Molten salt reactors (MSRs) are one of several next generation (Gen IV) nuclear reactor designs under development today. Like light water reactors (LWRs), MSRs use nuclear fission to generate heat. However, instead of using water for coolant, MSRs use liquid fluoride or chloride salt mixtures, a.k.a. salts.

MSR designs have a number of potential advantages in safety, efficiency, waste management over LWRs. Interest in commercializing MSR has grown since 2002 when the Generation IV International Forum (GIF) selected MSR as one of six most promising fourth generation nuclear reactor designs for international R&D collaboration.

There are two primary types of MSR: dissolved and solid fuel. In the first type, coolant and the fuel are the same fluid. The nuclear fuel, which is either uranium, plutonium, or thorium is dissolved in the molten salt. The second type of MSR is known as the fluoride salt-cooled high-temperature reactor (FHR). In FHR, the fuel is made of solid particles less than 1 mm wide that is suspended in the molten fluoride coolant. The fuel remains solid even as the fission reaction proceeds.

The heat from MSRs and other advanced nuclear reactors is amenable for a diverse range of applications including process heat, chemical processing, desalination and hydrogen production. Because MSRs are smaller than LWRs, the military has envisioned their use for marine propulsion.

The MSR is one of thirteen prototype nuclear reactors that was conceived and tested at. Oak Ridge National Laboratory (ORNL). At the time, a task force under the Atomic Energy Commission determined that “the highest probability of achieving technical feasibility.” The laboratory successfully ran its Molten Salt Reactor Experiment (MSRE) between 1965 and 1969. This reactor produced 7.4 MW of thermal power

MSRs have higher thermal efficiencies than LWRs because they operate at higher temperatures (700C to 800C) than conventional reactors (300C). MSRs with a closed-cycle turbine can reach 45% electrical power conversion efficiency. In contrast, LWRs, which use traditional steam turbines, only achieves 33% efficiency.

In spite of the early success in demonstrating MSR, the MSRE was shut down due to a combination of limited budget and the Atomic Energy Commission’s preference in pursuing the alternative sodium-cooled fast breeder reactor design.

Unlike the nuclear accidents at Fukushima Daiichi, Chernobyl, and Three Mile Island, MSRs fundamentally cannot meltdown because the fuel is already in liquid form. MSR employs passive safety by the nature of the coolant. When temperature rises, the salt coolant expands, slowing down the nuclear fission reaction.

Furthermore, if the reactor overheats, a plug melts at the bottom of the reactor and the liquid fuel is immediately evacuated into a catch basin below the reactor where the salt expands and becomes a solid as it cools to below 500C. In the event that the reactor vessel, pump, or pipe is ruptured, the salt simply solidifies and the nuclear fuel cannot contaminate the environment.

Because MSRs operate at atmospheric pressure, there is no risk of the reactor exploding. Unlike LWRs, the coolant is not pressurized and therefore a containment dome is not needed.

MSR generates less waste than conventional nuclear plants because these reactors do not use fuel rods that are used in LWRs. In addition, the reprocessing of highly radioactive fuel salts is not needed with FHR because it is efficient at burning transuranic elements. FHR can be configured to use the spent fuel from conventional reactors. In France and Russia, researchers are developing MSR that breed in the uranium or thoriums fuel cycles.

While MSRs have numerous design advantages in safety, waste management, cost, and efficiency over LWRs, the concept requires further development and validation.

Molten salts have a corrosive effect on the structural materials used in the reactor vessel and heat exchangers. A major area of development is fine tuning the coolant to control its corrosive effects on reactor components (pipes, pumps, et al.) Another focus is on developing metal alloys and structural ceramics that are resilient to radiation or chemical reactions.

Another challenge for MSR is tritium control. It is the only isotope that could escape during normal operating conditions. Tritium stripping technology from irradiated salt still needs to be adequately demonstrated. Researchers are also experimenting with double-walled heat exchanger designs.

The robustness of the MSR control system and related instrumentation also needs to be tested to ensure passive and active safety requirements are met. Instrumentation like flow meters must be made to be tolerant to high temperatures and corrosive materials. To date, researchers are still tackling individual technical challenges expected in a MSR reactor. No startup has yet designed a complete MSR plant for regulator approval.

Lastly, with each MSR reactor type, the fuel cycle must be developed. Specifically, this means how the reactor is refueled and what happens to the waste.

All aspects of the plant equipment and operation then must be qualified to the regulator. The regulators also must come up with a safety and licensing approach that accommodates historical requirements that are appropriate to regulate the unique features of MSR technology.

MSR and other Generation IV nuclear designs are intended for commercialization after 2030. The US Nuclear Regulatory Commission (NRC) expects at least two of the advanced nuclear reactor designs, including MSR, to reach technical maturity by the early 2030s.

In 2016, the International Atomic Energy Agency (IAEA) established a collaborative platform consisting of 17 member countries for MSR development. The US, Canada, China, Russia, France, India, Norway, and Denmark have active programs to commercialize MSR.

Among them, China is making the most aggressive effort to realize MSR for commercial production. The Shanghai Institute of Applied Physics (SINAP), which has a workforce of 500 researchers, is developing both MSR and FHR for demonstration in 2025 and deployment in the 2030s.

In the US, there are at least nine publicly and privately funded MSR efforts underway in universities and companies. The Department of Energy (DOE) is supporting MSR development through the GAIN MSR Technical Working Group that consists of six companies, as well as ORNL and Argonne National Lab. The DOE is also funding the development of FHR through a tri-university consortium that consists of Massachusetts Institute of Technology (MIT), University of California at Berkeley, and University of Wisconsin.