Impact of Eliminating Mercury Removal Pretreatment on the Performance of a High-Level Radioactive Waste Melter Offgas System

摘要

The Defense Waste Processing Facility (DWPF) at the Savannah River Site processes high-level radioactive wastenfrom the processing of nuclear materials that contains dissolved and precipitated metals and radionuclides. Vitrificationnof this waste into borosilicate glass for ultimate disposal at a geologic repository involves chemicallynmodifying the waste to make it compatible with the glass melter system. Pretreatment steps include removal ofnexcess aluminum by dissolution and washing, and processing with formic and nitric acids to (1) adjust the reduction–noxidation (redox) potential in the glass melter to reduce radionuclide volatility and improve melt rate; (2) adjustnfeed rheology; and (3) reduce, by steam stripping, the amount of mercury that must be processed in the melter.nElimination of formic acid pretreatment has been proposed to eliminate the production of hydrogen in the pretreatmentnsystems; alternative reductants would be used to control redox. However, elimination of formic acidnwould result in significantly more mercury in the melter feed; the current specification is no more than 0.45 wt%,nwhereas the maximumexpected before pretreatment is about 2.5wt%.An engineering study has been undertaken tonestimate the effects of eliminating mercury removal on the melter offgas system performance. A homogeneous gasphasenoxidation model and an aqueous phase model were developed to study the speciation of mercury in thenDWPF melter offgas system. The model was calibrated against available experimental data and then applied tonDWPF conditions. The gas-phase model predicted the Hg2tn2 =Hg2t ratio accurately, but some un-oxidized Hg0nremained. The aqueous model, with the addition of <1mM Cl2 showed that this remaining Hg0 would benoxidized such that the final Hg2tn2 =Hg2t ratios matched the experimental data. Results of applying the model tonDWPF show that due to excessive shortage of chloride, only 6% of the mercury fed is expected to be chlorinated,nmostly as Hg2Cl2, whereas the remaining mercury would exist either as elemental mercury (90%) or HgO (4%).