Thermodynamic Investigations of the LiF-CsF and NaF-CsF Pseudo-Binary Systems

The present study describes the thermodynamic assessment of two pseudo-binary systems of relevance to CsF solubility in molten fluoride salts: LiF-CsF and NaF-CsF. Prior to this work, there was limited experimental data published in the open literature on these systems, some of which were inconsistent and had some knowledge gaps. New equilibrium data have been obtained using Differential Scanning Calorimetry on both systems. The motivation for this experimental campaign was threefold: 1) to corroborate previously reported data, 2) to elucidate prior irregularities, and 3) fill some knowledge gaps. These measurements also revealed evidence of another stoichiometric phase, LiCsF2−β, which was not captured in previous assessments. In addition to new experimental measurements, thermodynamic treatments of the two binary systems have been revised using the CALPHAD method with the thermodynamic software FactSage and Optima. This combined experimental and computational campaign provides greater confidence in CsF thermochemical behaviour in Molten Salt Reactor systems.