Formation energetics of ceramic waste materials for the disposal of surplus weapons plutonium

Abstract

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, [special characters omitted] at 298.15 K can be obtained and combined with the third law entropy, S[special characters omitted], to determine the Gibbs free energy of formation, [special characters omitted] 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 CeAl₁₂O_19.918) 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, ΔH_sol, 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.