Background: Molten fluorides are used as a fuel carrier and coolant in the Molten Salt Reactor (MSR) and have been proposed to be the primary and secondary coolants for Advanced High Temperature Reactor (AHTR) and Very High Temperature Reactor (VHTR) in recent years. KF-ZrF4 (58-42 mol%) (FKZr) is of great potential for the above applications. Thermophysical properties of FKZr such as density

liquidus temperature

viscosity

thermal conductivity have important influences on the reactor physics and thermal hydraulic design. Few reports can be found in literature

and obvious discrepancies exist among the data obtained by different researchers. Purpose: On the basis of preparation of qualified FKZr salt

thermophysical properties such as melting point

enthalpy of fusion

density and thermal stability were measured and characterized by experiments. Methods: Differential Scanning Calorimetry (DSC)

Archimedes and thermal gravimetric (TG) were used to measure the melting point

enthalpy of fusion

density and thermal stability of FKZr. Results: The measured melting point of FKZr is 693.6 K which is identical with the results in literature; the enthalpy of fusion is 71.8 J?g?1; the density is ??3.3841?0.8954×10?3 T (753?923 K) with the uncertainty less than 0.005 g?cm?3; no apparent weightlessness appears below 883 K; at 973 K

the weight loss is about 0.1%

and increases dramatically at 1053 K. Conclusion: The melting point of FKZr is appropriate as coolant in MSR. FKZr shows good stability and no obvious volatiles below 873 K；but at higher operating temperature

the problems caused by vapor pressure need to be investigated further. Abstract