annealed or irradiated) have been measured by SIMS [3]. The high resolution ... We pointed out the ability of the implanted chlorine to migrate [3]. It was shown ...
Mater. Res. Soc. Symp. Proc. Vol. 985 © 2007 Materials Research Society
0985-NN05-03
Chlorine Diffusion in Uranium Dioxide : Thermal Effects versus Radiation Enhanced Effects Yves Pipon1, Nelly Toulhoat1,2, Nathalie Moncoffre1, Nicolas Bererd1, Henri Jaffrezic1, Marie France Barthe3, Pierre Desgardin3, Louis Raimbault4, Andre M. Scheidegger5, and Gaelle Carlot6 1 Université de Lyon / Université Claude Bernard Lyon 1 / IUT A Chimie, CNRS/IN2P3/IPNL, 4 rue Enrico Fermi, Villeurbanne, 69622, France 2 Commissariat a l'Energie Atomique, DEN, CEN Saclay, Gif sur Yvette cedex, 91191, France 3 CNRS, Centre d'Etudes et de Recherches par Irradiation, 3A rue de la Ferollerie, Orleans cedex2, 45071, France 4 Ecole des Mines, Centre d'Informatique Geologique (CIG), 35 rue Saint Honore, Fontainebleau cedex, 77305, France 5 Paul Scherrer Institut, Nuclear Energy and Safety Department (NES), Laboratory for Waste Management, Villigen, 5235, Switzerland 6 Commissariat a l'Energie Atomique, DEN/DEC/SESC/LLCC, Centre de Cadarache, Saint Paul lez Durance, 13108, France
ABSTRACT During reactor operation, chlorine (35Cl), an impurity of the nuclear fuel, is activated into 36 Cl, a long lived mobile isotope. Because of its long half life and its mobility, this isotope may contribute significantly to the instant release fraction under disposal conditions. Thermal annealing of Cl implanted UO2 sintered pellets show that it is mobile from temperatures as low as 1273 K (Ea = 4.3 eV). Chlorine diffusion induced by irradiation with fission products preserves a thermally activated contribution. The radiation induced defects significantly enhance chlorine migration. INTRODUCTION Chlorine is present as an impurity in the nuclear fuel (< 5 ppm). During reactor operation, Cl is activated into 36Cl, a long-lived isotope (T = 3.01.105 years). In the case of interim storage or disposal of the spent fuel, 36Cl may significantly contribute to the instant release of activity, thereby contaminating the bio/geosphere [1]. The evolution of spent nuclear fuel with time before water access to the waste form in a repository must be assessed, because it may influence the subsequent radionuclide release. This study provides information on the migration behavior of chlorine in a sintered depleted uranium dioxide. During in-reactor life, part of the 36Cl may be displaced from its original position, due to recoil or to collisions with fission products. In order to study the behavior of the displaced chlorine, 37Cl has been implanted into UO2 pellets. The thermal effects on the migration of the implanted and initially present (or pristine) Cl (35Cl = 75.78 % and 37Cl = 24.22 %) have been investigated on one hand [2-4]. Moreover, preliminary results on the effects of the irradiation defects induced by fission fragments have been studied [5]. In this work, we discuss the respective effects of temperature and defects (remaining from polishing or induced by 37Cl implantation or heavy ion irradiation) on the migration of Cl. 35
EXPERIMENTAL Sintered depleted UO2 (0.3 % of 235U) pellets have been provided by the CEA Cadarache (8.2 mm in diameter, mean grain size = 18 µm, one face polished to micron, O/U ratio = 2.0051, density = 10.76). The pellets have been annealed at 1673 K for 4 h in a reducing atmosphere (Ar + 5 % H2). 37Cl+ has been implanted into the polished surface at the 400 kV ion implanter of IPNL. 1 mm thick implanted pellets have been implanted at an energy of 270 keV (Rp around 150 nm), and an ion fluence of 1013 ions.cm-2. The maximum 37Cl content at the mean projected range (Rp) is around 6 ppm (atomic). This value, about 60 times higher than the 37Cl content of the pristine Cl, provides a good signal to noise ratio for SIMS analysis. Thermal annealings have been performed on these pellets in secondary vacuum, at a H2 pressure of 10-4 Pa and in the 1273-1473 K temperature range [3]. 200 µm thick pellets have also been used for post Cl implantation irradiations with fission products, performed at different temperatures. This thickness is needed to enable heating by Joule effect during irradiation. The implantation energy of 37Cl was of 360 keV (Rp around 200 nm), and the fluence was of 1013 ions.cm-2. The samples have been irradiated with 63.5 MeV 127I at the 15 MV TANDEM facility of the Institute of Nuclear Physics of Orsay, France. The irradiations have been performed in a dedicated cell (in secondary vacuum, at a H2 pressure of 5.10-4 Pa), at two fluxes (1.5.1010 and 4.5.1010 ions.cm-2.s1 ) and two temperatures (300 and 510 K), with a fixed irradiation fluence (5.1014 ions.cm-2) [5]. The spatial Cl distributions within the samples (as-received, as-implanted, implanted and annealed or irradiated) have been measured by SIMS [3]. The high resolution CAMECA IMS 6f SIMS facility of the “Ecole des Mines de Paris” in Fontainebleau, France was used for SIMS analysis. Positron Annihilation Spectroscopy (PAS), a technique sensitive to open volume defects, was used to investigate the polishing or implantation related defects on as-received and on implanted samples. A 37Cl implantation energy of 800 keV was achieved (Rp around 430 nm) in order to avoid the overlap of the zone where the implantation defects concentration is maximum with the more superficial zone dominated by the defects related to sample preparation. The maximum of the displaced atoms profile calculated with SRIM [6] is located at a depth around 320 nm. The positron-electron pair momentum distribution has been measured at 293 K by recording the Doppler broadening of the 511 keV annihilation line characterized by the low (S) and high (W) momentum annihilation fraction in the energy ranges 511 ± 0.66 keV and 504.36 – 505.43 keV, 513.57 – 517.64 keV respectively. The energy resolution of the Ge detector was 1.72 keV at 1.28 MeV. To investigate the depth dependence of S and W, the slow positron beam [7] at the CNRS/CERI, Orléans, France has been used to record the curves S(E) and W(E) as a function of the positron energy E in the 0.5 to 25 keV range. The positron mean implantation depth varied from 1 nm to 570 ± 250 nm. RESULTS Thermal diffusion We pointed out the ability of the implanted chlorine to migrate [3]. It was shown, that the diffusion starts from temperatures as low as 1273 K. After 10 hours annealing at 1473 K, most of the implanted chlorine has effused from the sample. SIMS mapping shows, that prior to effusion,
