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Helium behaviour in UO2 through low fluence ion implantation studies
Nuclear Instruments and Methods in Physics Research B 327 (2014) 113–116
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Helium behaviour in UO2 through low fluence ion implantation studies P. Garcia a,⇑, E. Gilabert b, G. Martin a, G. Carlot a, C. Sabathier a, T. Sauvage c, P. Desgardin c, M.-F. Barthe c a
CEA – DEN/DEC, Bât. 352, 13108 Saint-Paul-Lez-Durance Cedex, France Centre d’Et´udes Nucleáires de Bordeaux-Gradignan, Le Haut Vigneau, 33175 Gradignan, France c CNRS-CEMHTI, UPR3079, 45071 Orleáns, France b
a r t i c l e
i n f o
Article history: Received 10 October 2013 Received in revised form 15 November 2013 Accepted 15 November 2013 Available online 28 February 2014 Keywords: Helium Uranium dioxide Thermal desorption spectroscopy
a b s t r a c t In this work we focus on experiments involving implantation of 500 keV 3He ions in sintered polycrystalline material. Samples are implanted at low fluences (2 1013 ions/cm2) and subsequently isothermally annealed in a highly sensitive thermal desorption spectrometry (TDS) device PIAGARA (Plateforme Interdisciplinaire pour l’Analyse des GAz Rares en Aquitaine). The helium fluencies studied are two to three orders of magnitude lower than previous Nuclear Reaction Analysis (NRA) experiments carried out on identical samples implanted at identical energies. The fractional release of helium obtained in the TDS experiments is interpreted using a three-dimensional axisymmetric diffusion model which enables results to be quantitatively compared to previous NRA data. The analysis shows that helium behaviour is qualitatively independent of ion fluency over three orders of magnitude: helium diffusion appears to be strongly inhibited below 1273 K within the centre of the grains presumably as a result of helium bubble precipitation. The scenario involving diffusion at grain boundaries and in regions adjacent to them observed at higher fluencies is quantitatively confirmed at much lower doses. The main difference lies in the average width of the region in which uninhibited diffusion occurs. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Helium behaviour in actinide oxide fuels is highly relevant to the overall behaviour of fuel elements of nuclear power reactors. It is produced through a decay of actinides, ternary fissions or 17O(n, a)14C reactions and has in comparison to other rare gases being the subject of much fewer studies. In recent years, diffusion or release studies have been carried out which involved implanting single crystal or polycrystalline uranium dioxide material with 3 He ions at 500 keV and maximum concentrations of approximately 0.3 at% (1016 ions=cm2 ) at 1 lm from the sample surface [1,2]. Samples were subsequently characterised prior to, during and following isothermal annealing between 973 and 1373 K using three different types of Nuclear Reaction Analysis (NRA) techniques based on the 3He(d,a)p reaction. The fraction of helium released from samples was measured during annealing at high temperature as a function of time. After each annealing sequence, helium depth profiles were obtained [3]. In some cases, samples were characterised over small areas (roughly 0.1 0.1 mm), using a micrometer size deuteron beam [4]. At temperatures at or above 1023 K, helium release was shown to occur as a two stage process involving unhindered fast migration at and around the grain ⇑ Corresponding author. Tel.: +33 442254188. E-mail address: [email protected] (P. Garcia). http://dx.doi.org/10.1016/j.nimb.2013.11.042 0168-583X/Ó 2014 Elsevier B.V. All rights reserved.
boundaries. Helium depletion was seen to extend several microns into the grains. Within the grain, diffusion appeared to proceed at much slower rates, it was surmised as a result of helium bubble precipitation [5] or helium trapping. Moreover, the analysis of the fractional release data with a three-dimensional diffusion model which treated grain boundaries as perfect sinks, whence release occurs instantaneously, confirmed that grain boundaries constituted effective short circuits for the movement of helium atoms. Diffusion coefficients were inferred from this data analysis which characterised the behaviour of helium in the grain periphery. In the present work, we have carried out TDS experiments using a highly sensitive mass spectrometry device PIAGARA, developed at CNRS/CENBG and dedicated to rare gas studies, which enables us to investigate desorption from samples implanted with 3He+ ions at fluences three orders of magnitude below our NRA experiments. Under these conditions, radiation defects inevitably left over from the implantation process, play less of a role than in the higher fluence NRA experiments and their effect is thus investigated [6]. The first part of this paper is devoted to describing the materials, their preparation and the experimental setup. In the second part are presented and discussed results of isothermal annealing experiments performed under vaccum at five temperatures between 1023 and 1273 K. The helium release curves are interpreted using the same multi dimensional model as in the previous
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P. Garcia et al. / Nuclear Instruments and Methods in Physics Research B 327 (2014) 113–116
high fluence study and the inferred diffusion coefficients and associated mechanisms compared.
temperature data was not analysed because maximum release in this case is obtained after annealing for 60 min.
