Separation of fission products from high-level waste salt systems by partial crystallization: CsCl-NaCl-LiCl-KCl study

Abstract

Pyroprocessing refers to a series of high-temperature molten-salt processes used to manage and recycle nuclear materials to reduce the waste volume and hazardous environmental impact of the final waste products. Fission products with high halide affinity, including cesium (Cs-137) with a half-life of 30 years, are dissolved into a molten-salt bath. The separation and recovery of cesium can benefit waste-stream minimization because Cs-137—a strong gamma-ray emitter—is a potential source of environmental contamination. This work proposes the recovery of Cs+ from a chloride-salt matrix using a thermally controlled solid-liquid separation. Thermodynamic calculations for the CsCl-NaCl-LiCl-KCl system were performed to predict the partitioning of Cs+ into liquid and solid phases as functions of salt composition and temperature. The selected composition simulated the formulation of high-level salt waste from used fuel treatment. These calculations showed that once the solidus temperature was reached, Cs+ partitions to the liquid phase. According to the thermodynamic calculations, a thermally controlled liquid-solid separation holds the potential to recover up to 92.8 % of the initial cesium chloride. The phase transition temperatures of the sample, including the solidus and liquidus temperatures, were determined using differential scanning calorimetry. The phase composition of the system was studied at various temperatures using high-temperature X-ray diffraction. A laboratory-scale crystallization apparatus was used to melt the sample partially, allowing the collection of the liquid fraction for elemental analysis. All results demonstrated that Cs+ in this system concentrates in the liquid phase and confirmed the viability of recovering CsCl through thermally controlled solid-liquid separation.