High-level waste raffinate

Vitrification is the process of choice for separated highly radioactive wastes in virtually every reprocessing nation. (Donald et al., 1997; Ojovan and Lee, 2005; Vienna, 2005; Donald, 2010) Vitrification is:

• a proven process,

• tolerant to a wide range of waste compositions,

• a fast continuous process,

• generates no fine particulates, and

• the US Environmental Protection Agency (EPA, 2009) best demon­strated available technology (BDAT). Vitrification produces a waste form of good performance that is well understood (including many natural and ancient man-made analogs).

While vitrification into a borosilicate glass is the reference process, the next generation waste forms for HLW with potential benefits over vitrifica­tion are being developed. For example, glass composite materials (GCMs) including glass ceramics may allow for significantly higher waste loading than possible in typical borosilicate glasses (Ojovan and Lee, 2011). There are three primary limitations to the loading of HLW in glass: (i) decay heat, (ii) solubility of waste components (e. g., MoOtt, and (iii) noble metals. GCMs could allow for higher heat as the crystalline portions may be much more thermally stable. They also are expected to tolerate significantly higher concentrations of components that are sparsely soluble in the glass melt. The noble metals limit would depend on the processing methods, but, will not be more restrictive for GCMs. Crum et al. (2012) developed durable, radiation — resistant glass ceramics that could be processed using existing melter tech­nologies containing roughly double the waste loading of typical glasses. Figure 5.6 shows graphically the potential for the increases in waste loading.

Other potential waste forms for HLW are crystalline ceramic waste forms, which show promise for high loading and high chemical durability (Burakov et al., 2010). Development of the synthetic rock (Synroc) types of waste forms began in the 1950s; the term Synroc was coined by Ringwood et al. in 1979 when the most concerted waste form development and testing on these forms began (Ringwood et al. , 1979). Ranges of silicate, aluminate, and phosphate ceramics were developed in the 1960 to 1980s. Excellent reviews of these waste forms already exist (Lutze and Ewing, 1988; Donald, 2010; Burakov et al., 2010). A number of recent advancements in these materials have shown that complicated processes such as alkoxide

precipitation and hot isostatic pressing could be replaced with a simpler melt-cast-type process (Vance et al., 1996; Advocat et al., 1997; Stefanosky et al., 2009).

With the advanced separations methods currently available, it is possible to subdivide the HLW raffinate into streams with similar chemical proper­ties such as lanthanides, alkali and alkaline-earths, transition metal fission products, etc. Each of these streams could be separately immobilized in a form specifically design for the waste chemistry and disposal environment. A cost-benefit analysis was performed to evaluate the value of separating the HLW raffinate into constituent streams (Gombert et al., 2009 ). It was concluded that, aside from the noble metals, there was not a strong cost driver to further segregate the HLW. In the case of noble metals, there was a case for treating them separately under some circumstances. Thus, any further separations would be implemented for reasons other than cost.

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