to 1014 fission.s-1. The activity in the target, after 90 days of continuous
irradiation, will be about 4 1014 Bq, where more than 65 % (~ 3 1014 Bq) will be in a volatile
state at 2000 °C, the heating temperature of the target. The containment of this volatile
activity is crucial for the nuclear safety to avoid the transfer of the contamination in the
facility, namely to the experimental areas. The cryotrap is the technological solution
developed to limit the migration of non-ionised atoms into the facility. So it is considered as
an important element for nuclear safety (Elément Important pour la Sûreté) for the future
installation of SPIRAL2.
This work will be considered as a good experience perfectly applied for several
projects as SPIRAL2 and EURISOL, where the problem of the contamination will normally
be more constraining and more severe.
.
1.
Validation of simulation tool (Movak3D)
Movak3D is a computer program dedicated to the transport probabilities calculus of
particles in a definable vacuum system under molecular regime. The code is valid only for
large Knudsen numbers, which correspond to the high and ultra high vacuum ranges. The
validation study goal was to verify the coherence of Movak3D simulation results with the
principles of vacuum physics namely as they are given in the literature and with experimental
studies
1.1. Theoretical validation
a. Cylindrical tube
For theoretical validation, we chose simple and complicated shapes and geometries for
which we calculated by Movak3D the transmission probability of molecules and compared
them to theoretical estimation. The example of a cylindrical tube with constant section is
interesting. Santler1 (1986) tried to give an analytical formula to evaluate the transmission
probability as a function of the ratio between length (L) and the diameter (D) for this
geometrical shape. Movak3D reproduces exactly the predictions from the expression of
Santler (figure 1).
3
1

 6 L 
L'  L 1  1 /  3 

 7 D 

0,25
0,2
Pr
 3 L' 
Pr   1 
:

 4 D
Santler expression where
MOVAK3D
0,15
Santler
0,1
0,05
0
0
10
20
30
40
L/D
Figure 1 : Comparison of Movak3D calculation to analytical formula
b. Cylindrical annulus
Davis (1960) and Levenson et al. (1960; 1963) as well as Carette et al. (1983) used the
Monte-Carlo calculational method for determining the conductance of simple and complex
shapes. From calculation results, Levenson tried to give an analytic formula of the
transmission probability of cylindrical annulus. Thus he proposed the following expression 2:
Pr  1.33 K 0
D0  Di
L  1.33 D 0  D i 
where K0 is an experimental factor depending on the ratio Di/D0. K0 is given for few values of
Di/D0 (figure 6.). By fitting experimental points we propose an empirical formula giving K0 as
a function of diameters ratio:
4
3
2
D 
D 
D 
D 
K 0  3.565  i   5.2457  i   2.5688  i   0.1034  i   1.0002
 D0 
 D0 
 D0 
 D0 
Using these results, we calculate the transmission probability according to Levenson
of a cylindrical annulus for various values of Di/D0, where Di is the interior diameter and D0 is
the exterior one. We see that the MOVAK3D calculation results are in a very good agreement
with the Levenson formula results for small ratios Di/D0 (figure 2). The relative error is less
than 10 % for small ratios and it increases when the ratio Di/D0 increases.
4
relative error (%)
120
100
0.8
80
0.7
0.6
60
0.4
40
0.1
20
0
40
60
80
100
pipes lenght (au)
120
140
Figure 2 : Relative error between the Levenson formula and MOVAK3D for the transmission
probability given for several Di/D0 ratios (from 0.1 to 0.8)
1.2. Experimental validation
For experimental studies, several experiments were carried out in the vacuum
laboratory of GANIL to determine experimentally the transmission probability of a gas flow
and to compare it to the Movak3D simulations. The following figure gives an example of an
experimental set-up, where we studied the transmission of two gases, nitrogen and argon.
The tube to stimulate the condensation is cooled to 20 K by means of a cryogenerator
(industrial reference CTI-1020). So it presents the region of condensation. The transmission
probability is given by the ratio of the pressure of the gas at the exit and the entrance of the
cooled tube, respectively.
The vacuum is produced by using a turbo molecular pump with a pumping speed of 550 l.s -1
(Varian V-550). The pressure at the entrance and the exit of the system was measured using a
Balzers vacuum gauge sensor (industrial reference : IKR 050) and a Pfeiffer gauge (industrial
reference : PBR 260). The pressure in the tube was about 1.3 10-6 mbar.
