Decontamination of Molten Salt Wastes for Pyrochemical Reprocessing of Nuclear Fuels
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Decontamination of Molten Salt Wastes for Pyrochemical Reprocessing of Nuclear Fuels Martin C. Stennett*, Matthew L. Hand, and Neil C. Hyatt. Department of Materials Science and Engineering, The University of Sheffield, Sheffield, S13JD, United Kingdom. ABSTRACT Pyrochemical reprocessing of nuclear fuels, in which electrochemical separation of actinides and fission products is mediated by a molten alkali chloride salt (typically a LiCl-KCl eutectic) is of interest for future nuclear energy cycles. A key challenge in the management of pyrochemical reprocessing wastes is decontamination and recycling of the molten salt medium to remove entrained actinides and radioactive lanthanide fission products. Since pyrochlore oxides are promising candidates for the immobilisation of lanthanides and actinides, we sought to use the “problematic” molten salt to our advantage as a reaction medium for low temperature synthesis of titanate pyrochlores. Through control of reaction time and temperature, we demonstrated the synthesis of lanthanide pyrochlores at temperatures as low as 700 oC in 1 h, compared to 1350 oC in 36 h for conventional solid state synthesis. The importance of this study is in demonstrating the potential feasibility for decontamination of pyrochemical reprocessing wastes by simple addition of TiO2 to form lanthanide and actinide pyrochlores by rapid molten salt assisted reaction at moderate temperature. INTRODUCTION Conventional solid state synthesis (SSS) of mixed metal oxides is generally achieved by reaction between metals oxides and/or carbonate reagents at high temperature. SSS is controlled by diffusion of chemical species and long reaction times at high temperature, with intermediate milling steps, are often required to obtain single phase products [1, 2]. The long reaction times typically promote grain growth which may be un-favourable for certain applications such as high strength components. In addition it has been demonstrated that repeated cycling at very high temperatures can lead to decomposition of some mixed metal oxides [1] and volatilization of some reaction components, leading to product non-stochiometry, and potential environmental discharges, which are undesirable [3]. These disadvantages have lead to the development of other synthesis methods which including co-precipitation, molten salt, sol-gel, hydrothermal, liquid-phase and gas-phase reactions [1]. The prime advantage of these methods is that they reduce the diffusion distances required for phase formation which has the effect of reducing the synthesis temperature and time required. The alternative route under investigation here is the molten salt synthesis (MSS) route which utilises low melting point, water-soluble salts, as a liquid medium in which one or more of the reactant species may dissolve. This liquid salt assists rapid diffusion of reactant species at low temperature, which affords a chemically homogeneous product in relatively short reaction times [1, 2, 4, 5]. MSS generally uses a mixture of readily available and inexpensive
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