A comparison of ceramic and carbon‐based reductants for vitrification of low‐activity waste

Sucrose is the current baseline additive at the Hanford Waste Treatment and Immobilization Plant in Washington to control foaming during waste feed to glass transitions and the redox state of the glass melt. Alternative reductants are being investigated to alleviate strain on effluent treatment from toxic acetonitrile production from incomplete combustion of sucrose. This study evaluates ceramic additive options including B4C, B6Si, SiC, and VB2 in simulated low‐activity waste feed, as well as coke dust, probing the feed volume expansion during melting as well as the gas evolution. All alternative reductant options examined significantly reduced acetonitrile production; however, there was variability in their effectiveness as foam‐reducing agents. VB2 and coke matched the performance of sucrose in controlling foam volume and glass redox state, but with notably less acetonitrile production. B4C, B6Si, and SiC demonstrated improved foam control and very little acetonitrile production; however, the final glasses were over‐reduced, that is, Fe2+/FeT ≥ 0.5. These alternative reductant studies provide operational flexibility to the operation of the vitrification plant, as well as options for alternative raw materials in industrial glass melting.

What is it about?

The study examined alternative reductants to sucrose for use in vitrification processes at the Hanford site, focusing on reducing foam formation and toxic gas emissions. It utilized thermally stable ceramic reductants, such as boron nitride (BN), coke dust, and various refractory-like ceramics, in simulated melter feeds. BN was particularly effective, demonstrating a 90% reduction in acetonitrile production and improved redox control. The research found that coke dust and VB₂ were as effective as sucrose in foam control but did not exhibit low-temperature foaming. Additives like B₄C, SiC, and B₆Si also reduced foaming, although they led to over-reduction in the final glass products. The study suggested that an optimal X/N ratio might be necessary for achieving comparable results to sucrose.

Why is it important?

This study is important as it addresses the critical issue of nuclear waste management by exploring alternative ceramic reductants to improve the vitrification process. By investigating materials that reduce foaming and toxic gas emissions, the research contributes to safer and more efficient handling and storage of radioactive waste. The introduction of thermally stable ceramic compounds as reductants provides a new avenue for minimizing environmental hazards and operational risks associated with traditional organic reductants like sucrose. These findings hold significant implications for enhancing the sustainability and safety of nuclear waste treatment facilities, potentially influencing future policies and practices in waste management.

Key Takeaways:

1. Reduced Toxic Emissions: The study indicates that alternative ceramic reductants significantly lower the off-gassing of toxic acetonitrile compared to sucrose, suggesting a safer approach for nuclear waste vitrification processes.

2. Improved Foam Control: While coke and VB₂ showed similar effectiveness in controlling foaming as sucrose, other ceramic reductants like B₄C, SiC, and B₆Si reduced maximum foaming and total foam volume, although they led to over-reduction in the final glass.

3. Enhanced Material Stability: The research highlights that using refractory-like ceramics, which are thermally stable, can effectively control foam and redox conditions without the low-temperature foaming observed with sucrose, offering a robust alternative for industrial applications.