Pressureless Sintering of Sodalite Waste-forms for the Immobilization of Pyroprocessing Wastes
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Pressureless Sintering of Sodalite Waste-forms for the Immobilization of Pyroprocessing Wastes M. R. Gilbert AWE, Aldermaston, Reading, RG7 4PR, UK. ABSTRACT Sodalite (Na8[AlSiO4]6Cl2), a naturally occurring Cl-containing mineral, has long been regarded as a potential immobilization matrix for the chloride salt wastes arising from pyrochemical reprocessing operations, as it allows for the conditioning of the waste salt as a whole without the need for any pre-treatment. Here the consolidation and densification of Smdoped sodalite (as an analogue for AnCl3) has been investigated with the aim of producing fully dense (i.e. > 95 % t.d.) ceramic monoliths via conventional cold-press-and-sinter techniques at temperatures of < 1000 °C. Microstructural analysis of pressed and sintered sodalite powders under these conditions is shown to produce poorly sintered, porous, inhomogeneous pellets. However, by the addition of a sodium aluminophosphate glass sintering aid, fully dense Smsodalite ceramic monoliths can successfully be produced by sintering at temperatures as low as 800 °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 radionuclide chlorides 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 immobilize the radionuclide chlorides in a ceramic waste-form based upon mineral phases with naturally high chloride contents, of which sodalite is one such example. Natural sodalite, Na8[AlSiO4]6Cl2, is a naturally chlorine-containing mineral, containing up to 7.3 wt. % Cl [5]. It is a crystalline aluminosilicate formed of a framework of allcorner-linked tetrahedra. Sodalite is formed by the fusing of the 4-membered rings of the ȕcages, such that its framework structure consists solely of ȕ-cages, creating a microporous structure with a typical pore diameter of 4 Å [6]. Work carried out at Argonne National Laboratory on glass-bonded sodalites has demonstrated them to be effective for immobilizing wastes from NaCl-KCl based reprocessing, though preparation temperatures range from 850 – 1000 oC [7-9]. In addition, work at Idaho National Laboratory formed sodalite waste-forms by first incorporating the
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