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Xenon migration in UO2 under irradiation studied by SIMS profilometry
Journal of Nuclear Materials 440 (2013) 562–567
Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Presented at the NuMat 2012 Conference, 22–25 October 2012, Osaka, Japan
Xenon migration in UO2 under irradiation studied by SIMS profilometry B. Marchand a,b, N. Moncoffre a, Y. Pipon a,c,⇑, N. Bérerd a,c, C. Garnier b, L. Raimbault d, P. Sainsot e, T. Epicier f, C. Delafoy b, M. Fraczkiewicz b, C. Gaillard a, N. Toulhoat a, A. Perrat-Mabilon a, C. Peaucelle a a
Université de Lyon, CNRS/IN2P3, Université Lyon 1, Institut de Physique Nucléaire de Lyon, 4 rue Enrico Fermi, F-69622 Villeurbanne cedex, France AREVA, AREVA NP, 10 rue Juliette Récamier, F-69456 Lyon, France c Université de Lyon, Université Lyon 1, IUT Lyon 1, 43 bd du 11 novembre 1918, 69 622 Villeurbanne cedex, France d Ecole des Mines de Paris, Centre de Géosciences, 35 rue Saint Honoré, F-77305 Fontainebleau cedex, FRANCE e Université de Lyon, Université Lyon 1, LaMCoS, INSA-Lyon, CNRS UMR5259, F-69621 Villeurbanne cedex, France f Université de Lyon, INSA Lyon, MATEIS, UMR CNRS 5510, 7, avenue Jean-Capelle, 69621 Villeurbanne cedex, France b
a r t i c l e
i n f o
Article history: Available online 15 April 2013
a b s t r a c t During Pressurized Water Reactor operation, around 25% of the created Fission Products (FP) are Xenon and Krypton. They have a low solubility in the nuclear fuel and can either (i) agglomerate into bubbles which induce mechanical stress in the fuel pellets or (ii) be released from the pellets, increasing the pressure within the cladding and decreasing the thermal conductivity of the gap between pellets and cladding. After fifty years of studies on the nuclear fuel, all mechanisms of Fission Gas Release (FGR) are still not fully understood. This paper aims at studying the FGR mechanisms by decoupling thermal and irradiation effects and by assessing the Xenon behavior for the first time by profilometry. Samples are first implanted with 136Xe at 800 keV corresponding to a projected range of 140 nm. They are then either annealed in the temperature range 1400–1600 °C, or irradiated with heavy energy ions (182 MeV Iodine) at Room Temperature (RT), 600 °C or 1000 °C. Depth profiles of implanted Xenon in UO2 are determined by Secondary Ion Mass Spectrometry (SIMS). It is shown that Xenon is mobile during irradiation at 1000 °C. In contrast, thermal treatments do not induce any Xenon migration process: these results are correlated to the formation of Xenon bubbles observed by Transmission Electron Microscopy. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The standard fuel used in pressurized water nuclear reactors (PWR) is uranium oxide (UO2) enriched at about 4% in 235U. After 50 years of studies to understand the evolution of the PWR fuel under irradiation, the research is still very active. On the industrial point of view, progresses in understanding of the basic mechanisms allow: (i) reducing the overconservatism imposed in fuel design justifications, (ii) developing new safe and optimized products. One of the technological barriers is the fission gas release (FGR) concerning mainly Xenon release and Kr release to a lesser extent. Indeed, it has been observed that the FGR strongly accelerates above a burn-up threshold and the involved mechanisms are not clearly identified. The noble fission gases are well known for their low solubility in UO2. They do not interact with UO2 as well as with any other fission product, and, as a result, Xenon segregates and forms bubbles [1,2]. These bubbles have an important impact because they act as traps preventing Xenon and Kr migration. FGR ⇑ Corresponding author at: Université de Lyon, CNRS/IN2P3, Université Lyon 1, Institut de Physique Nucléaire de Lyon, 4 rue Enrico Fermi, F-69622 Villeurbanne cedex, France. Tel.: +33 4 72 43 10 57. E-mail address: [email protected] (Y. Pipon). 0022-3115/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.04.005
would therefore be controlled by Xenon bubble evolution. One of the processes considered in modeling the FGR in UO2 is the gas apparent diffusion from the grain to the grain boundaries taking into account the competitive mechanisms which are the precipitation of single atoms into intragranular bubbles and the resolution of gas atoms from bubbles. Also, during post-irradiation annealing, it is suggested that a thermal vacancy gradient may be responsible for the intragranular bubble diffusion towards the grain boundaries [3]. Previous experiments [2,4–9] have determined apparent diffusion coefficients by measuring Xenon release and by applying the Booth model [10] which is an indirect way to quantify migration. The aim of our work is to perform a ‘‘direct’’ measurement of the Xenon migration into sintered and polished UO2 samples. For that purpose, we aimed at using the SIMS technique in order to determine Xenon depth profiles. This technique had been used by Desgranges and Pasquet [11] to measure Xenon in UO2, however, they did not convert the SIMS signals into depth profiles mainly because of the different orientations of the UO2 grains inducing a varying sputtering during SIMS analyses. This problem was highlighted in the paper of Marchand et al. [12] where depth profiles were obtained using the RSF method described in [13]. In order to solve
B. Marchand et al. / Journal of Nuclear Materials 440 (2013) 562–567
