David Arcilesi

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.

The molten salt reactor (MSR) is a prominent Generation IV nuclear reactor concept that offers substantial advantages over conventional solid-fueled systems, including enhanced fuel utilization, inherent passive safety features, and significant reductions in long-lived radioactive waste. Central to its safety strategy is a reliance on natural circulation (NC) mechanisms, which eliminate the need for active pumping systems and enhance system reliability during normal and off-normal conditions. However, the challenges associated with molten salts, such as their high melting points, corrosivity, and material compatibility issues, render experimental investigations inherently complex and demanding. Therefore, the use of high-Pr-number surrogate fluids represents a practical alternative for studying molten salt behavior under safer and more accessible experimental conditions. In this study, a single-phase natural circulation loop setup at the University of Idaho’s Thermal–Hydraulics Laboratory was employed to investigate NC behavior under various operating conditions. The RELAP5-3D code was initially validated against water-based experiments before employing Therminol-66, a high-Prandtl-number surrogate for molten salts, in the natural circulation loop for the first time. The RELAP5-3D results demonstrated good agreement with both steady-state and transient experimental results, thereby confirming the code’s ability to model NC behavior in a single-phase flow regime. The results also highlighted certain experimental limitations that should be addressed to enhance the NC loop’s performance. These include increasing the insulation thickness to reduce heat losses, incorporating a dedicated mass flow measurement device for improved accuracy, and replacing the current heater with a higher-capacity unit to enable testing at elevated power levels. By identifying and addressing the main causes of these limitations and uncertainties during water-based experiments, targeted improvements can be implemented in both the RELAP5 model and the experimental setup, thereby ensuring that tests using a surrogate fluid for MSR analyses are conducted with higher accuracy and minimal uncertainty.