- Email: [email protected]
The use of NRA to study thermal diffusion of helium in (U, Pu)O2
Nuclear Instruments and Methods in Physics Research B 267 (2009) 2250–2254
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
The use of NRA to study thermal diffusion of helium in (U, Pu)O2 Y. Pipon a,c,*, C. Raepsaet b,c, D. Roudil a,c, H. Khodja b,c a b c
CEA, DEN, DTCD/SECM/LMPA, F-30207 Bagnols-sur-Cèze Cedex, France CEA, IRAMIS, Laboratoire Pierre Süe, F-91191 Gif-sur-Yvette, France CNRS, UMR9956, Laboratoire Pierre Süe, F-91191 Gif-sur-Yvette, France
a r t i c l e
i n f o
Available online 12 March 2009 PACS: 66.30.Dn 81.70.q Keywords: Helium Diffusion Radioactive samples NRA
a b s t r a c t A lot of work has been already done on helium atomic diffusion in UO2 samples, but information is still lacking about the fate of helium in high level damaged UOX and MOX matrices and more precisely their intrinsic evolutions under alpha self irradiation in disposal/storage conditions. The present study deals with helium atomic diffusion in actinide doped samples versus damage level. The presently used samples allow a disposal simulation of about 100 years of a UOX spent fuel with a 60 MW d kg1 burnup or a storage simulation of a MOX spent fuel with a 47.5 MW d kg1 burnup. For the first time, nuclear reaction analysis of radioactive samples has been performed in order to obtain diffusion coefficients of helium in (U, Pu)O2. Samples were implanted with 3He+ and then annealed at temperatures ranging from 1123 K to 1273 K. The evolution of the 3He depth profiles was studied by the mean of the non-resonant reaction: 3He(d, p)4He. Using the SIMNRA software and the second Fick’s law, thermal diffusion coefficients have been measured and compared to the 3He thermal diffusion coefficients in UO2 found in the literature. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction A large quantity of helium is expected to form in nuclear spent fuels (UOX and MOX) under long term storage conditions, leading to potential effects on physical, chemical and structural properties. These changes can induce an increase of the radionuclide source term release. The main factor of evolution is linked to the presence of actinides (formed by neutron capture on U atoms) within the fuel matrix [1]. The alpha decay of the actinides will lead to helium formation and production of defects created mainly by the recoil nuclei (this is the so-called alpha self-irradiation). These effects are particularly enhanced for fuels with higher concentrations of alpha emitters such as MOX fuels and high burnup UOX fuels. It is therefore important to assess the atomic diffusion of helium in the fuel to extrapolate its behaviour under storage/disposal conditions. In order to get information on helium atomic diffusion, only few techniques are suitable. One of them for instance is related to Knudsen-cell measurements. Ronchi and Hiernaut [2] have measured helium release from UO2, (U, Pu)O2 and PuO2 matrices containing up to some 100 at ppm helium. From these measurements and with a complex scheme of He release mechanisms, atomic diffusion
* Corresponding author. Address: Université de Lyon, Institut de Physique Nucléaire de Lyon (IPNL), 4, Rue Enrico FERMI, 69622 Villeurbanne Cedex, France. Tel.: +33 (0)4 72 43 10 57. E-mail address: [email protected] (Y. Pipon). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.03.025
coefficients have been deduced. However, usually, atomic diffusion is checked by Ion Beam Analysis (IBA) methods. Elastic Recoil Detection Analysis (ERDA) with heavy ions and Nuclear Reaction Analysis (NRA) methods are well suited to detect small quantities of He with a slight preference for NRA due to its lower detection limit. Therefore many studies, including those presented in [3–7], have been conducted by NRA to measure the thermal and under irradiation diffusion coefficients of He in unirradiated UO2 matrices. These experiments have shown that the thermal diffusion component is irrelevant to the conditions of storage and disposal. There remains the problem of the alpha self-irradiation, not considered in the literature [3–7], and which can modify the helium mobility by the presence of the defects. This paper presents the first results of helium atomic diffusion in radioactive samples (UO2 pellets doped with plutonium) studied by NRA. The nuclear microprobe has been used to access the helium spatial distribution and depth profiles before and after annealing in order to obtain atomic diffusion coefficients. They have been compared to previous values measured for non irradiated UO2 samples in order to check the impact of the alpha self-irradiation. 2. Sample selection The samples, named GIGONDAS, were elaborated in 1985 at CEA, Cadarache. They are made of UO2 pellets doped with 24.5 wt.% of plutonium (mainly 239Pu). Multi-analysis characterizations amongst which SEM (Scanning Electron Microscope),
Y. Pipon et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2250–2254
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micro-hardness and XRD (X-ray diffraction) have lead to the following conclusions: – the measured average density of each sample is 10.421 g cm3, – the initial stoichiometry has been determined with a ratio O/ (U+Pu) of 1.983, – the chemical composition is similar to a MOX fuel containing Pu aggregates. Alpha decay of the GIGONDAS samples actinides since 1985 leads to an integrated fluence of about 1.4 1019 He cm3 (around 0.02 at.%). By calculating defects created by the recoil nuclei, one can estimate the maximum sample damage around 0.3 displacements per atom (dpa). From the He accumulation value and the matrix damage estimation, it appears that the GIGONDAS samples are comparable to: (i) a UOX spent fuel with a burnup of 60 MW d kg1 which would have been stored for about 50-100 years or, (ii) a MOX spent fuel with a burnup of 47.5 MW d kg1 which would have been stored for a few years.
