Evaluation of Processing Conditions for the Reduction of Electrochemical Salt Waste Using Phosphate-Based Dechlorination

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Electrochemical processing of used nuclear fuel in molten chloride salts generates complex radioactive salt waste. Dechlorination of waste salt using phosphate compounds at elevated temperatures (∼600 °C) reduces the volume of material for disposal while evolving gaseous chlorine compounds that could be used to transform metallic uranium into UCl 3 . In this study, the effects of processing temperature, environment (air or argon), and H 3 PO 4 precursor-to-chlorine ratio on the dechlorination efficacy of a simple alkali salt mixture (SSM) and a salt waste simulant (ERV3) were evaluated. For both salts, the highest chlorine release was concurrent with the water boiling. For the SSM, atmospheric O 2 ( g ) significantly affected dehydration and condensation reactions of phosphoric acids processed at <300 °C, which resulted in ∼17 wt % less dechlorination in argon than in air. Processing at >500 °C led to the formation of alkali metaphosphate glasses with <1 wt % chlorine, irrespective of atmosphere. ERV3 samples required 600 °C and slightly higher H 3 PO 4 /Cl ratios than the theoretical value (>1) to consistently dechlorinate <1 wt %. Monazite was detected in samples with P/Cl = 1. Samples were X-ray amorphous when P/Cl = 1.21. Contamination from the silica crucible was detected in the SSM and ERV3 dechlorinated samples, which also affected crucible integrity.