Evolving Defect Chemistry of (Pu,Am)O_2±

The β decay of ²⁴¹Pu to ²⁴¹Am 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± 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± 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± , 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.