Ab initio Study of Point Defects in Uranium Oxides

Uranium dioxide (UO₂) is naturally occurring, which is also a central component in the production of nuclear energy. Although highly studied, there are still questions regarding the degradation process of UO₂ when exposed to the environment (i.e., humidity, oxygen-rich conditions, high pressure, heat, radiation, etc). We carried out density functional theory (DFT) calculations to investigate the structural and chemical changes in UO₂. In order to see the path taken, we introduced different types of defects within the baseline UO₂ structure. Those defects included Frenkel, Schottky, vacancy, and interstitial, all of which we tested independently. Among the point defects we investigated, we found that the interstitial defect of oxygen had the lowest formation energy and occurred spontaneously leading to the formation of U₄O₉. This formation is consistent with previous theoretical and experimental findings. Further oxidation to U₄O₁₀ may occur under oxygen-rich conditions. The structure that provided the best results was a combination of defects (interstitial and Schottky) leading to a U₃O₈ structure, corroborating previous experimental observations.