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

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 UCl3. In this study, the effects of processing temperature, environment (air or argon), and H3PO4 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 O2(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 H3PO4/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.