Impact of Impurities and Degradation Products during Chemical Processing of High Level Waste in the Defense Waste Processing Facility

SRNL-STI-2017-00221

Impact of Impurities and Degradation Products During Chemical Processing of High Level Waste in the Defense Waste Processing Facility

Dan P. Lambert, Wesley H. Woodham, Matthew S. Williams, J. David Newell, Jack Zamecnik, Savannah River National Laboratory, Aiken, SC, USA

Abstract

Testing was completed to develop a chemical processing flowsheet for the Defense Waste Processing Facility (DWPF), designed to vitrify and stabilize high level radioactive waste.  DWPF processing uses a reducing acid (formic acid) and an oxidizing acid (nitric acid) to rheologically thin the slurry and complete the necessary acid base and reduction reactions (primarily mercury and manganese).  Formic acid reduces mercuric oxide to elemental mercury, allowing the mercury to be removed during the boiling phase of processing through steam stripping.  In runs with active catalysts, formic acid can decompose to hydrogen and nitrate can be reduced to ammonia, both flammable gases, due to rhodium and ruthenium catalysis.  Replacement of formic acid with glycolic acid eliminates the generation of rhodium- and ruthenium-catalyzed hydrogen and ammonia. In addition, mercury reduction is still effective with glycolic acid. Hydrogen, ammonia and mercury are discussed in the body of the report.

However, other flammable degradation products from the antifoam and from impurities in the glycolic acid combine to add additional flammable gases. Studies were completed to understand the impact of impurities and reaction products in the glycolic acid (formic acid, formaldehyde, methoxyacetic acid, and methanol) and the degradation products in the antifoam (propanal, hexamethyldisiloxane, and trimethylsilanol).