Study of the atomic rare gas behaviour by ab initio calculations

The atomic behaviour of helium and krypton in uranium dioxide has been studied using an ab initio simulation technique. Incorporation energies and solution energies of these two rare gases have been calculated. Krypton atoms are found to be insoluble in this nuclear fuel whatever the trap considered and their presence in the lattice induces swelling when they are located in interstitial sites or in oxygen vacancies. Due to its small atomic size, the predicted helium behaviour is very different. Indeed, helium is found to be soluble in stoichiometric and hyperstoichiometric uranium dioxide in the presence of uranium vacancies or divacancies constituted by one uranium and one oxygen vacancy. Moreover helium atoms induce a lattice parameter contraction except in interstitial sites where a slight expansion is calculated.