(140e) Acid Consumption of Savannah River Site High Level Waste during Vitrification Pretreatment Processing

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, 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, 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. Minimizing excess formic acid, and associated hydrogen production, requires an accurate prediction of the acid requirement.

During a qualification test with a real waste sample, hydrogen was produced in excess of both the DWPF process limit and the levels during non-radioactive simulant testing. Studies were conducted to better understand the acid consumption during the pretreatment processes in order to gain a better understanding of the process chemistry and to ensure that hydrogen generation rates in non-radioactive simulations bound those of real waste testing. The current equation used to determine the acid addition during pretreatment contains an experimentally determined “stoichiometric multiplier” that varies for each sludge batch. The current equation focuses on the anionic demand (hydroxide, carbonate, and nitrite) along with the requirement for Mn reduction.

Experimental studies indicated that the coefficients for both Mn reduction and nitrite decomposition reactions in the current stoichiometric equation should be increased. Acid consumption from Ca and Mg is not accounted for in the current equation by any term. The acid consumption associated with carbonate destruction is coupled to other measures in the equation resulting in an error due to partial double counting of the acid requirement. Three alternative acid equations were investigated. One alternative was a minimal revision of the current anion-based equation coefficients. A second alternative was to consider the acid requirement from a cationic demand perspective. The third alternative was to combine the best features of both alternatives into a combined anionic-cationic based stoichiometric acid requirement equation.