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International audience; • SIMS and TEM techniques were combined to compare the thermal behaviour of Cs and Xe in UO 2 . • Both elements form bubbles with different growth kinetics. • At 1600 °C, caesium is found to be highly mobile in the UO 2 matrix while Xe distribution does not evolve.

International audience; This study aims to evaluate the influence of a metallic fission product, molybdenum, on the behaviour of caesium in UO2 at high temperature (1600°C) under reducing atmosphere. Both elements were successively introduced by ion implantation (firstly Mo, then Cs) at room temperature at a fluence of 1016 ions/cm². We show that in these co-implanted samples, the defects created by the Mo implantation render possible the nucleation of nanometric size bubbles during Cs implantation at room temperature. SIMS and TEM techniques were coupled in order to characterize the migration of both elements, as well as the UO2 microstructure evolution after annealing at 1600°C. We found that Mo is more mobile in presence of Cs, which may be related to an increase of available vacancies in the material produced by Cs implantation. On the contrary, the concentration profiles of Cs remain quite similar than when it is solely implanted, and its release percentage remain the same both in absence and presence of Mo. After annealing, both elements are found to be associated as Mo metallic precipitates and Cs bubbles distributed over the same depth (~150 nm). Comparing with the Cs bubbles distribution in absence of Mo, it is clear that the formation of Mo metallic precipitates hinders the Cs bubble migration. Our study highlights the role of dislocations on the nucleation of bubbles and metallic precipitates.

Molybdenum is an abundant element produced by fission in the nuclear fuel UO2 in a pressurized water reactor. Although its radiotoxicity is low, this element has a key role on the fuel oxidation and other fission products migration, in particular in the case of an accidental scenario. This study aims to characterize the behavior of molybdenum in uranium dioxide as a function of environmental conditions (oxygen partial pressure, high temperature, UO2 oxidation) typical of an accidental scenario. To do so, molybdenum was introduced in UO2 or UO2+ x pellets by ion implantation, a technique that allows us to mimic the production of Mo in the nuclear fuel by fission. Then, thermal treatments at high temperature and different oxygen partial pressures were carried out. The mobility of Mo in UOX samples was followed by secondary ion mass spectrometry (SIMS), while the Mo chemical speciation was investigated by spectroscopic techniques (XANES, Raman). In parallel, ab initio calculations were performed showing the effect of interstitial oxygen atoms on the Mo incorporation sites in UO2. We show that the Mo mobility is directly connected to its chemical state, which in turn, is linked to the redox conditions. Indeed, under reducing atmosphere, Mo is present in UO2 or UO2+ x samples under a metallic state Mo(0). Its mobility, being quite low, is driven by a diffusion mechanism. An increase of pO2 entails the UO2 and Mo oxidation and, as a consequence, a strong release of this element. We show an increase of the Mo release rate with the increase of the UO2+ x hyper-stoichiometry x. After thermal treatment, Mo remaining in the samples is located in the grains under the MoO2 form. Our experimental results are assessed by ab initio calculations showing that in the presence of oxygen Mo atoms adopt in UO2 a local structure close to the octahedral local geometry of Mo oxides.