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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.