this problem, a new data processing was developed by Marchand [15] which takes into account the different sputtering yields varying from one grain to another in UO2. Therefore, cartography of the sputtered crater is performed after SIMS which allows the determination of different sputtering yields present within the analyzed area. Thus, depth profile can be reconstructed by deconvolution of the SIMS signal. The aim of this paper is to study the synergistic effects of temperature and irradiation using swift heavy ions and thus a high electronic stopping power. In order to decouple the effects of temperature and irradiation, the first part of the paper will be dedicated to a thermal study of Xenon migration. It is important to underline that Xenon distribution profiles in UO2 are determined for the first time by the SIMS technique. This allows for the first time to measure Xenon concentration inside a sample instead of Xenon which is released outside. Therefore we determine directly apparent Xenon diffusion coefficient. 2. Material and method Depleted (0.3 at.% of 235U) UO2 cylindrical pellets (8 mm diameter/1 mm height) were provided by Areva NP. These pellets were sintered at 1700 °C which resulted in (7 ± 2) lm grain size and a density of (94.9 ± 0.3) % of the theoretical density. The samples were polished on one-side at the CSNSM-Orsay (Centre de Spectroscopie Nucléaire et de Spectroscopie de Masse) on an EcoMet 3000Ò polishing machine inside a glove box. In order to analyze the surface texture after polishing, interferometry measurements were performed at INSA, Lyon (Institut National des Sciences Appliquées) on a non-contact 3D Optical Profiler (Neox) of SensofarÒ. The measured mean roughness was found to be 4 nm. After polishing, all samples were annealed at 1000 °C for 10 h in a PECKLYÓ tubular furnace in a high secondary vacuum (105 Pa) in order to degas particles adsorbed on the surface. An additional high temperature post-polishing annealing at 1600 °C during 4 h in a reducing atmosphere (Ar + 5% H2) was performed in a NABERTHERMÒ tubular furnace in order to anneal all the defects created during polishing. This last statement was verified by performing Doppler Broadening Positron Annihilation Spectroscopy (PAS-DBS) measurements [14]. The pellets were then implanted with 800 keV 136Xe. The implanted pellets were divided into two groups. The first group was annealed at 1400 °C and 1600 °C. The second group was irradiated with swift heavy ions. All these pellets were analyzed, before and after annealing or irradiation, by Secondary Ion Mass Spectrometry (SIMS) to follow the evolution of the Xenon depth profiles. Transmission Electron Microscopy (TEM) observations were also carried out in order to characterize the UO2 microstructure and to follow the Xenon bubble evolution.
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3. Experimental details 3.1. Xenon implantation The samples were implanted with 800 keV 136Xe2+ ions at IPNL (Institut de Physique Nucléaire of Lyon) using the IMIO400 accelerator facility with a cooling device maintaining the sample temperature at 15 °C. The implantation fluence of 1 1016 Xe cm2 leads to a Xenon concentration similar to the mean Xenon concentration in the nuclear fuel with a 50 GWd t1 U burnup as estimated from SRIM data (see [15] for calculation details). Simulation of the Xenon implantation was performed with the SRIM.2008.4 software [16]. In the full cascade mode, SRIM software calculated the creation of 70 dpa (displacements per atom) with a distribution peaking Rd around 80 nm. The corresponding maximum Xenon concentration was calculated to be about 1 at.% at the projected range, RXenon , around 140 nm. Implantations were performed at 15 °C, and it was shown by TEM and by PAS [14], that in these conditions, no Xenon bubbles are formed. This result was previously shown by TEM observation after Xe implantation with a 8 1015 Xe cm2 fluence [17]. Therefore Xe is mainly present under atomic state in UO2 bulk just after implantation. It has been shown by lattice location experiments that Xe atoms substitute U atoms [18] and ab initio calculations predict that Xe must be trapped in divacancies or even trivacancies [19,20]. 3.2. Thermal annealings After implantation, the samples were annealed in the NABERTHERM furnace during up to 32 h at 1400 °C and up to 16 h at 1600 °C under reducing atmosphere (Ar + 5% H2) to ensure that the UO2 stoichiometry is kept stable. The sample stoichoimetry was determined by using the 16O(a,a)16O resonant nuclear reaction at 7.5 MeV on the 4 MV Van De Graaff accelerator at IPNL. The results indicate that no oxidation occurs at 1400 °C and that a slight oxidation is observed at 1600 °C, resulting in a maximum stoichiometry of UO2.1 after a 16 h annealing. 3.3. Irradiation A dedicated cell was developed at IPNL in order to irradiate the samples at temperatures as high as 1000 °C. This cell is illustrated in Fig. 1. The target is the implanted UO2 sample which is placed on a pyrolitic BN plate. A tungsten resistance is enclosed in the plate and can heat the target by Joule effect up to 1100 °C. A variable power supply allows controlling the intensity into the resistance and fixing the target temperature. A C thermocouple (Tungsten5% Rhenium vs. Tungsten-26% Rhenium; 0–2320 °C) is used to
Fig. 1. Irradiation cell elaborated at IPNL.
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Fig. 2. Xenon depth profiles measured by SIMS before and after annealing at 1400 °C (a) and at 1600 °C (b).
