Precipitation of cerianite crystals and its effect on the rheology of a simplified nuclear glass melt

In France, high-activity level wastes resulting from nuclear fission are conditioned in a homogeneous sodium-aluminoborosilicate glass by high-temperature vitrification. The tolerance of even a small fraction of crystals could enable an increase in the waste loadings, in addition to promoting process flexibility. If the waste loading were to be increased in French nuclear glass, cerianite (CeO₂) crystals could precipitate. In this study, we investigated the cerianite crystallization in a simplified nuclear glass melt at different temperatures, Ce₂O₃ wt%, and shear conditions. Furthermore, the evolution of the viscosity along with cerianite precipitation was followed. It was found that Ce₂O₃ is highly soluble in the glass melt, as even for a Ce₂O₃ wt% as high as 10% wt, the cerianite fraction in dynamic conditions at 1100°C after 8 h of crystallization was less than 1% vol. In addition, shear strongly accelerates cerianite crystallization and a high Ce₂O₃ content can engender the precipitation of highly branched dendrites. The evolution of the cerianite fraction did not significantly affect the viscosity of the glass melt. Finally, unlike what has been observed in the well-known platinum group metal (PGM)-bearing melts, a glass melt containing .8 vol% of cerianite crystals remains Newtonian.