Atomic insights into the initial oxidation of uranium and the protective role of oxide layer

The pursuit for long-term uranium storage, handling and disposal of spent nuclear fuels has driven extensive explorations on uranium oxidation over the past decades. Experimentally, the oxidative kinetics of different stages were proposed, but an atomic insight into the oxidation process remains elusive. In this work, by combining particle swarm optimization algorithm with first-principles calculations, we systematically investigate the initial oxidation behaviors of γ-U(001) surface and the physicochemical properties of the grown oxide layer. It is found that the oxidation initiates with the chemisorption of O on the surface, and with increased O coverage, O atoms penetrate into the surface that ultimately result in the formation of a well-ordered, compact UO₂(111) film. Calculations on the adsorption of O₂ reveal that compared to pure γ-U(001) surface, the growth of a UO₂(111) film greatly weakens the molecule-surface interaction. Remarkably, a distinct chemisorption-to-physisorption transition is revealed with increasing thickness of the oxide layer, highlighting its protective role against oxidation. Effects of the defect in oxide are also discussed, and the underlying electronic properties are analyzed. Our work not only provides fundamental insights into the initial oxidation of uranium in the context of nuclear industry, but also sheds light on future design and fabrication of corrosion-resistant metals/alloys for diverse applications.