Thermodynamics of the Rbf - Uf4 System: A Potential Fuel Candidate for Molten Salt Reactor

Abstract

Thermodynamic properties of the RbF-UF4 system, a potential fuel for Molten Salt Reactors (MSR), have been investigated for the first time combining experimental data gained by Differential Scanning Calorimetry (DSC), comparing literature data with those obtained by modelling. A comprehensive understanding of phase changes across various compositions and temperatures is presented together with the enthalpy and entropy related parameters of the considered phases. This study underscores the importance of the purity of compounds and the equilibrium between coexisting phases. The use of performant custom-designed light-weight crucibles provides reliable phase diagram reconstruction. In addition to the transformation enthalpies, heat capacities from solid to liquid transitions are measured for the most relevant compositions of the MSR fuel e.g. eutectic. In the absence of complete experimental data for intermediate compounds, the study explores the potential of estimating the heat capacities of individual compounds using Neumann-Kopp approach as a pragmatic way to describe the properties of the system. Emphasis is placed on the role of RbmUnF4n+m solid complexes that play a relevant role on phase properties during phase transitions and three eutectics, offering a higher actinide solubility and a more thermodynamically stable fuel than the more commonly explored singular eutectic LiF – UF4 and NaF – UF­­4 systems.

Keywords: RbF - UF4 system, Differential Scanning Calorimetry, Thermodynamic Optimisation, Phase diagram, Heat capacity, Molten salt reactor fuel