First-principles study of oxygen atom interactions with pristine and defective UN2 and UN(001), (110), and (111) surfaces

Understanding oxygen interactions with uranium nitride surfaces is essential for evaluating the corrosion resistance as accident-tolerant fuels. We systematically investigate oxygen adsorption and diffusion on pristine and defective low-index UN 2 and UN surfaces by first-principles method. The (001) surface exhibits the strongest oxygen adsorption in both systems, with the bridge site as the most stable configuration. The most favorable adsorption sites on the UN 2 (110) and (111) surfaces and UN(110) and (111) surfaces, are the U-top, B-U, bridge and fcc sites, respectively. Defects significantly enhance adsorption by reconstructing the local coordination and modulating the electronic structure. Diffusion analysis reveals that the B2(vacancy) →vacancy path on the N vac_ UN 2 (001) surface exhibits the highest diffusion coefficient with an energy barrier of 0.38eV in the UN 2 surfaces whereas the B(replace)→B1(replace) pathway on the O sub_ UN(001) surface demonstrates the highest diffusion coefficient, associated with an energy barrier of 0.698 eV in the UN surfaces. Oxygen atom accumulates at defect sites due to limited outward mobility, promoting the formation of an oxygen-rich surface layer. This study quantifies the influence of crystallographic orientation and defects on oxygen behavior in UN 2 and UN, providing theoretical insights and engineering guidance for designing oxidation-resistant coatings and assessing fuel-performance safety.