Results of the measurement listed below show an acceptable agreement between simulation
and experiment. The following table presents the transmission probability of the gases.
N2
Ar
Measurement
Movak3D
0.21  0.063
0.19  0.057
0.12  0.01
0.12  0.01
5
Figure 3 : Experimental set-up as used in the vacuum laboratory in GANIL. The gas flow was
transmitted at 20 K. Results of the experiments are compared to Movak3D calculations
6
2. Cryotrapping system for SPIRAL2
2.1. Function and efficiency of the cryotraps
To confine the radioactive contamination and to limit its migration into beam lines in
the future installation of SPIRAL2, a cryotrapping system was developed (it is called
CRYOTRAPS) and is presently under construction (figure 4). Many experimental and
simulation studies were conducted to dimension the system and to validate its conceptional
design. These studies showed that for the most penalizing gases (fission products), the
cryotrap has an efficiency of about 99.9 %, which means that 99.9 % of the incoming
radioactivity will be trapped on the cryotraps surfaces. With such efficiency, and for a
cryotraps established at about 3 meters far from the source, the transferred activity to the
beam line after the cryotraps will be about 109 Bq which is less than the expected activity of
radioactive beams.
Figure 4 : The cryotraps as designed by the SPIRAL2 team
2.2. Principle of the cryotraps
The cryotraps is designed as an assembly of 5 condensation areas (see figure 5) :
The most external areas (areas 1 and 5, in blue colour in figure 5) are dedicated to the group 1
of volatile gases, which the condensation temperature is less than 80 °K. This group is
constituted of almost only the fission products, so they present more than 85% of the total
gaseous activity.
The areas 2 and 4 are for the second group of gases whose the condensation
temperature is less than 20 °K. It is mainly constituted of rare gases.
The 3d group with tritium and hydrogen needs a specific solid absorber (like the activated
charcoal) to be condensed at 20°K. This group will be mainly condensed in the area 3, which
is covered by activated charcoal.
Figure 5 : Areas and groups of
the cryotraps
7
2.3. Activated charcoal behaviour
Experiments were conducted in GANIL to verify the behaviour of activated charcoal
under low temperature. The developed experimental set up allowed to compare the
condensation of some gases (nitrogen and hydrogen) in metallic surfaces (copper) and
metallic surfaces covered by activated charcoal (figure 6).
Activated charcoal exhibits good properties to stimulate the condensation of some
gases like hydrogen. To check this characteristic, we used two gases, nitrogen and hydrogen.
Then we measured the gas adsorption as a function of the cryogenic temperature.
We show that in order to trap nitrogen molecules, the metallic surface has to be cooled
to very low temperatures, while a surface coated with activated charcoal can act as an
absorber up to 100 K (figure 7). The experiments showed that hydrogen is much more
difficult to be absorbed compared to nitrogen. The cooling temperature does not exceed 50 K
even with activated charcoal (figure 8).
Activated charcoal
Figure 6 : Experimental set of activated charcoal under development in the Vacuum
Laboratory of GANIL
8
Figure 7 : Nitrogen adsorption on metallic surface and on activated charcoal
Figure 8 : Adsorption of hydrogen and nitrogen on activated charcoal
9
3. Source term of the contamination transfer
The activation of the uranium target under nominal operation (5 mA of 40 MeV
deuterons on a graphite converter) after 90 days of irradiation represents the source term of
the contamination transfer.
3.1. Neutron production
The fission reaction of a natural uranium target with fast neutrons will allow for the
production of a broad range of exotic neutron-rich nuclei. The aim is to obtain a high fission
rate within the interval [5 × 1013 ; 1014 ] fissions.s-1. The primary beam (deuterons of 40 MeV
and 5 mA) is converted by the stripping interaction in a graphite converter, into fast neutrons.
The converter is a graphite cylinder of 1 cm of thickness and 8 cm of diameter. The density is
1.8 g.cm-3. The beam is entirely stopped in the converter [3].
