Partitioning behavior of cesium in an alkali fluoride and its implications for cesium separation via melt-crystallization

Liquid-fueled molten-salt reactors (MSRs) are advanced nuclear-fission systems in which fuel is dissolved in molten salt. So far, all successfully demonstrated MSRs have used fluoride-based salts. Our investigation focuses on recovering non-radioactive cesium from a fluoride-salt surrogate (CsF-LiF-KF-NaF). FLiNaK, the eutectic mixture of LiF-KF-NaF, is widely used as a surrogate to study the physical and chemical behavior of fluoride salts for nuclear-energy applications and serves as the base salt in this work. Cesium-137, a high-yield fission product with a 30-year half-life, emits beta particles and gamma radiation, posing significant contamination risks due to its high water solubility and mobility. The separation of fission products from the salt can substantially reduce the volume of long-term nuclear waste. This study investigates a thermally controlled solid-liquid separation process based on melt-crystallization to effectively separate CsF from the rest of the matrix. Our study shows that cesium preferentially concentrates in the liquid phase during melt-crystallization. This behavior is supported by high-temperature (HT) X-ray diffraction (XRD), which shows no detectable Cs-containing crystalline phases above the solidus, implying that Cs predominantly partitions into the liquid. Furthermore, the calorimetric data shows that the addition of 10 wt.% of CsF to FLiNaK barely affects the system's melting temperature compared to that of pure FLiNaK. This study reveals how fission products might affect the thermal behavior of a fluoride-based fuel salt in MSRs and highlights the potential of employing melt-crystallization to effectively separate cesium from a fluoride matrix.