the implanted chlorine accumulates near the surface into “hot spots”, whereas the pristine chlorine (represented by 35Cl) remains homogeneously distributed [3]. The pristine chlorine is not affected by the thermal diffusion because it is initially homogeneous in UO2. The general transport equation has been used to deduce a simple model for the migration of the implanted 37 Cl concentration [3]. The apparent diffusion coefficients D have been calculated and plotted in Figure 1. The activation energy Ea and the pre exponential factor D0 of the deduced Arrhenius law are respectively of 4.3 eV and 49 cm2.s-1. A transport term (mean velocity around 3.10-10 cm.s-1 at 1473 K) reflects the accelerated migration of chlorine towards the surface. The drift force could be explained by the vacancy gradient induced by Cl implantation. In order to verify this hypothesis and to understand the trapping of Cl near the surface, slow PAS measurements have been performed. Three types of samples have been compared: i) an as received (or virgin) sample, annealed at 1673 K for 4 h under Ar + H2; ii) two samples annealed (as previously) and successively implanted at two fluences of 2.1013 at.cm-2 and 1016 at.cm-2 at an energy of 800 keV; iii) a reference sample (B23) annealed at 1973 K for 24 h under Ar + H2 and therefore free of polishing defects. Figures 2a and 2b display respectively the low (S) and high (W) momentum annihilation fraction variation plotted against the incident positron energy. In Figure 2c, S values are plotted against W values. The following conclusions can be inferred from these experiments: - the peak on the S curve and corresponding valley on the W curve at low positron energy (0.5 6 keV) shows that annealing at 1673 K of the as received pellets does not remove the vacancies created by polishing (fig. 2a, b) as it has been already demonstrated for other pellets prepared in the same conditions [8]. Positrons still detect large vacancy defects formed by agglomeration of polishing induced vacancies during the 1673 K annealing; - 37Cl implantation results in vacancy defects along the track on the as implanted samples. The shapes of the S and W curves are different after Cl implantation from those of the as received one (fig. 2a, b). First, for positron energy higher than 8 keV, implantation leads to the increase of S related to the decrease of W. It indicates that positrons detect vacancy defects induced by Cl implantation. In the S versus W plot the (S,W) values measured on the as implanted sample are aligned for positron energies in the range 6 - 25 keV (fig. 2c) and this line DVimp goes through the S and W values measured for the B23 reference. It indicates that positron detect the same type of vacancy defects whatever the depth and the fluence in the as-implanted samples. From 16 keV, the S value increases and respectively the W value decreases while the implantation fluence increases, indicating that vacancy defect concentration increases with the Cl fluence. Secondly at low positron energy, the peak on the S curve related to polishing defects is less intense for the low implantation fluence and disappears for the high fluence. It can be explained by the competition for positron trapping between the vacancies resulting from polishing and the vacancies related to implantation. The positron trapping in the polishing related defects can be observed in fig.2c. The (S,W) points corresponding to the positron energies lower that 6 keV, are situated above the line Dvimp indicating their difference of nature. The implantation vacancy defects have a smaller free volume comparable to the one detected in 1 MeV 3He implanted samples [9]. In conclusion, PAS experiments show the existence of two kinds of defects: the high free volume vacancies resulting from polishing and annealing, located at about 20-30 nm under the surface, and deeper vacancies induced by 37Cl implantation, that are different from the former pre existing defects. The implantation vacancies have a smaller free volume but are more abundant. They constitute potential paths allowing a rapid diffusion or transport of Cl towards the surface when annealing is performed. Finally Cl can be trapped into the vacancies of high free volume resulting from polishing near the surface, where it accumulates into “hot spots”, identified by SIMS before its
release at higher temperature or long annealing durations [3]. Complementary µ-XAS analyses, performed at the Cl K-edge (E = 2.822 keV) on the LUCIA beamline (PSI/SLS, Villigen, Switzerland) [4], revealed the existence of chloride and/or oxychloride compounds for the hot spot Cl as well as for the pristine Cl. No Cl gas bubbles have been identified. It is interesting to note that for a high implantation fluence of 1016 at.cm-2, we did not observe any accumulation nor trapping in the high free volume vacancies near the surface [2]. In this case, the quick direct release of the implanted Cl might be related to the concentration of vacancies induced by implantation (the quantity of displaced atoms calculated with SRIM is around 500 times higher for the high implantation fluence than for the low fluence).
