Engineering Oxidation-Resistant Uranium Mononitride Surfaces via Thermodynamically Stable (111) Facets

Uranium mononitride (UN) is a promising advanced nuclear fuel, yet its deployment is hindered by rapid oxidation in hydrothermal accident scenarios. Current mitigation strategies compromise its advantageous properties. Here, using first-principles calculations within the framework of Hubbard-corrected density-functional theory, we reveal a facet-dependent oxidation resistance in UN that provides an intrinsic solution. We find that the N-terminated (111) surface is thermodynamically dominant over the commonly studied (100) facet under ambient conditions, exhibiting a remarkably low surface energy of 0.36 J/m². More critically, oxygen adsorption is thermodynamically forbidden on this (111) facet (adsorption energy: +0.68 eV), in stark contrast to the strongly binding (100) surface (-1.57 eV). This inertness originates from a surface reconstruction that strengthens U-N bonding and induces a U⁵⁺-like electronic state. Our work establishes a foundational design principle: engineering a strong ⟨111⟩ texture is the key to fabricating oxidation-resistant UN fuels without sacrificing performance.