Electronic properties and stability of interstitial oxygen in UO₂ grain boundaries: An ab initio study

The oxidation of UO₂ is primarily governed by the diffusion of oxygen through the lattice. Oxygen diffusion is significantly influenced by defects and interfaces, with grain boundaries being particularly relevant in spent nuclear fuel due to their increasing concentration at the periphery of fuel rods. While experimental studies on bulk uranium suggest defects enhance oxygen diffusion, the role of grain boundaries remains contentious, with discrepancies between theoretical predictions of enhanced diffusion and experimental observations. This study employs density functional theory (DFT+U) to investigate the electronic properties and stability of interstitial oxygen in two coincident site lattice grain boundaries, Σ3 {111} and Σ5 {210}, in UO₂. We compare stoichiometric and non-stoichiometric grain boundary models, examining their formation energies, defect interactions, and local structural distortions. The interstitial oxygen defects cause an expansion of the oxygen cage and a contraction of the U-O bonds, both in bulk and at the grain boundaries. The Σ3 grain boundary showed potential for defect accumulation, while the Σ5 grain boundary did not demonstrate reduced defect formation energies relative to the bulk. Our findings contribute to the understanding the UO₂ oxidation process, in an attempt to address inconsistencies between theoretical and experimental studies on oxygen diffusion in grain boundaries.