PNNL

Abstract

Foaming during vitrification of radioactive waste in Joule-Heated Ceramic Melters (JHCM) is exacerbated by trapping of evolving gases, such as CO2, NOx and O2, beneath a highly viscous reaction layer. Foaming restricts heat transfer during melting. Sucrose is employed as the baseline reductant additive at the Hanford site in Washington State, USA to reduce foaming. Alternative carbon-based reductant additives were explored in simulated inactive Hanford high-iron HLW-NG-Fe2 feeds, for both their effect on foaming and to give insight to the behaviour of multivalent species in glass melts under different redox conditions. Graphite, coke (93% C), formic acid and HEDTA additives were compared with sucrose and a feed with no additive. Graphite and coke additions proved most effective in reducing the maximum feed porosity by 11 and 13%, respectively, compared with 3% for sucrose. Lower foaming could result in more efficient vitrification in JHCMs. Reductants also affected redox ratios in the multivalent species present in the feed. The order of reduction, Cr6+/Cr3+ > Ce3+/Ce4+ > Mn3+/Mn2+ > Fe3+/Fe2+ was as predicted on the basis of their redox potentials. There is less reduction overall, particularly in the Fe3+ ? Fe2+, than predicted by the calculations, which is attributed to the oxygenated atmosphere of the experiments.