Ab initio modeling of oxygen impurity atom incorporation into uranium mononitride surface and sub-surface vacancies

Abstract

The incorporation of oxygen atoms has been simulated into either nitrogen or uranium vacancy at the UN(001) surface, sub-surface or central layers. For calculations on the corresponding slab models both the relativistic pseudopotentials and the method of projector augmented-waves (PAW) as implemented in the VASP computer code have been used. The energies of O atom incorporation and solution within the defective UN surface have been calculated and discussed. For different configurations of oxygen ions at vacancies within the UN(001) slab, the calculated density of states and electronic charge re-distribution was analyzed. Considerable energetic preference of O atom incorporation into the N-vacancy as compared to U-vacancy indicates that the observed oxidation of UN is determined mainly by the interaction of oxygen atoms with the surface and sub-surface N vacancies resulting in their capture by the vacancies and formation of O-U bonds with the nearest uranium atoms.

Ab initio simulations of oxygen interaction with surfaces and interfaces in uranium mononitride

“…On the basis of these experimental observations, we have chosen the ferro-magnetic state for all our calculations performed for the self-consistent (relaxed) atomic magnetic moments without spin-orbit effects. Additional information on computational parameters in our calculations can be found elsewhere [20][21][22][23].…”

“…Considering oxygen-rich conditions and the formation of UO x N y layers, we suppose that the oxygen chemical potential equals [23,35,36]. Thus, we rely on the approach suggested in [37], in order to improve the calculated value of chemical potential of O atom ( 0 () O T…”

“…It was shown, in particular, that O impurities affect visibly UN properties, e.g., mobility of intrinsic defects and related thermal creep [18]. First ab initio calculations on the (001) surface and its reactivity have been performed only recently [19][20][21][22][23]. We considered moderate concentrations of N vacancies and O atoms incorporated therein which allowed us to study the early stages of UO x N y formation.…”

“…We considered moderate concentrations of N vacancies and O atoms incorporated therein which allowed us to study the early stages of UO x N y formation. Our previous calculations on O reactivity upon the (001) surface clearly have shown a possibility of spontaneous breaking of the O 2 chemical bond after molecular adsorption [21], strong O adatom chemisorption atop the surface U atoms [20] (typical for metallic surfaces), and energetically favorable incorporation of O adatoms into surface nitrogen vacancies [23]. In these calculations, defect energetics, atomic structures, effective atomic charges, electronic charge densities and density of states were calculated and analyzed.…”

First Principles Simulations on Surface Properties and Oxidation of Nitride Nuclear Fuels

“…This Chapter is based on our own theoretical studies [5][6][7][8][9][10] as well as available theoretical and experimental literature.…”