The UK currently holds the world’s largest stockpile of plutonium. For storage the Pu was converted into PuO2 and sealed in inert steel canisters under argon. Since sealing, some canisters exhibit signs of becoming pressurised raising concerns over their viability over anticipated lifetimes. A possible explanation for this pressurisation is the evolution of hydrogen gas from corrosion of the surface by entrained water.
Conditions in the canisters may not appear sufficiently oxidising to promote corrosion, at least not by water. However, recent experiments suggest the existence of a hyperstocihiometric PuO2+x phase that may act as a precursor to corrosion. Oxidation may be driven by radiolysis of water or by changes in the defect chemistry of PuO2 due to radioactive decay, in particular accumulation of Americium. Americium exhibits complex chemistry with multiple oxidation states some of which may promote formation of PuO2+x.
This project will employ density functional theory (DFT) to understand the defect chemistry of PuO2 and how incorporation of Am will influence reactions at the surface. The first objective will be to determine formation energies for the intrinsic defects in bulk PuO2. The DFT data will be combined with simple thermodynamics to generate Brouwer diagrams that will facilitate prediction of the defect chemistry in PuO2 as a function of key environmental variables including temperature and oxygen partial pressures present in the canisters.
Finally, the energies for americium incorporation into PuO2 lattice will be determined for all possible oxidations states. This will allow a prediction of the mode of Am incorporation, its oxidation state and the presence of any charge compensating defects. This detailed description of the defect chemistry of the Am bearing PuO2 will facilitate a prediction of the corrosion rate and hence the gas production rate. This information will prove critical to understanding the extent of future pressurisation in the canisters and allow a realistic assessment of the service life of the existing canisters.
Academic Lead: Sam Murphy
Researcher: William Neilson
Location: Lancaster University
