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PRINCIPAL PUBLICATION AND AUTHORS

Evolution of the perovskite phase in UO2-based samples under conditions representative of a severe nuclear accident by XANES, C. Le Gall (a), V. Klosek (a), F. Audubert (a), J. Léchelle (a), D. Drouan (a), J. C. Richaud (a), Y. Pontillon (a), M.O.J.Y. Hunault (b), P.L. Solari (b), M. Rovezzi (c), J.L. Hazemann (d), J. Solid State Chem. 319, 123792 (2022); https:/doi.org/10.1016/j.jssc.2022.123792 (a) CEA, DES, IRESNE, DEC, Cadarache (France) (b) Synchrotron SOLEIL, Gif-sur-Yvette (France) (c) OSUG, UAR 832 CNRS-Université Grenoble Alpes, Grenoble (France) (d) Institut Néel, UPR 2940 CNRS-Université Grenoble Alpes, Grenoble (France)

REFERENCES

[1] E. Geiger, PhD thesis, Université Paris-Saclay (2016). [2] H. Kleykamp, J. Nucl. Mater. 131, 221-246 (1985). [3] H. Kleykamp, J. Nucl. Mater. 206, 82-86 (1993). [4] E. Geiger et al., J. Nucl. Mater. 471, 25-33 (2016). [5] E. Geiger et al., J. Nucl. Mater. 495, 49-57 (2017). [6] M. Barrachin et al., J. Nucl. Mater. 453, 340-354 (2014).

form MoO2 (Figure 118). Mo depletion is also observed at temperatures as low as 900°C in the Mo Ru Rh Pd precipitates, which confirms Mo behaviour calculated by thermodynamics and observed in irradiated fuels [1-3]. In the same range of temperature, a reaction between Mo and Ba at the periphery of the oxide precipitates leads to the partial reaction of (Ba, Sr)ZrO3 with MoO2, leading to BaMoO4, ZrO2, and probably SrMoO4. This behaviour is also in agreement with thermodynamic calculations and with the experimental observations on spent fuels and in the frame of integral tests [2, 4-6].

An enhanced diffusion of Mo in these conditions should kinetically favour this reaction: at high temperature, in oxidising conditions, metallic Mo oxidises as MoO2, which would first dissolve in the UO2+x matrix, allowing Mo atoms to migrate to the periphery of the oxide precipitates driven by a gradient of O concentration (Figure 119). Mo would then react with the Ba contained in the perovskite phase, forming first BaMoO3 and finally BaMoO4.

In conclusion, a combination of XANES and SEM/EDX on SIMFUEL samples highlighted the reaction between Mo from the white metallic inclusions and Ba from the oxide phases and confirmed the formation of BaMoO4 described in the mechanisms proposed in the early stage of a severe nuclear accident sequence. These results will be useful to validate the theoretical models used in calculation codes and to assess thermodynamic databases, which are one of the main sources of errors when calculating a severe nuclear accident sequence involving molten fuel.

Fig. 119: Schematic mechanism for the evolution of the metallic and oxide precipitates at ΔGO2 = 290 kJ.mol(O2)-1 at 1000°C.