The Fluoride Salt-Cooled High-Temperature Reactor (FHR) is a distinct reactor concept that combines two proven technologies: molten fluoride salt coolant (typically FLiBe, a mixture of lithium fluoride and beryllium fluoride) with solid TRISO particle fuel. Unlike true molten salt reactors where fissile material is dissolved in the circulating salt, an FHR uses the salt purely as a heat transfer medium while the fuel remains in solid form as TRISO pebbles or compacts. This hybrid approach captures the advantages of both technologies: the excellent heat transfer and low-pressure operation of molten salts, and the robust fission product containment of TRISO fuel, while avoiding the materials science challenges of circulating radioactive fuel salt.
Kairos Power is the definitive FHR developer and the most advanced in the advanced reactor licensing pipeline. Their KP-FHR design uses FLiBe molten salt coolant circulating through a pebble bed of TRISO fuel particles. The Hermes low-power demonstration reactor (35 MWth, non-power) in Oak Ridge, Tennessee received the first NRC construction permit for an advanced reactor in December 2023, representing a landmark regulatory achievement. The first safety-related concrete was poured on May 7, 2025, and the reactor is targeting operation in 2027. Kairos finalized its HALEU fuel contract in January 2026, securing the enriched uranium feedstock for TRISO pebble production. The operating license application for Hermes is being prepared concurrently with construction.
Kairos's commercial ambitions are anchored by a 500 MW corporate PPA with Google, the first-ever U.S. corporate SMR fleet deal, with the first commercial KP-FHR reactor targeted for 2030. Each commercial module is designed to produce 75 MWe. The FHR concept delivers coolant outlet temperatures in the range of 600-700 degrees Celsius, lower than helium-cooled HTGRs but well above the 300 degrees Celsius range of water-cooled reactors, enabling efficient power conversion and potential process heat applications. With over $300 million in funding raised, Kairos has positioned the FHR as a credible near-term deployment pathway, benefiting from the combination of demonstrated TRISO fuel performance (validated commercially in China's HTR-PM), established salt chemistry knowledge from decades of molten salt research, and a construction permit already in hand.