Thermodynamic stability of actinide pyrochlore minerals in deep geologic repository environments

摘要

Crystalline phases of pyrochlore (e.g., CaPuTi_2O_7, CaUTi_2O_7) have been proposed as a durable ceramic waste form for disposal of high level radioactive wastes including surplus weapons-usable plutonium. In this paper, we use a linear free energy relationship to predict the Gibbs free energies of formation of pyrochlore phases (CaMTi_2O_7). The Pu-pyrochlore phase is predicted to be stable with respect to PuO_2, CaTiO_3, and TiO_2 at room temperatures. Pu-pyrochlore is expected to be stable in a geologic repository where silica and carbonate components are absent or limited. We suggest that a repository in a salt formation be an ideal environment for disposal of high level, pyrochlore-based ceramic wastes. In such environment, adding CaO as a backfill will make pyrochlore minerals thermodynamically stable and therefore effectively prevents actinide release from these mineral phases.