Molten Salt Synthesis of Zirconolite Polytypes

Abstract

Zirconolite (CaZrTi₂O₇), a durable and compositionally flexible titanate ceramic for the immobilization of separated actinides, is currently the UK’s preferred candidate phase for the immobilization of plutonium dioxide arising from aqueous reprocessing. Here, its suitability as a waste-form for actinide chlorides arising from pyrochemical reprocessing is investigated through synthesis via a molten salt mediated reaction using a number of different salt eutectics (MgCl₂:NaCl, CaCl₂:NaCl and KCl:NaCl). It is found that the effectiveness of the molten salt synthesis of zirconolite is governed by the solubility of ZrO₂ in the salt medium used; the synthesis proceeding via the formation of a perovskite (CaTiO₃) intermediate which then reacts with ZrO₂ to form zirconolite via a solution-diffusion mechanism. Most notably, in the KCl:NaCl eutectic different zirconolite polytypes are formed at different synthesis temperatures, with zirconolite-3T forming at 900 °C, giving way to zirconolite-2M at 1200 °C.