| A pyrochlore solid solution in the Gd2Ti2O 7, CaHfTi2O7, CaUTi2O7, and CaPuTi2O7 system is under consideration for the disposition of surplus weapons plutonium. Thermodynamic characterization is a key to modeling the performance of this waste material in a geologic repository. Because these end-member phases are refractory the thermodynamics of formation for these materials have not been determined. Using high-temperature oxide-melt solution calorimetry, the enthalpy of formation from the elements, DfHom , at 298.15 K can be obtained and combined with the third law entropy, S om , to determine the Gibbs free energy of formation, DfGo m. A full thermodynamic characterization of the formation energetics of zirconium and hafnium zirconolite (CaZrTi2O7 and CaHfTi2O7) from 0 to 1500 K, and partial characterizations of cerium pyrochlore, CaZrTi2O7, (an analogue for plutonium pyrochlore CaPuTi2O7) and two cerium aluminates (CeAlO 3 and CeAl12O19.918) are reported.; Oxides containing cerium are of great interest in the chemistry of modern lamp materials as well as being surrogates for materials containing plutonium. Pure Ce-pyrochlore, CaCeTi2O7, has been found to be thermodynamically metastable with respect to decomposition to constituent oxides and to a mixture of CaTiO3 perovskite, CeO2, and TiO2.; Trends in the thermodynamics of formation for oxide materials have been observed and are reported for the zirconolite/pyrochlores (e.g. CaZrTi 2O7, CaHfTi2O7 and CaCeTi2O 7) and the brannerites (e.g. CeTi2O6 and ThTi 2O6). These trends allow the estimation of the enthalpies of formation for the pyrochlores of Th, U, and Pu and the brannerites, UTi 2O6 and PuTi2O6. Each is predicted to be metastable as is the currently proposed waste composition for a pyrochlore based waste ceramic.; An additional trend in the solution energetics of the lanthanide(III) and actinide(IV) oxides over a variety of differing solvents and temperatures indicates that the enthalpy of solution, DeltaHsol, in differing solvents can be predicted for materials not yet studied. Application of this trend to the disposal of high level radio active waste in a borosilicate glass can lead to better estimates for thermodynamic modeling of the performance of a vitreous waste form. |
