Equilibrium and thermodynamic properties of the PuO2+x solid solution

Free energies for forming a fluorite-related NO, 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 AGO values indicate that the PuO2 + H2O reaction spontaneously forms oxide compositions up to PuO25. 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 O-2 pressures for the Pu+O-2 reaction suggest that 2+x at the upper phase boundary in air falls below 21.5 at 600-700 K and approaches 2.0 above 850 K. Results indicate that H-2 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 O-2 pressures upon heating. (C) 2002 Elsevier Science B.V. All rights reserved.