Modernization of the design of a measuring cell for determining the thermal diffusivity of salt melts using the laser flash method

In this work, a well-known cell for measuring the diffusivity of fluoride salt melts by the laser flash method has been modernized. Alkali metal halide melts, such as the eutectic mixture FLiNaK (46.5 mol% LiF – 11.5 mol% NaF – 42 mol% KF), are considered promising materials for use in nuclear power engineering, particularly in molten salt reactors (MSRs), where they act as coolants and actinide fission media. This makes the study of their thermophysical properties extremely important for the design of reactor cores and heat transfer systems. However, as data from the literature show, measurements of the thermal diffusivity of FLiNaK melts are accompanied by significant discrepancies associated with the influence of unaccounted for heat transfer factors and errors in experimental techniques. The laser flash method is one of the preferred methods for studying the thermal diffusivity of salt melts at high temperatures due to its ability to account for convective and radiative heat transfer. However, this method using a known cell leads to overestimated values of thermal diffusivity due to dissipated heat flow. In order to modernize this cell, a numerical model was built in COMSOL Multiphysics, which allowed us to study the influence of materials (Ni, BN, Au) and cell geometry on the heat transfer processes. Data analysis allowed to obtain an optimized cell design that minimized the fraction of heat flow lost, reduced the time to reach the temperature peak, eliminated the need for calibration measurements and extended the temperature range of measurements. Experimental validation of the improved cell was carried out using Netzsch LFA 467 HT HyperFlash equipment. The data obtained confirmed the possibility of more accurate measurement of FLiNaK diffusivity in the temperature range of 550-800°C. Particularly, using of the modernized measuring cell improves the reproducibility of the results and reduces the data scatter, reducing measurement error from 33,8 to 2,6%. These widens the prospects for further studies of high temperature melts, contributing to the development of new generation MSR technologies.