2. Experimental details
3. Data analysis, results and discussion
2.1. Sample preparation and implantation
In our previous high fluence NRA experiments [2], we used a diffusion model to rationalise experimental results. In this model, grains are idealised as 9lm radius, semi-infinite cylinders. The following diffusion equation was solved using a finite element solver (Cast3M [9])
Polycrystalline samples were initially cut from pellets 8 mm in diameter into cylinders roughly 1 mm thick and annealed at 1700 C for 24 h in a humidified flowing gas mixture of Ar/5% H2 (H2O/H2 ratio of 1.7%). This guarantees that the stoichiometry of the samples was close to 2. In fact, deviation from stoichiometry may be estimated from the correlations established by Perron [7] at a value between 2.001 and 2.003, so to all intents and purposes 2 considering the sensitivity of thermogravimetric measurements. Samples were then polished with decreasing grain sizes. The last polishing stage involved a colloidal suspension known as OPU the grain size of which is circa 50 nm. The average grain radius was determined from optical microscopy at roughly 9 lm. Specimens were subsequently implanted at room temperature with 500 keV 3 He ions on the van de Graaff accelerator at CEMHTI (Orleáns) to fluences of approximately 2 1013 ions/cm2. 2.2. High sensitivity TDS device (PIAGARA) and sample analysis The experiment involves heating a sample in a small furnace to induce helium movement and release and using mass spectrometry to monitor the amount of rare gases released as a function of time. The device is sensitive to roughly 108 rare gas atoms with an estimated error on the quantity of gas measured of about 2:5%. The device is made up of four distinct zones: the gas extraction or release system, a purification and separation zone, the calibration and analysis of the gas released. The setup is supplemented by a set of pumping units which guarantees ultrahigh vacuum conditions (107 Pa). More details may be found in Ref. [8]. There are three essential features which contribute to the very high performance of the system. The first is the high level of vacuum ensured by three ion pumps (two for the extraction area and one for the mass spectrometer). The second is the high level of purity of the gas obtained from using chemical traps that adsorb undesirable gases such as oxygen, nitrogen, carbon dioxide, water vapour and hydrogen. The third is the calibration method. Gases extracted from the targets are split into several well-defined volume fractions. One fraction is first analyzed to provide an estimate of the helium content. Then, an aliquot of the remaining gas kept in a volume known with high precision is mixed with a calibrated gas sample containing 3He and 4He in a well defined isotopic 4He/3He ratio (0.50409 ± 0.00057). Isotopic ratios measured in the mixture provide a precise determination of the concentration of the gas released from the sample. The advantage of this method is to provide a measurement of absolute concentrations that are not affected by potential changes in the mass spectrometer sensitivity because calibration and isotopic analyses are performed simultaneously. The actual gas analysis system is made up of an ion source in which electrons are used to ionise the collected rare gases, an analyser tube which enables ions to be selected depending upon the magnetic field imposed and an electron multiplier system for increased sensitivity. Samples are placed in the furnace part of the system and heated under vacuum to the desired temperature at a maximum ramp rate of 30 K per minute. A characterisation of the gas released from the sample is performed every 30 min. Five samples were studied (S1 through S5) and annealed at 1023, 1073, 1123, 1173 and 1273 K. for 14, 4, 8, 4 and 3 h respectively. However, the highest
@Cðr; z; tÞ ~ Cðr; z; tÞÞ m @Cðr; z; tÞ ¼ r:ðDðrÞr @t @z
ð1Þ
where DðrÞ is the space-dependent apparent helium diffusion coefficient (r and z are the distances from the grain centre and the sample surface respectively), and m a transport term. The initial helium concentration is that given by the as-implanted high fluence depth profile obtained from NRA. The boundary conditions are chosen such that the helium concentration goes to zero at the grain boundary, sample surface and for infinite values of z, for all values of t. It is further assumed that once a gas atom has reached the grain boundary it is released from the sample in a time negligible with respect to the annealing time. In accord with NRA (both using a micro-beam and in a TDS mode) and TEM observations [2], grain boundary helium bubble formation is not accounted for. It was shown that in order to reproduce the observed experimental data, and particularly the high helium mobility in the vicinity of the grain boundaries, the helium diffusion coefficient may be assumed to depend upon the distance from the grain centre r. This coefficient changes from a low value to a high value noted Da and Db respectively at a radius noted Rig, so the three main model parameters are Da; Db and Rig. Figs. 1 and 2 illustrate the capacity of the model to reproduce all three types of data obtained for high 3He fluences. Note also that micro-NRA analyses have shown that depletion occurred over a width approximately equal to 5.2 ± 2.1 mm compared to a calculated width of approximately 6 mm. Because the release curves (see Figs. 3 and 4) are similar to those observed and interpreted in the high fluence NRA study [2], it appears reasonable to base the interpretation of the low fluence experiments reported in this work on the same model although of course only an integral timedependent quantity is available here, i.e. the fractional helium release. The initial 3He distribution was that determined from NRA in high fluence samples but scaled to the lower fluence (i.e. 2 1013 ions/cm2). The result of best-fit determinations of the model parameters are also given in Figs. 3 and 4 and demonstrates the ability of the model to faithfully reproduce the low fluence data. The best fit triplets (Da; Db; Rig) are indicated in Table 1. Also included in the
Fig. 1. Best fit analysis to NRA desorption data for a high 3He fluence sample, annealed at 1173 K for 1 h.
