Synthesis, Characterization, and Enthalpies of Formation of Uranium Substituted Zirconolites

Uranium substituted zirconolite samples based on CaZrTi2O7 have been synthesized by a conventional solid-state method in air and structurally characterized by powder X-ray diffraction. The uranium substituted compounds crystallize in a monoclinic C2/m structure like the parent phase. The oxidation state of uranium has been confirmed to be mostly UVI by X-ray absorption spectroscopy (XAS). The enthalpies of formation from oxides (Delta H-Lox degrees) determined by high temperature oxide melt solution calorimetry of uranium substituted compounds are more exothermic than that of the parent CaZrTi2O7. The enthalpies of decomposition of uranium substituted zirconolites into perovskite (CaTiO3), UO3, and other binary oxides are endothermic by about 20 kJ/mol, implying that decomposition is probably thermodynamically unfavorable. The results show that zirconolite is a thermodynamically stable host for uranium in high level nuclear waste.