High-Temperature Oxidation Mechanism of Uranium Mononitride: A Study from Ab Initio Molecular Dynamics

Uranium mononitride (UN) is the promising advanced fuel for future reactors. However, the surface oxidation of metallic UN might jeopardize its safety and utility in storage and in use. The knowledge of the initial oxidation behavior of UN has been limited by now. Based on the van der Waals density functional (vdW-DF) scheme, we have carried out the ab initio molecular dynamics (AIMD) simulations to explore the surface oxidation dynamics and mechanism of UN. Our studies have revealed that UN can be readily oxidized due to the strong affinity of U and O atoms. During the initial oxidation at high temperatures, U will segregate out of UN surface to bond with O, forming an ultrathin uniform UO₂ film. The residual N atoms will migrate into the deep layers of UN driven by high temperatures. The current theoretical conclusions have strong implications to the interpretation of experimental observations.