(393f) Catalytic Hydrogen Generation In Dwpf Chemical Processing Cell
The Defense Waste Processing Facility (DWPF) at the Savannah River Site vitrifies High Level Waste (HLW) for repository internment. The process consists of three major steps: waste pretreatment, vitrification, and canister decontamination/sealing. The HLW consists of insoluble metal hydroxides (primarily iron, aluminum, magnesium, manganese, and uranium) and soluble sodium salts (carbonate, hydroxide, nitrite, nitrate, and sulfate). The pretreatment process in the Chemical Processing Cell (CPC) adds nitric and formic acids to the sludge to lower pH, destroy nitrite and carbonate, and reduce mercury and manganese. The ratio of nitric to formic acids is balanced to control glass redox. During this process, hydrogen can be produced by noble metal catalyzed decomposition of excess formic acid.
During a qualification test with a real waste sample, hydrogen was produced in excess not only of what was seen in non-radioactive simulant testing, but also of the scaled DWPF process limit. To gain better understanding of the process chemistry and to ensure that hydrogen generation rates in non-radioactive simulations bound those of real waste testing, a hydrogen research program was expanded and accelerated. The real waste test was repeated without exceeding the DWPF limits, though hydrogen generation was still in excess of that seen in simulant testing. It has been hypothesized that a stagnant region in the vessel contributed to the high hydrogen generation rate in the original test.
Additionally, work is in progress to learn more how the speciation and morphology of noble metals and the presence or absence of mercury affects the rate of hydrogen generation. A complimentary program evaluating the equation used to determine the amount of acid to add is also in progress with the intent of lessening sensitivity to analytical error in difficult to measure parameters.