High-Entropy Li-Na-K Fluoride-Chloride Melts for MSRs

This study theoretically evaluates the thermophysical properties of high-entropy molten salt mixtures composed of lithium, sodium, and potassium fluorides and chlorides, exhibiting enhanced thermodynamic stability as a result of the high configurational entropy. The equimolar mixture, ${Li}_{1/6}{Na}_{1/6}{K}_{1/6}{F}_{1/4}{Cl}_{1/4}$, maximizes configurational entropy. This mixture's configurational entropy ($\frac{1}{2}R\ln 24\approx 1.59R$ = 13.21 J mol^-1 K^-1) approaches the high-entropy threshold of Rln5 (13.38 J mol^-1 K^-1). These high-entropy mixtures, with entropy significantly exceeding that of FLiNaK (6.9 J mol^-1 K^-1), are promising coolants for molten salt reactors due to improved thermodynamic stability. Two eutectic compositions with reduced lithium content, derived from the phase diagram (melting points 606 and 630 °C), are also analyzed. Density, heat capacity, thermal conductivity, and viscosity are calculated from liquidus to 1000 °C using phenomenological superposition-extrapolation, thermodynamic perturbation theory (TPT), and molecular dynamics with machine learning interatomic potentials (MLIPs).