Initial oxidation behavior of α-U and γ-U surfaces

Uranium (U) is widely used in the nuclear industry and exhibits poor oxidation behavior in the atmosphere due to its high surface activity. However, the limited understanding of the initial oxidation process at the atomistic scale hinders the improvement of U oxidation resistance. Here, the interactions between O atoms and different facets of α-U and γ-U are systematically investigated by using density functional theory (DFT) calculations, and the relationships between energies and configurations are analyzed under high O coverage. O2 molecule is found to spontaneously dissociate on the α-U and γ-U surfaces, due to the strong O-U chemical interaction, the high coverage of O atoms over the surfaces is easily reached, meanwhile, the surface relaxations of U atoms are site-dependent. The initial oxidation layer and high-valent uranium oxide can be formed with an increase of the O coverage (> 1 ML), where the pre-adsorbed O atoms penetrate the surfaces at the initial oxidation stage. Further, the initial oxidation layers are comparable with the single layer of UO2 surfaces (e.g., (100) and (110)). These results indicate that the initial oxidation of α-U and γ-U surfaces are facet-independent, suggesting a facet-controlled strategy for improving the oxidation resistance of U metal could not work. In addition, the effect of surface strain and twin structure on the tuning of the initial oxidation of U surfaces are discussed. Our results provide new insights into the understanding of initial oxidative corrosion of U surfaces.