Advanced Ceramics and Glass-Ceramics for Immobilisation of ILW and HLW
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Advanced Ceramics and Glass-Ceramics for Immobilisation of ILW and HLW E. R. Vance, M. W. A. Stewart and S. Moricca, ANSTOsynroc, Australian Nuclear Science and Technology Organisation, Kirrawee DC, NSW 2232, Australia ABSTRACT Since the 1970s there has been a steady increase in research on candidate ceramic and glassceramics for immobilisation of HLW and ILW, both from the aspects of crystal-chemical design and processing technology. The variety of ceramics and glass-ceramics designed for different types of HLW and ILW will be presented, notably those which are problematic for vitrification. Several of these materials are optimally processed by hot isostatic pressing (HIP), a technology which can consolidate calcined intermediate-level and high-level nuclear waste. Thus we are targeting such wastes for development of alternative waste forms. The essential process steps during the HIP cycle will be outlined. Effective consolidation of a wide variety of tailored glassceramic and ceramic waste forms has been demonstrated. The principal advantages of the HIP technology include negligible offgas during the high temperature consolidation step, relatively small footprint, and high waste/volume loadings. While it can be argued that the “nuclear waste problem” is essentially solved technically, at least with current regulatory guidelines, different perceptions of the “best” waste form and processing method for a given waste, together with the general current lack of agreed locations for final repositories, or even interim storage sites, create uncertainties. INTRODUCTION As early as 1953, researchers were showing concern about the need to immobilise radioactive wastes arising from the recently-constructed nuclear reactors[1]. The initial concept was incorporation in clay minerals, an approach initially favored at Chalk River, Canada, later that decade. However from the 1960s the favored method for high-level nuclear waste (waste arising from used nuclear power plant fuel or primary fuel reprocessing waste) was incorporation in borosilicate glasses that could be melted and poured at temperatures of 1000-1200oC. Thus calcined waste was added to glass frit and vitrified. Table I shows the composition of typical reprocessing waste. The advantages of borosilicate glass were that most fission products and process chemical wastes after calcination could be incorporated in the glass structure and the glass was reasonably resistant to leaching by groundwaters characteristic of geological repositories, such repositories being generally agreed by the 1970s as the best way to deal with vitrified high-level waste. However in the mid-1970s, university researchers devised the idea of atomically incorporating waste radionuclides in the crystalline lattices of certain minerals that were known to be very resistant to water leaching, as such minerals that incorporated small amounts of natural radioactivity in their structures could be shown to have survived in hot, wet environments for millions of years [2]. Thus supercalcine, in which additi
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