Fig. 1. Example of a GIGONDAS proton spectrum measured by NRA (Ed = 1250 keV).
3.3. Nuclear reaction analysis 3. Experimental procedure To our knowledge, the only available facility in the world to perform IBA on non-contaminant radioactive samples is the hot line CASIMIR [8] at the Pierre Süe Laboratory (CEA Saclay). The 3He(d, p)4He nuclear reaction is used in this study to measure implanted 3He distribution in the GIGONDAS samples. The proton detection allows us to work at depth >5 lm in order to resolve the surface effects as it has been observed by Martin et al. [7]. Moreover this reaction is sensitive to helium concentration down to 100 at ppm. This is suitable for the simulation of the He accumulation in nuclear fuel during the first period of storage. 3.1. Helium implantation Samples were implanted with the nuclear microprobe [9] at the Pierre Süe Laboratory. A 2.9 MeV 3He microbeam was used, corresponding to a projected range (Rp) of 6.3 lm according to the SRIM-2008 code [10]. The ion beam, which size is 20 20 lm2, was swept over an area of 1 1 mm2 in order to obtain homogeneous implantation. Two samples, PA and PB, were implanted at a 5 1015 at cm2 fluence resulting in a maximum concentration of around 0.12 at.% at Rp (as predicted by SRIM). The vacancies induced by the implantation are evaluated by SRIM below 0.01 dpa in the range from 0 to 5 lm and below 0.1 dpa in the range from 5 to 7 lm. These last results show that the defects produced by alpha selfirradiation prevail on the defects induced by the 3He implantation.
NRA l-analysis was performed at nuclear microprobe of the the Pierre Süe Laboratory using the 3He(d, p)4He nuclear reaction. Beam was normal to the sample surfaces and proton detection was performed at 170° using an annular 1500 lm thick surface barrier silicon detector. The detector solid angle was measured with reference samples to (120 ± 10) msr. Backscattered deuterons were stopped using a 50 lm thick Mylar foil in front of the active surface. Data were acquired using the MPA-Win (Fast-Comtec) software. Non-annealed and annealed samples were imaged by electrostatic beam scanning in order to get helium spatial distribution, for an incident 20 20 lm2 beam of 1250 keV (the maps are not presented in the present paper). To probe the entire 3He distribution, the incident deuteron energy was progressively reduced from 1700 to 950 keV in variable steps. In the resulting proton spectrum presented in Fig. 1, the 13.2 MeV peak produced by the 3He(d, p) reaction is totally separated from other d-induced reaction contributions. Fig. 1 also displays a background that does not appear on the NRA spectra measured on UO2 samples. This contribution comes from the decay of alpha particles with an energy of 5.17 MeV (emitted by 239Pu and 240Pu) and 5.5 MeV (emitted by 238Pu and 241Am). The 20 20 lm2 beam was swept over an area of 50 50 lm2 for each energy step. The beam current density was quite high with a maximum value of 200 lA cm2. Each energy step was consequently done on a different area to limit any potential helium migration under beam irradiation. 4. NRA data processing and calculation of the diffusion coefficient
3.2. Annealing conditions The implanted samples were annealed at ATALANTE (CEA Marcoule) in the C19-503 shielded cell. The temperature and annealing duration selection were based on previous experiments [4] in order to get a direct comparison of the helium diffusion coefficients in non irradiated UO2 and in GIGONDAS matrices. The annealing was done in Ar, 4% H2 which allows the conservation of the stoichiometry. The first sample (PA) was annealed at 1273 K during 3 h and the second one (PB) at 1023 K during 10 h. The control of the stoichiometry was done by weighting the samples before and after each annealing (with a 1 mg precision). No significant mass variation was observed during the process.
