Résumé
The development of controlled spontaneously crystallizing glass-ceramic materials is an attractive way to increase the waste loading for glasses used to immobilize nuclear waste. This approach has been already demonstrated for the vitrification of waste solutions enriched in MoO3 in France, and as an alternative for immobilizing non-fissionable products obtained from aqueous reprocessing in the US. Design of the waste form and its evolution during processing, however, demands greater attention to ensure the required glass quality (durability, thermal stability). The mechanisms of phase separation and crystallization have to be carefully understood and controlled at each step in the processing. In this poster, we show illustrations of the main mechanisms of phase separation and crystallization as obtained by nucleation and growth. We discuss the evolution of the morphologies obtained after different processes and the different steps of equilibrium in the free energy-composition diagram at a temperature above the phase separation (immiscibility) temperature. Moreover, phase separation temperatures are determined by in-situ in temperature experiments (Environmental Scanning Electron Microscopy, viscosimetry) for a series of simplified borosilicate glasses enriched in MoO3.