measure the target temperature (Fig. 1a) and the surface temperature is also measured thanks to a bichromatic pyrometer. The whole set is encapsulated in stainless steel covered by a molybdenum layer to protect steel from temperature increase. The target holder is cooled down using water cooling tubes (Fig. 1b). To guarantee the beam line safety, an aluminum window (1.5 lm) is placed between the cell and the beam line. The cell is designed to work under vacuum (106 mbar). The irradiation experiments were performed at the 15 MV TANDEM accelerator facility at Orsay, on the 410 beamline. In order to enhance high electronic stopping power effects, irradiations were performed with 182 MeV Iodine ions. According to SRIM, these conditions correspond to an electronic stopping power (dE/dx)e of 30 keV nm1 at the UO2 surface. A total fluence of around 1015 I cm2 was achieved and the mean current value was around 90 nA during irradiation. This corresponds to a mean flux of around 1010 I cm2 s1. The irradiations were performed at three temperatures: RT, 600 °C and 1000 °C. 3.4. SIMS data processing and TEM experiments SIMS analysis were performed on a CAMECA IMS 6f facility at the ‘‘Ecole des mines de Paris’’ in Fontainebleau. A focused primary
2 beam of Oþ 2 was rastered over an area of 150 150 lm on the UO2 sample surfaces. Secondary ions were collected on the crater central part (62 lm diameter) to avoid crater-edge effects. The depth scale was determined by measuring the crater depth by optical interferometry at the INSA of Lyon. As already mentioned, a new data processing was previously developed [15] to follow the Xenon depth profile evolution, taking into account the different sputtering yields during SIMS analysis. Therefore, cartography of the sputtered crater is performed after SIMS analysis which allows the determination of different sputtering yields present within the analyzed area. Thus, depth profiles can be reconstructed by deconvolution of the SIMS signal. The microscope slides were prepared using the Focused Ion Beam (FIB) technique by the company SERMA technology. TEM analyses were performed at INSA Lyon on a 200 keV JEOL 2010F microscope in conventional mode and in STEM (Scanning Transmission Electron Microscope) mode. This last mode used a HAADF (‘‘High Angle Annular Dark Field’’) detector to detect the electrons scattered at wide angles. With such a detector, the detected intensity is directly proportional to the density of atoms contained in the sample thickness, which eliminates the interferences detected in the conventional mode.
Fig. 3. TEM observations of a UO2 sample implanted with 1016 Xe cm2 and annealed at 1600 °C during 12 h. Left figure was under-focused and right figure was over-focused. Xenon (black line) and defects profiles (red line) calculated by SRIM are also displayed. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
B. Marchand et al. / Journal of Nuclear Materials 440 (2013) 562–567
4. Results and discussion 4.1. Xenon thermal migration 4.1.1. Xenon depth profile evolution Fig. 2 displays the implanted Xenon depth profiles before and after annealing at 1400 °C (a) and at 1600 °C (b). At 1400 °C, no evolution of the Xenon distribution is observed. This result is in agreement with the work of Zacharie et al. [8] who performed post-irradiation annealing between 1130 and 1715 °C for UO2 samples containing Xenon concentration around 0.5 at.%. Indeed they observed a Xenon release inferior to 2% after a 5 h annealing at 1410 °C. Within the experimental uncertainty of the measurements at 1600°C, it appears that: (i) no significant Xenon release occurs after a 16 h anneal, and (ii) no broadening of the Xenon profiles is evidenced so no diffusion occurs. Zacharie et al. observed a release of around 8 % after a 10 h annealing at 1545 °C. This seems to indicate that in our experimental conditions, Xenon is more trapped than in Zacharie’s work. To confirm this assessment, TEM experiments were performed on these UO2 samples.
4.1.2. Evolution of the UO2 microstructure with temperature Fig. 3 shows TEM images obtained on a UO2 sample implanted with 1016 Xe cm2 and annealed at 1600 °C during 12 h. The left picture is obtained with under-focused conditions and the right picture with over-focused conditions. The interest to change the focus conditions is clearly to reveal the presence of Xenon bubbles. Results of SRIM calculations have been added next to the TEM observations in order to correlate the observed microstructure to the Xenon concentration and to the defects created by implantation. Fig. 3 shows the presence of three distinct zones: – At depths lower than about 40 nm (zone 1), no bubbles are observed, – At depths in between 40 nm and 110 nm (zone 2), a large number of small bubbles (around 2 nm in diameter) can be observed. By comparing with the SRIM profile, it appears that this area corresponds to the maximum of the defect profile, – The third zone displays two bubble populations. The first population has the same size than the bubbles present in zone 2. The bubble size of the second population is significantly larger (up to around 10 nm).
Fig. 4. STEM observation of a UO2 sample implanted with 1016 Xe cm2 and annealed at 1600 °C during 12 h.
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A STEM micrograph is presented in Fig. 4. It highlights the Xenon bubbles more clearly. It appears that the largest bubbles are located mainly near dislocations which are predominantly in zone 3. TEM micrographs obtained on the samples annealed at 1400 °C (not shown here) show only small sized bubbles (around 2 nm). The presence of these bubbles could explain that no Xenon migration occurs even after annealing at 1600 °C during 16 h. Moreover, concerning Xe thermal resolution, this can only occur if the bubble is overpressurized [21]. It was shown by Martin et al. [22] that at high temperature (over 1400 °C) non pressurized aggregates are observed. So in our experiments, Xe thermal resolution is unlikely. The bubble sizes measured after 1400 °C and 1600 °C annealing are in agreement with literature data, in particular, with those of Michel et al. [23] obtained in Xenon implanted UO2 samples. Indeed they observed 1 nm sized bubbles at 600 °C, which could reach 3 nm at 1400 °C. 4.2. Xenon migration under irradiation 4.2.1. Xenon depth profile evolution Fig. 5 shows the Xenon depth profiles before and after irradiation. At RT, almost no evolution occurs. At 600 °C, a slight Xenon release together with a kind of retention zone near the surface (the first 30 nm) are noticed. At 1000 °C, an important broadening of the Xenon profile together with a decrease of the total area and a displacement of the profile towards the bulk are observed. The most likely hypothesis to explain the different Xenon behavior at 600 °C and at 1000 °C is the mobility of uranium vacancies which occurs only from 800 °C [24] thus accelerating Xenon apparent diffusion in our 1000 °C irradiation conditions. The different processes observed for the sample irradiated at 1000 °C can be described according to the Fick’s second law such as: 2
dcðx; tÞ d cðx; tÞ dcðx; tÞ ¼D þm þ k cðx; tÞ 2 dt dx dx
ð1Þ
where D is the apparent diffusion coefficient (cm2 s1), v the transport velocity of Xenon within UO2 (cm s1) and k the release rate (s1). It is necessary to define the appropriate limit conditions. Two types of conditions can be considered: – Either conditions of the Neumann type for which the surface is impermeable which means that the Xenon flux is equal to zero and can be expressed by Eq. (2).
