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Published: 2004

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The Hanford Site in southeastern Washington State has been used extensively to produce nuclear materials for the U.S. strategic defense arsenal by the U.S. Department of Energy (DOE). A large inventory of radioactive and mixed waste has accumulated in 177 single- and double-shell tanks. Liquid waste recovered from the tanks will be pre-treated to separate the low-activity fraction from the high-level and transuranic wastes. Currently, the DOE Office of River Protection (ORP) is evaluating several options for immobilization of low-activity tank wastes for eventual disposal in a shallow subsurface facility at the Hanford Site. A significant portion of the waste will be converted into immobilized low-activity waste (ILAW) glass with a conventional Joule-heated ceramic melter. In addition to ILAW glass, supplemental treatment technologies are under consideration by the DOE to treat a portion of the low activity waste. The reason for using this alternative treatment technology is to accelerate the overall cleanup mission at the Hanford site. The ORP selected Bulk Vitrification (BV) for further development and testing. Work in FY03 on engineered and large scale tests of the BV process suggested that approximately 0.3 to as much as 3 wt% of the waste stream 99Tc inventory would end up in a soluble form deposited in a vesicular layer located at the top of the BV melt and in the sand used as an insulator after vitrification. In the FY03 risk assessment (RA) (Mann et al., 2003), the soluble Tc salt in the BV waste packages creates a 99Tc concentration peak at early times in the groundwater extracted from a 100-meter down-gradient well. This peak differs from the presently predicted baseline WTP glass performance, which shows an asymptotic rise to a constant release rate. Because of the desire by regulatory agencies to achieve essentially equivalent performance to WTP glass with supplemental treatment technologies, the BV process was modified in FY04 in an attempt to minimize deposition of soluble 99Tc salts by including a castable refractory block (CRB) in place of a portion of the refractory sand layer and using a bottom-up melting technique to eliminate the vesicular glass layer at the top. However, the refractory block is still porous and there is the potential for leachable 99Tc to deposit in the pores of the CRB. The purpose of this progress report is to document the status of a laboratory testing program being conducted at Pacific Northwest National Laboratory (PNNL) for CH2M Hill Hanford Group in support of the LAW Supplemental Treatment Technologies Demonstration project. The objective of these tests was to provide an initial estimate of the leachable fraction of key contaminants of concern (Cs, Re [chemical analogue for 99Tc], and 99Tc) that could condense within the BV CRB. This information will be used to guide development of additional modifications to the BV process to further reduce the soluble 99Tc levels in the BV waste package.