Defect chemistry of bulk and nanoscale PuO2

Neilson, William and Murphy, Samuel (2022) Defect chemistry of bulk and nanoscale PuO2. PhD thesis, Lancaster University.

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Abstract

The management of large stockpiles of Pu, separated from spent nuclear fuel or nuclear weapons programmes and stored as the oxide PuO2, requires an understanding of the material’s aging behaviour during interim storage. This includes characterising the effect of radiogenic impurities that accumulate at significant concentration over storage time periods, as well as insight into the segregation of both radiogenic and non-radiogenic species between accommodation as defects in the oxide powder or in the storage container headspace above it. Point defects play a crucial role in the properties of crystalline materials. Modern first principles atomistic simulation techniques, such as density functional theory (DFT), are now widely employed for the simulation of point defects of both intrinsic and extrinsic origin. However, it is only through the careful use of thermodynamics that the defect energies obtained in these simulations can be exploited to provide a realistic description of a system under specific operating conditions, such as those present in PuO2 storage containers. This thesis has developed the Defect Analysis Package (DefAP), an open-source Python code that is designed to combine DFT data with established thermodynamic relationships to provide new insight into the defect chemistry of materials. Aided by DefAP, PuO2 under interim storage conditions has been studied. The results show that the intrinsic defect chemistry of PuO2±x is dominated by oxygen vacancies and interstitials and that PuO2+x is thermodynamically very unfavourable. Radiogenic Am occupies Pu sites in (Pu,Am)O2±x with an evolving ratio of the +III and +IV oxidation states, dependent upon temperature, oxygen partial pressure, and the concentration of the Am itself. It is observed that even small concentrations of Am(III) impact significantly on the material’s properties: it promotes a reducing environment and acts as a p-type dopant, elevating the concentration of holes in the valence band leading to increased electrical susceptibility and a postulated increase in surface reactivity. He from alpha-decay was found to be preferentially accommodated in PuO2±x on the interstitial site, but that the impact of Am is large enough to, under certain conditions, alter this accommodation mechanism to an oxygen vacancy. The reproduction of the available experimental data lends confidence to the model’s accuracy. In the final part of the thesis, it is explored whether simulations of nanoparticles — instead of the bulk material — may offer a better representation of the stored PuO2 powder. Whilst the simulated nanoparticles display many bulk-like features, it was seen that unique characteristics are present as a consequence of the under-coordinated atoms located at the particle’s surface. Surface configurations promoting reduced Pu or oxidised O ions have been observed and with thermodynamic relationships the environmental conditions where these differing surface configurations become favourable were predicted. The improved scientific understanding of PuO2 presented in the thesis is essential information in the long-term move towards its storage and will inform future disposition programmes. An improved understanding also has the potential to reduce some of the pessimisms built into stores’ safety cases.