Uranium nitride (UN) is considered a promising candidate material for advanced nuclear fuels in Generation IV reactors due to its high thermal conductivity and excellent fission product retention capability. However, the surface corrosion behavior of UN in oxygen-containing environments severely limits its practical application. In this work, first-principles calculations based on density functional theory (DFT) were employed to systematically investigate the adsorption and dissociation behaviors of O₂ molecules and O atoms on the UN(100), (110), and (111) surfaces. The electronic structure mechanisms underlying the adsorption were elucidated through analysis of surface work function, projected density of states (PDOS), Bader charge, and charge density difference. The calculated results show that O₂ molecules undergo thermodynamically highly favorable dissociative chemisorption on all three UN surfaces at 0 K, with the O–O bond length stretched to 1.44–1.50 Å, characteristic of a peroxo-like (O₂²⁻) species, which is intermediate between the superoxide (O₂⁻, ~1.33 Å) and complete dissociation. The most stable adsorption configurations on each surface are: (100)-H site (−4.05 eV), (110)-B(2) site (−4.81 eV), and (111)-H site (−5.03 eV), with the adsorption strength governed by the surface coordination environment. The adsorption energies of O atoms (−4.5 to −8.2 eV) are significantly higher than those of O₂ molecules. The order of the most stable O atom adsorption sites is (110)-B(2) site (−8.22 eV) > (100)-H site (−6.97 eV) > (111)-T(U) site (−5.09 eV), which differs from that of O₂, revealing the efficient O atom trapping effect of the groove structure on the (110) surface. Electronic structure analysis indicates that upon O atom adsorption, the work functions of the (100) and (110) surfaces increase by 0.62 eV and 0.39 eV, respectively, while that of the (111) surface decreases by 0.57 eV, suggesting that the (111) surface is more susceptible to further oxidation. In terms of bonding mechanisms, the interaction between O and U is primarily dominated by p–d hybridization between O-2p and U-6d orbitals. The U-5f orbital participates indirectly through f–d coupling with U-6d, while direct p–f hybridization is enhanced in the case of isolated O atom adsorption. This study provides an atomic-scale theoretical basis for understanding the initial oxidation mechanism of UN surfaces and offers important guidance for the surface protection design of UN-based nuclear fuels.

Metallic uranium has important applications in nuclear fuels, nuclear materials, and related engineering fields because of its distinctive nuclear physical properties. However, its high chemical reactivity makes it susceptible to hydriding corrosion in hydrogen-containing environments. The substantial volumetric expansion associated with hydride formation, together with the resulting cracking and pulverization, severely compromises structural integrity and poses significant risks to long-term storage and safe service. Focusing on the role of grain boundaries in the hydriding corrosion of metallic uranium, this review summarizes current understanding of hydrogen ingress through the surface oxide layer, hydrogen segregation and migration at grain boundaries, and the regulation of these processes by elemental segregation. Defects and grain boundaries within the oxide layer can provide preferential pathways for hydrogen transport. In the metallic matrix, local structural distortion, excess free volume, and characteristic electronic structures at grain boundaries promote hydrogen trapping and accumulation, potentially facilitating hydride nucleation and intergranular damage by weakening interfacial cohesion. Metallic solutes affect the hydriding process primarily by modifying phase stability, hydrogen accumulation, and grain boundary cohesion, whereas the effects of non-metallic impurities depend on their states of occurrence, segregation tendencies, and co-segregation interactions with hydrogen. Grain boundary structure and local chemistry therefore jointly govern hydrogen ingress, accumulation, and damage evolution, making them central to understanding and regulating localized hydriding corrosion in metallic uranium.