P. Garcia et al. / Nuclear Instruments and Methods in Physics Research B 327 (2014) 113–116
Fig. 2. Measured and calculated depth profile for a high annealed at 1173 K for 1 h.
3
He fluence sample,
Fig. 3. Best fit analyses to TDS desorption data for a low 3He fluence samples, annealed at 1023 and 1123 K.
115
Additional calculations were carried out which demonstrated that it is the value of the Rig parameter (i.e. the model parameter that determines the extent of the zone over which enhanced diffusion is considered) that mainly controls the overall level of release. The release kinetics are more sensitive to the diffusion coefficients, and particularly Db. An a-dimensional analysis would no doubt reveal that the level of release is actually sensitive to the ratio of Rig over the grain radius. One should also note that the data relating to the experiment carried out at 1273 K was not interpreted with the model because the helium release fraction increased too sharply, precluding an accurate determination of the model parameters. It is interesting to compare the low fluence data reported in this work with the results of NRA experiments performed at fluences three orders of magnitude greater. As mentioned already the release kinetics are quite similar in high and low helium concentration experiments. The main differences in terms of helium behaviour lie in the overall level of release which is higher at low helium concentrations. The model however is capable of reproducing the data quite adequately in all situations (i.e. high and low helium fluences) which suggests that the helium release scenario is similar in both cases: rapid release from regions located around the grain boundary and extending several microns into the grain and slower apparent diffusion of helium from regions located at the centre of the grains. In our initial interpretation, it was suggested that the diffusion coefficient derived in the region located around the grain boundaries and which explained the fast-release kinetics was the physical parameter most characteristic of the intrinsic behaviour of helium in the material. The other diffusion coefficient (Da) being related to a combination of phenomena is more difficult to interpret and involves no doubt bubble nucleation and growth amongst other phenomena [2,5]. In order to pursue this comparison, we have indicated in Fig. 5 the data obtained from our previous studies and those relevant to the present low helium fluence study. The first point is that the model parameters obtained in this low fluence study are consistent with those obtained at higher fluences [2]. This would appear to indicate that the particular model parameter reported in Fig. 5 is indeed independent of the level of damage in the material. The main effect of a high helium fluence appears to lie in the different overall level of release which translates into greater values for parameter Rig. Unfortunately, we cannot compare parameters Rig for the low and high fluence experiments because there is some uncertainty in relation to the exact initial fluence in the present study. The only data point that appears to lie outside its expected range of values in Fig. 5 is that corresponding to the 1123 K experiment. No obvious explanation can be given
Fig. 4. Best fit analyses to TDS desorption data for a low 3He fluence samples, annealed at 1073 and 1173 K.
Table 1 Result of best-fit determination of model parameters for samples S1 through S4. Annealing conditions
Da (m2/s)
1023 K, 14 h
3:1 10
1073 K, 4 h
5 1018
1123 K, 8 h 1173 K, 4 h
Db (m2/s)
Rig (lm)
2:5 0:4 1017
7.4
1:9 0:4 1016
6.8
1018
1 0:2 1016
6.2
1:5 1017
1 0:4 1015
5.7
18
figures are the results of calculations performed with slightly different values of Db. One notes that Db is determined with a good degree of confidence.
Fig. 5. Arrhenius representation of diffusion coefficients in regions adjacent to grain boundaries for low fluence (2 1013 ions/cm2) and high fluence (1016 ions/ cm2) helium release experiments.
