The research into the Molten Salt Reactor (MSR) started at Oak Ridge National Laboratory (ORNL) in the 1950’s with the Aircraft Reactor Experiment (ARE), which ran successfully for 100 hours at a power up to 2.5 MWth and an outlet temperature up to 860°C. The ARE showed that the UF4 was chemically stable in the salt and that the gaseous fission products were removed automatically by the circulation pumps. The fuel salt had a strong negative temperature coefficient, and the reactor power could be manipulated from zero to full power without control rods by changing the power demand.

Afterwards the ORNL focused on graphite moderated reactors working with the thorium-uranium fuel cycle. Neutrons leaking from the primary salt were captured in the blanket salt to produce U-233. This uranium could easily be recovered by fluorination of the UF4 in the salt to the volatile UF6. This process is nowadays used to produce UF6 for uranium enrichment and is an important step in the fuel processing of the MSFR as well.
The research at ORNL culminated in the Molten Salt Reactor Experiment (MSRE), which ran successfully for five years until December 1969. The MSRE had a thermal power of 8 MW and operated either with U-233, U-235 or Pu-239. However, the fuel salt did not contain any thorium. During operation, uranium was removed from the fuel salt through fluorination.

The experience gained was used in the design of the Molten Salt Breeder Reactor (MSBR), which had a large core to reduce neutron leakage and a low power density to reduce irradiation damage to the graphite moderator. To achieve net breeding, the produced U-233 was removed by fluorination, and a process flow sheet was designed to separate the thorium from the lanthanides. Both salt loops were connected to drain tanks via freeze plugs made of solid salt cooled by air. This plug could thaw in the events of overheating or operator intervention. Unfortunately, the MSBR was never built and the freeze plug and chemical fuel salt processing were never applied.

Inspired by Generation-IV, the world regained interest in the Molten Salt Fast Reactor (MSFR) due to its prevailing characteristics in the fields of nuclear safety, optimal waste management and sustainability. The MSFR is the only Generation-IV reactor with a liquid salt to carry the fuel and to transport the heat. This special feature enables the MSFR to use innovative safety features, like the freeze plug and the mechanism to drain the fuel salt in fail-safe storage tanks underneath the reactor core.