High local oxygen coverage causes initial oxidation of UN(001) surface

Uranium mononitride (UN) is a promising nuclear fuel, however, during its storage and operation, the surface oxidation or the initial corrosion of UN always takes place. The limited understanding of atomistic corrosion mechanism hinders the improvement of UN corrosion resistance. Here, the interactions between adsorbates (O₂ and H₂O molecules and O adatoms) and the pristine UN(001) surface, the UN(001) surface with N vacancies (N_vac_UN(001)), and the UN(001) surface with N vacancies occupied by O atoms (O2N_UN(001)) are investigated by using density functional theory calculations. O₂ molecule is found to spontaneously dissociate on these surfaces, while the H₂O molecule is weakly adsorbed on the top site of U atom. Due to the attractive O[sbnd]O lateral interactions, high local O coverage (e.g., 4O coordinated to a U atom) is easily resulted, where significant relaxation of surface U atoms is observed. The maximum vertical displacement of U atoms is more than 1 Å at medium O coverage (e.g., 3O coordinated to a U atom). N vacancy and O-doping on the UN(001) surface do not change the vertical displacement of U atoms, but alter the adsorption energies of O atoms. The competition between surface U-O and subsurface U-N interactions leads to the formation of uranium cavities in the surface, preluding the dissolution as oxides of U atoms away from the surface. Our results provide new insights for the understanding of initial oxidative corrosion of UN surface.