Fig. 5. Xenon depth profiles measured by SIMS before and after irradiation.
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The deduced apparent diffusion coefficient (9.5 1016 cm2 s ) is one order of magnitude higher than the one given by Turnbull [25] who studied the fission gas release of in-pile irradiated UO2 pellets and who found around 1 1016 cm2 s1 at 1000 °C. The differences between our results and Turnbull results could be mainly explained by the fact that Turnbull deduced his results from in-pile experiments whereas we performed irradiation on UO2 samples implanted with Xenon. More precisely, Turnbull experiments were made with a neutron flux of 4 1016 m2 s1 generating fission products with a wide energy range up to 110 MeV. In our experiments, irradiation was performed with Iodine ions of a single energy (186 MeV) with a pretty high flux of 5 1019 I m2 s1. Moreover, Turnbull measured Xe release and had to use the Booth model to determine Xe diffusion coefficient. In this last model, several hypotheses are made, the major ones being a spherical shape of grains and no possible accumulation of Xenon at the grain surface. Our diffusion coefficient is obtained after solving the Fick’s equation without any hypothesis on the grain geometry. 1
Fig. 6. Fit of the Xenon profile irradiated at 1000 °C.
dC j ¼0 dx surface
ð2Þ
– Or conditions of the Dirichlet type with a constant Xenon concentration at the surface expressed by Eq. (3).
Cð0; tÞ ¼ constant
ð3Þ
We chose Neumann conditions since we observed a slight increase of Xenon concentration at the surface for the profiles of the samples irradiated at 600 °C and at 1000 °C. In order to simulate the evolution of the Xenon concentration profiles, as-implanted profiles were first fitted with Gaussian shaped curves. The evolution of these curves was then simulated by using the one dimensional finite difference method. Therefore, the total depth profile was discretized into 1.5 nm slices. D, v, k parameters were thus deduced from successive iterations until the final profile is correctly fitted. It is important to keep in mind that each migration mechanism induces a particular modification of the profile shape: a broadening is characteristic of a diffusion process, a profile shift is significant of a transport process and an area decrease means a release mechanism. Consequently, only one set of parameters can allow a correct fit of the final profile. This method was applied for the sample irradiated at 1000 °C and the fitted spectrum is presented in Fig. 6. The obtained values are respectively:
D ¼ 9:5 1016 cm2 s1 ; 5
k ¼ 2:5 10
1
s :
v ¼ 3:1 1010 cm s1
and
4.2.2. Evolution of the UO2 microstructure with the irradiation Fig. 7 shows TEM and STEM micrographs of the UO2 sample irradiated at 1000 °C. Only small sized bubbles can be observed. In contrast with the images obtained by STEM on 1600 °C annealed samples (Fig. 4) in which Xenon bubbles were not aligned, here a clear bubble alignment is put forward all along the Iodine ion beam path (Fig. 7) on irradiated samples. Similar results have been observed at 600 °C but no bubbles have been observed at 15 °C. This means that the bubble alignments are induced by a coupling effect of the high energy heavy ion irradiation and temperature. The bubbles alignment phenomenon has been already observed for in-pile irradiated UO2 pellets, between 1300 and 1800 °C by Baker [26]. He observed 5 nm size intragranular bubbles, aligned along the fission product path called fission tracks. In our irradiation conditions, temperature is much lower (1000 °C), but the FP energy is much higher and accordingly, the electronic stopping power is more important. This tends to show that a threshold for bubble alignment along the fission tracks axis exists. In the literature, the fission tracks have been observed from given thresholds which values depend on the considered materials. In UO2, Matzke [27] has determined a threshold value between 22 and 29 keV nm1 and more recently Toulemonde [28] proposed a lower value around 11 keV nm1. In all cases, our conditions are far above these thresholds, implying the formation of fission tracks in our samples.
Fig. 7. STEM observation of a UO2 sample implanted with 1016 Xe cm2 and irradiated at 1000 °C.
B. Marchand et al. / Journal of Nuclear Materials 440 (2013) 562–567
To conclude on the irradiation results, we see an enhanced Xenon apparent diffusion under irradiation at 1000 °C revealing the main role played by temperature and irradiation energy. 5. Conclusion In this paper, we have studied for the first time, the Xenon depth profiles evolution under irradiation. We have also highlighted that the Xenon mobility in UO2 was clearly induced by a synergistic effect of both temperature and irradiation (Radiation Enhanced Diffusion). The proposed mechanism suggests that after thermal annealing, Xenon is kept trapped into bubbles preventing any mobility. Under irradiation at 600 °C and 1000 °C, point defects are created. At 600 °C, only vacancies are mobile near the surface which could explain the partial Xenon migration observed in these conditions. At 1000 °C, all point defects are mobile and allow the atomic Xenon migration on the whole implanted zone. These processes can occur in our experimental conditions when irradiating with high energy heavy ions above the threshold value allowing the creation of fission tracks. We assume that the fission tracks would lead to the resolution of Xenon bubbles (at least partly) making possible such Xenon enhanced apparent diffusion. Acknowledgements The authors are very grateful to M. Toulemonde for fruitful discussions on fission track formation mechanisms. They also thank S. Gavarini for his contribution to the irradiation chamber elaboration and the Tandem staff for its very efficient help during irradiation experiments. References [1] R.M. Cornell, J. Nucl. Mater. 38 (1971) 319–328. [2] W. Miekeley, F.W. Felix, J. Nucl. Mater. 42 (1972) 297–306. [3] J.H. Evans, J. Nucl. Mater. 225 (1995) 302–307.