3.2. Geometry configuration of the target
The target consists of 19 stacks of natural uranium carbide (UC2) pellets in a
hexagonal arrangement within a matrix of carbon (figure 9). The diameter of each pellet is 1.5
cm and its thickness is 0.1 cm. The density of UC2 is 11 g.cm-3. The pellets are mounted in a
distance from each other of about 0.03 cm. The active length of the target is 8 cm. A safety
window is placed between the converter and the target to stop the deuteron beam if the
converter is accidentally lost or destroyed (figure 10)4. This system was modelled using the
code MCNPX5.
Figure 9 : Uranium carbide target in reality (left), and as modelled (right)
10
UC2 target is covered
by tantalum and
carbon cylinders
4 cm
Deuterons
40 MeV ; 5 mA
8,4 cm
UC2 target
Converter (graphite)
Safety windows
(carbon)
Figure 10 : Geometry configuration of the converter and the target as modelled
An important fraction of the radioactivity is created in the target by the neutrons
induced from the converter (~1013 n.cm-2.s-1). After 90 days of irradiation, the total activity in
the target is about 4.4 × 1014 Bq. About 70 % of this activity originates from fission products,
10 % from actinides, 8% from halogens (iodine) and 8% from rare gases. The target loses
about 73 % of this activity after 8 hours of cooling (to 1.5 × 1014 Bq). After one year of
cooling, the activity is 9.4 1011 Bq and it reaches 5.9 × 1010 Bq after 10 years. The activation
was calculated by the code DARWIN PEPIN2[6].
3.3. Volatile gases in the uranium target
Since the ISOL method will be used to extract the ions, the target will need to be
heated at high temperature (~2000 °C). We assume that all elements produced in the target
with a melting temperature less than 2000 °C are in gaseous state and therefore constitute the
source term of the radioactive contamination transfer “feeding” the cryotraps. The following
table presents these elements with their melting temperature. The total activity of these gases
after 3 months of irradiation is about 3.0 1014 Bq. The table 2 gives the activity after cooling.
Table 1 : Gaseous elements in the target
Designation
Melting (°C)
Designation
Melting (°C)
Te
450
Y
1523
Ba
725
Ce
795
Sr
790
Pr
935
Sb
630
Rh
1966
Np
640
Nd
1010
Zr
1852
Sn
232
Designation
In
Se
Sm
Eu
Cd
Pd
11
La
920
As
Sublimation
à 613 °C
Ag
Melting (°C)
Designation
Melting (°C)
Designation
Fusion (°C)
Designation
Melting (°C)
Designation
Melting (°C)
Designation
Melting (°C)
156
Ge
937
Pu
639
Fe
1535
Ta
303
Kr
-157
217
Th
1750
Ni
1453
Be
1278
Ru
39,6
Xe
-112
1072
Zn
420
Tm
1545
Mn
1245
Cs
28,5
3
H
-256
822
Dy
1420
Yb
824
Po
254
I
113,7
321
Ho
1470
Co
1495
Ra
700
Ga
29,9
1552
Tb
1360
Li
180
Pb
327
T
-259
Table 2 : Activity of gases (Bq) in the target just after irradiation (0 hour) and after some
periods of cooling
Activity (Bq)
0 hour
3.0 1014
8 hours
6.2 1013
5 days
2.6 1013
12
3 months
4.9 1012
1 year
1.1 1012
962
Er
1522
Lu
1656
Bi
271
Br
-7,1
4. Accommodation
4.1. Interest in the accommodation coefficient
Accommodation is an important phenomenon for molecule transfer under the regime of
molecular flow. The accommodation is badly known and experimentally there are few
experiments which allowed to measure the accommodation coefficient for some surfaces and
some gases. It was necessary for us to develop theoretical models predicting this coefficient.
It is evident that if we do not take into account this phenomenon in the transfer calculations,
results would be very penalizing of the dimensioning of the installation.
The accommodation coefficient is defined as the ratio of the energy actually
transferred between impinging gas molecules and a surface, and the energy which would be
theoretically transferred if the impinging molecules reached complete thermal equilibrium
with the surface7. We can consider the example given by the figure 11 to show the importance
of the accommodation phenomenon. The example shows the separator with two pumps, a
pumping system on the beam line and at the end of this line we set the cryotraps. The source
is heated at 2000 °C and the beam line wall is at the ambient temperature (20°C). In this
condition, and without accommodation, 0.2 % of the cesium is transferred to the cryotraps. If
we take into account the accommodation phenomenon with a coefficient of 0.7, we find by
calculation that only 0.022 % will be transferred.
