Evolving Defect Chemistry of (Pu,Am)O2±x

The beta decay of Pu-241 to Am-241 results in a significant ingrowth of Am during the interim storage of PuO2. Consequently, the safe storage of the large stockpiles of separated Pu requires an understanding of how this ingrowth affects the chemistry of PuO2. This work combines density functional theory (DFT) defect energies and empirical potential calculations of vibrational entropies to create a point defect model to predict how the defect chemistry of PuO2 evolves due to the incorporation of Am. The model predicts that Am occupies Pu sites in (Pu,Am)O-2 +/- x in either the +III or +IV oxidation state. High temperatures, low oxygen-to-metal (O/M) ratios, or low Am concentrations favor Am in the +III oxidation state. Am (+III) exists in (Pu,Am)O-2 +/- x as the negatively charged (Am-Pu(1-)) defect, requiring charge compensation from holes in the valence band, thereby increasing the conductivity of the material compared to Am-free PuO2. Oxygen vacancies take over as the charge compensation mechanism at low O/M ratios. In (Pu,Am)O-2 +/- x, hypo- and (negligible) hyperstoichiometry is found to be provided by the doubly charged oxygen vacancy (V-O(2+)) and singly charged oxygen interstitial (O-i(1-)), respectively.

Automated summary

This study predicts the effects of Am incorporation on the chemistry of PuO2 by establishing a point defect model, showing that high temperatures, low oxygen-to-metal ratios, or low Am concentrations favor the formation of Am in the +III oxidation state, increasing the material's conductivity.