Melting behaviour of simulated radioactive waste as functions of different redox iron-bearing raw materials

Highlights

• Fe²⁺ iron raw material reduces foaming in the HLW-NG-Fe2 feed during melting.

• O₂ evolution is suppressed with an Fe²⁺ iron raw material during melting.

• Structural changes occur at different temperatures with Fe³⁺ and Fe²⁺ raw materials.

• Large releases of CO₂ prior to foam collapse cause reduction in overall foaming.

• Iron raw material selection may have a structural effect on the final glass product.

Abstract

Improved understanding of the mechanisms by which foaming occurs during vitrification of high-level radioactive waste feeds prior to operation of the Waste Treatment and Immobilization Plant at the Hanford Site, USA, will help to obviate operational issues and reduce the duration of the clean-up project by enhancing the feed-to-glass conversion. The HLW-NG-Fe2 high-iron simulated waste feed has been shown to exhibit excessive foaming, and the most recent predictive models overestimate the feed melting rate. 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 effects on foaming during melting, to improve understanding of the mechanisms of foam production. A decrease 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 °C. This feed was tested in a Laboratory Scale Melter with no excessive foaming or feeding issues. Analysis of the final glass products indicates 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 h, suggesting a transient structure slightly different to that of the baseline in the glass produced using the reduced raw material.