Oxygen diffusion and migration in clean and defective uranium nitride UN (0 0 1) surfaces

Abstract

This study focuses on the diffusion of atomic and molecular oxygen through the uranium mono-nitride UN (0 0 1) surface. The adsorption of oxygen at the most favorable sites has been checked on different surface states namely: clean surface and surfaces containing defects such as inclusion atoms.

Inclusions atoms are positioned at a uranium U atom vacancy or at a nitrogen N atom vacancy location of the UN (0 0 1) surface. Neptunium, plutonium, protactinium, silver and neodymium which are the most probable nuclear reactions (n, U) products have been selected as U atom substitute. Some light elements such as carbon, chromium and silicon were used to replace an N atom.

The first principle calculation, based on Density Functional Theory (DFT) was used, taking into account the Generalized Gradient Approximation (GGA) and the Projector-Augmented Wave (PAW) to describe the exchange-correlation functional.

The purpose of this work is to verify the oxygen adsorption energy variations performed across all the studied surfaces. The most favorable sites of UN (0 0 1) clean and defective surfaces to oxygen O atom diffusion were preliminary identified. In the second step, atomic dynamical Potential Energy Surface (PES) was used to study the interaction between O atom and UN (0 0 1) surfaces at these sites. Finally, Nudged Elastic Band (NEB) method was used in order to investigate the migration of O atom through the UN (0 0 1) surfaces.

The results show that at the bridge site, the adsorption and incorporation energies of oxygen atom on and in the UN (0 0 1) surfaces respectively, do not substantially vary with the type and position of the studied impurities. But, at the N vacancy site, the adsorption energy of the O atom decreases practically when UN (0 0 1) surfaces contain inclusion atoms compared to the clean surface case.

Furthermore, the NEB calculations show discrepancies for the Minimum Energy Path (MEP) during the migration of the O atom at the bridge site through the studied UN (0 0 1) surfaces and depending on the type and position of the added impurities. Among the studied MEPs, protactinium is found to be the most suitable barrier to the diffusion of oxygen through the UN surface as an inclusion on UN (0 0 1)