Preventing fuel failure for a beyond design basis accident in a fluoride salt cooled high temperature reactor

Abstract

The fluoride salt-cooled high-temperature reactor (FHR) combines high-temperature coated-particle fuel with a high-temperature salt coolant for a reactor with unique market and safety characteristics. This combination can eliminate large-scale radionuclide releases by avoiding major fuel failure during a catastrophic Beyond Design Basis Accident (BDBA). The high-temperature core contains liquid salt coolant surrounded by a liquid salt buffer; these salts limit core heatup while decay heat drops. The vessel insulation is designed to fail during a BDBA. The silo contains a frozen BDBA salt designed to melt and surround the reactor vessel during a major accident to accelerate heat transfer from the vessel. These features provide the required temperature gradient to drive decay heat from core to the vessel wall and to the environment below fuel failure temperatures. A 1047 MWth FHR was modeled using the STAR-CCM+ computational fluid dynamics package. Peak temperatures and heat transfer phenomena were calculated, focusing on feasibility of melting the BDBA salt that improves heat transfer from vessel to silo. A simplified wavelength-independent radiation model was examined to approximate the heat transfer capability with radiation heat transfer. The FHR BDBA system kept peak temperatures below the fuel failure point in all cases. Reducing the reactor vessel-silo gap size minimized the time to melt the BDBA salt. Radiation heat transfer is a dominant factor in the high-temperature accident sequence. It keeps peak fuel temperatures hundreds of degrees lower than with convection and conduction only; it makes higher core powers feasible. The FHR's atmospheric pressure design allows a thin reactor vessel, ensuring the high accident temperatures reach the vessel's outer surface, creating a large temperature difference from the vessel to the frozen salt. This greatly accelerates the heat transfer over current reactor designs with thick, relatively cool accident outer vessel temperatures. The frozen BDBA salt in the FHR places a limit on the upper temperature at the vessel outer boundary for significant time; it is a substantial heat sink for the accident duration. Finally, surrounding the FHR vessel, the convection of hot air, and circulating salt later in the accident, preferentially transports heat upward in the FHR; this provides a conduction path through the concrete silo to the atmosphere above the FHR.