Uranium nitride (UN) is a promising nuclear fuel for space reactors and accident-tolerant fuel (ATF) applications due to its high uranium density, high thermal conductivity, and high melting point. However, its rapid oxidation in oxygen-containing environments severely limits its practical use. Experimental observations have shown that UN oxidation proceeds through a U₂N₃ transition phase before eventually forming UO₂, but the underlying atomic-scale mechanisms have not been fully elucidated.

In this work, we systematically investigate the initial oxidation behavior of UN, the metastable structural characteristics of U₂N₃, and the energetic and kinetic evolution during oxidation by combining first-principles density functional theory (DFT) and molecular dynamics (MD) simulations. UN surface models are constructed to study oxygen adsorption and vacancy migration using the nudged elastic band (NEB) method. For U₂N_3±x, we examine three representative compositions, including stoichiometric U₂N₃, hypo-stoichiometric U₂N_2.6, and hyper-stoichiometric U₂N_3.4. The randomness of point defects is quantified by the coefficient of variation, and the formation energies are calculated to assess thermodynamic stability. Diffusion coefficients of N and O are computed using mean square displacement (MSD) for U₂N₃O_x system at 600 ℃.

Our results show that the UN surface acts as a trap for nitrogen vacancies. Additionally, the diffusion of oxygen atoms into the bulk through a vacancy-mediated mechanism is kinetically feasible. Within the investigated compositions (U₂N₃, U₂N_2.6 and U₂N_3.4), all exhibit high configurational flexibility and rich metastability. In U₂N_2.6, the additional structural vacancies prefer a random distribution over clustering. In U₂N_3.4, the interstitial N atoms show no preference for specific positions and are highly flexible. Thermodynamically, the stability of U₂N₃O_x increases monotonically with oxygen content, indicating that the oxidation process is spontaneous. Electronically, oxygen introduction increases the oxidation states of both U and N, with U being more sensitive. Kinetically, the diffusion coefficients of both N and O in U₂N₃O_x decrease gradually with increasing oxygen content, although the decrease is modest and remains within the same order of magnitude (approximately 10^–8 cm²/s). Oxygen diffuses slightly faster than nitrogen. In contrast, UN exhibits complex three-dimensional migration pathways involving both \langle 100\rangle and dual-atom cooperative \langle 100\rangle migration mechanisms, while U₂N₃ shows a much simpler vacancy-diffusion mechanism strictly along the \langle 100\rangle direction due to its regular network of structural vacancies.

Collectively, our findings elucidate that the widespread presence of U₂N₃ during the initial stage of UN oxidation arises not from its thermodynamic stability, but from its exceptional configurational flexibility, abundant metastable states, and low migration barriers. This work provides a microscopic understanding of the transition role of U₂N₃ in UN oxidation and offers atomic-scale parameters for developing oxidation kinetic models of UN fuels.