Crystallization and rheological study of a Nd-oxyapatite-bearing melt

Up to this day the composition of the French nuclear waste glasses is designed to avoid the precipitation of crystals during the vitrification process because they can affect different properties of the glass melt, mainly its viscosity. The ability to handle a crystal-bearing melt by understanding its rheological behavior potentially allows higher waste loadings and promotes a greater process flexibility. Only a few studies experimentally address the subject, mainly because the rheological study of a crystal-bearing melt is challenging, as the crystal fractions is a function of both time and temperature and it cannot be assessed in situ. Rare-earth (RE) oxy-apatites are among the crystals susceptible to be precipitated in the nuclear glass, and due to its prismatic morphology, they are expected to have a significant effect on the melt viscosity. In this work, we used a simplified Nd-rich nuclear glass to study the rheological behavior of an apatite-bearing melt, for which the apatite equilibrium fraction was already partly known. We studied the evolution of viscosity during crystallization for different conditions of shear and performed steady-state and transient studies on the crystallized glass melt. The curve of evolution of viscosity during crystallization showed an Avrami-like shape, and an evident acceleration of crystallization kinetics was observed as the shear was increased. In addition, the equilibrium viscosity after reaching the equilibrium fraction was inversely proportional to the applied shear. The apatite-bearing melt showed a very shear thinning behavior from an apatite fraction above 3 vol%. We were able to determine the effective maximum packing fraction for different conditions of shear.