Crystallization of CaHf1-xZrxTi2O7 (0 ≤ x ≤1) zirconolite in SiO2-Al2O3-CaO-Na2O-TiO2-HfO2-ZrO2-Nd2O3 glasses

Glass-ceramics containing (Hf,Zr)-zirconolite crystals (nominally CaHf1-x Zr (x) Ti2O7 with 0 <= x <= 1) were envisaged to immobilize minor actinides and plutonium. Such materials were prepared in this study by controlled crystallization of glasses belonging to the SiO2-Al2O3-CaO-Na2O-TiO2-HfO2-ZrO2-Nd2O3 system. Neodymium was used as trivalent actinides surrogate. The effect of total or partial substitution of ZrO2 by HfO2 (neutron poison for fission reactions) on glass crystallization in the bulk and near the surface is presented. It appeared that Hf/Zr substitution had not significant effect on nature, structure, and composition of crystals formed both on glass surface (titanite + anorthite) and in the bulk (zirconolite). This result can be explained by the close properties of Zr4+ and Hf4+ ions and by their similar structural role in glass structure. However, strong differences were observed between the nucleation rate I-Z of zirconolite crystals in glasses containing only HfO2 and in glasses containing only ZrO2. Hf-zirconolite (CaHfTi2O7) crystals were shown to nucleate only very slowly in comparison with Zr-zirconolite (CaZrTi2O7) crystals. Composition changes - by increasing either HfO2 or Al2O3 concentration or by introducing ZrO2 in parent glass - were performed to increase I-Z in hafnium-rich glasses. The proportion of Nd3+ ions incorporated in the zirconolite phase was estimated using ESR.