- Email: [email protected]
XAS characterisation of xenon bubbles in uranium dioxide
Available online at www.sciencedirect.com
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 2887–2891 www.elsevier.com/locate/nimb
XAS characterisation of xenon bubbles in uranium dioxide P. Martin a,*, P. Garcia a, G. Carlot a, C. Sabathier a, C. Valot a, V. Nassif b, O. Proux c, J.-L. Hazemann d a CEA Cadarache, DEN/DEC/SESC/LLCC, Baˆt. 130, 13108 St. Paul Lez Durance, France CEA Grenoble, DSM/DRFMC/SP2M/NRS, 17 Avenue des Martyrs, 38054 Grenoble Cedex 9, France c Laboratoire de Ge´ophysique Interne et Tectonophysique, UMR CNRS/Universite´ Joseph Fourier, 1381 rue de la Piscine, Domaine Universitaire, 38400 Saint-Martin-D’He`res, France d Institut Ne´el, CNRS, 25 Avenue des Martyrs, BP 166, 38042 Grenoble Cedex 9, France b
Available online 24 March 2008
Abstract X-ray absorption spectroscopy experiments were performed on a set of uranium dioxide samples implanted with 1017 xenon cm 2 at 800 keV (8 at.% at 140 nm). EXAFS measurements performed at 12 K showed that during implantation the gas forms highly pressurised nanometre size inclusions. Bubble pressures were estimated at 2.8 ± 0.3 GPa at low temperature. Following the low energy xenon implantation, samples were annealed between 1073 and 1773 K for several hours. Stability of nanometre size highly pressurized xenon aggregates in UO2 is demonstrated up to 1073 K as for this temperature almost no modification of the xenon environment was observed. Above this temperature, bubbles will trap migrating vacancies and their inner pressure is seen to decrease substantially. Ó 2008 Elsevier B.V. All rights reserved. PACS: 61.10.Ht; 64.75.+g; 66.30.Jt Keywords: EXAFS; XANES; Xenon; UO2; Precipitation
1. Introduction One of the difficulties encountered when modelling the release of fission gases in UO2 lies in predicting the proportion of rare gas atoms which are isolated in the matrix and that of atoms which form bubbles. In-pile, the accepted picture is that trapping by bubbles of diffusing gas atoms or irradiation induced bubble nucleation is offset by a radiation induced re-solution phenomenon. However in out of pile annealing experiments, radiation effects can hardly be invoked. In this case, modelling fission gas bubbles as behaving as perfect sinks leads to an underestimation of the fraction of fission gases released. The fact that the gas contained in intra-granular bubbles is highly pressurised could be of primary importance. Furthermore, evi-
*
Corresponding author. Tel.: +33 4 42253866; fax: +33 4 42253285. E-mail address: [email protected] (P. Martin).
0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.03.180
dence of intra-granular bubbles of a few nanometres in size in irradiated UO2 fuels has been reported for a wide range of burn-ups [1–3], but very few data exist relative to the pressure inside those nanometric bubbles [1,3]. The most exhaustive study is due to Nogita et al. who examined using TEM [3] samples taken from the outer part of pellets irradiated to average burn-ups of 30, 44 and 83 MW d kg 1. Bubble sizes ranged between 1 nm at low burn-up to 10 nm for the higher burn-up samples. On larger bubbles (4–10 nm) using a nano-EDX analysis, they estimated xenon densities in the 3.8–6 g cm 3 range. The upper values quoted by the authors would suggest solid xenon particles. But more precise data are lacking relative to both xenon pressures within the bubbles and their thermal stability. In this paper we proposed to follow by X-ray absorption spectroscopy both size and gas pressure within rare gas atom inclusions in UO2. Indeed, we showed in a previous work [4] that the average pressure in Xe aggregates in UO2 could be estimated using XAS measurements at the
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Table 1 Best fit parameters from xenon K edge XAFS data and aggregate internal pressure values deduced using Asaumi’s equation of state [8] (see text for details) Thermal treatment
Best-fit parameters from xenon K edge EXAFS data
Temperature (K)
Duration (h)
Number of nearest neighbours Xe N
Xe–Xe bond ˚) length R (A
Debye–Waller ˚ 2) factor r2 (A
Fit Rfactor
_ 1073 1673 1673 1773 1773
_ 12 2 12 2 12
3.6 ± 0.5 3.3 ± 0.5 4.8 ± 0.5 5.7 ± 0.5 7.0 ± 0.5 5.9 ± 0.5
3.97 ± 0.02 4.04 ± 0.02 4.39 ± 0.01 4.39 ± 0.01 4.39 ± 0.01 4.32 ± 0.01
0.025 ± 0.004 0.024 ± 0.004 0.008 ± 0.001 0.008 ± 0.001 0.009 ± 0.001 0.006 ± 0.001
0.013 0.004 0.018 0.020 0.014 0.012
xenon K edge. Our experimental study is based on ion implantation of xenon ions in uranium dioxide sintered pellets. Following a low energy xenon implantation (800 keV), samples were annealed between 1073 and 1773 K for several hours. Based on XAS results, data on bubble evolution and stability has been gathered. 2. Experimental 2.1. Sample preparation Measurements were carried out on depleted (0.3% of uranium) UO2 polycrystalline sintered discs cut from dense UO2 pellets (98% of theoretical density). All samples were first polished and annealed at 1673 K for 4 h in a reducing atmosphere (Ar + H2 5%) so as to remove surface defects produced by the polishing process and insure the samples were stoichiometric. The samples were then implanted with xenon ions at the Institut de Physique Nucle´aire de Lyon (IPNL). Doses and energies were chosen so that the xenon concentration reached a value of around 8 at.% at approximately 140 nm from the sample surface (1017 Xe cm 2 at 800 keV). The SRIM code [5] was used in order to evaluate the ion projected ranges (Rp = 140 nm) as well as the number of displacements per atom (dpa = 590) created at Rp at the final fluence. These calculations were performed with a target density of 10.74 g cm 3 and displacement threshold energies of 40 eV for U atoms and 20 eV for O atoms. The thermal treatments were performed under a reducing atmosphere (Ar + H2 5%) for 1–12 h with temperatures ranging form 1073 to 1773 K.
