Differences in the Existence States of Hydrogen in UO2 and PuO2 from DFT + U Calculations

Abstract

First-principles DFT + U methods are performed to calculate the formation energy and to determine the relative stability of hydrogen at the different sites of UO2 and PuO2. Twenty-one incorporation sites for hydrogen, i.e., along the pathway from its first nearest-neighboring oxygen to the octahedral interstitial site, are considered. The results indicate that hydrogen in UO2 energetically prefers to exist as a hydride ion ([(UO2)n]+H–) rather than forming a hydroxyl group ([UnO2n–1]+[OH]−). The negative formation energy of hydrogen at the octahedral interstitial site of UO2 shows that hydrogen is soluble and can oxidize uranium ion to the higher valence states. However, hydrogen in PuO2 is relatively stable in the form of [PunO2n–1]+[OH]− with comparison to [(PuO2)n]+H–. The slightly positive formation energy of hydrogen in the form of the hydroxyl group in PuO2 reveals that hydrogen is either insoluble or just lies on the edge of solubility. The differences in the existence states of atomic hydrogen in ...