(66e) Impact of Noble Metals and Mercury on Hydrogen Generation during High Level Waste Pretreatment at the Savannah River Site

The Defense Waste Processing Facility (DWPF) at the Savannah River Site vitrifies radioactive High Level Waste (HLW) for repository internment. The process consists of three major steps: waste pretreatment, vitrification, and canister decontamination/sealing. HLW consists of insoluble metal hydroxides (primarily iron, aluminum, calcium, 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 ions, and reduce mercury and manganese. Glass formers are added, and the batch is concentrated to the final solids target prior to vitrification. During these processes, hydrogen can be produced by catalytic decomposition of excess formic acid. The waste contains silver, palladium, rhodium, ruthenium, and mercury, but silver and palladium have been shown to be insignificant factors in catalytic hydrogen generation.

A statistically-designed experimental study was used to evaluate the impact of varying levels of rhodium, ruthenium and mercury on hydrogen generation using a non-radioactive HLW simulant. Findings from this study were:

?« Rh controlled the maximum hydrogen generation rate in the first two hours after acid addition.

?« Ru controlled the maximum hydrogen generation rate after the period of Rh control had passed, typically 6-8 hours later.

Increasing the ratio of Hg/Rh shifted the time of the maximum hydrogen generation rate from the earlier Rh period to the later Ru period when holding Ru at its fission yield ratio to Rh.

A previously documented inhibiting effect of Hg on hydrogen generation apparently requires very little mercury in terms of moles Hg/mole Rh (or Ru). Additional increases in Hg concentration produce only a minimal inhibition in hydrogen generation rates.

Low Hg runs do not necessarily bound high Hg runs for the maximum hydrogen generation rate over the full CPC cycle. Two of the four Rh-Ru combinations had a cross-over point where the hydrogen generation rate in the high Hg run went from always lower to always higher than in the low Hg run.

Maximum hydrogen generation rates in the high Hg runs could exceed the maximum hydrogen generation rates from the low Hg runs.