![CAnes logo[96]](https://canes.mit.edu/hs-fs/hubfs/CAnes%20logo%5B96%5D.png?width=300&height=91&name=CAnes%20logo%5B96%5D.png "CAnes logo[96]")
Request Copy
Report Date: January 2014
Appendices: No
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 potentially eliminate large-scale radionuclide releases by avoiding major fuel failures during a catastrophic Beyond Design Basis Accident (BDBA). Fuel failure temperatures are above 1600°C while coolant boiling points are above 1400°C. These properties with appropriate system design enable peak fuel temperatures to remain below fuel failure temperatures by (1) a system heat capacity that slows the temperature rise in an accident thus providing time for the decay heat rate to decrease and (2) transfer of decay heat from fuel to the environment through the reactor silo structure.
The BDBA system operates if all other reactor cooling systems fail and reactor temperatures increase beyond design limits. The high-temperature reactor core contains liquid salt coolant surrounded by a liquid salt buffer. The heat capacity of the fuel, coolant, and buffer slow core heat up while the decay heat drops. The reactor is in a silo with a silo cooling system. The vessel insulation system is designed to fail upon high temperatures enabling efficient heat transfer from reactor vessel to the silo cooling system. The FHR has a thin-wall high-temperature vessel resulting in high vessel wall surface temperatures for efficient radiation heat transfer to the silo wall. In most cases the accident would be stopped at this point. If the vessel fails, there is sufficient salt to keep the reactor core covered. The salt freezing point is above 400°C and thus the salt can’t leak out of the silo because it freezes and thus seals any cracks.
The silo contains a low-cost frozen BDBA salt. If the silo cooling system fails, this BDBA salt melts and fills the space between the reactor vessel and silo. Raising the temperature of the salt to its melting point, melting the salt and further increases in salt temperature absorbs large amounts of decay heat and provides time for the decay heat rate to decrease. Circulation of the salt and radiation heat transfer efficiently moves heat from the vessel wall to the silo wall. This results in a small temperature drop from the fuel to the silo wall and makes available a large temperature drop (>1000°C) to drive decay heat by conduction through the silo structure to the environment to provide long-term decay heat removal.
A 1047 MWth FHR was modeled using the STAR-CCM+ computational fluid dynamics package. Peak temperatures and heat transfer phenomena were calculated, focusing on the feasibility of melting the BDBA salt that improves heat transfer from vessel to silo. A simplified wavelength-independent radiation model was used to approximate the heat transfer capability. The BDBA system kept peak temperatures below fuel failure temperature in all cases. Reducing the reactor vessel-silo gap size minimized the time to melt the BDBA salt. Radiation heat transfer is the dominate factor in high-temperature accident sequences. It keeps peak fuel temperatures hundreds of degrees lower than with convection and conduction only; it enables higher reactor power levels without fuel failure in a BDBA. Convection of hot air and circulating salt later in the accident between the vessel wall and silo wall preferentially transports heat upward in the FHR. The conduction path to the atmosphere is primarily in the upward direction.
It is the combination of a high-temperature fuel and coolant that prevents fuel failures even if large-scale structural failures. This is the first design study of such a BDBA system. Only a subset of the design options has been explored. There are significant uncertainties.
Program: ANP : Advanced Nuclear Power Program
Type: TR
RPT. No.: 151