U and Xe transport in UO2±x: Density functional theory calculations

The detrimental effects of the fission gas Xe on the performance of oxide nuclear fuels are well known. However, less well known are the mechanisms that govern fission gas evolution. Here, to better understand bulk Xe behavior (diffusion mechanisms) in UO2 +/- x we calculate the relevant activation energies using density functional theory techniques. By analyzing a combination of Xe solution thermodynamics, migration barriers, and the interaction of dissolved Xe atoms with U, we demonstrate that Xe diffusion predominantly occurs via a vacancy-mediated mechanism. Since Xe transport is closely related to the diffusion of U vacancies, we have also studied the activation energy for this process. To best reproduce experimental data for the Xe and U activation energies, it is critical to consider the active charge-compensation mechanism for intrinsic defects in UO2 +/- x. Due to the high thermodynamic cost of reducing U4+ ions, any defect formation occurring at a fixed composition, i.e., no change in UO2 +/- x stoichiometry, always avoids such reactions, which, for example, implies that the ground-state configuration of an O Frenkel pair in UO2 does not involve any explicit local reduction (oxidation) of U ions at the O vacancy (interstitial).