2 pumps
Separator
Beam line
Pumps
Cryotraps entrance
Figure 11 : Beam line in a configuration near from SPIRAL2 configuration
13
4.2. Accommodation models
Two models of accommodation were developed in this work :
The first (called M1) is based on a simple formulation of elastic scattering. In this
model we define two configurations, M1C1 where the accommodation coefficient is
considered as constant during the thermalisation of the molecule and M1C2, where it varies as
a function of the temperature during the thermalisation process of the molecule. In that case,
the accommodation coefficient is calculated as the average of all accommodation coefficients
depending on the temperature.
The second model uses the formalism developed by Manson8, which takes into
account differential coefficients of reflection and incidence. Two configurations are
considered : first a one-dimensional calculation (M2D1) and second a three-dimensional
calculation (M2D3)9.
The comparison of calculated values to measured ones (figure 12) shows clearly that
the calculation results often underestimate the experimental values, however experimental and
theoretical trends seems to be identical (same trends of the variation). These results show the
difficulty of the quantification of the accommodation coefficient. Since it is not easy to
quantify exactly this coefficient, it would be of great interest to predict it theoretically with
different approaches and to choose the adequate values depending on the objective of the
calculation and the dimensioning. The result of the efforts towards this goal are shown in the
next section.

1,2
1
Accomodation coefficient
0,8
0,6
experience
M1C1
0,4
M1C2
M2D1
0,2
M2D3
0
0
0,2
0,4
0,6
0,8
1
1,2
Mg/Mw
Figure 12 Accommodation coefficient as a function of the ration of molecular masses of the
gas and the wall, respectively.
14
4.3. SCA code
We developed the SCA code (Scattering Coefficient of Accommodation) in the aim to
calculate easily the accommodation coefficient. With this code we can choose the model of
the calculation (M1C1, M1C2, M2D1 or M2D3) (figure 13).
The required input data are the atomic mass of the gaseous species, the atomic mass of the
wall material, the gas temperature at the source, the wall temperature and the condensation
temperature of the gaseous element.
The code is publicly available and can be downloaded from the presently constructed new
website of GANIL.
Figure 13 : Graphical User Interface windows of the SCA code, developed by GANIL
15
5. Release measurement
It is important to note that in a near domain, experiments were conducted during 2007
at the ISOLDE facility at CERN to measure releases of some key isotopes. These experiments
were conducted by a team including the “Experts Group” of SPIRAL2 and physicists from
GANIL working for the SPIRAL2 project.
We present here still preliminary results, since the analysis of the data are has not yet
been completely finalized. The detailed study and analysis of these results will allow to
understand better the phenomena of diffusion and desorption and especially to have
experimental data about the behaviour of some elements under extreme conditions (fire for
example) in the contamination transfer. These cases are of great interest for the nuclear safety.
Two campaigns of experiments were conducted in ISOLDE at CERN to study the
implantation of volatile radioisotopes on different metal surfaces in the frame of the study of
the contamination transfer and releases, which is highly related to the radioactivity
confinement. Six metals were used for these experiments : Cu, Al, Inox, W, Sn and Zr.
Radioactive beams used for implantation (at 30 keV) were 85Kr-m, 91Sr, 128Sb, 132Te, 132I,
135
Xe, 135I, 86Rb, 135Xe, 136Cs, 140Ba, 140La, 141Ce. These masses were chosen since they are
the most penalizing in SPIRAL2 .
The second experiment used samples on Al, Cu, W, Zr, Ti, Inox, Au, Pt, DLC, C, Quartz,
Re and Rh with following beams : 79Kr, 84Rb, 123I, 125Xe and 132Cs.
Nuclei of several masses (86, 135, 136, 140 and 141) were implanted at 30 keV in a sample of
zirconium. Preliminary results show that there is no release at ambient temperature.
Figure 14 : Activity as a function of the time after implantation : measured (observed) and
estimated with the hypothesis that there is no release (expected)
16
Other measurements of releases were done for several heating temperatures (up to 1200
°C). Samples, after implantation, were heated during one hour under a pressure of about 4.10-5
mbar. Preliminary results show the strong dependence between the release mechanism and the
temperature of the sample. This is the case for example for rubidium on copper (figure 15) :
the release fraction is 10 % at 400 °C and 100 % at 600 °C. For caesium, it is about 35 % at
400 °C and 75 % at 800 °C. For cerium and barium we see that the heating, for this interval of
temperature, has no effect on the release.
