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, however, 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 firstprinciples 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, include stoichiometric U₂N₃, hypo-stoichiometric U₂N_2.6, and hyperstoichiometric 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 600ௗ°C.
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 U₂N₃.4), 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, introduction of oxygen increases the oxidation states of U and N, with U being more sensitive. Kinetically, the diffusion coefficients of both N and O in U₂N₃O_x gradually decrease as the oxygen content increases, although the decrease is modest and within the same order of magnitude (~10^-8 cm²/s). Oxygen diffuses slightly faster than nitrogen. In contrast, UN exhibits complex three-dimensional migration pathways involving both <100> and <110> mechanisms, while U₂N₃ shows a much simpler vacancy-diffusion mechanism strictly along the <100> 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.