Towards a Silicate Matrix for the Immobilisation of Halide-rich Wastes
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Towards a Silicate Matrix for the Immobilisation of Halide-rich Wastes M. R. Gilbert1 1 AWE, Aldermaston, Reading, RG7 4PR, UK. ABSTRACT Single-phase calcium chlorosilicate and sodalite, two potential ceramic waste-forms for the immobilisation of CaCl2-based pyroprocessing wastes, have been fabricated at temperatures below the volatilisation point of CaCl2. Solid solutions doped with Sm3+ as an inactive analogue for trivalent actinides have been fabricated and characterised. XRD analysis shows both phases will successfully accommodate Sm3+, with the sodalite in particular remaining single-phase. Fabrication of Sm-doped calcium chlorosilicate in air results in the formation of SmOCl and Ca(Si2O5) secondary phases, however, calcination in an inert atmosphere is shown to successfully retard the formation of SmOCl allowing for higher levels of doping. 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 immobilise the radionuclide chlorides contained within. However, halide-rich wastes such as these can be problematic to immobilise, 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 immobilise the radionuclide chlorides in a ceramic waste-form based upon mineral phases with naturally high chloride contents. Current research at AWE is focused on wastes arising from pyroprocessing in CaCl2 salt, limiting the processing temperatures to a maximum of 800 oC to prevent substantial loss of chlorine via volatilization. Potential candidates under investigation include chloroapatite, calcium chlorosilicate, rondorfite, sodalite and quadridavyne, of which calcium chlorosilicate and sodalite are presented here. Calcium chlorosilicate (Ca3(SiO4)Cl2) has one of the highest Cl contents of any chlorinecontaining silicate at ~ 25 wt. %. It has a monoclinic structure consisting of an approximately cubic close-packed arrangement of SiO44- tetrahedra and Cl- anions, with the Ca2+ cations occupying interspersed octahedral sites in such a way that a distorted NaCl-type structure results [5-8]. Calcium chlorosilicate has been investigated as a potential waste-form for the immobilisation of precipitated chlorine [9] and is seen as potentially highly suitable for those wastes resulting from CaCl2 based processes. Sodalite (Na8
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