Glassy Wasteforms for Nuclear Waste Immobilization
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INTRODUCTION
THE choice of wasteform to use for nuclear waste immobilization is a difficult decision, and durability is not the sole criterion. In any immobilization process where radioactive materials are used, the process and operational conditions can become complicated, particularly if operated remotely and equipment maintenance is required. Therefore, priority is given to reliable, simple, rugged technologies and equipment, which may have advantages over complex or sensitive equipment. A variety of matrix materials and techniques is available for immobilization. The choice of the immobilization technology depends on the physical and chemical nature of the waste and the acceptance criteria for the longterm storage and disposal facility to which the waste will be consigned. A host of regulatory, process, and product requirements has led to the investigation and adoption of a variety of matrices and technologies for waste immobilization. The main immobilization technologies that are available commercially and have been demonstrated to be viable are cementation, bituminization, and vitrification.[1] Vitrification is a particularly attractive immobilization route because of the high chemical durability of the glassy product. This characteristic was used by industry for centuries. The chemical resistance of glass can allow it to remain in a corrosive environment for many thousands and even millions of years. In most countries, High-level radioactive waste (HLW) has been incorporated into alkali borosilicate or phosphate vitreous waste forms for many years, and vitrification is an established technology. Large streams of low- and intermediate-level radioactive waste (LILW) are planned to be vitrified in the United States, South Korea, and Russia. Vitreous waste forms represent one MICHAEL I. OJOVAN, Assistant Professor, is with the Department of Materials Science and Engineering, Immobilisation Science Laboratory, University of Sheffield, Sheffield, S1 3JD, United Kingdom. Contact e-mail: M.Ojovan@sheffield.ac.uk WILLIAM E. LEE, Professor, is with the Department of Materials, Imperial College London, London SW7 2AZ, United Kingdom. Manuscript submitted May 12, 2010. Article published online November 17, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
end of the spectrum of the HLW waste forms shown in Table I.[2,3] At the other end of the spectrum shown in Table I, the use of predominantly crystalline ceramic wasteforms (ceramication) has also been proposed including singlephase ceramics such as zircon to accommodate a limited range of active species such as Pu and multiphase systems such as Synroc to accommodate a broader range of active species.[4] To date, these systems have not been used extensively to immobilize active waste. Recently, however, there has been a trend to systems intermediate between the ‘‘completely’’ glassy or crystalline materials.[5] An illustration of nuclear waste forms used and developed for industrial application is given in Figure 1.[6] Typically, nondurable crystals are Na2SO4, Na2MoO4 2H2O, CaM
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