Two possible methods exist to determine the helium distribution. We can either analyze the proton spectrum measured for a given deuteron incident energy with the SIMNRA program [11] (or NDF code [12]) or use the excitation curve method (ECM) [13]. In this paper, we will focus on the first method. The SIMNRA 6.04 simulation program was used to reconstruct the experimental spectra. The nuclear reaction cross-sections (for He, O and C) are described in [14–16] with a deuteron stopping power in UO2 estimated from [10]. In the first step of the data processing, the radioactive decay background is removed from the spectra. The calculated spectrum intensity was next fitted using the contribution of the 16O(d, p)17O nuclear reaction.
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Fig. 2. Helium yield on each (50 50) lm2 region for: (a) non-annealed sample, (b) annealed sample.
Fig. 3. Depth profiles fitted by a Gaussian curve for PA sample: (a) before annealing, (b) after 3 h annealing at 1273 K.
The distribution of He was simulated by a multilayer representation. The thickness of each layer was optimized with the aid of the RESOLNRA 1.0 code [17], which calculates, at different depths and for each deuteron energy, the depth resolution. The calculated entire He profile was approximated by a Gaussian curve as it is frequently done for implanted profiles. The next step consists of a comparison between the as-implanted and the annealed helium distributions. The distribution
Fig. 4. Depth profiles fitted by a Gaussian curve for the PB sample: (a) before annealing, (b) after a 30 h annealing at 1123 K.
broadening due to annealing can be calculated from the onedimensional model of the second’s Fick law [18] (1)
@C @2C ¼D 2 @t @x
ð1Þ
with C: elemental concentration of helium and D: diffusion coefficient.
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Y. Pipon et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2250–2254 Table 1 Helium thermal diffusion coefficients (cm2 s1) from this study and from literature.
1123 K 1173 K 1273 K 1373 K
[2] (U0.9, 238Pu0.1)O2 samples Measurements by 4He desorption [4He] = 0.04 at%
This study (U0.75, 239Pu0,25)O2 samples [4He]max = 0.02 at% [3He]max = 0.1 at% Rp = 6.3 lm
[4] UO2 samples [3He]max = 0.05 at% Rp = 6.3 lm
8.5 1012 2.0 1011 9.7 1011 3.7 1010
9.2 1014
2.4 1014 4.8 1014 2.3 1013
1.6 1012
The analytical solution of this equation is a Gaussian equation centered around xc (2)
! C0 ðx xc Þ2 : Cðx; tÞ ¼ pffiffiffiffiffiffiffiffiffiffiffiffi exp 4Dt 4pDt
ð2Þ
The width of the Gaussian distribution is given by relation (3)
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffi hðx xc Þ2 i ¼ 2Dt:
ð3Þ
The diffusion coefficient is related to the width of the profiles by expression (4)
r2f ¼ r2i þ 2Ds
ð4Þ
where rf is the standard deviation of the distribution obtained after annealing for a duration s at temperature T and ri, the standard deviation of the He distribution corresponding to the as-implanted sample. 5. Results and discussion 5.1. Helium spatial distribution The maps are used first to monitor discrepancies on helium spatial distribution after sample annealing. Secondly, they also validate the possibility to compare the depth profiles performed on different areas of size 50 50 lm2 (from as-implanted and annealed samples surface). For as-implanted sample implanted He is homogeneously distributed. For the annealed sample map shows less homogeneous He distribution and several depleted zones of small size (<20 20 lm2). Fig. 2 displays the number of detected particles for each zone of 50 50 lm2. As expected, the as-implanted sample (2.a) is very homogeneous and the helium distribution of the annealed sample (2.b) is also quite regular. This is of importance because this shows that the number of protons does not change (or only a little) for different (50 50) lm2 zones and that depth profiles performed on these areas can be compared. 5.2. Helium depth profiles Each depth profile has been made with a 10 layer-representation by SIMNRA. The depth profiles obtained for PA sample (before and after annealing) are shown in Fig. 3 and the ones for PB sample in Fig. 4. A single depth profile has too few points to be fitted by a Gaussian curve. That is the reason why several profiles for different incident deuteron energies have been simulated with SIMNRA. One can see that after annealing at 1123 K and at 1273 K, the asimplanted signal has become much broader which results from atomic diffusion. 5.3. Helium diffusion coefficients Diffusion coefficients were determined from the broadening of the as-implanted 3He distributions and are compared with the values from the literature (Table 1).