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Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Presented at the NuMat 2012 Conference, 22–25 October 2012, Osaka, Japan
Xenon migration in UO2 under irradiation studied by SIMS profilometry B. Marchand a,b, N. Moncoffre a, Y. Pipon a,c,⇑, N. Bérerd a,c, C. Garnier b, L. Raimbault d, P. Sainsot e, T. Epicier f, C. Delafoy b, M. Fraczkiewicz b, C. Gaillard a, N. Toulhoat a, A. Perrat-Mabilon a, C. Peaucelle a a
Université de Lyon, CNRS/IN2P3, Université Lyon 1, Institut de Physique Nucléaire de Lyon, 4 rue Enrico Fermi, F-69622 Villeurbanne cedex, France AREVA, AREVA NP, 10 rue Juliette Récamier, F-69456 Lyon, France c Université de Lyon, Université Lyon 1, IUT Lyon 1, 43 bd du 11 novembre 1918, 69 622 Villeurbanne cedex, France d Ecole des Mines de Paris, Centre de Géosciences, 35 rue Saint Honoré, F-77305 Fontainebleau cedex, FRANCE e Université de Lyon, Université Lyon 1, LaMCoS, INSA-Lyon, CNRS UMR5259, F-69621 Villeurbanne cedex, France f Université de Lyon, INSA Lyon, MATEIS, UMR CNRS 5510, 7, avenue Jean-Capelle, 69621 Villeurbanne cedex, France b
a r t i c l e
i n f o
Article history: Available online 15 April 2013
a b s t r a c t During Pressurized Water Reactor operation, around 25% of the created Fission Products (FP) are Xenon and Krypton. They have a low solubility in the nuclear fuel and can either (i) agglomerate into bubbles which induce mechanical stress in the fuel pellets or (ii) be released from the pellets, increasing the pressure within the cladding and decreasing the thermal conductivity of the gap between pellets and cladding. After fifty years of studies on the nuclear fuel, all mechanisms of Fission Gas Release (FGR) are still not fully understood. This paper aims at studying the FGR mechanisms by decoupling thermal and irradiation effects and by assessing the Xenon behavior for the first time by profilometry. Samples are first implanted with 136Xe at 800 keV corresponding to a projected range of 140 nm. They are then either annealed in the temperature range 1400–1600 °C, or irradiated with heavy energy ions (182 MeV Iodine) at Room Temperature (RT), 600 °C or 1000 °C. Depth profiles of implanted Xenon in UO2 are determined by Secondary Ion Mass Spectrometry (SIMS). It is shown that Xenon is mobile during irradiation at 1000 °C. In contrast, thermal treatments do not induce any Xenon migration process: these results are correlated to the formation of Xenon bubbles observed by Transmission Electron Microscopy. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The standard fuel used in pressurized water nuclear reactors (PWR) is uranium oxide (UO2) enriched at about 4% in 235U. After 50 years of studies to understand the evolution of the PWR fuel under irradiation, the research is still very active. On the industrial point of view, progresses in understanding of the basic mechanisms allow: (i) reducing the overconservatism imposed in fuel design justifications, (ii) developing new safe and optimized products. One of the technological barriers is the fission gas release (FGR) concerning mainly Xenon release and Kr release to a lesser extent. Indeed, it has been observed that the FGR strongly accelerates above a burn-up threshold and the involved mechanisms are not clearly identified. The noble fission gases are well known for their low solubility in UO2. They do not interact with UO2 as well as with any other fission product, and, as a result, Xenon segregates and forms bubbles [1,2]. These bubbles have an important impact because they act as traps preventing Xenon and Kr migration. FGR ⇑ Corresponding author at: Université de Lyon, CNRS/IN2P3, Université Lyon 1, Institut de Physique Nucléaire de Lyon, 4 rue Enrico Fermi, F-69622 Villeurbanne cedex, France. Tel.: +33 4 72 43 10 57. E-mail address: [email protected] (Y. Pipon). 0022-3115/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.04.005
would therefore be controlled by Xenon bubble evolution. One of the processes considered in modeling the FGR in UO2 is the gas apparent diffusion from the grain to the grain boundaries taking into account the competitive mechanisms which are the precipitation of single atoms into intragranular bubbles and the resolution of gas atoms from bubbles. Also, during post-irradiation annealing, it is suggested that a thermal vacancy gradient may be responsible for the intragranular bubble diffusion towards the grain boundaries [3]. Previous experiments [2,4–9] have determined apparent diffusion coefficients by measuring Xenon release and by applying the Booth model [10] which is an indirect way to quantify migration. The aim of our work is to perform a ‘‘direct’’ measurement of the Xenon migration into sintered and polished UO2 samples. For that purpose, we aimed at using the SIMS technique in order to determine Xenon depth profiles. This technique had been used by Desgranges and Pasquet [11] to measure Xenon in UO2, however, they did not convert the SIMS signals into depth profiles mainly because of the different orientations of the UO2 grains inducing a varying sputtering during SIMS analyses. This problem was highlighted in the paper of Marchand et al. [12] where depth profiles were obtained using the RSF method described in [13]. In order to solve