DE0 (eV) 1.6 ± 0.3 2.1 ± 0.3 3.9 ± 0.5 3.3 ± 0.5 2.3 ± 0.5 0.5 ± 0.5
Aggregate internal pressure (GPa)
2.8 ± 0.3 2.0 ± 0.3 _ _ _ 0.07 ± 0.03
oscillations from the raw absorption spectra. As described in our previous work, the methodology applied was as follows: a first estimation of Xe–Xe bond length was obtained by comparison on calculated Xe XANES spectra using the FEFF8.20 ab initio full multi-scattering code [8]. Whenever extended fine structure was available in our data, metric parameters (neighbouring atomic distances (R), mean squared radial displacement or Debye–Waller factors (r2) and coordination numbers (N)) were obtained from the EXAFS data. Curve fitting were performed in k2 for R-values using the ARTEMIS [7] software based on FEFF8.20 calculations with the Xe cell parameter deduced from the first calculation step. Experimental EXAFS spectra were Fourier transformed using a Kaiser–Bessel window over ˚ 1 and corrected to account a k space range of 2.0–7.0 A for the phase component. Indeed, distances read in Fourier transforms figures correspond to actual atomic distances. The ‘‘R-factors” reported in Table 1 are indicative of the overall quality and suitability of the fits. A value of 0.02 signifies that the average deviation between the theory and the data is two percent. During the fitting process, the amplitude factor (S 20 ) was fixed at 0.88 which is the value calculated by FEFF8.20. As the aim of these measurements was to estimate the internal pressure inside solid xenon aggregates, we used the Birch–Murnaghan type equation of state (EOS) for solid xenon at around 0 K determined by Asaumi [9], as in our previous paper [4]. 3. Results 3.1. As implanted sample
2.2. Xenon K edge XAS analysis and pressure estimation XAS measurements were performed on the FAME (BM30B) beam line of the European Synchrotron Radiation Facility (Grenoble, France). For each sample, XAS spectra were recorded at xenon K edge (34.561 keV) using a 30 elements fluorescence detector [6]. In order to obtain quantitative data on the Xe local environment, spectra were collected at temperatures well below the xenon melting temperature (161.4 K) using a helium cryostat (i.e. at temperatures of 12 K). ATHENA software [7] was used for normalizing XANES spectra and extracting EXAFS
As we can see in Fig. 1(a), the XANES spectrum collected presents three resonances. Those resonances indicate that a discernable fraction of xenon has a structured environment and is not randomly distributed in the UO2 matrix [10]. In addition, a full multiple scattering calculation performed with the FEFF8.20 code accurately reproduces experimental spectra using a face cubic centred xenon clus˚ . This result ter of 45 atoms with a cell parameter of 5.58 A indicates that no Xe–O or Xe–U contributes to the XAS signal. The EXAFS results given in Table 1 confirm this hypothesis as the fit (Cf. Fig. 1(b)) gives a Xe local environ-
P. Martin et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2887–2891
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3.2. Annealed samples
a
b
As observed in Fig. 2(a), the XAFS signal collected after a thermal treatment of 12 h at 1073 K is almost identical to the one corresponding to the as implanted sample. The values given in Table 1 confirm this observation with a xenon local environment comprising one coordination shell with ˚ . Therefore, only an increase 3.6 ± 0.5 Xe at 4.04 ± 0.02 A ˚ of 0.07 A in Xe–Xe distance is observed whereas coordination number and structural disorder remain identical. In this case, annealing does not induce any measurable modification of aggregate size but the internal pressure estimated at 2.0 ± 0.3 GPa indicates a small depressurization of Xe aggregates. In our previous study of UO2 samples implanted with xenon ions under identical conditions, XAFS results revealed the existence of nanometre size xenon aggregates with an estimated pressure of 2.1 ± 0.3 GPa after annealing for 12 h at 873 K [4]. Fourier transforms obtained for samples annealed at high temperature (1673 K and 1773 K) are displayed in Fig. 2(b). Compared to the as implanted signal, a clear modification of xenon environment can be observed with
Fig. 1. (a) XANES spectrum obtained on a implanted sample compared to calculated spectrum for Xe at 3.2 GPa (0 energy corresponds to xenon K edge). (b) EXAFS fit result.