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P. Garcia et al. / Nuclear Instruments and Methods in Physics Research B 327 (2014) 113–116
for this. Also, we have excluded from the activation energy and diffusion coefficient prefactor, the results obtained at 1023 K to remain consistent with the high fluence experiments. Indeed these showed only an incipient effect of grain boundaries at that temperature which must correspond to a defect annealing stage. The interpretation with the model, which reflects observations and enables the data to be analysed quantitatively, points to grain boundaries and free surfaces having a different role with respect to helium migration and possibly defect annealing. The reason for this remains unclear but work is ongoing using micro-X-ray-diffraction as a means of characterising the presence of defects and how they anneal out in the vivinity of grain boundaries as a function of temperature [10]. A detailed comparison of helium diffusion coefficients reported in the literature with the results of our NRA studies has been given elsewhere [2]. Because the present TDS data are consistent with them the same discussion applies in this case. The main point is that only the results of Ronchi and Hiernaut [11] lie above those reported in the present work. Their study involved monitoring radiogenic helium release from 238Pu doped polycrystalline material which had been stored several years to allow the helium inventory to build up. All other helium diffusion coefficients are reported to lie roughly three orders of magnitude below the values presented here. In particular, Nakajima et al.’s data [12] was obtained from helium infusion experiments of UO2 single crystal powders. Despite the fact that activation energies derived in our study and that of Nakajima et al. are similar (roughly 1.4 and 2 eV respectively), the latter authors derived much lower diffusion coefficients than in our study. At least two reasons can be put forward. The first is that is that the initial microstructure of the material relevant in the infusion study is necessarily very different from that studied in the present case. In addition to the presence of grain boundaries, the role of which is now substantially documented, equilibrium point defect concentrations will necessarily differ because of different stoichiometries and impurety concentrations. A 102 change in stoichiometry would modify electronic, anion or cation defect concentrations over several oders of magnitude. Also, it is possible that radiation defects that are necessarily present in our study and indeed in Ronchi’s, interact with migrating helium atoms and enhance this migration as they are eliminated at the grain boundaries during the annealing sequence. More parametric studies, notably using materials equilibrated under different oxygen partial pressures are required to tackle this. 4. Conclusion A set of isothermal TDS experiments have been carried out on 500 keV 3He implanted sintered polycrystalline uranium dioxide disks. The helium fluence was roughly 2 1013 ions/cm2 and
the annealing temperature ranged between 1023 and 1273 K. The experimental novelty lies in the high senstivity device used which enabled very low fluences to be investigated equivalent to maximum helium concentrations of 0.5 ppm. The helium release curves show release kinetics very similar to those of previous high fluence experiments characterised using NRA so the data relating to the present study were analysed with the same diffusion model. The analysis confirms the scenario put forward for the high fluence experiments: in regions lying close to grain boundaries, helium migration proceeds unhindered and the diffusion coefficient derived in this area is considered to be characteristic of the material; whilst in the grain centres helium movement appears to be inhibited presumably as it interacts with radiation defects left over from the helium implantation stage. The diffusion coefficient corresponding to helium movement around the grain boundary is practically identical to that derived in the same region from high fluence NRA experiments. The details behind such a complex behaviour no doubt involve interactions between radiation induced defects and grain boundaries, be it during annealing or the actual ion implantation stage. The analysis of the diffusion coefficients above 1023 K provides an activation energy for diffusion and a pre-exponential factor of roughly 1.4 eV and 6 1010 m2 s 1 respectively. Acknowledgments This work was supported by the MATAV nuclear ceramics basic research programme of the Nuclear Energy Division at CEA under the auspices of which most of this work was carried out. References [1] G. Martin, Ph.D. thesis, Université d’Orléans, 2007. [2] P. Garcia, G. Martin, P. Desgardin, G. Carlot, T. Sauvage, C. Sabathier, E. Castelier, H. Khodja, M.-F. Barthe, J. Nucl. Mater. 430 (2012) 156. [3] G. Martin, T. Sauvage, P. Desgardin, P. Garcia, G. Carlot, Nucl. Instrum. Meth. Phys. Res. B 258 (2007) 471. [4] G. Martin, P. Desgardin, T. Sauvage, P. Garcia, G. Carlot, H. Khodja, M.-F. Barthe, Nucl. Instrum. Meth. Phys. Res. B 249 (2006) 509. [5] P. Garcia, G. Martin, C. Sabathier, G. Carlot, A. Michel, P. Martin, B. Dorado, M. Freyss, M. Bertolus, R. Skorek, et al., Nucl. Instrum. Meth. Phys. Res. B 277 (2012) 98. [6] G. Martin, C. Sabathier, G. Carlot, P. Desgardin, C. Raepsaet, T. Sauvage, H. Khodja, Nucl. Instrum. Meth. Phys. Res. B 273 (2012) 122. [7] P.O. Perron, Thermodynamics of Non-stoichiometric Uranium Dioxide, Report AECL-3072, 1968. [8] A. Michel, Ph.D. thesis, Université de Caen, 2011. [9] Cast3m Finite Element Package. Available from: http://www-cast3m.cea.fr. [10] A. Richard, H. Palancher, E. Castelier, J.-S. Micha, M. Gamaleri, G. Carlot, H. Rouquette, P. Goudeau, G. Martin, F. Rieutord, et al., J. Appl. Crystallogr. 45 (2012) 826. [11] C. Ronchi, J.-P. Hiernaut, J. Nucl. Mater. 325 (2004) 1. [12] K. Nakajima, H. Serizawa, N. Shirasu, Y. Haga, Y. Arai, J. Nucl. Mater. 419 (2011) 272.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Helium behaviour in UO2 through low fluence ion implantation studies P. Garcia a,⇑, E. Gilabert b, G. Martin a, G. Carlot a, C. Sabathier a, T. Sauvage c, P. Desgardin c, M.-F. Barthe c a
