(32e) Selection and Preliminary Evaluation of Alternate Reductants for DWPF SRAT Processing

The Defense Waste Processing Facility (DWPF) at the Savannah River Site vitrifies High Level Waste (HLW) for repository internment. The process consists of three major steps: waste pretreatment, vitrification, and canister decontamination/sealing. The HLW consists of insoluble metal hydroxides (primarily iron, aluminum, magnesium, manganese, and uranium) and soluble sodium salts (carbonate, hydroxide, nitrite, nitrate, and sulfate). The pretreatment process in the Chemical Processing Cell (CPC) adds nitric and formic acids to the sludge to lower pH, destroy nitrite and carbonate, and reduce mercury and manganese. The ratio of nitric to formic acids is balanced to control glass REDuction/OXidation potential (REDOX). During this process, hydrogen can be produced by noble metal catalyzed decomposition of excess formic acid.

SRNL has been investigating the use of alternate reductants to formic acid to mitigate the hydrogen generation issue at DWPF. Several reductants were selected for testing during Sludge Receipt and Adjustment Tank (SRAT) processing. The reductants fall into two general categories: reducing acids and non-acidic reducing agents. Reducing acids were selected as direct replacements for formic acid to reduce mercury in the SRAT, to acidify the sludge, and to balance the melter REDOX. Non-acidic reductants were selected as melter reductants and would not be able to reduce mercury in the SRAT.

Based on the testing performed, the only viable short-term path to mitigating hydrogen generation in the CPC is replacement of formic acid with a mixture of glycolic and formic acids. An experiment using glycolic acid blended with formic on an 80:20 molar basis was able to reduce mercury, while also targeting a predicted (REDOX) of 0.2 expressed as Fe2+/"Fe.