Ab initio modelling of UN grain boundary interfaces

TL;DR

This study uses ab initio DFT calculations to investigate oxygen behavior at UN grain boundaries, providing insights into oxidation mechanisms relevant for nuclear fuel stability.

Contribution

It presents the first detailed ab initio analysis of oxygen interactions within UN grain boundaries, extending understanding beyond surface processes.

Findings

Oxygen incorporation energies at grain boundaries are comparable to surface energies.

N vacancy formation energies are lower at grain boundaries than on surfaces.

Oxygen solution energies suggest grain boundaries as active sites for oxidation.

Abstract

The uranium mononitride (UN) is a material considered as a promising candidate for Generation-IV nuclear reactor fuels. Unfortunately, oxygen in air affects UN fuel performance and stability. Therefore, it is necessary to understand the mechanism of oxygen adsorption and further UN oxidation in the bulk and at surface. Recently, we performed a detailed study on oxygen interaction with UN surface using density functional theory (DFT) calculations. We were able to identify an atomistic mechanism of UN surface oxidation consisting of several important steps, starting with the oxygen molecule dissociation and finishing with oxygen atom incorporation into vacancies on the surface. However, in reality most of processes occur at the interfaces and on UN grain boundaries. In this study, we present the results of first DFT calculations on O behaviour inside UN grain boundaries performed using GGA exchange-correlation functional PW91 as implemented into the VASP computer code. We consider a simple interface (310)[001](36.8{\deg}) tilt grain boundary. The N vacancy formation energies and energies of O incorporation into pre-existing vacancies in the grain boundaries as well as O solution energies were compared with those obtained for the UN (001) and (110) surfaces.