CEA – DEN/DEC, Bât. 352, 13108 Saint-Paul-Lez-Durance Cedex, France Centre d’Et´udes Nucleáires de Bordeaux-Gradignan, Le Haut Vigneau, 33175 Gradignan, France c CNRS-CEMHTI, UPR3079, 45071 Orleáns, France b
a r t i c l e
i n f o
Article history: Received 10 October 2013 Received in revised form 15 November 2013 Accepted 15 November 2013 Available online 28 February 2014 Keywords: Helium Uranium dioxide Thermal desorption spectroscopy
a b s t r a c t In this work we focus on experiments involving implantation of 500 keV 3He ions in sintered polycrystalline material. Samples are implanted at low fluences (2 1013 ions/cm2) and subsequently isothermally annealed in a highly sensitive thermal desorption spectrometry (TDS) device PIAGARA (Plateforme Interdisciplinaire pour l’Analyse des GAz Rares en Aquitaine). The helium fluencies studied are two to three orders of magnitude lower than previous Nuclear Reaction Analysis (NRA) experiments carried out on identical samples implanted at identical energies. The fractional release of helium obtained in the TDS experiments is interpreted using a three-dimensional axisymmetric diffusion model which enables results to be quantitatively compared to previous NRA data. The analysis shows that helium behaviour is qualitatively independent of ion fluency over three orders of magnitude: helium diffusion appears to be strongly inhibited below 1273 K within the centre of the grains presumably as a result of helium bubble precipitation. The scenario involving diffusion at grain boundaries and in regions adjacent to them observed at higher fluencies is quantitatively confirmed at much lower doses. The main difference lies in the average width of the region in which uninhibited diffusion occurs. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Helium behaviour in actinide oxide fuels is highly relevant to the overall behaviour of fuel elements of nuclear power reactors. It is produced through a decay of actinides, ternary fissions or 17O(n, a)14C reactions and has in comparison to other rare gases being the subject of much fewer studies. In recent years, diffusion or release studies have been carried out which involved implanting single crystal or polycrystalline uranium dioxide material with 3 He ions at 500 keV and maximum concentrations of approximately 0.3 at% (1016 ions=cm2 ) at 1 lm from the sample surface [1,2]. Samples were subsequently characterised prior to, during and following isothermal annealing between 973 and 1373 K using three different types of Nuclear Reaction Analysis (NRA) techniques based on the 3He(d,a)p reaction. The fraction of helium released from samples was measured during annealing at high temperature as a function of time. After each annealing sequence, helium depth profiles were obtained [3]. In some cases, samples were characterised over small areas (roughly 0.1 0.1 mm), using a micrometer size deuteron beam [4]. At temperatures at or above 1023 K, helium release was shown to occur as a two stage process involving unhindered fast migration at and around the grain ⇑ Corresponding author. Tel.: +33 442254188. E-mail address: [email protected] (P. Garcia). http://dx.doi.org/10.1016/j.nimb.2013.11.042 0168-583X/Ó 2014 Elsevier B.V. All rights reserved.
boundaries. Helium depletion was seen to extend several microns into the grains. Within the grain, diffusion appeared to proceed at much slower rates, it was surmised as a result of helium bubble precipitation [5] or helium trapping. Moreover, the analysis of the fractional release data with a three-dimensional diffusion model which treated grain boundaries as perfect sinks, whence release occurs instantaneously, confirmed that grain boundaries constituted effective short circuits for the movement of helium atoms. Diffusion coefficients were inferred from this data analysis which characterised the behaviour of helium in the grain periphery. In the present work, we have carried out TDS experiments using a highly sensitive mass spectrometry device PIAGARA, developed at CNRS/CENBG and dedicated to rare gas studies, which enables us to investigate desorption from samples implanted with 3He+ ions at fluences three orders of magnitude below our NRA experiments. Under these conditions, radiation defects inevitably left over from the implantation process, play less of a role than in the higher fluence NRA experiments and their effect is thus investigated [6]. The first part of this paper is devoted to describing the materials, their preparation and the experimental setup. In the second part are presented and discussed results of isothermal annealing experiments performed under vaccum at five temperatures between 1023 and 1273 K. The helium release curves are interpreted using the same multi dimensional model as in the previous
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P. Garcia et al. / Nuclear Instruments and Methods in Physics Research B 327 (2014) 113–116
high fluence study and the inferred diffusion coefficients and associated mechanisms compared.
temperature data was not analysed because maximum release in this case is obtained after annealing for 60 min.
