A Whole-Core Thermal Hydraulic Model for Pin-Fueled Fluoride-Salt-Cooled Reactors
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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 (HTGRs) 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 detailed (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 thermal hydraulic model is developed for the solid pin-fueled design to analyze the steady-state and transient behavior of the core. A finite volume model is used to compute temperatures in the solid regions in the core. The coolant flowing through the pin bundles in the core is modeled using the conventional subchannel methodology. For the solid pin fuel configuration, a steady-state 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. Whole-core steady-state temperature, pressure, and flow profiles for different power profiles and flow rates are presented and discussed. The subchannel-based thermal hydraulic model is then extended to analyze the annular pin-fueled core configuration for steady-state scenarios. A transient CFD model is developed for the solid pin-fueled configuration to perform code-to-code comparison with the subchannel-based model. The transient thermal hydraulic 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. For the two protected accident scenarios involving loss of heat sink and loss of forced flow investigated in this study, the peak fuel and coolant temperatures are generally well within the allowable safety limits for this FHR configuration. The results from the unprotected transient over power accident simulation with an assumed power profile show that the peak fuel temperature during the transient is within the maximum allowable temperature for the coated particle TRISO fuel. However, for accident scenarios with more severe power excursions, the peak fuel temperature could exceed the maximum allowable TRISO temperature, and an optimization of core design might be necessary to provide better thermal margins against more severe U-TOP accidents. Insights from these simulations can guide the optimization of core design, and analysis of core safety during accident scenarios.