˚ . Such a ment consisting of 3.6 ± 0.5 Xe at 3.97 ± 0.02 A bond distance corresponds to a cell parameter of ˚ whence a pressure of 2.8 ± 0.3 GPa at 12 K 5.61 ± 0.02 A can be inferred based on Asaumi’s EOS. Furthermore, in a perfect face centred cubic Xe crystal, one would expect 12 nearest neighbours for the first shell. The 3.6 ± 0.5 value measured indicates that the xenon atoms have precipitated and form nanometre size aggregates. A value of approximately 50% is theoretically expected for a 1 nm radius spherical inclusion [4]. ˚2 But the Debye–Waller factor value of 0.025 ± 0.004 A is indicative of substantial structural disorder in the Xe– Xe shell. By taking 55 K as a value for the xenon Debye temperature [11], the expected thermal Debye–Waller value calculated in the Debye approximation for the first Xe–Xe ˚ 2. bond for a perfect xenon crystal equals to 0.008 A The XAFS results demonstrate nucleation of nanometre size xenon aggregates with an internal pressure estimated at 2.8 ± 0.3 GPa at 12 K.
Fig. 2. Fourier transforms of k2 EXAFS signals at xenon K edge before and after thermal treatments.
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a more intense peak shifted to higher distances. In each case, only one Xe–Xe distance is observed. Numerical values derived from the EXAFS best fit are given in Table 1. For samples annealed at 1673 K for 2 or 12 h, the Xe– ˚ ) corresponds to the value Xe distance (4.39 ± 0.01 A expected for a stress free xenon crystal. Moreover, the ˚ 2) confirms Debye–Waller factor value (0.008 ± 0.001 A this result as it is also equal to the value calculated (in the Debye approximation) with known Debye temperature. The only difference between the two results is the coordination number which goes from 4.8 to 5.7 after annealing for 2 and 12 h. Those values are lower than the 12 nearest neighbours. Two phenomena could explain such an evolution, the first one is that xenon bubbles remain of nanometre size; the second one is that the 1–2 nm bubbles previously characterized have grown which leads to a decrease in gas pressure. When the sample is cooled down, Xe crystallites then precipitates at the bubble surface. Nevertheless, the increase in number of nearest neighbours indicates bubbles growth. Following annealing at 1773 K for 2 h, similar values are obtained for Xe–Xe distances and disorder: 4.39 ± ˚ and 0.009 ± 0.001 A ˚ 2. On the other hand, the neigh0.01 A bour number increases further to reach 7 which indicates that the bubble radius has increased (or that the cavities contain more gas). But after annealing for 12 h, an inversion of the tendency is observed with a reduction in the number of neighbours, Xe–Xe distance and structural dis˚ compared order. The measured distance of 4.32 ± 0.01 A ˚ indicates that the xenon bubbles are still to 4.39 ± 0.01 A pressurized with an internal pressure estimated at 0.07 ± 0.03 GPa. The low value of Debye–Waller factor (0.006 ˚ ) confirms this conclusion as only an increase of xenon A Debye temperature due to the overpressure within the aggregates can explain such a low value [12]. 4. Discussion The first result of this study is that no Xe–O or Xe–U bonds are observed whatever the annealing sequence. This does not contradict previous theoretical studies relating to xenon trapping sites in uranium dioxide. Catlow et al. [13] found using atomistic computer simulations that in stoichiometric UO2 and for very low Xe concentrations (10 6 at.%), that Xe atoms are trapped in trivacancy sites at 1673 K and should therefore have a local environment consisting of U and O atoms. It may be the case in our study that the contribution to the EXAFS and XANES spectra of Xe–U or X–O bonds is too weak to be observed. The sensitivity of the technique required working at high concentrations (8 at.%) to be able to collect XAS signals on annealed samples. At such high concentrations only a Xe–Xe bonds are observed. The second result is that precipitation of nanometre size bubbles containing gas at 2.8 ± 0.3 GPa is liable to occur in the absence of any annealing simply as a result of irradiation induced movement. Recent TEM work on xenon
