Abstract
The Molten Salt Reactor (MSR) is an innovative Generation-IV design that presents significant advantages over conventional solid-fueled systems. It incorporates passive safety features, achieves high fuel burn-up for better resource utilization, and minimizes nuclear waste. Since passive safety systems primarily rely on natural circulation (NC) principles, they are given priority to enhance the reliability of inherent safety features in advanced reactors, particularly in MSRs. In this context, numerous studies have been conducted, both numerically and experimentally, using different natural circulation loops to investigate the behavior of NC. This dissertation presents preliminary outcomes from the validation of RELAP5-3D and CFD/ANSYS FLUENT codes using experimental data obtained from the Molten Salt Natural Circulation Loop (MSNCL) setup at the University of Idaho's Thermal-Hydraulics Laboratory. Different working fluids, including gases, water, and Therminol-66 (Th-66) as a surrogate fluid for molten salt, have been used to explore a range of Prandtl numbers and evaluate their heat transfer characteristics. Similarity techniques were employed to evaluate the feasibility of using Th-66 as a simulant fluid for thermal-hydraulic (TH) analysis of prototypical MSR systems, including the Molten Salt Fast Reactor (MSFR) and the FUJI-233Um reactor. The results demonstrated the capability of both system-level TH codes and CFD models to accurately reproduce NC behavior across a range of operating conditions. Furthermore, the application of scaling laws and similarity techniques confirmed that Th-66 effectively replicates the heat transfer characteristics of MSFR and FUJI reactor systems. Its suitability as a surrogate fluid is reinforced by its ability to match the Prandtl number (Pr) ranges of various candidate molten salts, thereby ensuring reliable experimental and numerical investigations of heat transfer phenomena in MSR designs. Additionally, new insights into the feasibility of alternative simulant fluids have been developed, offering a viable substitute for actual molten salts while mitigating their associated challenges. This advancement is particularly significant for the research and development (R&D) of molten salt reactors (MSRs), as it enables experimental and numerical studies to be conducted under more accessible and controllable conditions while preserving the key TH characteristics of molten salt systems.