(372b) Pilot-Scale Testing of the Glyocolic-Nitric Acid Flowsheet

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 ~4 million gallons of waste have been processed and 3,800 two-ton canisters of glass (~15 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.  A laboratory-directed research and development grant was awarded to investigate potential flowsheet improvements through process intensification.

Process modeling and proof of concept testing were completed to demonstrate the viability of replacing the existing semibatch process with a continuous process.  Three process constraints make the development of a continuous process challenging:

1. Chemical reactions destroying carbonate and nitrite produce 50 L gas per L of slurry
2. The waste stream is a non-newtonian slurry and must be well mixed throughout processing
3. The reduction of mercury and separation of mercury from the slurry has a low yield in current processing

Several continuous reactors, suitable for slurry processing, were evaluated.  All needed modification to handle the high offgas generation rates.  The best candidate was tested in lab-scale experiments to demonstrate the feasibility of continuous processing.  The paper will describe the process flowsheet development and the recommended path forward in development of a continuous process for DWPF.