[5] UO2 samples [3He]max = 0.2 at% Rp = 1.9 lm
[7] UO2 samples [3He]max = 0.3 at% Rp = 1.9 lm
<6 1013
1.5 1012
The diffusion coefficients measured in this study for (U, 239Pu)O2 matrices are five times larger than those measured in UO2 samples in [4] for very close experimental conditions. Also for a non-irradiated UO2 sample, Guilbert et al. [5] and Martin et al. [7] measured a diffusion coefficient at 1373 K which is respectively smaller and equal to our value at a lower temperature (1273 K). Consequently the alpha self-irradiation is proved to enhance the helium atomic diffusion. This is confirmed by the 1010 cm2 s1 value (at 1273 K) found by Ronchi and Hiernaut [2] who have worked on (U, 238Pu)O2 samples with even more damage created by alpha self irradiation (around 0.7 dpa). Furthermore our increase of helium atomic diffusion appears to be thermally activated and can not induced subsequent gas release during the simulated period. 6. Conclusion and outlook Thermal atomic diffusion was measured for the first time on radioactive samples by NRA. The nuclear microprobe of the Pierre Süe Laboratory was successfully used to perform helium mappings and depth profiles of the GIGONDAS samples. We have also highlighted by NRA that the helium thermal diffusion seemed to be affected by defects created by alpha self-irradiation by comparing our diffusion coefficients to the values found in the literature. Nevertheless this assumption needs to be confirmed. We will soon measure 3He diffusion coefficients on GIGONDAS samples, which were annealed prior to 3He implantation in order to remove the defects created by alpha decay. If the defects induced by alpha self-irradiation play a role on the helium migration, the diffusion coefficients will be smaller than those displayed in this study. If no difference will be observed, it will demonstrate that in a sample close to the chemical composition of a MOX fuel with Pu agglomerates, the diffusion of helium is still larger than in a UO2 matrix. Other samples which are (U, Cm)O2 matrices will also be studied to identify a possible effect of the chemical composition on the helium migration. At last, to complete these experiments, diffusion coefficients will be measured at temperatures lower than 1073 K to reproduce better disposal/storage conditions. Acknowledgments The authors are indebted for the technical support to M. Desir, R. Caraballo, D. Ancri, B. Charles (from CEA Marcoule), J. Hoarau, D. Guiot, D. Guillier, F. Saillant and Y. Kilisky (from CEA Saclay). Financial support for this research was provided under the CEAPRECCI program (T. Lieven, C. Ferry, M. Firon) as part of CEA-EDF agreement (J.M. Gras). References [1] J.-M. Gras, R.D. Quang, H. Masson, T. Lieven, C. Ferry, C. Poinssot, M. Debes, J.-M. Delbecq, J. Nucl. Mater. 362 (2007) 383. [2] C. Ronchi, J.P. Hiernaut, J. Nucl. Mater. 325 (2004) 1.
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[3] P. Trocellier et al., Nucl. Instr. and Meth. B 206 (2003) 1077. [4] D. Roudil, X. Deschanels, P. Trocellier, C. Jégou, S. Peuget, J.-M. Bart, J. Nucl. Mater. 325 (2004) 148. [5] S. Guilbert et al., J. Nucl. Mater. 327 (2004) 88. [6] T. Sauvage et al., Nucl. Instr. and Meth. B 240 (2005) 271. [7] G. Martin, P. Garcia, H. Labrim, T. Sauvage, G. Carlot, P. Desgardin, M.F. Barthe, J.P. Piron, J. Nucl. Mater. 357 (2006) 198. [8] C. Raepsaet et al., Nucl. Instr. and Meth. B 267/12-13 (2009) 2245. [9] H. Khodja, E. Berthoumieux, L. Daudin, J.P. Gallien, Nucl. Instr. and Meth. B 181 (2001) 83. [10] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Matter, Pergamon, New York, 1985.
