Heat Capacity and Enthalpy of the Binary Eutectic MgCl₂-NaCl Molten Salt System from 400 °C to 800 °C

Molten salt systems have been under study for the past few decades due their applications in future molten salt reactors (MSRs) and pyroprocessing of spent metallic nuclear fuel. The high vaporization temperature and low vapor pressure of some molten salt systems make them excellent mediums for nuclear fuel, reactor coolant, and thermal energy storage. The eutectic composition of MgCl₂-NaCl has been proposed as a heat transfer fluid and fuel carrier component for MSRs as it is both abundant and inexpensive. Understanding the thermochemical and thermophysical properties of molten salt systems is critical before implementation. In this work, thermochemical properties including melting point, enthalpy of fusion (△H_fus), and heat capacity (C_p) at high temperatures were investigated by using high temperature drop calorimetry and differential scanning calorimetry (DSC). The melting point and △H_fus of the eutectic were measured by DSC to be 427.89 °C and 43.84 ± 2.28 kJ/mol respectively. C_p data were derived from drop calorimetric experiments, which were performed on physical mixtures and pre-melted mixtures at 400 °C, 500 °C, 600 °C, 700 °C, and 800 °C. The experimental enthalpy and C_p values of the pre-melted were found to be consistent with theoretical calculations and reported NIST literature values, but higher than values reported in experimental literature. This suggests that the formation of intermediate line compounds at high temperatures may contribute to measured C_p values, which needs to be avoided or corrected in calorimetric investigations.