Figure 1. Arrhenius plot for the thermal and irradiation induced diffusion of the implanted 37Cl. Data for I, Cs thermal [10-11] and U athermal diffusion [12] are also presented.
Figure 2. Plot of the low (S) and high (W) momentum annihilation fraction versus positron energy (a and b, respectively) and plot of S as a function of W (c) in sintered UO2 pellets.
Diffusion under heavy ion irradiation Previous preliminary results [5] have shown that the main process responsible for the implanted 37Cl migration, at 300 K and 510 K, is diffusion and accumulation near the sample surface. Four apparent diffusion coefficients have been measured and are plotted on figure 1. The same values have been obtained for both 127I fluxes and are about 5.10-15 cm2.s-1 at 300 K and 3.10-14 cm2.s-1 at 510 K. They are of the same order of magnitude as the thermal diffusion coefficients measured at respectively 1373 K and 1473 K [3]. The activation energy is of 0.1 eV. Moreover, at 510 K, a global gain of 24 % (and 80 % in the peak close to the sample surface) has been measured for the chlorine content in the probed depth. In comparison, the gain at 300 K is only of 3 % for the total chlorine content and around 10 % close to the sample surface. This is illustrated in figure 3 which displays the depth profiles of 35Cl and 37Cl as a function of depth, for the highest flux. This means that pristine chlorine has migrated from the bulk towards the surface along the iodine track, and that this effect is enhanced by the temperature. The 35Cl/37Cl isotopic ratio measured near the sample surface is close to the natural one. This means that the 37Cl implanted area is also enriched in 35Cl, and could correspond to the migration of pristine chlorine from the bulk.
Figure 3. a : Plot of the 37Cl depth profiles of the as-implanted and irradiated samples (for T = 300 and 510 K, flux = 4.5.1010 ions.cm-2.s-1, fluence = 5.1014 ions.cm-2); b : 37Cl and 35Cl depth profiles after irradiation (flux = 4.5.1010 ions.cm-2.s-1, T = 510 K). The insert represents the vacancy distributions induced by I irradiation and Cl implantation.
DISCUSSION AND CONCLUSION The comparison of the data obtained on the thermal diffusion of the implanted chlorine with those reported in the literature for mobile fission products such as cesium or iodine (Figure 1), shows the greater thermal mobility of chlorine in uranium dioxide. Considering in first approximation that the diffusion length L can be expressed as a function of the diffusion coefficient D and time t by : L = (D.t)1/2, and taking D at 1473 K around 3.10-14 cm2.s-1, L ∼ 17 µm after 3 years. Note that the role of the transport over time is still an unresolved issue and will therefore not be discussed here. Under fission product irradiation, the diffusion coefficient, that preserves a thermally activated contribution, is of the same order at 510 K. It results that there is a great probability for the chlorine contained in the UO2 grains to have reached the grain
boundaries after 3 years, in the core of the fuel rod as well as at its periphery. The migration of chlorine to the grain boundaries where it concentrates has been shown in [3]. The behavior of the pristine chlorine under irradiation needs to be confirmed. However, in a first approach, it seems that fission product irradiation induces a high mobility of the pristine chlorine at 510 K. Indeed, a great quantity of vacancies are produced along the fission product track (figure 3). The “thermal spike” associated to each fission product induces a strong heating along its path, with the simultaneous formation of a thermoelastic pressure field, and contributes to the formation of defect clusters [13]. As Frenkel anion type of disorder dominates in UO2+x [14], the diffusion of oxygen vacancies/chlorine pairs is probably increased by the thermal and mechanical effects. In storage or disposal conditions, diffusion will be activated by self irradiation processes due to recoil nuclei. The accumulation of damage due to the recoil nuclei will probably play a less important role than that of the fission products in activating diffusion. All these results indicate therefore, that, during reactor operation and after, the majority of 36Cl is likely to have moved to grain boundaries, rim and gap. This fraction might then significantly contribute to the rapid or instant release of chlorine. This could have important consequences for safety assessment. REFERENCES 1.
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