Equilibrium and thermodynamic properties of the PuO2+x solid solution

Free energies for forming a fluorite-related PuO2+x solid solution via the oxide–water reaction at 300–800 K are obtained using estimated partial molar free energy isotherms based on bounding conditions and property data. Derived ΔG° values indicate that the PuO2+H2O reaction spontaneously forms oxide compositions up to PuO2.5 at 300 K and are consistent with diffraction data for fluorite-related solids formed by PuO2 and amorphous Pu(OH)4 in aqueous solutions. Free energies of formation for PuO2+x at 300 K vary systematically from −246.8±2.0 kcal mol−1 for x=0.1 to −273.9±6.0 kcal mol−1 for x=0.5 and become increasingly positive at elevated temperatures. Estimated equilibrium O2 pressures for the Pu+O2 reaction suggest that 2+x at the upper phase boundary in air falls below 2.5 at 600–700 K and approaches 2.0 above 850 K. Results indicate that H2 pressures generated by the oxide–water reaction in storage containers are not limited by equilibrium and suggest that low firing of oxygenated precipitates forms PuO2+x, a product with enhanced solubility and potential for generating high O2 pressures upon heating.