implanted UO2 polycrystalline [14] and single crystal [15] samples with local concentration around 2 at.% showed no precipitation following implantation. In both cases, the authors observed a tangled dislocation network of interstitial type as observed in irradiated UO2 fuel [16,17]. In our case the same defect type is expected but at a higher concentration as the xenon fluence induced an estimated 590 dpa instead of the 63 dpa in Sathonay et al.’s [15] or Sabathier’s [14] work. Concerning the temperature effect on xenon aggregates, two behaviours are revealed: below 1073 K and at high temperature (1673 and 1773 K). At low temperature (873 K [4] and 1073 K) nanometre size aggregates appear to be stable and remain highly pressurized at roughly 2.0 ± 0.3 GPa. But compared to the as implanted sample a pressure decrease of 0.8 GPa is measured. The usual explanation for this behaviour is that rare gas aggregates act as defect sinks and observed depressurization is due to vacancy annihilation towards the bubble surface. But, if this mechanism is valid, a pressure decrease should appear between 873 and 1073 K as sink efficiency increases with temperature. A more valid hypothesis could be the existence of an oxygen short range network rearrangement of the UO2 matrix around aggregates in order to minimize UO2 internal stress, as for these temperatures oxygen thermal diffusion is activated [18]. At high temperatures (1673 K and 1773 K), large, defect free and non pressurized aggregates are observed. In this case, bubbles seem to trap migrating vacancies and therefore act as defect sinks as would be expected. Moreover in this temperature range thermal diffusion of xenon is probably activated [19]. At the highest temperature, the measured pressure appears to increase with annealing time. This can be interpreted in terms of the existence of a bimodal bubble size distribution. The first class of bubbles corresponds to the original nanometre size aggregates, and the second class to the larger un-pressurised bubbles. It is possible that the larger bubbles obtained for the longest annealing time have grown enough to intersect the sample surface. The contribution to the XAS signal of the gas contained in these bubbles would disappear, leaving only a contribution of the xenon contained in the much smaller highly pressurised aggregates. 5. Conclusion X-ray absorption spectroscopy has been applied to characterize xenon aggregate pressure in uranium dioxide samples implanted with 1017 xenon cm 2 at 800 keV (8 at.% at 140 nm). The pressure evolution with thermal treatment was studied in the 873–1773 K temperature range. EXAFS measurements performed at 12 K show that during implantation gas atoms precipitate to form nanometre size inclusions with an estimated internal pressure of 2.8 ± 0.3 GPa. Data obtained on thermally treated samples show that nanometre size xenon aggregates in UO2 remain highly pressurised up to 1073 K. At higher temperature (1663
P. Martin et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2887–2891
and 1773 K) larger un-pressurised bubbles are present. The fact that xenon bubbles do not grow at low temperature would indicate, either that they are inefficient sinks for mobile gas atoms or that individual gas atoms that are not contained in bubbles are trapped at various defect sites in the matrix. To finalize the bubble characterization, TEM experiments are scheduled in order to observe and to characterize the bi-modal bubble size distribution suggested. Acknowledgements We acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities. This work was supported by the ISMIR programme which is devoted to modelling and understanding radiation effects in insulators and is co-funded by CEA and CNRS. References [1] L.E. Thomas, in: S.E. Donnely, J.H. Evans (Eds.), Fundamental Aspects of Inert gases in Solids, NATO ASI Series B: Physics, Vol. 279, Plenum Publishing Corporation, New York, London, 1991, p. 431. [2] S. Kashibe, K. Une, K. Nogita, J. Nucl. Mater. 206 (1993) 22. [3] K. Nogita, K. Une, Nucl. Instr. and Meth. B 141 (1998) 481.
