Phosphate-Based Dechlorination of Electrorefiner Salt Waste using a Phosphoric Acid Precursor
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Authors
Murray, Paige N
Issue Date
2024
Type
Thesis
Language
en_US
Keywords
Dechlorination , Electrochemical processing , Phosphate glass , Pyroprocessing
Alternative Title
Abstract
Electrochemical processing of spent nuclear fuel in molten chloride salts results in radioactive salt waste. A growing global energy demand has revitalized interest in nuclear energy technology. With increasing quantities of used nuclear fuel (UNF), the Department of Energy (DOE) launched the “Converting UNF Radioisotopes Into Energy” (CURIE) program in 2022, providing resources for projects focusing on advancing UNF recycling technologies, including advanced non-aqueous methods like electrochemical reprocessing. Projects supported by the CURIE program will aim to reduce the volume of high-level waste (HLW) that will require permanent disposal and will also potentially provide feedstock that could be used in domestic advanced reactors. To that end, salt partitioning is an effective way to reduce HLW volume before long-term repository storage.Chlorine removal from the salt waste has been identified as an effective and efficient first step in the management of this and volume reduction of this HLW. Cl removal from the salt waste stream also aides in the waste form process as Cl solubility in the baseline oxide glasses like borosilicate glass tends to be very low, and the capacity of baseline mineral waste form options such as sodalite and apatite have very limited Cl capacities.
In this work, a simple salt was dechlorinated with a phosphoric acid phosphate precursor, resulting in a glassy dechlorinated product. Because it is unknown if these processes would be conducted in a hot cell with an air or argon environment, dechlorination efficacy was evaluated in both environments. It was found that dechlorination was possible in both air and argon environments, although atmospheric oxygen appeared to play a significant role in the process. To evaluate temperature and atmosphere dependence on the extent of dechlorination, scanning electron microscopy (SEM)/energy dispersive X-ray spectroscopy (EDS) and powder X-ray diffraction (XRD) was performed on the intermediate products formed under different temperature conditions for each atmosphere. Thermogravimetric analysis (TGA) and evolved gas analysis (EGA) were used to understand the off-gas behavior during dechloination under different processing conditions (e.g. heating profile and environment).
This work serves as an initial step to advance the Technological Readiness Level of H3PO4-based dechlorination step towards implementation of iron phosphate waste forms to immobilize electrochemical fuel reprocessing salt waste streams. Sections of this thesis are verbatim reproduced from the study published by the American Chemical Society in the journal ACS Omega (“Phosphate-Based Dechlorination of Electrorefiner Salt Waste using a Phosphoric Acid Precursor”).