[11] M. Mayer, SIMNRA user’s guide, Technical Report IPP 9/113, Max-PlanckInstitute für Plasmaphysik, Garching, 1997. [12] N.P. Barradas, C. Jeynes, R.P. Webb, Appl. Phys. Lett. 71 (1997) 291. [13] P.M. G Damen, Hj. Matzke, C. Ronchi, J.P. Hiernault, T. Wiss, R. Fromknecht, A. van Veen, F. Labohm, Nucl. Instr. and Meth. B 191 (2002) 571. [14] H.S. Bosch, G.M. Hale, Nucl. Fusion 32 (1992) 611. [15] R.F. Seiler, C.H. Jones, W.J. Anzick, D.F. Herring, K.W. Jones, Nucl. Phys. 45 (1963) 647. [16] M. Kokkoris et al., Nucl. Instr. and Meth. B 249 (2006) 77. [17] M. Mayer, Nucl. Instr. and Meth. B 266 (2008) 1852. [18] J. Philibert, Diffusion et Transport de Matière dans les Solides, editions de la physique, Paris, 1985.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
The use of NRA to study thermal diffusion of helium in (U, Pu)O2 Y. Pipon a,c,*, C. Raepsaet b,c, D. Roudil a,c, H. Khodja b,c a b c
CEA, DEN, DTCD/SECM/LMPA, F-30207 Bagnols-sur-Cèze Cedex, France CEA, IRAMIS, Laboratoire Pierre Süe, F-91191 Gif-sur-Yvette, France CNRS, UMR9956, Laboratoire Pierre Süe, F-91191 Gif-sur-Yvette, France
a r t i c l e
i n f o
Available online 12 March 2009 PACS: 66.30.Dn 81.70.q Keywords: Helium Diffusion Radioactive samples NRA
a b s t r a c t A lot of work has been already done on helium atomic diffusion in UO2 samples, but information is still lacking about the fate of helium in high level damaged UOX and MOX matrices and more precisely their intrinsic evolutions under alpha self irradiation in disposal/storage conditions. The present study deals with helium atomic diffusion in actinide doped samples versus damage level. The presently used samples allow a disposal simulation of about 100 years of a UOX spent fuel with a 60 MW d kg1 burnup or a storage simulation of a MOX spent fuel with a 47.5 MW d kg1 burnup. For the first time, nuclear reaction analysis of radioactive samples has been performed in order to obtain diffusion coefficients of helium in (U, Pu)O2. Samples were implanted with 3He+ and then annealed at temperatures ranging from 1123 K to 1273 K. The evolution of the 3He depth profiles was studied by the mean of the non-resonant reaction: 3He(d, p)4He. Using the SIMNRA software and the second Fick’s law, thermal diffusion coefficients have been measured and compared to the 3He thermal diffusion coefficients in UO2 found in the literature. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction A large quantity of helium is expected to form in nuclear spent fuels (UOX and MOX) under long term storage conditions, leading to potential effects on physical, chemical and structural properties. These changes can induce an increase of the radionuclide source term release. The main factor of evolution is linked to the presence of actinides (formed by neutron capture on U atoms) within the fuel matrix [1]. The alpha decay of the actinides will lead to helium formation and production of defects created mainly by the recoil nuclei (this is the so-called alpha self-irradiation). These effects are particularly enhanced for fuels with higher concentrations of alpha emitters such as MOX fuels and high burnup UOX fuels. It is therefore important to assess the atomic diffusion of helium in the fuel to extrapolate its behaviour under storage/disposal conditions. In order to get information on helium atomic diffusion, only few techniques are suitable. One of them for instance is related to Knudsen-cell measurements. Ronchi and Hiernaut [2] have measured helium release from UO2, (U, Pu)O2 and PuO2 matrices containing up to some 100 at ppm helium. From these measurements and with a complex scheme of He release mechanisms, atomic diffusion
* Corresponding author. Address: Université de Lyon, Institut de Physique Nucléaire de Lyon (IPNL), 4, Rue Enrico FERMI, 69622 Villeurbanne Cedex, France. Tel.: +33 (0)4 72 43 10 57. E-mail address: [email protected] (Y. Pipon). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.03.025
