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

Findlay, J. and Cheneler, D. and Degueldre, C. (2026) Experimental thermodynamic modelling study of RbF-CeF3 system and its application to the RbF-PuF3 system. Journal of Molecular Liquids, 444: 129140. ISSN 0167-7322

Abstract

The thermodynamic characterisation of the RbF-PuF3 system was undertaken using CeF3 as a non-radioactive surrogate for PuF3, a promising candidate fuel component for Molten Salt Reactors (MSRs). The UK's existing plutonium stockpile, if converted to PuF3, represents a viable option for molten salt reactor fuels. Incorporating PuF3 into multicomponent systems may optimise both thermophysical and neutronic properties, making the binary RbF-PuF3 system a key foundation for more complex fuel salt compositions. Novel DSC data from the RbF-CeF3 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 [MF6]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(MF3) = 0.33. This structural information serves as an important reference for the reliable optimisation of more complex RbF-MF3 systems in future thermodynamic modelling. CeF3 was selected over LaF3 in this study due to its closer similarity to PuF3 in melting point, enthalpy of fusion, and ionic radius. Its use revealed eutectics at lower temperatures and higher actinide solubility than comparable AlkF-MF3 (Alk = Li, Na; M = Ce, Pu) systems. Finally, RbmMnF3n+m solid complexes were shown to strongly affect phase behaviour and eutectics, offering greater thermodynamic stability relative to single-eutectic systems such as LiF-PuF3 and NaF-PuF3, albeit with slightly higher melting points. The optimised RbF-PuF3 liquid solution was further assessed with an Ellingham diagram to evaluate redox potentials and possible corrosion resistance improvements.