Molten Salt Reactors (MSRs) use molten salt compounds, typically fluorides or chlorides, as the primary coolant and in some designs as the fuel carrier itself, with fissile material dissolved directly in the circulating salt. Originally demonstrated at Oak Ridge National Laboratory with the Molten Salt Reactor Experiment (MSRE) in the 1960s, MSR technology has experienced a dramatic resurgence driven by its inherent safety advantages: molten salts operate at atmospheric pressure (eliminating the high-pressure failure modes of water-cooled reactors), have high boiling points (well above operating temperatures), and enable passive safety through freeze-plug drain systems that passively relocate fuel to a sub-critical geometry during abnormal conditions. MSRs can operate in either thermal or fast neutron spectra, offering flexibility for different fuel cycles including uranium, plutonium, and thorium.
The MSR category encompasses two major subcategories with distinct designs. Fluoride salt-cooled high-temperature reactors (FHRs), such as Kairos Power's KP-FHR, use molten fluoride salt (FLiBe) as a coolant with solid TRISO fuel, combining the high-temperature heat transfer properties of salt with the proven containment performance of TRISO particles. Kairos received the first NRC construction permit for an advanced reactor in December 2023 for its Hermes demonstration reactor in Oak Ridge, Tennessee, with first safety-related concrete poured in May 2025 and operation targeted for 2027. Kairos also signed a landmark 500 MW corporate PPA with Google, the first U.S. corporate SMR fleet deal, with the first reactor targeted for 2030. True molten salt reactors, where fuel is dissolved in the salt, include Terrestrial Energy's IMSR (195 MWe), which completed CNSC Phase 2 Vendor Design Review with no fundamental barriers identified, and Copenhagen Atomics' thorium MSR designed in a shipping-container form factor.
Other MSR developers include Moltex Energy, whose SSR-W (Stable Salt Reactor-Wasteburner) uses a novel design with molten salt fuel in static tubes rather than a pumped circulation loop, partnering with NB Power in New Brunswick, Canada. Elysium Industries is developing a Molten Chloride Salt Fast Reactor (MCSFR) that uses chloride salts to enable a fast neutron spectrum without solid fuel. The diversity of MSR approaches reflects the technology's fundamental versatility: different salt chemistries, fuel forms, and neutron spectra can be combined to optimize for electricity generation, process heat, waste burning, or thorium fuel cycle utilization, making MSRs one of the most active areas of advanced reactor innovation.