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Clotilde Gaillard1 Lola Sarrasin1 Clémentine Panetier1 Yves Pipon1 2 Nathalie Moncoffre1

1, Institut de Physique Nucléaire de Lyon (CNRS), Villeurbanne, , France
2, Université de Lyon, Villeurbanne, , France

In case of a nuclear reactor accident, one of the biggest problems in term of safety is the release of radioactive elements in the environment. It is thus important to be able to evaluate the nature, quantity and release kinetics of these elements from the nuclear fuel UO2. Our work is focused on two fission products, caesium and molybdenum, which are abundant and whose behaviour is suspected to be interconnected to each other and to the fuel chemistry. Mo is chemically reactive and is present in the UO2 fuel under several chemical forms, metallic, oxides or as compounds with other FP like Cs. Moreover, it has a direct influence on the UO2 oxidation and it is often described as a buffer to the fuel oxidation. Cs is a volatile species whose release rate may be influence by the presence of Mo in the fuel.
The aim of our work is to study the effect of high temperature and irradiation on the release of Cs and Mo from uranium dioxide. In particular, we investigate the effect of the fuel stoichiometry on the migration of Mo and Cs and also, on the behaviour of the matrix under these extreme conditions. To do so, UO2+x pellets of different stoichiometry are prepared by wet oxidation of UO2. Then, Cs and Mo are introduced, either separately or together, by ion implantation in UO2 or UO2+x samples at a mean concentration ranging from 0.1 to 1 at%. High temperature treatments at 1600°C or irradiations are then led. Different irradiation regimes are studied, involving the production of different kind of defects in the material: ballistic defects, electronic defects or a combination of ballistic and electronic defects. The latter case is representative of what occurs in the fuel during the FP production. After treatments, Mo and Cs concentration profiles in samples are obtained by SIMS (Secondary Ions Mass Spectrometry) in order to measure the element release and if possible, their diffusion coefficients. X-ray absorption spectroscopy is used to characterize the Mo chemical form in the matrix. In parallel, an investigation of the microstructure evolution of UO2 and UO2+x is followed by combination of µ-Raman spectroscopy and X-ray diffraction.
On the FP behaviour, we confirm the strong correlation between Mo and Cs migration when these two elements are present at the same time. We also show that the combined effect of irradiation and temperature leads to a species migration, especially in high electronic stopping regime. This can be relied to the defects created in the matrix by the irradiation and can be explained by the thermal spike model. The effect of the UO2 hyper-stoichiometry will be discussed, in term of FP release and matrix micro-structure resistance to irradiation.

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