A Simplified Thermal Hydraulic Model for Solid Pin-Fueled Molten Salt Reactors Under Low-Flow Accident Scenarios

Abstract

Fluoride-salt-cooled high-temperature reactors (FHRs) are an emerging category of next generation reactors. 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 postulated accident scenarios is essential to enable the further development of preconceptual FHRs. While accident scenarios that involve high core inlet flow rates, such as loss of heat sink and reactivity insertion accidents can be analyzed using simpler flow models, accidents involving low-flow situations such as loss of forced flow due to coolant pump failure typically require more complex models with tight coupling between momentum and energy equations due to the buoyancy dominated flows in these postulated accident scenarios. This study develops a core-level thermal hydraulic model with simplifications to provide a conservative estimate for core temperatures during loss of forced flow accidents. The key simplification is that the model neglects reversed and recirculating flows that could exist in buoyancy-driven flows, which have the net effect of reducing the transverse temperature gradient in the fuel assembly pin bundle regions, thus reducing the core temperatures encountered during the natural circulation accident. The objective of this simplified model is to provide a first-pass, conservative estimate of the peak fuel, graphite, and coolant temperatures, which is particularly useful when evaluating different safety system designs. Further optimization of a few down selected safety systems could use more complex models that would incur a substantially higher computational expense.