(32c) Impact of Acid Addition On Mercury Stripping Rate During High Level Waste Pretreatment at the Savannah River Site

The Defense Waste Processing Facility (DWPF) at the Savannah River Site vitrifies radioactive High Level Waste (HLW) for repository internment. The process consists of three major steps: waste pretreatment, vitrification, and canister decontamination/sealing. HLW consists of insoluble metal hydroxides (primarily iron, aluminum, calcium, magnesium, manganese, and uranium) and soluble sodium salts (carbonate, hydroxide, nitrite, nitrate, and sulfate). In addition, the waste contains oxides and hydroxides of silver, palladium, rhodium, ruthenium, and mercury. The pretreatment process in the Chemical Processing Cell (CPC) consists of two process tanks, the Sludge Receipt and Adjustment Tank (SRAT) and the Slurry Mix Evaporator (SME) as well as a melter feed tank. During SRAT processing, nitric and formic acids are added to the sludge to lower pH, destroy nitrite and carbonate ions, and reduce mercury and manganese. Once reduced, the mercury is steam stripped from the waste by reflux evaporation through a decanter designed to collect the condensed mercury. Startup testing of the DWPF process indicated that one pound of mercury could be removed from the process for every 750 pounds of water evaporated. Recent studies during qualification of Sludge Batch 5 (SB5) indicated that the removal rate is dependent on the amount of acid added during pretreatment. Results from the SB5 laboratory testing with simulants as well as radioactive samples of the waste will be compared to DWPF processing data. In addition, results from Sludge Batch 6 laboratory tests will be presented.