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[4] P. Garcia, P. Martin, G. Carlot, E. Castelier, M. Ripert, C. Sabathier, C. Valot, F. D’Acapito, J.-L. Hazemann, O. Proux, V. Nassif, J. Nucl. Mater. 352 (2006) 136. [5] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. [6] O. Proux, X. Biquard, E. Lahera, J.-J. Menthonnex, A. Prat, O. Ulrich, Y. Soldo, P. Tre´visson, G. Kapoujvan, G. Perroux, P. Taunier, D. Grand, P. Jeantet, M. Deleglise, J.-P. Roux, J.-L. Hazemann, Phys. Scripta 115 (2005) 970. [7] B. Ravel, M. Newville, J. Synchrotron Radiat. 12 (2005) 537. [8] A.L. Ankudinov, B. Ravel, J.J. Rehr, S.D. Conradson, Phys. Rev. B 58 (1998) 7565. [9] K. Asaumi, Phys. Rev. B 29 (1984) 7026. [10] D.C. Koningsberger, X-ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS and XANES, Wiley-Interscience Publication, New York, 1988. [11] B.M. Powell, G. Dolling, in: M.L. Klein, J.A. Venables (Eds.), Rare Gas Solid, Academic Press, London – New York – San Francisco, 1977, p. 921. [12] G. Faraci, A.R. Pennisi, A. Terrasi, S. Mobilio, Phys. Rev. B 38 (1988) 13468. [13] S. Nicoll, H.J. Matzke, C.R.A. Catlow, J. Nucl. Mater. 226 (1995) 51. [14] C. Sabathier, L. Vincent, P. Garcia, F. Garrido, G. Carlot, L. Thome, P. Martin, C. Valot, Nucl. Instr. and Meth. B 266 (2008) 3027. [15] G. Sattonnay, L. Vincent, F. Garrido, L. Thome, J. Nucl. Mater. 355 (2006) 131. [16] A.D. Whapham, B.E. Sheldon, Philos. Mag. 12 (1965) 1179. [17] A.D. Whapham, Philos. Mag. 26 (1967) 39. [18] J. Belle, J. Nucl. Mater. 30 (1969) 3. [19] G.T. Lawrence, J. Nucl. Mater. 71 (1978) 195.
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 2887–2891 www.elsevier.com/locate/nimb
XAS characterisation of xenon bubbles in uranium dioxide P. Martin a,*, P. Garcia a, G. Carlot a, C. Sabathier a, C. Valot a, V. Nassif b, O. Proux c, J.-L. Hazemann d a CEA Cadarache, DEN/DEC/SESC/LLCC, Baˆt. 130, 13108 St. Paul Lez Durance, France CEA Grenoble, DSM/DRFMC/SP2M/NRS, 17 Avenue des Martyrs, 38054 Grenoble Cedex 9, France c Laboratoire de Ge´ophysique Interne et Tectonophysique, UMR CNRS/Universite´ Joseph Fourier, 1381 rue de la Piscine, Domaine Universitaire, 38400 Saint-Martin-D’He`res, France d Institut Ne´el, CNRS, 25 Avenue des Martyrs, BP 166, 38042 Grenoble Cedex 9, France b
Available online 24 March 2008
Abstract X-ray absorption spectroscopy experiments were performed on a set of uranium dioxide samples implanted with 1017 xenon cm 2 at 800 keV (8 at.% at 140 nm). EXAFS measurements performed at 12 K showed that during implantation the gas forms highly pressurised nanometre size inclusions. Bubble pressures were estimated at 2.8 ± 0.3 GPa at low temperature. Following the low energy xenon implantation, samples were annealed between 1073 and 1773 K for several hours. Stability of nanometre size highly pressurized xenon aggregates in UO2 is demonstrated up to 1073 K as for this temperature almost no modification of the xenon environment was observed. Above this temperature, bubbles will trap migrating vacancies and their inner pressure is seen to decrease substantially. Ó 2008 Elsevier B.V. All rights reserved. PACS: 61.10.Ht; 64.75.+g; 66.30.Jt Keywords: EXAFS; XANES; Xenon; UO2; Precipitation
1. Introduction One of the difficulties encountered when modelling the release of fission gases in UO2 lies in predicting the proportion of rare gas atoms which are isolated in the matrix and that of atoms which form bubbles. In-pile, the accepted picture is that trapping by bubbles of diffusing gas atoms or irradiation induced bubble nucleation is offset by a radiation induced re-solution phenomenon. However in out of pile annealing experiments, radiation effects can hardly be invoked. In this case, modelling fission gas bubbles as behaving as perfect sinks leads to an underestimation of the fraction of fission gases released. The fact that the gas contained in intra-granular bubbles is highly pressurised could be of primary importance. Furthermore, evi-
*
Corresponding author. Tel.: +33 4 42253866; fax: +33 4 42253285. E-mail address: [email protected] (P. Martin).