2. Experimental details
3. Data analysis, results and discussion
2.1. Sample preparation and implantation
In our previous high fluence NRA experiments [2], we used a diffusion model to rationalise experimental results. In this model, grains are idealised as 9lm radius, semi-infinite cylinders. The following diffusion equation was solved using a finite element solver (Cast3M [9])
Polycrystalline samples were initially cut from pellets 8 mm in diameter into cylinders roughly 1 mm thick and annealed at 1700 C for 24 h in a humidified flowing gas mixture of Ar/5% H2 (H2O/H2 ratio of 1.7%). This guarantees that the stoichiometry of the samples was close to 2. In fact, deviation from stoichiometry may be estimated from the correlations established by Perron [7] at a value between 2.001 and 2.003, so to all intents and purposes 2 considering the sensitivity of thermogravimetric measurements. Samples were then polished with decreasing grain sizes. The last polishing stage involved a colloidal suspension known as OPU the grain size of which is circa 50 nm. The average grain radius was determined from optical microscopy at roughly 9 lm. Specimens were subsequently implanted at room temperature with 500 keV 3 He ions on the van de Graaff accelerator at CEMHTI (Orleáns) to fluences of approximately 2 1013 ions/cm2. 2.2. High sensitivity TDS device (PIAGARA) and sample analysis The experiment involves heating a sample in a small furnace to induce helium movement and release and using mass spectrometry to monitor the amount of rare gases released as a function of time. The device is sensitive to roughly 108 rare gas atoms with an estimated error on the quantity of gas measured of about 2:5%. The device is made up of four distinct zones: the gas extraction or release system, a purification and separation zone, the calibration and analysis of the gas released. The setup is supplemented by a set of pumping units which guarantees ultrahigh vacuum conditions (107 Pa). More details may be found in Ref. [8]. There are three essential features which contribute to the very high performance of the system. The first is the high level of vacuum ensured by three ion pumps (two for the extraction area and one for the mass spectrometer). The second is the high level of purity of the gas obtained from using chemical traps that adsorb undesirable gases such as oxygen, nitrogen, carbon dioxide, water vapour and hydrogen. The third is the calibration method. Gases extracted from the targets are split into several well-defined volume fractions. One fraction is first analyzed to provide an estimate of the helium content. Then, an aliquot of the remaining gas kept in a volume known with high precision is mixed with a calibrated gas sample containing 3He and 4He in a well defined isotopic 4He/3He ratio (0.50409 ± 0.00057). Isotopic ratios measured in the mixture provide a precise determination of the concentration of the gas released from the sample. The advantage of this method is to provide a measurement of absolute concentrations that are not affected by potential changes in the mass spectrometer sensitivity because calibration and isotopic analyses are performed simultaneously. The actual gas analysis system is made up of an ion source in which electrons are used to ionise the collected rare gases, an analyser tube which enables ions to be selected depending upon the magnetic field imposed and an electron multiplier system for increased sensitivity. Samples are placed in the furnace part of the system and heated under vacuum to the desired temperature at a maximum ramp rate of 30 K per minute. A characterisation of the gas released from the sample is performed every 30 min. Five samples were studied (S1 through S5) and annealed at 1023, 1073, 1123, 1173 and 1273 K. for 14, 4, 8, 4 and 3 h respectively. However, the highest
@Cðr; z; tÞ ~ Cðr; z; tÞÞ m @Cðr; z; tÞ ¼ r:ðDðrÞr @t @z
ð1Þ
where DðrÞ is the space-dependent apparent helium diffusion coefficient (r and z are the distances from the grain centre and the sample surface respectively), and m a transport term. The initial helium concentration is that given by the as-implanted high fluence depth profile obtained from NRA. The boundary conditions are chosen such that the helium concentration goes to zero at the grain boundary, sample surface and for infinite values of z, for all values of t. It is further assumed that once a gas atom has reached the grain boundary it is released from the sample in a time negligible with respect to the annealing time. In accord with NRA (both using a micro-beam and in a TDS mode) and TEM observations [2], grain boundary helium bubble formation is not accounted for. It was shown that in order to reproduce the observed experimental data, and particularly the high helium mobility in the vicinity of the grain boundaries, the helium diffusion coefficient may be assumed to depend upon the distance from the grain centre r. This coefficient changes from a low value to a high value noted Da and Db respectively at a radius noted Rig, so the three main model parameters are Da; Db and Rig. Figs. 1 and 2 illustrate the capacity of the model to reproduce all three types of data obtained for high 3He fluences. Note also that micro-NRA analyses have shown that depletion occurred over a width approximately equal to 5.2 ± 2.1 mm compared to a calculated width of approximately 6 mm. Because the release curves (see Figs. 3 and 4) are similar to those observed and interpreted in the high fluence NRA study [2], it appears reasonable to base the interpretation of the low fluence experiments reported in this work on the same model although of course only an integral timedependent quantity is available here, i.e. the fractional helium release. The initial 3He distribution was that determined from NRA in high fluence samples but scaled to the lower fluence (i.e. 2 1013 ions/cm2). The result of best-fit determinations of the model parameters are also given in Figs. 3 and 4 and demonstrates the ability of the model to faithfully reproduce the low fluence data. The best fit triplets (Da; Db; Rig) are indicated in Table 1. Also included in the
Fig. 1. Best fit analysis to NRA desorption data for a high 3He fluence sample, annealed at 1173 K for 1 h.
