Modeling oxygen-void interactions in uranium nitride

TL;DR

This study develops a first-principles model to understand how oxygen impurities interact with voids in uranium nitride, revealing their role in reducing surface energy and promoting swelling under irradiation.

Contribution

The paper introduces a novel first-principles approach to quantify oxygen-void interactions in uranium nitride, elucidating their impact on swelling behavior.

Findings

Oxygen segregation at surface nitrogen sites significantly reduces surface energy.

Surface energy reduction is most pronounced for small cavities at intermediate temperatures.

Oxygen-induced surface energy reduction is crucial for explaining experimental swelling observations.

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. 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 ($R_v$ = 1--10 nm) at intermediate temperatures that coincide with the onset of 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 both oxygen concentration and cavity size, but is largely insensitive to porosity. Our results suggest that oxygen-induced surface energy reduction is essential for reconciling the mechanistic swelling model of UN with experimental observations.