Atomic scale simulation, based upon empirical potentials, was used to obtain insight into the helium diffusion. By varying the stoichiometry, together with the system temperature, the molecular dynamics simulations revealed two diffusion regimes. The first one involved a low activation energy (0.5eV) and it was suggested that oxygen vacancies assisted migration. This regime appeared to provide the main contribution to diffusion when structural defects (extrinsic defects imposed, for instance, by the stoichiometry) were present. The second regime involved a higher activation energy (about 2eV) and predominated in the higher temperature range, or for perfect stoichiometry; suggesting an intrinsic migration process. Oxygen diffusion was also considered in the differing stoichiometry domains, considering the dependence of He behaviour upon oxygen defects. Further investigations were made, using nudged elastic band calculations, to obtain a better understanding of the migration mechanisms operating for He and O.

Molecular Dynamics Simulation of Helium and Oxygen Diffusion in UO2± x. Govers, K., Lemehov, S., Hou, M., Verwerft, M.: Journal of Nuclear Materials, 2009, 395[1-3], 131-9