Extended defects-enhanced oxygen diffusion in ThO2

TL;DR

This study uses molecular dynamics simulations to demonstrate that extended defects such as dislocations, grain boundaries, and voids significantly enhance oxygen self-diffusion in ThO2, a nuclear fuel material, by lowering activation barriers.

Contribution

The paper provides a detailed quantitative analysis of how different types of extended defects influence oxygen diffusion in ThO2, highlighting the prominent role of grain boundaries.

Findings

Grain boundaries, especially Σ3 twin boundaries, greatly increase oxygen diffusion.

Extended defects reduce the activation barrier for oxygen migration.

Oxygen transport is significantly enhanced within nanometers of defects.

Abstract

Oxygen self-diffusion is key to understanding stoichiometry and defect structures in oxide nuclear fuels. Experimentally, low activation-barrier oxygen migration was found in ThO$_2$, a candidate nuclear fuel, possibly due to short-circuit diffusion mechanisms. Here, we perform extensive molecular dynamics simulations to show that various types of extended defects can enhance oxygen self-diffusion with a much-reduced activation barrier in ThO$_2$. In this work, we consider extended defects including 1D (dislocation), 2D (grain boundary), and 3D (void) defects. Due to the distinct characteristics of each type of extended defect, the modulation of oxygen diffusion varies. These results provide a quantitative description of oxygen transport, which is significantly enhanced within a close distance (nanometer scale) from the extended defects. Among all these defects, grain boundary, particularly the $\Sigma 3$ twin boundary with a low formation energy, exhibits the strongest effect on increasing oxygen transport.