THE BEHAVIOR OF THE NOBLE-GASES IN ALPHA-U3O8

Atomistic simulation techniques have been applied to the study of noble gas behaviour in alpha-U3O8. In stoichiometric U3O8 the most stable solution site for all noble gas atoms is the nine-coordinated interstitial site. However, in oxygen deficient U3O8-x solution is likely at either the nine-coordinated, O(3), or O(5) interstitial sites which are adjacent to an appropriate oxygen vacancy. We note that the calculated solution energies for the noble gases in U3O8-x are higher than those predicted for UO2+x but lower than those for UO2 or UO2-x. Migration of noble gas atoms occurs parallel to the O-U planes and, in U3O8-x, is assisted by plane oxygen vacancies; the activation energy for Xe migration is about 3.75 eV. We have also considered a migration mechanism assisted by chain oxygen vacancies which, due to their high formation energy, would only be formed by structural radiation damage. This radiation damage-assisted mechanism yields the much lower activation energy for Xe diffusion of 1.3 eV, which is in much better agreement with experimental results on irradiated samples.