P. Garcia et al. / Nuclear Instruments and Methods in Physics Research B 327 (2014) 113–116
Fig. 2. Measured and calculated depth profile for a high annealed at 1173 K for 1 h.
3
He fluence sample,
Fig. 3. Best fit analyses to TDS desorption data for a low 3He fluence samples, annealed at 1023 and 1123 K.
115
Additional calculations were carried out which demonstrated that it is the value of the Rig parameter (i.e. the model parameter that determines the extent of the zone over which enhanced diffusion is considered) that mainly controls the overall level of release. The release kinetics are more sensitive to the diffusion coefficients, and particularly Db. An a-dimensional analysis would no doubt reveal that the level of release is actually sensitive to the ratio of Rig over the grain radius. One should also note that the data relating to the experiment carried out at 1273 K was not interpreted with the model because the helium release fraction increased too sharply, precluding an accurate determination of the model parameters. It is interesting to compare the low fluence data reported in this work with the results of NRA experiments performed at fluences three orders of magnitude greater. As mentioned already the release kinetics are quite similar in high and low helium concentration experiments. The main differences in terms of helium behaviour lie in the overall level of release which is higher at low helium concentrations. The model however is capable of reproducing the data quite adequately in all situations (i.e. high and low helium fluences) which suggests that the helium release scenario is similar in both cases: rapid release from regions located around the grain boundary and extending several microns into the grain and slower apparent diffusion of helium from regions located at the centre of the grains. In our initial interpretation, it was suggested that the diffusion coefficient derived in the region located around the grain boundaries and which explained the fast-release kinetics was the physical parameter most characteristic of the intrinsic behaviour of helium in the material. The other diffusion coefficient (Da) being related to a combination of phenomena is more difficult to interpret and involves no doubt bubble nucleation and growth amongst other phenomena [2,5]. In order to pursue this comparison, we have indicated in Fig. 5 the data obtained from our previous studies and those relevant to the present low helium fluence study. The first point is that the model parameters obtained in this low fluence study are consistent with those obtained at higher fluences [2]. This would appear to indicate that the particular model parameter reported in Fig. 5 is indeed independent of the level of damage in the material. The main effect of a high helium fluence appears to lie in the different overall level of release which translates into greater values for parameter Rig. Unfortunately, we cannot compare parameters Rig for the low and high fluence experiments because there is some uncertainty in relation to the exact initial fluence in the present study. The only data point that appears to lie outside its expected range of values in Fig. 5 is that corresponding to the 1123 K experiment. No obvious explanation can be given
Fig. 4. Best fit analyses to TDS desorption data for a low 3He fluence samples, annealed at 1073 and 1173 K.
Table 1 Result of best-fit determination of model parameters for samples S1 through S4. Annealing conditions
Da (m2/s)
1023 K, 14 h
3:1 10
1073 K, 4 h
5 1018
1123 K, 8 h 1173 K, 4 h
Db (m2/s)
Rig (lm)
2:5 0:4 1017
7.4
1:9 0:4 1016
6.8
1018
1 0:2 1016
6.2
1:5 1017
1 0:4 1015
5.7
18
figures are the results of calculations performed with slightly different values of Db. One notes that Db is determined with a good degree of confidence.
Fig. 5. Arrhenius representation of diffusion coefficients in regions adjacent to grain boundaries for low fluence (2 1013 ions/cm2) and high fluence (1016 ions/ cm2) helium release experiments.