0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.03.180
dence of intra-granular bubbles of a few nanometres in size in irradiated UO2 fuels has been reported for a wide range of burn-ups [1–3], but very few data exist relative to the pressure inside those nanometric bubbles [1,3]. The most exhaustive study is due to Nogita et al. who examined using TEM [3] samples taken from the outer part of pellets irradiated to average burn-ups of 30, 44 and 83 MW d kg 1. Bubble sizes ranged between 1 nm at low burn-up to 10 nm for the higher burn-up samples. On larger bubbles (4–10 nm) using a nano-EDX analysis, they estimated xenon densities in the 3.8–6 g cm 3 range. The upper values quoted by the authors would suggest solid xenon particles. But more precise data are lacking relative to both xenon pressures within the bubbles and their thermal stability. In this paper we proposed to follow by X-ray absorption spectroscopy both size and gas pressure within rare gas atom inclusions in UO2. Indeed, we showed in a previous work [4] that the average pressure in Xe aggregates in UO2 could be estimated using XAS measurements at the
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P. Martin et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2887–2891
Table 1 Best fit parameters from xenon K edge XAFS data and aggregate internal pressure values deduced using Asaumi’s equation of state [8] (see text for details) Thermal treatment
Best-fit parameters from xenon K edge EXAFS data
Temperature (K)
Duration (h)
Number of nearest neighbours Xe N
Xe–Xe bond ˚) length R (A
Debye–Waller ˚ 2) factor r2 (A
Fit Rfactor
_ 1073 1673 1673 1773 1773
_ 12 2 12 2 12
3.6 ± 0.5 3.3 ± 0.5 4.8 ± 0.5 5.7 ± 0.5 7.0 ± 0.5 5.9 ± 0.5
3.97 ± 0.02 4.04 ± 0.02 4.39 ± 0.01 4.39 ± 0.01 4.39 ± 0.01 4.32 ± 0.01
0.025 ± 0.004 0.024 ± 0.004 0.008 ± 0.001 0.008 ± 0.001 0.009 ± 0.001 0.006 ± 0.001
0.013 0.004 0.018 0.020 0.014 0.012
xenon K edge. Our experimental study is based on ion implantation of xenon ions in uranium dioxide sintered pellets. Following a low energy xenon implantation (800 keV), samples were annealed between 1073 and 1773 K for several hours. Based on XAS results, data on bubble evolution and stability has been gathered. 2. Experimental 2.1. Sample preparation Measurements were carried out on depleted (0.3% of uranium) UO2 polycrystalline sintered discs cut from dense UO2 pellets (98% of theoretical density). All samples were first polished and annealed at 1673 K for 4 h in a reducing atmosphere (Ar + H2 5%) so as to remove surface defects produced by the polishing process and insure the samples were stoichiometric. The samples were then implanted with xenon ions at the Institut de Physique Nucle´aire de Lyon (IPNL). Doses and energies were chosen so that the xenon concentration reached a value of around 8 at.% at approximately 140 nm from the sample surface (1017 Xe cm 2 at 800 keV). The SRIM code [5] was used in order to evaluate the ion projected ranges (Rp = 140 nm) as well as the number of displacements per atom (dpa = 590) created at Rp at the final fluence. These calculations were performed with a target density of 10.74 g cm 3 and displacement threshold energies of 40 eV for U atoms and 20 eV for O atoms. The thermal treatments were performed under a reducing atmosphere (Ar + H2 5%) for 1–12 h with temperatures ranging form 1073 to 1773 K.
DE0 (eV) 1.6 ± 0.3 2.1 ± 0.3 3.9 ± 0.5 3.3 ± 0.5 2.3 ± 0.5 0.5 ± 0.5
Aggregate internal pressure (GPa)
2.8 ± 0.3 2.0 ± 0.3 _ _ _ 0.07 ± 0.03
oscillations from the raw absorption spectra. As described in our previous work, the methodology applied was as follows: a first estimation of Xe–Xe bond length was obtained by comparison on calculated Xe XANES spectra using the FEFF8.20 ab initio full multi-scattering code [8]. Whenever extended fine structure was available in our data, metric parameters (neighbouring atomic distances (R), mean squared radial displacement or Debye–Waller factors (r2) and coordination numbers (N)) were obtained from the EXAFS data. Curve fitting were performed in k2 for R-values using the ARTEMIS [7] software based on FEFF8.20 calculations with the Xe cell parameter deduced from the first calculation step. Experimental EXAFS spectra were Fourier transformed using a Kaiser–Bessel window over ˚ 1 and corrected to account a k space range of 2.0–7.0 A for the phase component. Indeed, distances read in Fourier transforms figures correspond to actual atomic distances. The ‘‘R-factors” reported in Table 1 are indicative of the overall quality and suitability of the fits. A value of 0.02 signifies that the average deviation between the theory and the data is two percent. During the fitting process, the amplitude factor (S 20 ) was fixed at 0.88 which is the value calculated by FEFF8.20. As the aim of these measurements was to estimate the internal pressure inside solid xenon aggregates, we used the Birch–Murnaghan type equation of state (EOS) for solid xenon at around 0 K determined by Asaumi [9], as in our previous paper [4]. 3. Results 3.1. As implanted sample