coefficients have been deduced. However, usually, atomic diffusion is checked by Ion Beam Analysis (IBA) methods. Elastic Recoil Detection Analysis (ERDA) with heavy ions and Nuclear Reaction Analysis (NRA) methods are well suited to detect small quantities of He with a slight preference for NRA due to its lower detection limit. Therefore many studies, including those presented in [3–7], have been conducted by NRA to measure the thermal and under irradiation diffusion coefficients of He in unirradiated UO2 matrices. These experiments have shown that the thermal diffusion component is irrelevant to the conditions of storage and disposal. There remains the problem of the alpha self-irradiation, not considered in the literature [3–7], and which can modify the helium mobility by the presence of the defects. This paper presents the first results of helium atomic diffusion in radioactive samples (UO2 pellets doped with plutonium) studied by NRA. The nuclear microprobe has been used to access the helium spatial distribution and depth profiles before and after annealing in order to obtain atomic diffusion coefficients. They have been compared to previous values measured for non irradiated UO2 samples in order to check the impact of the alpha self-irradiation. 2. Sample selection The samples, named GIGONDAS, were elaborated in 1985 at CEA, Cadarache. They are made of UO2 pellets doped with 24.5 wt.% of plutonium (mainly 239Pu). Multi-analysis characterizations amongst which SEM (Scanning Electron Microscope),
Y. Pipon et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2250–2254
2251
micro-hardness and XRD (X-ray diffraction) have lead to the following conclusions: – the measured average density of each sample is 10.421 g cm3, – the initial stoichiometry has been determined with a ratio O/ (U+Pu) of 1.983, – the chemical composition is similar to a MOX fuel containing Pu aggregates. Alpha decay of the GIGONDAS samples actinides since 1985 leads to an integrated fluence of about 1.4 1019 He cm3 (around 0.02 at.%). By calculating defects created by the recoil nuclei, one can estimate the maximum sample damage around 0.3 displacements per atom (dpa). From the He accumulation value and the matrix damage estimation, it appears that the GIGONDAS samples are comparable to: (i) a UOX spent fuel with a burnup of 60 MW d kg1 which would have been stored for about 50-100 years or, (ii) a MOX spent fuel with a burnup of 47.5 MW d kg1 which would have been stored for a few years.
Fig. 1. Example of a GIGONDAS proton spectrum measured by NRA (Ed = 1250 keV).
3.3. Nuclear reaction analysis 3. Experimental procedure To our knowledge, the only available facility in the world to perform IBA on non-contaminant radioactive samples is the hot line CASIMIR [8] at the Pierre Süe Laboratory (CEA Saclay). The 3He(d, p)4He nuclear reaction is used in this study to measure implanted 3He distribution in the GIGONDAS samples. The proton detection allows us to work at depth >5 lm in order to resolve the surface effects as it has been observed by Martin et al. [7]. Moreover this reaction is sensitive to helium concentration down to 100 at ppm. This is suitable for the simulation of the He accumulation in nuclear fuel during the first period of storage. 3.1. Helium implantation Samples were implanted with the nuclear microprobe [9] at the Pierre Süe Laboratory. A 2.9 MeV 3He microbeam was used, corresponding to a projected range (Rp) of 6.3 lm according to the SRIM-2008 code [10]. The ion beam, which size is 20 20 lm2, was swept over an area of 1 1 mm2 in order to obtain homogeneous implantation. Two samples, PA and PB, were implanted at a 5 1015 at cm2 fluence resulting in a maximum concentration of around 0.12 at.% at Rp (as predicted by SRIM). The vacancies induced by the implantation are evaluated by SRIM below 0.01 dpa in the range from 0 to 5 lm and below 0.1 dpa in the range from 5 to 7 lm. These last results show that the defects produced by alpha selfirradiation prevail on the defects induced by the 3He implantation.
