PNNL

Abstract

The mechanisms by which foaming occurs during vitrification of high-level radioactive waste feeds must be fully understood prior to operation of the Waste Treatment and Immobilization Plant at the Hanford Site, USA, in order to obviate safety issues and reduce the duration of the clean-up project by enhancing the feed-to-glass conversion. In a vigorously foaming high-iron simulated waste feed (HLW-NG-Fe2), the influence of delivering iron as a Fe2+-bearing raw material (FeC2O4·2H2O), rather than a Fe3+ (Fe(OH)3) material, was evaluated in terms of the effect on foaming during melting. A reduction of 50.0 ± 10.8 % maximum generated foam volume is observed using FeC2O4·2H2O as the iron source, compared with Fe(OH)3. This is determined to be due to a large release of CO2 before the foam onset temperature (the temperature above which the liquid phases forming are sufficiently viscous to trap the gases) and suppression of O2 evolution during foam collapse. Structural analyses of simulated waste feeds after different stages of melting show that the remaining Fe2+ in the modified feed is oxidised to Fe3+ at temperatures between 600 and 800 ?. This feed was tested in a Laboratory Scale Melter with no excessive foaming or feeding issues. Analysis of the final glass products indicated that the glasses produced using the original HLW-NG-Fe2 feed using Fe(OH)3 and the feed made with FeC2O4·2H2O are structurally similar but not identical: the difference in the structure converges when the glass is melted for 24 hours, suggesting a transient structural change in the glass using the reduced raw material.