2.2. Xenon K edge XAS analysis and pressure estimation XAS measurements were performed on the FAME (BM30B) beam line of the European Synchrotron Radiation Facility (Grenoble, France). For each sample, XAS spectra were recorded at xenon K edge (34.561 keV) using a 30 elements fluorescence detector [6]. In order to obtain quantitative data on the Xe local environment, spectra were collected at temperatures well below the xenon melting temperature (161.4 K) using a helium cryostat (i.e. at temperatures of 12 K). ATHENA software [7] was used for normalizing XANES spectra and extracting EXAFS
As we can see in Fig. 1(a), the XANES spectrum collected presents three resonances. Those resonances indicate that a discernable fraction of xenon has a structured environment and is not randomly distributed in the UO2 matrix [10]. In addition, a full multiple scattering calculation performed with the FEFF8.20 code accurately reproduces experimental spectra using a face cubic centred xenon clus˚ . This result ter of 45 atoms with a cell parameter of 5.58 A indicates that no Xe–O or Xe–U contributes to the XAS signal. The EXAFS results given in Table 1 confirm this hypothesis as the fit (Cf. Fig. 1(b)) gives a Xe local environ-
P. Martin et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2887–2891
2889
3.2. Annealed samples
a
b
As observed in Fig. 2(a), the XAFS signal collected after a thermal treatment of 12 h at 1073 K is almost identical to the one corresponding to the as implanted sample. The values given in Table 1 confirm this observation with a xenon local environment comprising one coordination shell with ˚ . Therefore, only an increase 3.6 ± 0.5 Xe at 4.04 ± 0.02 A ˚ of 0.07 A in Xe–Xe distance is observed whereas coordination number and structural disorder remain identical. In this case, annealing does not induce any measurable modification of aggregate size but the internal pressure estimated at 2.0 ± 0.3 GPa indicates a small depressurization of Xe aggregates. In our previous study of UO2 samples implanted with xenon ions under identical conditions, XAFS results revealed the existence of nanometre size xenon aggregates with an estimated pressure of 2.1 ± 0.3 GPa after annealing for 12 h at 873 K [4]. Fourier transforms obtained for samples annealed at high temperature (1673 K and 1773 K) are displayed in Fig. 2(b). Compared to the as implanted signal, a clear modification of xenon environment can be observed with
Fig. 1. (a) XANES spectrum obtained on a implanted sample compared to calculated spectrum for Xe at 3.2 GPa (0 energy corresponds to xenon K edge). (b) EXAFS fit result.
˚ . Such a ment consisting of 3.6 ± 0.5 Xe at 3.97 ± 0.02 A bond distance corresponds to a cell parameter of ˚ whence a pressure of 2.8 ± 0.3 GPa at 12 K 5.61 ± 0.02 A can be inferred based on Asaumi’s EOS. Furthermore, in a perfect face centred cubic Xe crystal, one would expect 12 nearest neighbours for the first shell. The 3.6 ± 0.5 value measured indicates that the xenon atoms have precipitated and form nanometre size aggregates. A value of approximately 50% is theoretically expected for a 1 nm radius spherical inclusion [4]. ˚2 But the Debye–Waller factor value of 0.025 ± 0.004 A is indicative of substantial structural disorder in the Xe– Xe shell. By taking 55 K as a value for the xenon Debye temperature [11], the expected thermal Debye–Waller value calculated in the Debye approximation for the first Xe–Xe ˚ 2. bond for a perfect xenon crystal equals to 0.008 A The XAFS results demonstrate nucleation of nanometre size xenon aggregates with an internal pressure estimated at 2.8 ± 0.3 GPa at 12 K.
Fig. 2. Fourier transforms of k2 EXAFS signals at xenon K edge before and after thermal treatments.
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a more intense peak shifted to higher distances. In each case, only one Xe–Xe distance is observed. Numerical values derived from the EXAFS best fit are given in Table 1. For samples annealed at 1673 K for 2 or 12 h, the Xe– ˚ ) corresponds to the value Xe distance (4.39 ± 0.01 A expected for a stress free xenon crystal. Moreover, the ˚ 2) confirms Debye–Waller factor value (0.008 ± 0.001 A this result as it is also equal to the value calculated (in the Debye approximation) with known Debye temperature. The only difference between the two results is the coordination number which goes from 4.8 to 5.7 after annealing for 2 and 12 h. Those values are lower than the 12 nearest neighbours. Two phenomena could explain such an evolution, the first one is that xenon bubbles remain of nanometre size; the second one is that the 1–2 nm bubbles previously characterized have grown which leads to a decrease in gas pressure. When the sample is cooled down, Xe crystallites then precipitates at the bubble surface. Nevertheless, the increase in number of nearest neighbours indicates bubbles growth. Following annealing at 1773 K for 2 h, similar values are obtained for Xe–Xe distances and disorder: 4.39 ± ˚ and 0.009 ± 0.001 A ˚ 2. On the other hand, the neigh0.01 A bour number increases further to reach 7 which indicates that the bubble radius has increased (or that the cavities contain more gas). But after annealing for 12 h, an inversion of the tendency is observed with a reduction in the number of neighbours, Xe–Xe distance and structural dis˚ compared order. The measured distance of 4.32 ± 0.01 A ˚ indicates that the xenon bubbles are still to 4.39 ± 0.01 A pressurized with an internal pressure estimated at 0.07 ± 0.03 GPa. The low value of Debye–Waller factor (0.006 ˚ ) confirms this conclusion as only an increase of xenon A Debye temperature due to the overpressure within the aggregates can explain such a low value [12]. 