Atomistic simulation techniques were used to study the behavior of noble gases in the α-phase. In stoichiometric samples, the most stable solution site for all noble-gas atoms was the 9-coordinated interstitial site. However, in O-deficient U3O8-x, dissolution was likely to occur at either the 9-coordinated, O(3), or at the O(5) interstitial sites which were adjacent to a suitable O vacancy. It was found that the calculated solution energies for noble gases in U3O8-x were higher than those predicted for UO2+x, but were lower than those for UO2 or UO2-x. The migration of noble-gas atoms occurred parallel to the O-U planes and, in U3O8-x, was assisted by plane O vacancies. The activation energy for Xe migration was equal to about 3.75eV. A migration mechanism which was assisted by chain O vacancies was also considered. These vacancies, due to their high formation energy, could only be created by structural radiation damage. This radiation damage-assisted mechanism gave a much lower activation energy for Xe diffusion (1.3eV). This result was in much better agreement with experimental data on irradiated samples.

R.G.J.Ball, R.W.Grimes: Philosophical Magazine A, 1992, 66[3], 473-90