NRA l-analysis was performed at nuclear microprobe of the the Pierre Süe Laboratory using the 3He(d, p)4He nuclear reaction. Beam was normal to the sample surfaces and proton detection was performed at 170° using an annular 1500 lm thick surface barrier silicon detector. The detector solid angle was measured with reference samples to (120 ± 10) msr. Backscattered deuterons were stopped using a 50 lm thick Mylar foil in front of the active surface. Data were acquired using the MPA-Win (Fast-Comtec) software. Non-annealed and annealed samples were imaged by electrostatic beam scanning in order to get helium spatial distribution, for an incident 20 20 lm2 beam of 1250 keV (the maps are not presented in the present paper). To probe the entire 3He distribution, the incident deuteron energy was progressively reduced from 1700 to 950 keV in variable steps. In the resulting proton spectrum presented in Fig. 1, the 13.2 MeV peak produced by the 3He(d, p) reaction is totally separated from other d-induced reaction contributions. Fig. 1 also displays a background that does not appear on the NRA spectra measured on UO2 samples. This contribution comes from the decay of alpha particles with an energy of 5.17 MeV (emitted by 239Pu and 240Pu) and 5.5 MeV (emitted by 238Pu and 241Am). The 20 20 lm2 beam was swept over an area of 50 50 lm2 for each energy step. The beam current density was quite high with a maximum value of 200 lA cm2. Each energy step was consequently done on a different area to limit any potential helium migration under beam irradiation. 4. NRA data processing and calculation of the diffusion coefficient
3.2. Annealing conditions The implanted samples were annealed at ATALANTE (CEA Marcoule) in the C19-503 shielded cell. The temperature and annealing duration selection were based on previous experiments [4] in order to get a direct comparison of the helium diffusion coefficients in non irradiated UO2 and in GIGONDAS matrices. The annealing was done in Ar, 4% H2 which allows the conservation of the stoichiometry. The first sample (PA) was annealed at 1273 K during 3 h and the second one (PB) at 1023 K during 10 h. The control of the stoichiometry was done by weighting the samples before and after each annealing (with a 1 mg precision). No significant mass variation was observed during the process.
Two possible methods exist to determine the helium distribution. We can either analyze the proton spectrum measured for a given deuteron incident energy with the SIMNRA program [11] (or NDF code [12]) or use the excitation curve method (ECM) [13]. In this paper, we will focus on the first method. The SIMNRA 6.04 simulation program was used to reconstruct the experimental spectra. The nuclear reaction cross-sections (for He, O and C) are described in [14–16] with a deuteron stopping power in UO2 estimated from [10]. In the first step of the data processing, the radioactive decay background is removed from the spectra. The calculated spectrum intensity was next fitted using the contribution of the 16O(d, p)17O nuclear reaction.
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Y. Pipon et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2250–2254
Fig. 2. Helium yield on each (50 50) lm2 region for: (a) non-annealed sample, (b) annealed sample.
Fig. 3. Depth profiles fitted by a Gaussian curve for PA sample: (a) before annealing, (b) after 3 h annealing at 1273 K.
The distribution of He was simulated by a multilayer representation. The thickness of each layer was optimized with the aid of the RESOLNRA 1.0 code [17], which calculates, at different depths and for each deuteron energy, the depth resolution. The calculated entire He profile was approximated by a Gaussian curve as it is frequently done for implanted profiles. The next step consists of a comparison between the as-implanted and the annealed helium distributions. The distribution
Fig. 4. Depth profiles fitted by a Gaussian curve for the PB sample: (a) before annealing, (b) after a 30 h annealing at 1123 K.
broadening due to annealing can be calculated from the onedimensional model of the second’s Fick law [18] (1)
@C @2C ¼D 2 @t @x
ð1Þ
with C: elemental concentration of helium and D: diffusion coefficient.
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Y. Pipon et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2250–2254 Table 1 Helium thermal diffusion coefficients (cm2 s1) from this study and from literature.
