Investigation of molybdenum and caesium behaviour in urania by ab initio calculations

A theoretical study of molybdenum and caesium solution in uranium dioxide is carried out. Calculations are performed using the density functional theory with the projector-augmented-wave method as implemented in the Vienna ab initio simulation package (VASP). Correlation effects are taken into account within the DFT+U approach. Molybdenum is preferentially inserted in uranium-oxygen divacancies for understoichiometric urania and uranium vacancies for overstoichiometric urania. The favourable sites for caesium solution are the Schottky defect for understoichiometric urania and U vacancies and U-O divacancies for overstoichiometric urania. Using the stability of many binary and ternary compounds in comparison to soluted atoms, we show that caesium and molybdenum are insoluble in uranium dioxide whatever the stoichiometric regime.