Figure 15 : Release fraction for some isotopes as a function of the heating temperature of the
samples
17
6. Conclusion and dissemination
There exists a major interest on the cryotraps in an installation as SPIRAL2. It will be
considered as an Important Element for the Safety “Element Important pour la Sûreté” since it
will be a barrier between the red and yellow radiological zones. With a significant efficiency
of ~99.9 %, the cryotraps will contain a great portion of the radioactive contamination and
avoid that it were transferred on the beam line system up to experimental areas.
The first experimental tests of the prototype of the cryotraps are planned for 2008.
The SCA code can be downloaded from the new website of GANIL, which is under
construction.
Finally, this work was presented at several workshops and meetings :
* SPIRAL2 week, GANIL (caen), November 2007
Two posters : 1. About the production rates and the activation of the uranium carbide target
of SPIRAL
2. Containment of volatile isotopes for SIRAL2
* Common EURISOL DS - EURONS Town Meeting, Helsinki, Finland, September 2007
Poster : Containment of volatile isotopes for SIRAL2
* 51st Workshop on Modern Problems & Capability of Vacuum Gas Dynamics, Värmdö
(Sweden), July 2007
Présentation : “Simulation and experimental studies for the containment of radioactive
volatile isotopes in SPIRAL2”
* XVth International Conference on Electromagnetic Isotope Separators and Techniques
related to their Applications (EMIS), Deauville (France), June 2007
Poster : “About the production rates and the activation of the uranium carbide target of
SPIRAL2”
* SAFERIB and EURISOL joint meeting (Safety and Radioprotection), Jüelich (Germany),
May 2007
Présentation : Conformity to legislation, comparative study in European Union
* SAFERIB and EURISOL joint meeting (Safety and Radioprotection), Warsaw (Poland),
June 2006
Présentation : Ganil progress report, WP 5 EURISOL and SAFERIB
* 14th Biennial Topical Meeting of the ANS Radiation Protection and Shielding Division,
April 2006, Carlsbad, New Mexico, USA
Présentations : 1. Neutronic perturbation of the uranium target activation in SPIRAL 2
2. Radioactive contamination transfer in Spiral 2 - Simulation and
experimental studies
* First Workshop on Actinide Target Development, Vancouver (Canada), April 2006
18
Présentations : 1. Neutronic perturbation of the uranium target activation in SPIRAL 2
2. Radioactive contamination transfer in Spiral 2
* Workshop on Operational Radiation Protection at High-Energy Accelerators, CERN,
Geneva, November 2005
Présentation : Radioactive contamination transfer in Spiral 2 - Simulation and experimental
studies
* SAFERIB and EURISOL joint meeting (Safety and Radioprotection), Saclay (France),
October 2005
Présentation : Definition and conception of a cryotrapping system for SPIRAL2
7. References
Santler, D.J., and M.D. Boeckmann, 1987b, Combining transmission probabilities of
different diameter tubes. J. Vac. Sci.&Technol
2
Levenson et al., 1963, Le Vide 18, Paris.
3
More information concerning SPIRAL2 can be found at http://www.ganil.fr.
4
M.Fadil et al, « About the production rates and the activation of the uranium carbide target
for SPIRAL 2” EMIS Conference, Deauville, France, 2007
1
5
Monte Carlo N-Particle Transport Code System for Multiparticle and High Energy Applications, Version 2.5.e.
Oak Ridge National Laboratory. Contributed by Los Alamos National Laboratory. USA
DARWIN version 2.1. Manuel, Commissariat à l’énergie atomique, CEA/Saclay,
DM2S/SERMA
7
A. Roth, Vacuum Technology, Elsevier Science
8
Ali Özer, J.R. Manson, Comparison of one-dimensional and three-dimensional models for
the energy accommodation coefficient, Surf. Sci. 502-503 (2002) 352-357
9
André Muis, J.R. Manson, Calculation of the energy accommodation coefficient using
classical scattering theory, Surf. Sci. 486 (2001) 82-94
6
19