SUMMARY

Fluoride-salt-cooled high-temperature reactors (FHRs) are an emerging category of reactors that combine the graphite-matrix coated-particle fuel developed for high temperature gas reactors with a high heat capacity, single-phase molten salt coolant. One of the potential configurations for the FHR core includes the pin bundle configuration in which the molten salt coolant flows parallel to an array of fuel and non-fuel pins. A thermal hydraulic modeling tool that can perform fluid flow and heat transfer analyses in the core region of the reactor during normal operation and under different postulated accident scenarios is essential to enable the further development of pre-conceptual pin-fueled FHR designs. To enable multiphysics coupling and the analysis of several different core design iterations for this FHR, the thermal hydraulic model must provide pin-level resolution across the entire core while incurring a modest computational overhead and providing fast simulation turnaround times. This requirement is addressed in the present study. A comprehensive subchannel-based thermal hydraulic model is developed to analyze the steady-state and transient behavior of the core. A computational fluid dynamics (CFD) model is developed for 1/12th of a single fuel assembly. The results from the CFD model are compared with the subchannel-based model to perform code-to-code comparison and preliminary verification of the subchannel model. The subchannel-based model is used for steady-state analyses of the pin-fueled core for different power profiles and coolant flow rates. The transient version of the subchannel model is then used to analyze accident scenarios that involve high (forced circulation) as well as low (natural circulation) coolant flow rates into the core. Insights from these simulations can guide the optimization of core design, and analysis of core safety during accident scenarios.