Process Development for Savannah River Site High Level Radioactive Waste

The Defense Waste Processing Facility (DWPF) treats legacy nuclear waste generated at the Savannah River Site (SRS) during production of plutonium and tritium required during the Cold War. Approximately 35 million gallons of nuclear waste is first treated via a complex sequence of controlled chemical reactions and then vitrified into a borosilicate glass form and poured into stainless steel canisters. Converting the nuclear waste into borosilicate glass canisters is a safe, effective way to reduce the volume of the waste and stabilize the radionuclides.

Radioactive processing in DWPF began in 1996.  To date 3.7 million gallons of waste have been processed and 2,562 two-ton canisters of glass (10 million pounds) have been produced.  However, higher facility throughput is needed to shorten the processing life of DWPF.  The current processing flowsheet is unable to handle this higher throughput.  In addition, there are safety (hydrogen generation) and processing issues (pH and anion depletion due to catalytic formate decomposition) that are impacting production.  Savannah River National Laboratory was asked to investigate potential flowsheet improvements.

Flowsheet development was completed in four stages, (1) identification of alternative reductants and acids in simple screening tests, (2) extensive testing of two best alternative flowsheets, (3) selection of the best alternative using a systems engineering evaluation, and (4) lab-scale testing to determine optimal processing conditions and to evaluate downstream processing impacts. 

Bench-scale flowsheet testing was performed to develop the new nitric-glycolic acid flowsheet as an alternative to the existing nitric-formic acid flowsheet currently used by the DWPF.  Testing established that the new flowsheet can effectively reduce mercury and remove it by steam stripping with minimal catalytic hydrogen generation.  All other processing objectives were met, including greatly reducing the Slurry Mix Evaporator (SME) product yield stress and increasing throughput compared to the baseline nitric/formic flowsheet. 

The vast majority of the experiments performed to date have been completed using nonradioactive simulants.  A successful demonstration of the flowsheet with radioactive waste was completed in August 2011.  The paper will describe the process flowsheet development and the remaining work required prior to implementing the flowsheet in DWPF.