4. Discussion The first result of this study is that no Xe–O or Xe–U bonds are observed whatever the annealing sequence. This does not contradict previous theoretical studies relating to xenon trapping sites in uranium dioxide. Catlow et al. [13] found using atomistic computer simulations that in stoichiometric UO2 and for very low Xe concentrations (10 6 at.%), that Xe atoms are trapped in trivacancy sites at 1673 K and should therefore have a local environment consisting of U and O atoms. It may be the case in our study that the contribution to the EXAFS and XANES spectra of Xe–U or X–O bonds is too weak to be observed. The sensitivity of the technique required working at high concentrations (8 at.%) to be able to collect XAS signals on annealed samples. At such high concentrations only a Xe–Xe bonds are observed. The second result is that precipitation of nanometre size bubbles containing gas at 2.8 ± 0.3 GPa is liable to occur in the absence of any annealing simply as a result of irradiation induced movement. Recent TEM work on xenon
implanted UO2 polycrystalline [14] and single crystal [15] samples with local concentration around 2 at.% showed no precipitation following implantation. In both cases, the authors observed a tangled dislocation network of interstitial type as observed in irradiated UO2 fuel [16,17]. In our case the same defect type is expected but at a higher concentration as the xenon fluence induced an estimated 590 dpa instead of the 63 dpa in Sathonay et al.’s [15] or Sabathier’s [14] work. Concerning the temperature effect on xenon aggregates, two behaviours are revealed: below 1073 K and at high temperature (1673 and 1773 K). At low temperature (873 K [4] and 1073 K) nanometre size aggregates appear to be stable and remain highly pressurized at roughly 2.0 ± 0.3 GPa. But compared to the as implanted sample a pressure decrease of 0.8 GPa is measured. The usual explanation for this behaviour is that rare gas aggregates act as defect sinks and observed depressurization is due to vacancy annihilation towards the bubble surface. But, if this mechanism is valid, a pressure decrease should appear between 873 and 1073 K as sink efficiency increases with temperature. A more valid hypothesis could be the existence of an oxygen short range network rearrangement of the UO2 matrix around aggregates in order to minimize UO2 internal stress, as for these temperatures oxygen thermal diffusion is activated [18]. At high temperatures (1673 K and 1773 K), large, defect free and non pressurized aggregates are observed. In this case, bubbles seem to trap migrating vacancies and therefore act as defect sinks as would be expected. Moreover in this temperature range thermal diffusion of xenon is probably activated [19]. At the highest temperature, the measured pressure appears to increase with annealing time. This can be interpreted in terms of the existence of a bimodal bubble size distribution. The first class of bubbles corresponds to the original nanometre size aggregates, and the second class to the larger un-pressurised bubbles. It is possible that the larger bubbles obtained for the longest annealing time have grown enough to intersect the sample surface. The contribution to the XAS signal of the gas contained in these bubbles would disappear, leaving only a contribution of the xenon contained in the much smaller highly pressurised aggregates. 5. Conclusion X-ray absorption spectroscopy has been applied to characterize xenon aggregate pressure in uranium dioxide samples implanted with 1017 xenon cm 2 at 800 keV (8 at.% at 140 nm). The pressure evolution with thermal treatment was studied in the 873–1773 K temperature range. EXAFS measurements performed at 12 K show that during implantation gas atoms precipitate to form nanometre size inclusions with an estimated internal pressure of 2.8 ± 0.3 GPa. Data obtained on thermally treated samples show that nanometre size xenon aggregates in UO2 remain highly pressurised up to 1073 K. At higher temperature (1663
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and 1773 K) larger un-pressurised bubbles are present. The fact that xenon bubbles do not grow at low temperature would indicate, either that they are inefficient sinks for mobile gas atoms or that individual gas atoms that are not contained in bubbles are trapped at various defect sites in the matrix. To finalize the bubble characterization, TEM experiments are scheduled in order to observe and to characterize the bi-modal bubble size distribution suggested. Acknowledgements We acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities. This work was supported by the ISMIR programme which is devoted to modelling and understanding radiation effects in insulators and is co-funded by CEA and CNRS. References [1] L.E. Thomas, in: S.E. Donnely, J.H. Evans (Eds.), Fundamental Aspects of Inert gases in Solids, NATO ASI Series B: Physics, Vol. 279, Plenum Publishing Corporation, New York, London, 1991, p. 431. [2] S. Kashibe, K. Une, K. Nogita, J. Nucl. Mater. 206 (1993) 22. [3] K. Nogita, K. Une, Nucl. Instr. and Meth. B 141 (1998) 481.
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