Compatibility of UN with refractory metals (V, Nb, Ta, Cr, Mo and W): An ab initio approach to interface reactions and diffusion behavior

Abstract

Uranium mononitride (UN)-uranium dioxide (UO2) composites are being considered as an innovative advanced technology fuel option for light water reactors, where an optimal balance between the chemical advantages of UO2 and the thermal and neutronic properties of UN is struck. However, the effect and extent of chemical interactions between UN and UO2 during sintering and operation are still open issues of importance. A possibility to avoid these interactions is to protect the UN phase before sintering the UN-UO2 composites by encapsulating the UN. This protective material must have a high melting point, high thermal conductivity, and reasonably low neutron cross-section. Among many candidates, the use of refractory metals is a promising option. In this study, density functional theory calculations (DFT) were performed to study the interactions and kinetics at the UN-X interfaces respectively (X = V, Nb, Ta, Cr, Mo, and W). The diffusion behaviors in UN and in the metal were studied using the self-consistent mean field (SCMF) theory. Generally, the diffusion of metal atoms in UN is slow compared to the diffusion of N atoms in the metals. Furthermore, the DFT calculations predict that Ta and V may react with UN to form UTaN2 and V8N at the UN-X interfaces, respectively. In some cases, the formation of these phases also promotes the formation of point defects in the UN and metal phases. The interaction between W and Mo with the UN phase is largely prohibited. According to this work, Mo and W can be regarded as highly promising candidate materials for the fabrication of stable UN-UO2 composite fuel.