The knowledge of the behaviour of fission products (FP) in the nuclear fuel is very important not only for nuclear safety considerations but also for understanding the evolution of the fuel properties under irradiation.
Due to the difficulty of performing and interpreting experiments on the nuclear fuel in extreme conditions, theoretical predictions are of special interest. In this work, we have focused mainly on the behaviour of caesium in UO2 through ab initio electronic structure calculations. The solubility of caesium at point defects in the matrix and its diffusion have been studied. We have also studied the relative stability of caesium dissolved in the fuel in comparison to the solid precipitates. The role of electronic correlation effects of the f electrons of uranium on these properties is particularly emphasized, its importance is first tested on the description of the defect free UO2 crystal. The formation energies of the main point defects in UO2 are calculated by ab initio methods while their concentration as a function of fuel stoechiometry and temperature is estimated within the point defect model. The migration barriers and migration paths for the self-diffusion of oxygen and uranium vacancies and oxygen interstitials in UO2 are evaluated and discussed. The solubility of caesium is found to be very low in UO2 in agreement with experimental measurements. The most favourable trapping sites are determined as a function of oxygen concentration in the fuel. Our results show that in the hyper-stoichiometric regime, the diffusion of caesium from its most favourable trapping site is limited by the uranium vacancy diffusion mechanism. We also considered the formation of the main solid phases of caesium resulting from its oxidation (Cs2O, Cs2O2 et CsO2) and from its interaction with the fuel (Cs2UO4), with the FP molybdenum (Cs2MoO4) and with zirconium (Cs2ZrO3). The possible formation of such phases, their solubility and their interdependence will affect the release behaviour of caesium.