Modeling oxygen-void interactions in uranium nitride

Abstract

Oxygen impurities in uranium nitride (UN) are reported to influence its swelling behavior under irradiation, yet the underlying mechanism remains unknown. In this work, we develop a first-principles model that quantifies the interaction of oxygen with voids and fission gas bubbles in UN, leading to a reduction in surface energy that can promote swelling. As an approximation, surface energetics are obtained from planar (001) slab calculations and applied to curved void surfaces. The analysis reveals that segregation of substitutional oxygen at surface nitrogen sites is the primary driver of surface energy reduction, $|\Delta \sigma |$, while oxygen in surface hollow sites plays a minor and sometimes counteracting role. $|\Delta \sigma |$ is most pronounced for small cavities ($R_v\lesssim$ a few nm) at intermediate temperatures that overlap the reported onset range for breakaway swelling in UN. Larger voids require higher temperatures for oxygen adsorption to significantly lower their surface energy. The temperature dependence of $|\Delta \sigma |$ exhibits three regimes: negligible reduction at low temperatures due to sluggish oxygen diffusion, a maximum at intermediate temperatures where oxygen incorporation is optimal, and a decline at high temperatures due to enhanced bulk solubility. A parametric analysis reveals that $|\Delta \sigma |$ depends strongly on oxygen concentration and cavity size, while its porosity dependence remains bounded because the geometric porosity prefactor is partly offset by the surface-site concentration. Our results suggest that oxygen-induced surface energy reduction is essential for reconciling the mechanistic swelling model of UN with experimental observations.