Accommodation of Nitrogen in PuO₂ Under Storage Conditions

The UK is currently in possession of one of the world's largest civilian stockpile of separated plutonium, which is currently stored as an oxide powder within gas-tight storage packages. To ensure safe long-term storage of the materials and to aid with future decision making with regards to reprocessing or disposition, it is imperative to understand the aging process of plutonium dioxide powder and the chemistry of the head space above the material. In general, the plutonium powder was placed inside a metal container and bagged out in a plastic bag under an argon atmosphere. This was then welded shut inside an outer metal container in air. Therefore, it is likely that species such as nitrogen and water are entrained inside the packages and these species may have an influence on the chemistry on the oxide power as it ages as well as the head space above it. During storage, plutonium-241 undergoes β-decay, resulting in the build-up of americium which alters the reactivity of the oxide. Increasing reactivity may lead to the evolution of gaseous species and a concomitant increase in pressurisation.<br/><br/>Density functional theory (DFT) simulations were executed to determine the accommodation energies for nitrogen in different sites in the PuO₂ lattice. These energies are then incorporated into a point defect model showing how nitrogen is accommodated in the PuO2 lattice under conditions relevant to the storage of plutonium, such as temperature and oxide stoichiometry. The results can be used to support the safe long-term storage of plutonium or future processing into new fuels.