Molten Salt Reactor

A molten salt reactor (MSR) is a class of nuclear fission reactors in which the primary coolant, or even the fuel itself is a molten salt mixture. MSRs run at higher temperatures than water-cooled reactors for higher thermodynamic efficiency, while staying at low vapor pressure.

Operating at near atmospheric pressures reduces the mechanical stress endured by the system, thus simplifying aspects of reactor design and improving safety. It should be possible to construct and operate molten salt reactors more cheaply than coal power plants.

The nuclear fuel may be solid or dissolved in the coolant itself. In many designs the nuclear fuel is dissolved in the molten fluoride salt coolant as uranium tetrafluoride (UF₄). The fluid becomes critical in a graphite core which serves as the moderator. Solid fuel designs rely on ceramic fuel dispersed in a graphite matrix, with the molten salt providing low pressure, high temperature cooling. The salts are much more efficient than water at removing heat from the core, reducing the need for pumping, piping, and reducing the size of the core.

The early Aircraft Reactor Experiment (1954) was primarily motivated by the small size that the design could provide, while the Molten-Salt Reactor Experiment (1965–1969) was a prototype for a thorium fuel cycle breeder reactor nuclear power plant. One of the Generation IV reactor designs is a molten salt-cooled, solid-fuel reactor; the initial reference design is 1000 MW_e with a deployment target date of 2040.

Another advantage of a small core is that it has fewer materials to absorb neutrons. In a reactor employing thorium fuel, the improved neutron economy makes more neutrons available to breed thorium-232 into uranium-233. Thus, the compact core makes the molten salt design particularly suitable for the thorium fuel cycle.