Competition Between Internal and Surface Crystallization in Glass-ceramics Developed for Actinides Immobilization

摘要

Glass-ceramic matrices containing zirconolite (nominally CaZrTi_2O_7) as the only crystalline phase in their bulk can be considered as good candidates for actinide-rich nuclear wastes (containing minor actinides or Pa) immobilization. In this study, three different methods are envisaged and compared to prepare such waste forms using neodymium as trivalent actinides surrogate. Independently on the preparation method, zirconolite is shown to be the only crystalline phase to nucleate in the bulk. However, crystallization of silicate phases (titanite CaTiSiO_5 + anorthite CaAl_2Si_2O_8) can occur from samples surface and can compete with zirconolite crystallization. The effect of the ciystal growth thermal treatment duration (2-300 h) at high temperature (1050-1200°C) on glass-ceramics structure and microstructure is studied. In the oxides system studied here, it appears that zirconolite is not thermodynamically stable in comparison with titanite but, for kinetics reasons, such transformation will not occur during waste forms disposal.