Structure-property relationships in actinide containing molten salts – A review: Understanding and modelling the chemistry of nuclear fuel salts

Molten salts have recently received increased attention worldwide as key materials for sustainable and low-carbon energy supply technologies (e.g. thermal energy storage, concentrated solar power technologies, fission and fusion nuclear reactors) thanks to their appealing thermo-physical properties. In particular, they are used as fuel and coolant in the Molten Salt Reactor (MSR), considered a breakthrough technology for the next generation of nuclear fission reactors. This review focuses on the thermochemical and thermo-physical properties of actinide bearing fluoride and chloride salts, used as nuclear fuel in MSRs, and more specifically on the structure–property relationships. In the last 15 years, the knowledge on the structural properties of actinide containing salts has grown quite significantly thanks to the development of dedicated high temperature in situ X-ray Absorption Spectroscopy measurements, and to the advancement of atomistic simulations. These have evidenced the formation of short-range order in the liquid, which contributes to storing energy in the salt and directly influences transport and thermodynamic excess properties. This interrelationship is illustrated in the present article, which covers both experimental and computational information, as well as most recent developments in the modelling methods at the mesoscale of the structural, thermodynamic, density and viscosity properties using the CALPHAD methodology.