Xenon-metal pair formation in UO2 investigated using DFT+U

TL;DR

This study uses DFT+U calculations to investigate how noble metal precipitates interact with xenon gas in UO2 fuel, revealing stable pair formations that explain experimental observations.

Contribution

It provides the first computational analysis of Xe-metal pair formation in UO2, identifying stable structures and their energetic favorability.

Findings

Xe-metal pairs are more stable than single defects.

Pd and Mo show the strongest binding with Xe.

Results explain the experimentally observed noble metal and fission gas pairing.

Abstract

A recent experimental study on a spent uranium dioxide (UO2) fuel sample from Belgium Reactor 3 (BR3) identified a unique pair structure formed by the noble metal phase (NMP) and fission gas (xenon [Xe]) precipitate. However, the fundamental mechanism behind this structure remains unclear. The present study aims to provide an understanding of the interaction between five different metal precipitates (molybdenum [Mo], ruthenium [Ru], palladium [Pd], technetium [Tc], and rhodium [Rh]) and the Xe fission gas atoms in UO2, by using density functional theory (DFT) in combination with the Hubbard U correction to compute the formation energies involved. All DFT+U calculations were performed with occupation matrix control to ensure antiferromagnetic ordering of UO2. The calculated formation and binding energies of the Xe and solid fission products in the NMP reveal that these metal precipitates form stable pair structures with Xe. Notably, the formation energy of Xe-metal pairs is lower than that of the isolated single defects in all instances, with Pd and Mo showing the most favourable binding energy, likely accounting for the observed pair structure formation.