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this problem, a new data processing was developed by Marchand [15] which takes into account the different sputtering yields varying from one grain to another in UO2. Therefore, cartography of the sputtered crater is performed after SIMS which allows the determination of different sputtering yields present within the analyzed area. Thus, depth profile can be reconstructed by deconvolution of the SIMS signal. The aim of this paper is to study the synergistic effects of temperature and irradiation using swift heavy ions and thus a high electronic stopping power. In order to decouple the effects of temperature and irradiation, the first part of the paper will be dedicated to a thermal study of Xenon migration. It is important to underline that Xenon distribution profiles in UO2 are determined for the first time by the SIMS technique. This allows for the first time to measure Xenon concentration inside a sample instead of Xenon which is released outside. Therefore we determine directly apparent Xenon diffusion coefficient. 2. Material and method Depleted (0.3 at.% of 235U) UO2 cylindrical pellets (8 mm diameter/1 mm height) were provided by Areva NP. These pellets were sintered at 1700 °C which resulted in (7 ± 2) lm grain size and a density of (94.9 ± 0.3) % of the theoretical density. The samples were polished on one-side at the CSNSM-Orsay (Centre de Spectroscopie Nucléaire et de Spectroscopie de Masse) on an EcoMet 3000Ò polishing machine inside a glove box. In order to analyze the surface texture after polishing, interferometry measurements were performed at INSA, Lyon (Institut National des Sciences Appliquées) on a non-contact 3D Optical Profiler (Neox) of SensofarÒ. The measured mean roughness was found to be 4 nm. After polishing, all samples were annealed at 1000 °C for 10 h in a PECKLYÓ tubular furnace in a high secondary vacuum (105 Pa) in order to degas particles adsorbed on the surface. An additional high temperature post-polishing annealing at 1600 °C during 4 h in a reducing atmosphere (Ar + 5% H2) was performed in a NABERTHERMÒ tubular furnace in order to anneal all the defects created during polishing. This last statement was verified by performing Doppler Broadening Positron Annihilation Spectroscopy (PAS-DBS) measurements [14]. The pellets were then implanted with 800 keV 136Xe. The implanted pellets were divided into two groups. The first group was annealed at 1400 °C and 1600 °C. The second group was irradiated with swift heavy ions. All these pellets were analyzed, before and after annealing or irradiation, by Secondary Ion Mass Spectrometry (SIMS) to follow the evolution of the Xenon depth profiles. Transmission Electron Microscopy (TEM) observations were also carried out in order to characterize the UO2 microstructure and to follow the Xenon bubble evolution.
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3. Experimental details 3.1. Xenon implantation The samples were implanted with 800 keV 136Xe2+ ions at IPNL (Institut de Physique Nucléaire of Lyon) using the IMIO400 accelerator facility with a cooling device maintaining the sample temperature at 15 °C. The implantation fluence of 1 1016 Xe cm2 leads to a Xenon concentration similar to the mean Xenon concentration in the nuclear fuel with a 50 GWd t1 U burnup as estimated from SRIM data (see [15] for calculation details). Simulation of the Xenon implantation was performed with the SRIM.2008.4 software [16]. In the full cascade mode, SRIM software calculated the creation of 70 dpa (displacements per atom) with a distribution peaking Rd around 80 nm. The corresponding maximum Xenon concentration was calculated to be about 1 at.% at the projected range, RXenon , around 140 nm. Implantations were performed at 15 °C, and it was shown by TEM and by PAS [14], that in these conditions, no Xenon bubbles are formed. This result was previously shown by TEM observation after Xe implantation with a 8 1015 Xe cm2 fluence [17]. Therefore Xe is mainly present under atomic state in UO2 bulk just after implantation. It has been shown by lattice location experiments that Xe atoms substitute U atoms [18] and ab initio calculations predict that Xe must be trapped in divacancies or even trivacancies [19,20]. 3.2. Thermal annealings After implantation, the samples were annealed in the NABERTHERM furnace during up to 32 h at 1400 °C and up to 16 h at 1600 °C under reducing atmosphere (Ar + 5% H2) to ensure that the UO2 stoichiometry is kept stable. The sample stoichoimetry was determined by using the 16O(a,a)16O resonant nuclear reaction at 7.5 MeV on the 4 MV Van De Graaff accelerator at IPNL. The results indicate that no oxidation occurs at 1400 °C and that a slight oxidation is observed at 1600 °C, resulting in a maximum stoichiometry of UO2.1 after a 16 h annealing. 3.3. Irradiation A dedicated cell was developed at IPNL in order to irradiate the samples at temperatures as high as 1000 °C. This cell is illustrated in Fig. 1. The target is the implanted UO2 sample which is placed on a pyrolitic BN plate. A tungsten resistance is enclosed in the plate and can heat the target by Joule effect up to 1100 °C. A variable power supply allows controlling the intensity into the resistance and fixing the target temperature. A C thermocouple (Tungsten5% Rhenium vs. Tungsten-26% Rhenium; 0–2320 °C) is used to
Fig. 1. Irradiation cell elaborated at IPNL.
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Fig. 2. Xenon depth profiles measured by SIMS before and after annealing at 1400 °C (a) and at 1600 °C (b).
