(608d) Investigating Fission Product Separation from High-Level Waste Chloride Salt Systems, with a Focus on Cesium, through Melt-Crystallization Techniques.

Nuclear fission reactors employ fissile materials, like U-235, to initiate controlled chain reactions, resulting in the generation of heat. Reprocessing of spent nuclear fuel plays a critical role in recovering fissile materials and reducing the volume of nuclear waste. One such reprocessing technique, pyroprocessing, involves dissolving spent nuclear fuel in a bath of molten salt and electrochemically separating the actinides using an electrorefiner. Typically, a LiCl-KCl eutectic salt system serves as the electrolyte and solvent. The fission products remain in the salt. Historically, the salt, along with the fission products, has been disposed of as solid waste. However, separating fission products from the salt is advantageous environmentally and economically, as it reduces the volume of waste produced. The present research proposes a novel, thermally controlled solid-liquid separation process for recovering Cs⁺ from a chloride-salt matrix (LiCl-KCl-CsCl-NaCl). Cesium-137 is a high-yield fission product, with a half-life of 30 years, and is a significant source of radioactive environmental contamination with high water solubility and mobility. The choice of the salt matrix, particularly the presence of NaCl can be explained by the fact that Idaho National Laboratory has a history of processing sodium-bonded fuels. The system was investigated using differential scanning calorimetry (DSC), high-temperature X-ray diffraction (HT-XRD), inductively coupled plasma optical-emission spectroscopy (ICP-OES), and thermodynamic modeling. The calculation and analytical results showed that Cs⁺ can be concentrated in the liquid phase during melt-crystallization. A novel two-heat-source melt-crystallizer was designed and used to collect liquid samples during the partial-crystallization process to demonstrate the concept.

A different method, employing a cold finger, has also been tested using this composition. Findings from the experiments corroborate the trends observed with the two-heat-source setup, with cesium being concentrated in the liquid phase. Notably, the cold finger setup offers an enhancement: it incorporates liquid mixing, facilitating the uniform distribution of temperature throughout the bulk molten salt, thus aiding in achieving thermal equilibrium more effectively.