Steady-State Thermal-Hydraulic Model for Fluoride-Salt-Cooled Small Modular High-Temperature Reactors

A whole-core, steady-state, thermal-hydraulic model for the cylindrical pin-type fluoride-salt-cooled small modular advanced high-temperature reactor (SmAHTR) is developed. In this preconceptual reactor design initially proposed by Oak Ridge National Laboratory, each fuel assembly in the graphite-moderated core has the FLiBe coolant flowing parallel to a hexagonal array of fuel and moderator pins. The present study considers a slightly modified fuel assembly design with a hexagonal inner housing compared to the original cylindrical housing. Burnable poison pins and control rods are also included in the fuel assembly considered here. The thermal-hydraulic model employs finite volumes to solve three-dimensional conduction in the pins and the hexagonal graphite reflector regions in the core. Heat transfer between the fuel assemblies is also addressed. The finite volumes in the fluid region are modeled using a subchannel approach in which the fluid is discretized into edge, corner, and interior subchannels and the resulting mass, momentum, and energy equations are systematically solved. The subchannel model also includes the transport between adjacent subchannels both due to radial pressure gradient–driven cross flow and turbulent mixing. Appropriate closure models from the literature are used to quantify axial and lateral flow resistances, heat transfer from solid to fluid, and turbulent mixing. The resulting thermal-hydraulic model provides detailed temperature and flow information for the entire core at a modest computational cost. Preliminary verification studies are also performed and reported.

Whole-core, steady-state results are presented for this SmAHTR core configuration for different power profiles. The effect of grid refinement and total mass flow rate into the core on the peak fuel temperature is also investigated. Fuel temperatures from a preliminary analysis with pin power distributions from a neutronic model are also included. The peak fuel temperature of ~1229°C in this illustrative case is below the steady-state operation limit for the SmAHTR.