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

International audienceIn France, High-activity Level Wastes (HLW) resulting from nuclear fission are conditioned in alumino-borosilicate glasses by high-temperature vitrification. Up to this day, the precipitation of crystals during the vitrification is avoided because it can directly affect the physical properties of the melt, especially its viscosity. The tolerance of even a small fraction of crystals could lead to a more flexible process and could allow the application of higher waste loadings. In the French nuclear glass, Cerianite (CeO2) is the crystalline phase stable at the highest temperatures, which means that an increase in the waste loading would potentially stabilize it at vitrification temperatures. In this work, we investigated the cerianite crystallization in a simplified nuclear glass matrix for different temperatures, Ce2O3 wt%, and flow conditions (quiescent and dynamic). Furthermore, we studied the evolution of the viscosity along with cerianite precipitation and compared the effect of cerianite particles on viscosity-shear dependency to the one showed by platinum group metal particles (PGM)-bearing melts. We found that shear strongly accelerates cerianite crystallization and that a high Ce2O3 content can engender the precipitation of dendritic-like crystals instead of euhedral cubic-shaped crystals, even at high temperatures (in this case, at 1100 and 1160°C). Even for a Ce2O3 content as high as 10%wt, the cerianite fraction after 8h of crystallization in dynamic conditions at 1100°C (the temperature of vitrification in the hot crucible melter) was less than 1%vol. The evolution of the cerianite fraction did not significantly affect the viscosity of the glass. Finally, a melt containing a cerianite volume fraction of 0.8 % remains Newtonian, in disparity of a melt containing the same amount of PGM particles