(34a) 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.