Molten Salt Synthesis of Zirconolite Polytypes
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Molten Salt Synthesis of Zirconolite Polytypes M. R. Gilbert AWE, Aldermaston, Reading, RG7 4PR, UK. 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. INTRODUCTION Pyrochemical reprocessing techniques enable the recovery of Pu metal from spent nuclear material without the need to convert it to PuO2 and back [1]. These methods utilise an electrorefining process, where the Pu is separated from the impurities in a molten chloride salt, most typically either CaCl2 or an equimolar mixture of NaCl-KCl, at temperatures of between 750 – 850 oC [2]. Post-reprocessing, this chloride salt must be replaced, as it now contains a number of different waste streams which will contaminate the cathode and affect the properties of the molten salt. This contaminated salt must be disposed of in such a way as to immobilize the radionuclides contained within. However, halide-rich wastes such as these can be problematic to immobilize, as not only are their solubilities in melts very low, but even in small quantities they can seriously affect the properties of the waste-form [3,4]. In addition, processing temperatures are often severely limited in order to prevent the volatilisation of the halides. One approach is to selectively immobilize the actinide cations from the chloride media in titanate ceramics, using the salt as a reaction medium for molten salt synthesis (MSS). In this method, a salt or eutectic mix of salts with low melting point is added to the reactants (typically 80-100 wt. % of the reactant mixture) and heated to above the melting point of the salt. The molten salt medium essentially acts as a solvent, assisting the diffusion of reactant species and thus reducing the temperature and time required for synthesis. There are two generally accepted mechanisms of product formation through MSS: solution-precipitation and solution-diffusion (sometimes referred to as “templating”). Which mechanism is dominant depends upon the solubility and relative dissolution rates of the reactants in the molten salt [5-7]. Nominally CaZrTi2O7, zirconolite exhibits a wide range of stoichiometri
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