Experimental thermodynamic modelling study of RbF-CeF3 system and its application to the RbF-PuF3 system

The thermodynamic characterisation of the RbF-PuF 3 system was undertaken using CeF 3 as a non-radioactive surrogate for PuF 3 , a promising candidate fuel component for Molten Salt Reactors (MSRs). The UK's existing plutonium stockpile, if converted to PuF 3 , represents a viable option for molten salt reactor fuels. Incorporating PuF 3 into multicomponent systems may optimise both thermophysical and neutronic properties, making the binary RbF-PuF 3 system a key foundation for more complex fuel salt compositions. Novel DSC data from the RbF-CeF 3 binary, combined with literature sources, enabled a systematic description of phase equilibria across different compositions and temperatures. Particular emphasis was placed on compound purity, phase coexistence, and the use of lightweight custom-designed crucibles, which proved essential for reliable phase diagram reconstruction. In the absence of complete experimental data for intermediate compounds, their thermodynamic properties were estimated using the Neumann–Kopp relation and the CALPHAD method. The liquid phase was described using the Modified Quasichemical Model in the Quadruplet Approximation (MQMQA), applied here for the first time to include liquid complexes of the type [MF 6 ] 3− (M = Ce, Pu). Raman spectroscopy has independently confirmed the presence of these species for M = La and Ce, revealing specific short-range ordering at X(MF 3 ) = 0.33. This structural information serves as an important reference for the reliable optimisation of more complex RbF-MF 3 systems in future thermodynamic modelling. CeF 3 was selected over LaF 3 in this study due to its closer similarity to PuF 3 in melting point, enthalpy of fusion, and ionic radius. Its use revealed eutectics at lower temperatures and higher actinide solubility than comparable AlkF-MF 3 (Alk = Li, Na; M = Ce, Pu) systems. Finally, Rb m M n F 3n+m solid complexes were shown to strongly affect phase behaviour and eutectics, offering greater thermodynamic stability relative to single-eutectic systems such as LiF-PuF 3 and NaF-PuF 3 , albeit with slightly higher melting points. The optimised RbF-PuF 3 liquid solution was further assessed with an Ellingham diagram to evaluate redox potentials and possible corrosion resistance improvements. • Thermodynamic properties of the RbF-CeF 3 and RbF-PuF 3 molten salt system and intermediate solid compounds were successfully determined using CeF 3 as the most suitable surrogate material to date. • Phase equilibria were determined by TGA/DSC using novel custom lightweight crucibles offering higher thermal sensitivity for clearer DSC signal interpretation. • The liquid phase was modelled with one [MF 6 ] 3− complex (M = Ce, Pu), ordering at X(MF 3 ) = 0.25 and confirmed by Raman spectroscopy in surrogate systems, to assist in constructing higher-order RbF-MF 3 systems. • C P data of MF 3 and RbF intermediate solid compounds were calculated using CALPHAD methodology and accurate C P functions from the Neumann Kopp rule. • Two eutectics were confirmed and redox potential was compared with other base salts and common alloy corrosion products.