Abstract

Natural circulation is a key passive heat removal mechanism in advanced reactor systems, including Molten Salt Reactors (MSRs). Owing to the high operating temperatures and material challenges associated with molten salts, surrogate fluids with Prandtl numbers comparable to those of molten salts have emerged as promising candidates for studying heat transfer phenomena in MSRs. The present study marks the first experimental and numerical investigation using Therminol-66 (Th-66) simulant oil as a surrogate fluid for molten salts in a natural circulation (NC) test loop setup at the University of Idaho Thermal-Hydraulics Laboratory. Experimental temperature measurements and energy-balance-based mass flow rate estimations were used to validate a three-dimensional computational fluid dynamics (CFD) model developed in ANSYS FLUENT. Two numerical configurations were considered: an adiabatic-wall model and a model incorporating distributed heat losses. The inclusion of heat losses significantly improved predictive accuracy, reducing the maximum relative error in heater outlet temperature to 16.7%. The largest deviation of 35.5% was observed at the heater inlet, primarily due to differences in power distribution and hydraulic resistance between the experimental system and the simplified numerical model. The CFD model systematically overpredicted the mass flow rate, mainly as a result of geometric simplifications (e.g., omission of flanges and minor loss elements) and the assumption that the total heater power was applied directly to the immersed heater rods. On the experimental side, distributed heat losses and indirect mass flow rate estimation introduced additional uncertainty. Nevertheless, the CFD model successfully captured the overall thermal and hydraulic trends across all operating conditions. The validated simulations further provided detailed insight into local and global temperature and velocity distributions within the heater and cooler sections. The results highlight the importance of accurately representing thermal losses and hydraulic resistance to achieve reliable prediction of natural circulation behavior in surrogate MSR systems.

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