Thermodynamic investigation of the NaCl-KCl salt system from 25 to 950 °C

NaCl-KCl molten salt system has been proposed as a primary component of several promising heat transfer eutectics and fuel host for Generation IV molten salt reactors (MSR). In this work, several key thermodynamic parameters, including high temperature enthalpies, isobaric heat capacities (Cp) and molar enthalpies of mixing (ΔHmix), of the NaCl-KCl system (NaCl, 75 mol% NaCl – 25 mol% KCl, 51 mol% NaCl – 49 mol% KCl, 25 mol% NaCl – 75 mol% KCl, and KCl) were measured by high temperature drop calorimetry (HTDC) utilizing laser sealed nickel and aluminum crucibles. Salts were checked for phase purity by X-ray diffraction (XRD), and water content by Karl Fischer coulometric titrimetry and thermogravimetric analysis (TGA). Thermo-mechanical analysis (TMA) utilizing custom-built boron nitride (BN) crucibles was employed to determine temperatures of phase transitions (solid → liquid) for the salts and plotted against the pseudobinary phase diagram for the NaCl-KCl system. Small deviations (3–10 %) were found among Cp values obtained from HTDC, differential scanning calorimetry (DSC), and NIST, comparable to the average uncertainty based on two standard deviations of measured data that are ∼6% for DSC and ∼3% for HTDC. Additionally, the measured enthalpies and Cp values of mixed NaCl-KCl salts (e.g., 51 mol% NaCl – 49 mol% KCl eutectic) were shown to behave as a nearly statistical mixtures of its pure endmembers, consistent with ΔHmix = −1.324 kJ/mol at 800 °C, and a regular interaction parameter of Ω = 4.77 ± 0.79 kJ/mol. This study represents a step towards improved accuracy and precision in determinations of thermodynamic parameters of molten salts by using DSC, TMA, and HTDC, in completion of the Molten Salt Thermal Properties Database (MSTDB).