Design considerations for insulation system for fluoride salt cooled high temperature reactor under beyond design basis accident conditions

The Fluoride-salt-cooled High-temperature Reactor (FHR) is a new nuclear power reactor concept being developed in the U.S. and China. It employs liquid salt as a primary coolant, which was originally developed for a molten salt reactor. The liquid salt utilized is called FLiBe whose melting and boiling temperatures are 459degC and 1433degC, respectively. This new concept reactor has improved safety features because the coated particle fuel pebbles has high temperature margin to retain fission products up to around 1650degC and the high boiling point of the coolant.. The Beyond Design Basis Accident (BDBA) system prevents fuel failure and large scale fission products release in case of a severe accident. The proposed BDBA system insulates the reactor core from the environment to minimize the heat loss during normal operations and utilizes high temperatures in a BDBA to degrade the insulation system to enable decay heat to escape to the environment. This change combined with the high-temperature capability of fuel and coolant enable removal of decay heat and avoidance of high-temperature fuel failure. In the present study, we examine two insulation systems that lose their insulative capabilities at higher temperatures to enable transfer of decay heat to the environment in a BDBA thus maintaining fuel temperatures below fuel failure temperatures. First is the mirror insulation system. This system is composed of multi-layer thin metal sheets. Ideally, these insulation sheets have radiation emissivity (ε) which is zero at 600degC but rapidly increases to one at over 700degC, and then decay heat is released to the earth. Secondly, the firebrick system may be employed with a BDBA salt. Firebrick insulation works because of its low thermal conductivity and voids in the firebrick. A BDBA salt can be included in the silo that melts at high temperatures. The salt penetrates the firebrick, fills the voids, and increases the thermal conductivity while absorbing decay heat. (author)