measure the target temperature (Fig. 1a) and the surface temperature is also measured thanks to a bichromatic pyrometer. The whole set is encapsulated in stainless steel covered by a molybdenum layer to protect steel from temperature increase. The target holder is cooled down using water cooling tubes (Fig. 1b). To guarantee the beam line safety, an aluminum window (1.5 lm) is placed between the cell and the beam line. The cell is designed to work under vacuum (106 mbar). The irradiation experiments were performed at the 15 MV TANDEM accelerator facility at Orsay, on the 410 beamline. In order to enhance high electronic stopping power effects, irradiations were performed with 182 MeV Iodine ions. According to SRIM, these conditions correspond to an electronic stopping power (dE/dx)e of 30 keV nm1 at the UO2 surface. A total fluence of around 1015 I cm2 was achieved and the mean current value was around 90 nA during irradiation. This corresponds to a mean flux of around 1010 I cm2 s1. The irradiations were performed at three temperatures: RT, 600 °C and 1000 °C. 3.4. SIMS data processing and TEM experiments SIMS analysis were performed on a CAMECA IMS 6f facility at the ‘‘Ecole des mines de Paris’’ in Fontainebleau. A focused primary
2 beam of Oþ 2 was rastered over an area of 150 150 lm on the UO2 sample surfaces. Secondary ions were collected on the crater central part (62 lm diameter) to avoid crater-edge effects. The depth scale was determined by measuring the crater depth by optical interferometry at the INSA of Lyon. As already mentioned, a new data processing was previously developed [15] to follow the Xenon depth profile evolution, taking into account the different sputtering yields during SIMS analysis. Therefore, cartography of the sputtered crater is performed after SIMS analysis which allows the determination of different sputtering yields present within the analyzed area. Thus, depth profiles can be reconstructed by deconvolution of the SIMS signal. The microscope slides were prepared using the Focused Ion Beam (FIB) technique by the company SERMA technology. TEM analyses were performed at INSA Lyon on a 200 keV JEOL 2010F microscope in conventional mode and in STEM (Scanning Transmission Electron Microscope) mode. This last mode used a HAADF (‘‘High Angle Annular Dark Field’’) detector to detect the electrons scattered at wide angles. With such a detector, the detected intensity is directly proportional to the density of atoms contained in the sample thickness, which eliminates the interferences detected in the conventional mode.
Fig. 3. TEM observations of a UO2 sample implanted with 1016 Xe cm2 and annealed at 1600 °C during 12 h. Left figure was under-focused and right figure was over-focused. Xenon (black line) and defects profiles (red line) calculated by SRIM are also displayed. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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4. Results and discussion 4.1. Xenon thermal migration 4.1.1. Xenon depth profile evolution Fig. 2 displays the implanted Xenon depth profiles before and after annealing at 1400 °C (a) and at 1600 °C (b). At 1400 °C, no evolution of the Xenon distribution is observed. This result is in agreement with the work of Zacharie et al. [8] who performed post-irradiation annealing between 1130 and 1715 °C for UO2 samples containing Xenon concentration around 0.5 at.%. Indeed they observed a Xenon release inferior to 2% after a 5 h annealing at 1410 °C. Within the experimental uncertainty of the measurements at 1600°C, it appears that: (i) no significant Xenon release occurs after a 16 h anneal, and (ii) no broadening of the Xenon profiles is evidenced so no diffusion occurs. Zacharie et al. observed a release of around 8 % after a 10 h annealing at 1545 °C. This seems to indicate that in our experimental conditions, Xenon is more trapped than in Zacharie’s work. To confirm this assessment, TEM experiments were performed on these UO2 samples.
4.1.2. Evolution of the UO2 microstructure with temperature Fig. 3 shows TEM images obtained on a UO2 sample implanted with 1016 Xe cm2 and annealed at 1600 °C during 12 h. The left picture is obtained with under-focused conditions and the right picture with over-focused conditions. The interest to change the focus conditions is clearly to reveal the presence of Xenon bubbles. Results of SRIM calculations have been added next to the TEM observations in order to correlate the observed microstructure to the Xenon concentration and to the defects created by implantation. Fig. 3 shows the presence of three distinct zones: – At depths lower than about 40 nm (zone 1), no bubbles are observed, – At depths in between 40 nm and 110 nm (zone 2), a large number of small bubbles (around 2 nm in diameter) can be observed. By comparing with the SRIM profile, it appears that this area corresponds to the maximum of the defect profile, – The third zone displays two bubble populations. The first population has the same size than the bubbles present in zone 2. The bubble size of the second population is significantly larger (up to around 10 nm).
Fig. 4. STEM observation of a UO2 sample implanted with 1016 Xe cm2 and annealed at 1600 °C during 12 h.
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A STEM micrograph is presented in Fig. 4. It highlights the Xenon bubbles more clearly. It appears that the largest bubbles are located mainly near dislocations which are predominantly in zone 3. TEM micrographs obtained on the samples annealed at 1400 °C (not shown here) show only small sized bubbles (around 2 nm). The presence of these bubbles could explain that no Xenon migration occurs even after annealing at 1600 °C during 16 h. Moreover, concerning Xe thermal resolution, this can only occur if the bubble is overpressurized [21]. It was shown by Martin et al. [22] that at high temperature (over 1400 °C) non pressurized aggregates are observed. So in our experiments, Xe thermal resolution is unlikely. The bubble sizes measured after 1400 °C and 1600 °C annealing are in agreement with literature data, in particular, with those of Michel et al. [23] obtained in Xenon implanted UO2 samples. Indeed they observed 1 nm sized bubbles at 600 °C, which could reach 3 nm at 1400 °C. 4.2. Xenon migration under irradiation 4.2.1. Xenon depth profile evolution Fig. 5 shows the Xenon depth profiles before and after irradiation. At RT, almost no evolution occurs. At 600 °C, a slight Xenon release together with a kind of retention zone near the surface (the first 30 nm) are noticed. At 1000 °C, an important broadening of the Xenon profile together with a decrease of the total area and a displacement of the profile towards the bulk are observed. The most likely hypothesis to explain the different Xenon behavior at 600 °C and at 1000 °C is the mobility of uranium vacancies which occurs only from 800 °C [24] thus accelerating Xenon apparent diffusion in our 1000 °C irradiation conditions. The different processes observed for the sample irradiated at 1000 °C can be described according to the Fick’s second law such as: 2
dcðx; tÞ d cðx; tÞ dcðx; tÞ ¼D þm þ k cðx; tÞ 2 dt dx dx
ð1Þ
where D is the apparent diffusion coefficient (cm2 s1), v the transport velocity of Xenon within UO2 (cm s1) and k the release rate (s1). It is necessary to define the appropriate limit conditions. Two types of conditions can be considered: – Either conditions of the Neumann type for which the surface is impermeable which means that the Xenon flux is equal to zero and can be expressed by Eq. (2).
