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Published: 1998

Total Pages: 28

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The DWPF at the Department of Energy's (DOE) Savannah River Site (SRS) began radioactive operations in December of 1995. The High Level Waste Tank Farm at SRS contains approximately thirty three million gallons of salt, supernate, and insoluble sludge wastes accumulated during more than three decades of weapons manufacture. In the DWPF, the radioactive components from this waste will ultimately be processed into a stable, borosilicate glass for long-term storage in a geological repository. The feeds to the DWPF are pretreated in a number of steps. Insoluble sludges, primarily aluminum, iron and other transition metals, are combined from several tanks, treated by caustic dissolution of aluminum and washed to remove soluble salts; these materials are removed to increase waste loading in the glass produced by the DWPF. The water soluble radioactive species in the salt and supernate, primarily cesium and actinides, are precipitated by sodium tetraphenylborate (NaTPB) or adsorbed onto sodium titanate. The resulting solids are also washed to remove excessive soluble salts before feeding to the DWPF. The soluble species removed by washing are disposed of as low level radioactive waste in a concrete form known as Saltstone. The presentation includes a brief overview of the High Level Waste system, pretreatment, and disposition of the various streams. The washed tetraphenylborate precipitates of cesium and potassium are hydrolyzed by copper catalyzed formic acid hydrolysis in the Salt Processing Cell (SPC) to yield soluble formates, boric acid, benzene and minor organic byproducts. The benzene and most of the organic byproducts are then steam stripped. The resulting aqueous hydrolysis product, including the still insoluble actinides adsorbed onto sodium titanate, is combined in the Chemical Processing Cell (CPC) with the insoluble sludge which has been treated with nitric acid and formic acid to remove mercury and to adjust the glass redox. Borosilicate glass frit is added and after assuring the melter feed meets glass quality and processing requirements, the slurry is fed to the melter producing glass which is poured into stainless steel canisters. The canisters are sealed, blasted to remove surface contamination, and welded prior to temporary storage in the Glass Waste Storage Building (GWSB). An overview of the DWPF process and its chemistry is included. The composition of the feeds is of primary importance to the DWPF. Critical factors determined by the feeds are related to safety, process design and operability, and glass quality. The Safety Analysis Report (SAR) source term, process shielding, potential for criticality, and generation of flammable gases are safety factors related to feed composition. Canister heat generation, NO(subscript x) emissions, and corrosive species are process design parameters determined by feed composition. Nitrite in the washed precipitate, glass insolubles, glass liquidus (temperature of complete melting) and glass melt viscosity are operability parameters determined by composition. And glass durability is the critical quality parameter which requires knowledge and control of the feed compositions. The basis for each of these composition related factors is presented and the system for specifying feed acceptance criteria is described. The composition, and thus the durability, of the glass is determined by the mixing ratios of sludge insolubles, aqueous hydrolysis product, and frit. The frit is a purchased raw material; naturally, its composition is essentially fixed. Also, the glass components in the aqueous hydrolysis product are essentially invariant because the cesium plus potassium to boron ratio is unity, essentially all of the water is evaporated, and the sodium titanate concentration is carefully controlled in the precipitation process. Therefore, the sludge composition is the primary source of feed variability. The combination of process and tank farm history, strategic tank samples, system waste removal plans, and process modeling which project sludge batch composition and evaluate process related parameters and glass durability is described. All the sludge batches, each of which can feed the DWPF for several years, is projected and evaluated through completion of waste removal. Finally, extensive sludge characterization through sampling and analysis is combined with small scale testing in the Shielded Cells of the Savannah River Technology Center (SRTC) to assure the sludge batch meets all the feed acceptance criteria.