Flowsheet Alternative for Chemical Processing of Savannah River Site High Level Radioactive Waste to Minimize Hydrogen Generation

Approximately 150 million gallons of High Level Radioactive Waste (HLW) was generated as a byproduct of tritium and plutonium production for nuclear weapons at the Savannah River Site (SRS). The insoluble solids or sludge portion of HLW is chemically treated in the SRS’s Defense Waste Processing Facility (DWPF) to be incorporated in the glass matrix for permanent stabilization. During chemical processing of the sludge, hydrogen is generated; requiring offgas instrumentation to measure hydrogen and a large air purge is needed to dilute the hydrogen below the Lower Explosive Limit (LEL). The Process Technology Programs (PTP) section of the Savannah River National Laboratory (SRNL) was requested to develop an alternative chemical processing flowsheet for DWPF to minimize hydrogen generation.

Hydrogen is generated in DWPF due to the noble metal catalyzed decomposition of formic acid. Since formic acid is a reducing acid, more than 20 reducing agents and reducing acids were evaluated as an alternative to formic acid. Glycolic acid was chosen as the best replacement for formic acid. A mixture of 80 mole % glycolic acid/20% nitric acid was used as the reducing acid for most of the experiments.

Flowsheet testing was performed to develop the nitric acid-glycolic acid-formic acid flowsheet as an alternative to the nitric acid-formic acid flowsheet currently being processed at the DWPF. This new flowsheet has shown that mercury can be removed in the Sludge Receipt and Adjustment Tank (SRAT) with minimal hydrogen generation. All processing objectives were also met, including greatly reducing the Slurry Mix Evaporator (SME) product yield stress as compared to the baseline nitric acid-formic acid flowsheet. Forty-six runs were performed in total, including the baseline run and the melter feed preparation runs. The results for this flowsheet development will be discussed.