Understanding and pathways to avoid major fuel failures and radionuclide releases in fluoride salt-cooled high-temperature reactor severe accidents

Fluoride salt-cooled High-temperature Reactors (FHRs) are a new type of power reactor that delivers heat to the power cycle between 600°C and 700°C. The FHR uses High-Temperature Gas-cooled Reactor (HTGR) graphite-matrix coated-particle fuel with failure temperatures of 1650°C. The FHR coolants are clean fluoride salts that have melting points above 350°C and boiling points above 1400°C. This combination may enable the design of a large FHR that will not have significant fuel failure and thus radionuclide releases to the environment even in a beyond-design-basis accident (BDBA) that includes failure of all cooling systems, the vessel, and containment systems. A first effort has been undertaken to understand FHR BDBAs and develop an FHR BDBA system to prevent major fuel failure if an accident occurs in a large FHR. Four design features limit BDBA fuel temperatures to lower than fuel failure temperatures. First, there is a large temperature drop to transfer decay heat from the fuel to the environment in a BDBA. Second, the large temperature difference between normal operating temperatures and fuel failure temperatures allows the use of increasing temperatures in an accident to degrade the insulation system and other barriers that prevent efficient transfer of decay heat from the reactor core to the environment in an accident. Third, the silo around the reactor vessel contains a BDBA salt that in an accident heats up, melts, and partly floods the silo to improve heat transfer from fuel to the environment. Fourth, the fuel and coolant retain fission products and actinides at high temperatures.