Thermal-Hydraulic System-Level Analysis of a Molten Salt Natural Circulation Loop

Molten salt reactors (MSRs) are a class of Generation IV nuclear reactors using molten salts as heat transfer fluids. MSRs bring a number of benefits, including low primary system working pressure, high working temperature, and enhanced safety due to the passive safety systems adopted. Although MSRs promise these benefits, a number of key technology needs, such as the accurate prediction of the thermal-hydraulic performance of the passive safety systems, which completely rely on natural circulation, are indispensable for MSR development, licensing, and future deployment. Therefore, this study develops the one-dimensional (1D) NAtural Circulation COde (NACCO) considering the buoyancy and radiative heat transfer effects in high-temperature molten salts for such predictions. The 1D code, developed using MATLAB, is then benchmarked with experimental data from three natural circulation flow experiments, where water, nitrate salt NaNO₃-KNO₃ (60–40 wt%), and fluoride salt LiF-BeF₂ (66–34 mol%, FLiBe) were used as the working fluids. Our analysis shows that (1) the buoyancy and radiative heat transfer effects need to be considered for high-temperature molten salt natural circulation flows, while the radiative heat transfer effect is negligible for low-temperature water flows in the natural circulation experiments investigated, and (2) the 1D code NACCO predicts salt temperature profiles reasonably well, with less than 18°C and 25°C discrepancies from experimental data for the pipe centerline temperature of NaNO₃-KNO₃ and FLiBe up to 450°C and 750°C, respectively.