Ab initio study of solution energy and diffusion of caesium in uranium dioxide

The behaviour of caesium in nuclear fuels is investigated using density functional theory (DFT). In a first step, the incorporation and solution energies of Cs in pre-existing trap sites of UO2 (vacancies, interstitials, U–O di-vacancy and Schottky trio defects) are calculated using the projector-augmented-wave (PAW) derived pseudopotentials as implemented in the Vienna ab initio simulation package (VASP). Correlation effects are taken into account within the DFT + U approach. The solubility of caesium is found to be very low, in agreement with experimental data. The migration of Cs is found to be highly anisotropic, it is controlled by uranium diffusion with an Arrhenius activation energy of 4.8 eV in hyperstoichiometric UO2+x, in good agreement with experimental values.