Cold cap glass-melt migration for radioactive waste vitrification at the Hanford site

With public and environmental pressure to clean-up the 200,000 m3 of radioactive wastes at the Hanford site, WA, USA, in a timely manner, many pathways to efficient processing of the wastes are being explored. One avenue, rife for exploration, is the waste feed-to-glass transition that occurs within a “cold cap” layer inside the melters, used to vitrify the wastes into stable glass wasteforms, for long-term immobilisation of the radioactive isotopes. Foaming beneath the cold cap, caused by the trapping of evolving gases in the glass-forming melt, can restrict heat-transfer from the glass melt to the reacting feed, thus reducing the efficiency of the melters, and the clean-up project. For a high-foaming, high-iron waste feed, HLW-NG-Fe2, a series of laboratory-scale representations of the cold cap were examined for the original feed, a reduced iron raw material (FeC2O4·2H2O), as well as added sucrose, graphite, coke, formic acid and HEDTA. The FeC2O4·2H2O raw material reduced foaming by 50 ± 2%, with minimal change to the final glass structure. Graphite and coke were most effective additives in foam reduction, reducing foaming by 51 ± 3 and 54 ± 3%, respectively. The effect of the reductants on the redox behaviour of Fe, Cr, Mn and Ce with temperature was explored, as well as the contribution to O2 evolution. Manipulation of the structure of the iron in the melt influenced the level of precipitation of Fe, Mn, Cr and Ni -bearing spinel crystals. The final graphite and coke glasses had Fe2+ content above the contract limit, and a structural change identified in most of the glasses, requiring further examination, appears to influence the chemical durability and glass transition temperature. Evidence is presented to suggest adoption of the reductants is feasible for mitigation of excessive foaming, pending further optimisation, although thus far, there is no indication that they offer improved melting rates.