1123 K 1173 K 1273 K 1373 K
[2] (U0.9, 238Pu0.1)O2 samples Measurements by 4He desorption [4He] = 0.04 at%
This study (U0.75, 239Pu0,25)O2 samples [4He]max = 0.02 at% [3He]max = 0.1 at% Rp = 6.3 lm
[4] UO2 samples [3He]max = 0.05 at% Rp = 6.3 lm
8.5 1012 2.0 1011 9.7 1011 3.7 1010
9.2 1014
2.4 1014 4.8 1014 2.3 1013
1.6 1012
The analytical solution of this equation is a Gaussian equation centered around xc (2)
! C0 ðx xc Þ2 : Cðx; tÞ ¼ pffiffiffiffiffiffiffiffiffiffiffiffi exp 4Dt 4pDt
ð2Þ
The width of the Gaussian distribution is given by relation (3)
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffi hðx xc Þ2 i ¼ 2Dt:
ð3Þ
The diffusion coefficient is related to the width of the profiles by expression (4)
r2f ¼ r2i þ 2Ds
ð4Þ
where rf is the standard deviation of the distribution obtained after annealing for a duration s at temperature T and ri, the standard deviation of the He distribution corresponding to the as-implanted sample. 5. Results and discussion 5.1. Helium spatial distribution The maps are used first to monitor discrepancies on helium spatial distribution after sample annealing. Secondly, they also validate the possibility to compare the depth profiles performed on different areas of size 50 50 lm2 (from as-implanted and annealed samples surface). For as-implanted sample implanted He is homogeneously distributed. For the annealed sample map shows less homogeneous He distribution and several depleted zones of small size (<20 20 lm2). Fig. 2 displays the number of detected particles for each zone of 50 50 lm2. As expected, the as-implanted sample (2.a) is very homogeneous and the helium distribution of the annealed sample (2.b) is also quite regular. This is of importance because this shows that the number of protons does not change (or only a little) for different (50 50) lm2 zones and that depth profiles performed on these areas can be compared. 5.2. Helium depth profiles Each depth profile has been made with a 10 layer-representation by SIMNRA. The depth profiles obtained for PA sample (before and after annealing) are shown in Fig. 3 and the ones for PB sample in Fig. 4. A single depth profile has too few points to be fitted by a Gaussian curve. That is the reason why several profiles for different incident deuteron energies have been simulated with SIMNRA. One can see that after annealing at 1123 K and at 1273 K, the asimplanted signal has become much broader which results from atomic diffusion. 5.3. Helium diffusion coefficients Diffusion coefficients were determined from the broadening of the as-implanted 3He distributions and are compared with the values from the literature (Table 1).
[5] UO2 samples [3He]max = 0.2 at% Rp = 1.9 lm
[7] UO2 samples [3He]max = 0.3 at% Rp = 1.9 lm
<6 1013
1.5 1012
The diffusion coefficients measured in this study for (U, 239Pu)O2 matrices are five times larger than those measured in UO2 samples in [4] for very close experimental conditions. Also for a non-irradiated UO2 sample, Guilbert et al. [5] and Martin et al. [7] measured a diffusion coefficient at 1373 K which is respectively smaller and equal to our value at a lower temperature (1273 K). Consequently the alpha self-irradiation is proved to enhance the helium atomic diffusion. This is confirmed by the 1010 cm2 s1 value (at 1273 K) found by Ronchi and Hiernaut [2] who have worked on (U, 238Pu)O2 samples with even more damage created by alpha self irradiation (around 0.7 dpa). Furthermore our increase of helium atomic diffusion appears to be thermally activated and can not induced subsequent gas release during the simulated period. 6. Conclusion and outlook Thermal atomic diffusion was measured for the first time on radioactive samples by NRA. The nuclear microprobe of the Pierre Süe Laboratory was successfully used to perform helium mappings and depth profiles of the GIGONDAS samples. We have also highlighted by NRA that the helium thermal diffusion seemed to be affected by defects created by alpha self-irradiation by comparing our diffusion coefficients to the values found in the literature. Nevertheless this assumption needs to be confirmed. We will soon measure 3He diffusion coefficients on GIGONDAS samples, which were annealed prior to 3He implantation in order to remove the defects created by alpha decay. If the defects induced by alpha self-irradiation play a role on the helium migration, the diffusion coefficients will be smaller than those displayed in this study. If no difference will be observed, it will demonstrate that in a sample close to the chemical composition of a MOX fuel with Pu agglomerates, the diffusion of helium is still larger than in a UO2 matrix. Other samples which are (U, Cm)O2 matrices will also be studied to identify a possible effect of the chemical composition on the helium migration. At last, to complete these experiments, diffusion coefficients will be measured at temperatures lower than 1073 K to reproduce better disposal/storage conditions. Acknowledgments The authors are indebted for the technical support to M. Desir, R. Caraballo, D. Ancri, B. Charles (from CEA Marcoule), J. Hoarau, D. Guiot, D. Guillier, F. Saillant and Y. Kilisky (from CEA Saclay). Financial support for this research was provided under the CEAPRECCI program (T. Lieven, C. Ferry, M. Firon) as part of CEA-EDF agreement (J.M. Gras). References [1] J.-M. Gras, R.D. Quang, H. Masson, T. Lieven, C. Ferry, C. Poinssot, M. Debes, J.-M. Delbecq, J. Nucl. Mater. 362 (2007) 383. [2] C. Ronchi, J.P. Hiernaut, J. Nucl. Mater. 325 (2004) 1.
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