(309a) Evaluation of Chemical Additives for Glycolate Destruction in High-Level Radioactive Waste Condensate in the Defense Waste Processing Facility

Savannah River National Laboratory (SRNL) has completed a work scope in support of the Savannah River Remediation (SRR) Nitric-Glycolic flowsheet. Glycolic acid will replace formic acid as a reducing agent in the pre-processing of High Level Waste sludge performed at the Defense Waste Processing Facility (DWPF). Glycolic acid will perform the same function as formic acid, namely reduction of mercury and adjustment of feed rheology and melter oxidation/reduction potential, while significantly reducing the potential for hydrogen generation in DWPF processing. Development testing has demonstrated glycolic acid virtually eliminates hydrogen production in the pre-processing steps.

The Nitric-Glycolic flowsheet significantly improves DWPF’s ability to address Savannah River Site’s key challenge - the incorporation of effluent received from the Salt Waste Processing Facility (SWPF). SWPF will deliver significant effluent volume to DWPF, resulting in a concurrent increase in DWPF effluent returned to the Concentration, Storage and Transfer Facilities (CSTF). This work scope demonstrates that glycolate can be destroyed under the Nitric-Glycolic flowsheet prior to returning the DWPF recycle stream to CSTF.

To avoid potential flammability issues due to thermolysis of glycolate in CSTF, SRR tasked SRNL to quantify and mitigate glycolate returns via DWPF recycle stream. The development of a strategy for glycolic mitigation initiated with a system’s engineering workshop. Various chemical and/or physical solutions for how to and where to destroy glycolate were considered – consistent with DWPF operational capabilities and process requirements. The workshop identified chemical oxidation of glycolate within the DWPF Recycle Collection Tank (RCT)^[1] as the most promising option with sodium permanganate and Fenton’s reagent (metallic ion catalyzed hydrogen peroxide) as the two most promising oxidants.

SRNL between Fenton’s reagent and permanganate could be made. A test matrix was generated to evaluate the impact of:

• Glycolate concentration,
• Oxidant type (sodium permanganate or Fenton’s reagent),
• Oxidant strike time (rate of delivery),
• Oxidant stoichiometry,
• RCT solution temperature, and
• RCT solution pH.

Testing conditions were selected to reflect DWPF process capability and requirements. Non-prototypic concentrations of process organics were added to the process simulant to challenge the methodology and better support the down select. Tests were conducted with discrete liquid sampling events and online offgas analysis for data generation.

For the test conditions performed and data obtained, three conclusions are drawn.

• Sodium permanganate was clearly superior to Fenton’s reagent as an oxidizing agent for destruction of glycolate in the RCT. Permanganate strikes consistently reduced glycolate concentrations from nominally 125-250 mg/L to below the target threshold of 50 mg/L. Corresponding strikes with Fenton’s reagent did not successfully mitigate glycolate.
• Successful permanganate mitigations of glycolate were demonstrated across broad range of RCT process conditions. These conditions include the following.

1. RCT solution starting pH: 3-13
2. RCT solution temperature: 15-50 ⁰C
3. Oxidant strike time: 10 minutes to 2 hours
4. Glycolate concentration: 125-250 mg/L (with exploratory testing up to 2000 mg/L)
5. Oxidant addition amount: 100-150% stoichiometric requirement for all organics

SRNL, in conjunction with SRR, is continuing the glycolate mitigation development effort. Additional testing include protocol testing intended to clearly define strike parameters for engineering and operational implementation, off-normal testing for remediating foam-over conditions and testing at larger scale or with tank waste. Results from the testing will be discussed.

[1] The RCT collects offgas condensate from pre-processing, vitrification, and other unit operations performed in DWPF and is the singular return vessel delivering recycle effluent back to CSTF.