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for this. Also, we have excluded from the activation energy and diffusion coefficient prefactor, the results obtained at 1023 K to remain consistent with the high fluence experiments. Indeed these showed only an incipient effect of grain boundaries at that temperature which must correspond to a defect annealing stage. The interpretation with the model, which reflects observations and enables the data to be analysed quantitatively, points to grain boundaries and free surfaces having a different role with respect to helium migration and possibly defect annealing. The reason for this remains unclear but work is ongoing using micro-X-ray-diffraction as a means of characterising the presence of defects and how they anneal out in the vivinity of grain boundaries as a function of temperature [10]. A detailed comparison of helium diffusion coefficients reported in the literature with the results of our NRA studies has been given elsewhere [2]. Because the present TDS data are consistent with them the same discussion applies in this case. The main point is that only the results of Ronchi and Hiernaut [11] lie above those reported in the present work. Their study involved monitoring radiogenic helium release from 238Pu doped polycrystalline material which had been stored several years to allow the helium inventory to build up. All other helium diffusion coefficients are reported to lie roughly three orders of magnitude below the values presented here. In particular, Nakajima et al.’s data [12] was obtained from helium infusion experiments of UO2 single crystal powders. Despite the fact that activation energies derived in our study and that of Nakajima et al. are similar (roughly 1.4 and 2 eV respectively), the latter authors derived much lower diffusion coefficients than in our study. At least two reasons can be put forward. The first is that is that the initial microstructure of the material relevant in the infusion study is necessarily very different from that studied in the present case. In addition to the presence of grain boundaries, the role of which is now substantially documented, equilibrium point defect concentrations will necessarily differ because of different stoichiometries and impurety concentrations. A 102 change in stoichiometry would modify electronic, anion or cation defect concentrations over several oders of magnitude. Also, it is possible that radiation defects that are necessarily present in our study and indeed in Ronchi’s, interact with migrating helium atoms and enhance this migration as they are eliminated at the grain boundaries during the annealing sequence. More parametric studies, notably using materials equilibrated under different oxygen partial pressures are required to tackle this. 4. Conclusion A set of isothermal TDS experiments have been carried out on 500 keV 3He implanted sintered polycrystalline uranium dioxide disks. The helium fluence was roughly 2 1013 ions/cm2 and
the annealing temperature ranged between 1023 and 1273 K. The experimental novelty lies in the high senstivity device used which enabled very low fluences to be investigated equivalent to maximum helium concentrations of 0.5 ppm. The helium release curves show release kinetics very similar to those of previous high fluence experiments characterised using NRA so the data relating to the present study were analysed with the same diffusion model. The analysis confirms the scenario put forward for the high fluence experiments: in regions lying close to grain boundaries, helium migration proceeds unhindered and the diffusion coefficient derived in this area is considered to be characteristic of the material; whilst in the grain centres helium movement appears to be inhibited presumably as it interacts with radiation defects left over from the helium implantation stage. The diffusion coefficient corresponding to helium movement around the grain boundary is practically identical to that derived in the same region from high fluence NRA experiments. The details behind such a complex behaviour no doubt involve interactions between radiation induced defects and grain boundaries, be it during annealing or the actual ion implantation stage. The analysis of the diffusion coefficients above 1023 K provides an activation energy for diffusion and a pre-exponential factor of roughly 1.4 eV and 6 1010 m2 s 1 respectively. Acknowledgments This work was supported by the MATAV nuclear ceramics basic research programme of the Nuclear Energy Division at CEA under the auspices of which most of this work was carried out. References [1] G. Martin, Ph.D. thesis, Université d’Orléans, 2007. [2] P. Garcia, G. Martin, P. Desgardin, G. Carlot, T. Sauvage, C. Sabathier, E. Castelier, H. Khodja, M.-F. Barthe, J. Nucl. Mater. 430 (2012) 156. [3] G. Martin, T. Sauvage, P. Desgardin, P. Garcia, G. Carlot, Nucl. Instrum. Meth. Phys. Res. B 258 (2007) 471. [4] G. Martin, P. Desgardin, T. Sauvage, P. Garcia, G. Carlot, H. Khodja, M.-F. Barthe, Nucl. Instrum. Meth. Phys. Res. B 249 (2006) 509. [5] P. Garcia, G. Martin, C. Sabathier, G. Carlot, A. Michel, P. Martin, B. Dorado, M. Freyss, M. Bertolus, R. Skorek, et al., Nucl. Instrum. Meth. Phys. Res. B 277 (2012) 98. [6] G. Martin, C. Sabathier, G. Carlot, P. Desgardin, C. Raepsaet, T. Sauvage, H. Khodja, Nucl. Instrum. Meth. Phys. Res. B 273 (2012) 122. [7] P.O. Perron, Thermodynamics of Non-stoichiometric Uranium Dioxide, Report AECL-3072, 1968. [8] A. Michel, Ph.D. thesis, Université de Caen, 2011. [9] Cast3m Finite Element Package. Available from: http://www-cast3m.cea.fr. [10] A. Richard, H. Palancher, E. Castelier, J.-S. Micha, M. Gamaleri, G. Carlot, H. Rouquette, P. Goudeau, G. Martin, F. Rieutord, et al., J. Appl. Crystallogr. 45 (2012) 826. [11] C. Ronchi, J.-P. Hiernaut, J. Nucl. Mater. 325 (2004) 1. [12] K. Nakajima, H. Serizawa, N. Shirasu, Y. Haga, Y. Arai, J. Nucl. Mater. 419 (2011) 272.