Molten salt reactor (MSR)

In a molten salt reactor (MSR) the coolant — and in several designs the fuel too — is neither water nor gas but a mixture of molten inorganic salts: fluorides of lithium, beryllium, zirconium and uranium in most thermal-spectrum designs, or chlorides of sodium and uranium in fast-spectrum ones. Those salts melt between 400 and 500°C, stay liquid and chemically stable well above 700°C, and at those temperatures their vapor pressure is essentially zero: the reactor runs at near-atmospheric pressure, without the thick vessel or high-pressure containment a water-cooled reactor needs. That combination, high temperature and low pressure, is what sets the MSR family apart from any PWR or BWR.

The feature that gives the concept its name, though, is not the salt itself but what is dissolved in it, and that is a frequent point of confusion. In a dissolved-fuel design, uranium or thorium is part of the salt as a fluoride or chloride (UF₄, ThF₄, UCl₃): there are no fuel elements, no cladding, no pellets, and the salt itself flows through the core, the heat exchanger and, if something goes wrong, a passive drain tank. That is the case for Oak Ridge's MSRE, China's TMSR-LF1 and the US MCRE. But not every molten salt implies this: several reactors under development use molten salt only as a coolant while the fuel stays solid — TRISO particles in a graphite matrix, as in an HTGR — and never dissolves. Kairos Power's Hermes, under construction in Oak Ridge, Tennessee, is of this second kind: the two families share coolant chemistry but differ sharply in core design, fission-product handling and licensing.

Thorium-232 is not itself fissile, but it is fertile: capturing a neutron turns it into thorium-233, which decays into protactinium-233 and then into uranium-233, which does fission with thermal neutrons. The catch is that protactinium-233 also captures neutrons and competes with breeding; in a solid-fuel reactor it has to decay inside the fuel element, at a cost in efficiency. A dissolved-fuel MSR can pull the protactinium out of the salt while the reactor runs, let it decay elsewhere, and feed clean uranium-233 back into the core: that is the technical reason the thorium cycle and the MSR are so closely linked. The MSRE itself proved the idea at small scale by running on uranium-233 in 1968, and China's TMSR-LF1 added thorium to its salt in 2024.

The precedent is the Molten-Salt Reactor Experiment (MSRE) at Oak Ridge National Laboratory, Tennessee. It grew out of a 1950s nuclear-aircraft program and was built as an 8 MWt reactor: uranium-235 at 33% enrichment, dissolved as UF₄ in a salt of lithium, beryllium and zirconium fluorides (LiF-BeF₂-ZrF₄) that melted at about 449°C and circulated between 600 and 650°C, moderated by a graphite core, with the salt-contact parts built in Hastelloy-N (then called INOR-8), a nickel alloy developed specifically to resist corrosion by molten fluorides. It reached criticality on June 1, 1965, logged more than 13,000 hours at full power through 1969, and on October 8, 1968 switched to running on uranium-233, the first reactor in history to use that fuel. It was shut down in December 1969 and designated a nuclear historic landmark in 1994.

No commercial molten salt unit operates today, but two experimental programs are active. In China, the 2 MWt TMSR-LF1, in Minqin, Wuwei (Gansu), received an operating licence in June 2023, reached criticality on October 11 that year and full power on June 17, 2024; in October 2024 it completed the world's first experiment loading thorium into the salt of an operating molten salt reactor, with UF₄ enriched below 20% in a FLiBe carrier salt with 99.95% lithium-7. According to China's National Nuclear Safety Administration, it is today the only thorium-fueled molten salt reactor operating in the world. In the United States, Southern Company leads a project with TerraPower and Idaho National Laboratory on the Molten Chloride Reactor Experiment (MCRE), under a five-year, $170 million cost-shared agreement with the Department of Energy signed in November 2021; unlike the fluoride designs, it uses a chloride salt and a fast spectrum. In 2025 the laboratory produced its first batch of fuel, converting 95% of metallic uranium feedstock into 18 kg of uranium chloride fuel salt, out of the 72 to 75 batches it needs to reach criticality; operation is planned for 2028, for about six months, at the laboratory's LOTUS facility.

The open problems are as central as the promise. Molten salt, fluoride or chloride, corrodes structural alloys at operating temperature; the Hastelloy-N that Oak Ridge developed for the MSRE remains the reference material, and materials qualification and corrosion testing are among the fronts Oak Ridge itself identifies as necessary before licensing a commercial design. Salts that contain lithium generate tritium: lithium-6, present even as a trace in natural lithium, captures neutrons and produces it, so FLiBe-based designs need lithium enriched in lithium-7 (TMSR-LF1 uses 99.95%) and systems to capture the tritium that is still produced and that permeates through hot metals. Keeping the salt's chemistry under control — redox state, impurities, dissolved fission products — takes online monitoring and adjustment, an operating discipline with no equivalent in a light-water reactor. And with all of that solved to varying degrees depending on the program, no molten salt unit runs at commercial scale today: the MSRE was an experiment closed down more than five decades ago, and the current Chinese and US projects are experimental or demonstration reactors.