Fig. 5. Xenon depth profiles measured by SIMS before and after irradiation.
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The deduced apparent diffusion coefficient (9.5 1016 cm2 s ) is one order of magnitude higher than the one given by Turnbull [25] who studied the fission gas release of in-pile irradiated UO2 pellets and who found around 1 1016 cm2 s1 at 1000 °C. The differences between our results and Turnbull results could be mainly explained by the fact that Turnbull deduced his results from in-pile experiments whereas we performed irradiation on UO2 samples implanted with Xenon. More precisely, Turnbull experiments were made with a neutron flux of 4 1016 m2 s1 generating fission products with a wide energy range up to 110 MeV. In our experiments, irradiation was performed with Iodine ions of a single energy (186 MeV) with a pretty high flux of 5 1019 I m2 s1. Moreover, Turnbull measured Xe release and had to use the Booth model to determine Xe diffusion coefficient. In this last model, several hypotheses are made, the major ones being a spherical shape of grains and no possible accumulation of Xenon at the grain surface. Our diffusion coefficient is obtained after solving the Fick’s equation without any hypothesis on the grain geometry. 1
Fig. 6. Fit of the Xenon profile irradiated at 1000 °C.
dC j ¼0 dx surface
ð2Þ
– Or conditions of the Dirichlet type with a constant Xenon concentration at the surface expressed by Eq. (3).
Cð0; tÞ ¼ constant
ð3Þ
We chose Neumann conditions since we observed a slight increase of Xenon concentration at the surface for the profiles of the samples irradiated at 600 °C and at 1000 °C. In order to simulate the evolution of the Xenon concentration profiles, as-implanted profiles were first fitted with Gaussian shaped curves. The evolution of these curves was then simulated by using the one dimensional finite difference method. Therefore, the total depth profile was discretized into 1.5 nm slices. D, v, k parameters were thus deduced from successive iterations until the final profile is correctly fitted. It is important to keep in mind that each migration mechanism induces a particular modification of the profile shape: a broadening is characteristic of a diffusion process, a profile shift is significant of a transport process and an area decrease means a release mechanism. Consequently, only one set of parameters can allow a correct fit of the final profile. This method was applied for the sample irradiated at 1000 °C and the fitted spectrum is presented in Fig. 6. The obtained values are respectively:
D ¼ 9:5 1016 cm2 s1 ; 5
k ¼ 2:5 10
1
s :
v ¼ 3:1 1010 cm s1
and
4.2.2. Evolution of the UO2 microstructure with the irradiation Fig. 7 shows TEM and STEM micrographs of the UO2 sample irradiated at 1000 °C. Only small sized bubbles can be observed. In contrast with the images obtained by STEM on 1600 °C annealed samples (Fig. 4) in which Xenon bubbles were not aligned, here a clear bubble alignment is put forward all along the Iodine ion beam path (Fig. 7) on irradiated samples. Similar results have been observed at 600 °C but no bubbles have been observed at 15 °C. This means that the bubble alignments are induced by a coupling effect of the high energy heavy ion irradiation and temperature. The bubbles alignment phenomenon has been already observed for in-pile irradiated UO2 pellets, between 1300 and 1800 °C by Baker [26]. He observed 5 nm size intragranular bubbles, aligned along the fission product path called fission tracks. In our irradiation conditions, temperature is much lower (1000 °C), but the FP energy is much higher and accordingly, the electronic stopping power is more important. This tends to show that a threshold for bubble alignment along the fission tracks axis exists. In the literature, the fission tracks have been observed from given thresholds which values depend on the considered materials. In UO2, Matzke [27] has determined a threshold value between 22 and 29 keV nm1 and more recently Toulemonde [28] proposed a lower value around 11 keV nm1. In all cases, our conditions are far above these thresholds, implying the formation of fission tracks in our samples.
Fig. 7. STEM observation of a UO2 sample implanted with 1016 Xe cm2 and irradiated at 1000 °C.
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To conclude on the irradiation results, we see an enhanced Xenon apparent diffusion under irradiation at 1000 °C revealing the main role played by temperature and irradiation energy. 5. Conclusion In this paper, we have studied for the first time, the Xenon depth profiles evolution under irradiation. We have also highlighted that the Xenon mobility in UO2 was clearly induced by a synergistic effect of both temperature and irradiation (Radiation Enhanced Diffusion). The proposed mechanism suggests that after thermal annealing, Xenon is kept trapped into bubbles preventing any mobility. Under irradiation at 600 °C and 1000 °C, point defects are created. At 600 °C, only vacancies are mobile near the surface which could explain the partial Xenon migration observed in these conditions. At 1000 °C, all point defects are mobile and allow the atomic Xenon migration on the whole implanted zone. These processes can occur in our experimental conditions when irradiating with high energy heavy ions above the threshold value allowing the creation of fission tracks. We assume that the fission tracks would lead to the resolution of Xenon bubbles (at least partly) making possible such Xenon enhanced apparent diffusion. Acknowledgements The authors are very grateful to M. Toulemonde for fruitful discussions on fission track formation mechanisms. They also thank S. Gavarini for his contribution to the irradiation chamber elaboration and the Tandem staff for its very efficient help during irradiation experiments. References [1] R.M. Cornell, J. Nucl. Mater. 38 (1971) 319–328. [2] W. Miekeley, F.W. Felix, J. Nucl. Mater. 42 (1972) 297–306. [3] J.H. Evans, J. Nucl. Mater. 225 (1995) 302–307.
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