(162a) Reducing Mercury Emissions and Increasing Mercury Recovery from Processing High Level Radioactive Waste in the Defense Waste Processing Facility at the Savannah River Site

The Defense Waste Processing Facility (DWPF) at the Savannah River Site (SRS) near Aiken, South Carolina, processes and immobilizes the legacy waste from the production of nuclear weapons materials. Although great progress has been made in processing and vitrifying high level waste (HLW) at SRS, approximately 100 million gallons of radioactive waste remains to be processed.

In this paper, we discuss a flowsheet alternative that is being considered for use in processing high level radioactive waste to increase condenser efficiency, which could potentially decrease mercury emissions from the DWPF stack. The flowsheet change aims to further decrease the air purge during chemical processing. The lower purge is possible due to the elimination of hydrogen, ammonia and three flammable decomposition gases as a result of replacement of the antifoam and the use of glycolic acid instead of formic acid.

Testing demonstrated that the lower air purge is feasible. However, the lower purge also decreased detection limits for offgas species and allowed the identification of carbon monoxide (CO) for the first time. CO is produced concurrently with carbon dioxide (CO₂). In addition, one of the tests used nitrogen as a purge gas in place of air and this led to almost complete recover of mercury during the DWPF simulation. This new discovery offers a potential improvement to recovering mercury during processing. At present, no mercury is recovered in DWPF as planned. The potential for this lower air purge both increasing the condenser efficiency, decreasing mercury release through the stack and increasing mercury recovery will be discussed.