Molten-salt reactor


A molten-salt reactor is a class of nuclear fission reactor in which the primary nuclear reactor coolant and/or the fuel is a mixture of molten salt with a fissile material.
Two research MSRs operated in the United States in the mid-20th century. The 1950s Aircraft Reactor Experiment was primarily motivated by the technology's compact size, while the 1960s Molten-Salt Reactor Experiment aimed to demonstrate a nuclear power plant using a thorium fuel cycle in a breeder reactor.
Increased research into Generation IV reactor designs renewed interest in the 21st century with multiple nations starting projects. On October 11, 2023, China's TMSR-LF1 reached criticality, and subsequently achieved full power operation, as well as thorium breeding.

Properties

MSRs eliminate the nuclear meltdown scenario present in water-cooled reactors because the fuel mixture is kept in a molten state. The fuel mixture is designed to drain without pumping from the core to a containment vessel in emergency scenarios, where the fuel solidifies, quenching the reaction. In addition, hydrogen evolution does not occur. This eliminates the risk of hydrogen explosions. They operate at or close to atmospheric pressure, rather than the 75–150 times atmospheric pressure of a typical light-water reactor. This reduces the need and cost for reactor pressure vessels. The gaseous fission products have little solubility in the fuel salt, and can be safely captured as they bubble out of the fuel, rather than increasing the pressure inside the fuel tubes, as happens in conventional reactors. MSRs can be refueled while operating while conventional reactors shut down for refueling. MSR operating temperatures are around, significantly higher than traditional LWRs at around. This increases electricity-generation efficiency and process-heat opportunities.
Relevant design challenges include the corrosivity of hot salts and the changing chemical composition of the salt as it is transmuted by the neutron flux.
MSRs, especially those with fuel in the molten salt, offer lower operating pressures, and higher temperatures. In this respect an MSR is more similar to a liquid metal cooled reactor than to a conventional light water cooled reactor. MSR designs are often breeding reactors with a closed fuel cycle—as opposed to the once-through fuel currently used in conventional nuclear power generators.
MSRs exploit a negative temperature coefficient of reactivity and a large allowable temperature rise to prevent criticality accidents. For designs with the fuel in the salt, the salt thermally expands immediately with power excursions. In conventional reactors the negative reactivity is delayed since the heat from the fuel must be transferred to the moderator. An additional method is to place a separate, passively cooled container below the reactor. Fuel drains into the container during malfunctions or maintenance, which stops the reaction.
The temperatures of some designs are high enough to produce process heat, which led them to be included on the GEN-IV roadmap.

Advantages

MSRs offer many potential advantages over light water reactors:
  • Passive decay heat removal is achieved in MSRs. In some designs, the fuel and the coolant are a single fluid, so a loss of coolant carries the fuel with it. Fluoride salts dissolve poorly in water, and do not form burnable hydrogen. The molten salt coolant is not damaged by neutron bombardment, though the reactor vessel is.
  • A low-pressure MSR does not require an expensive, steel core containment vessel, piping, and safety equipment. However, most MSR designs place radioactive fluid in direct contact with pumps and heat exchangers.
  • MSRs enable cheaper closed nuclear fuel cycles, because they can operate with slow neutrons. Closed fuel cycles can reduce environmental impacts: chemical separation turns long-lived actinides into reactor fuel. Discharged wastes are mostly fission products with shorter half-lives. This can reduce the needed containment to 300 years versus the tens of thousands of years needed by light-water reactor spent fuel.
  • The fuel's liquid phase can be pyroprocessed to separate fission products from fuels. This may have advantages over conventional reprocessing.
  • Fuel rod fabrication is replaced with salt synthesis.
  • Some designs are compatible with fast neutrons, which can "burn" transuranic elements such as, from LWRs.
  • An MSR can react to load changes in under 60 seconds.
  • Molten-salt reactors can run at high temperatures, yielding high thermal efficiency. This reduces size, expense, and environmental impacts.
  • MSRs can offer a high "specific power",, as demonstrated by ARE.
  • Potential neutron economy suggests that MSR may be able to exploit the neutron-poor thorium fuel cycle.

    Disadvantages

  • In circulating-fuel-salt designs, radionuclides dissolved in fuel contact equipment such as pumps and heat exchangers, potentially requiring fully remote maintenance.
  • Some MSRs require onsite chemical processing to manage core mixture and remove fission products.
  • Regulatory changes to accommodate non-traditional design features
  • Some MSR designs rely on expensive nickel alloys to contain the molten salt. Such alloys are prone to embrittlement under high neutron flux.
  • Corrosion risk. Molten salts require careful management of their oxidation state to manage corrosion risks. This is particularly challenging for circulating designs, in which a mix of isotopes and their decay products circulate through the reactor. Static designs benefit from modularising the problem: the fuel salt is contained within fuel pins whose regular replacement, primarily due to neutron irradiation, is normalized; while the coolant salt has a simpler chemical composition and does not pose a corrosion risk either to the fuel pins or to the reactor vessel. MSRs developed at ORNL in the 1960s were safe to operate only for a few years, and operated at only about. Corrosion risks include dissolution of chromium by liquid fluoride thorium salts at greater than, hence endangering stainless steel components. Neutron radiation can transmute common alloying agents such as Co and Ni, shortening lifespan. Lithium salts such as FLiBe warrant the use of to reduce tritium generation. ORNL developed Hastelloy N to help address these issues, while other structural steels may be acceptable, such as 316H, 800H, and Inconel 617.
  • Some MSR designs can be turned into a breeder reactor to produce weapons-grade nuclear material.
  • MSRE and ARE used high enriched uranium approaching weapons-grade. These levels would be illegal in most modern power plant regulatory regimes. Most modern designs employ lower-enriched fuels.
  • Neutron damage to solid moderator materials can limit the core lifetime. For example, MSRE was designed so that its graphite moderator had loose tolerances, so neutron damage could change them without consequences. "Two fluid" MSR designs do not use graphite piping because graphite changes size when bombarded with neutrons. MSRs using fast neutrons cannot use graphite, because it moderates neutrons.
  • Thermal MSRs have lower breeding ratios than fast-neutron breeders, though their doubling time may be shorter.

    Coolant

MSRs can be cooled in various ways, including using molten salts.
Molten-salt-cooled solid-fuel reactors are variously called "molten-salt reactor system" in the Generation IV proposal, molten-salt converter reactors, advanced high-temperature reactors, or fluoride high-temperature reactors.
FHRs cannot reprocess fuel easily and have fuel rods that need to be fabricated and validated, requiring up to twenty years from project inception. FHR retains the safety and cost advantages of a low-pressure, high-temperature coolant, also shared by liquid metal cooled reactors. Notably, steam is not created in the core, and no large, expensive steel pressure vessel. Since it can operate at high temperatures, the conversion of the heat to electricity can use an efficient, lightweight Brayton cycle gas turbine.
Much of the current research on FHRs is focused on small, compact heat exchangers that reduce molten salt volumes and associated costs.
Molten salts can be highly corrosive and corrosivity increases with temperature. For the primary cooling loop, a material is needed that can withstand corrosion at high temperatures and intense radiation. Experiments show that Hastelloy-N and similar alloys are suited to these tasks at operating temperatures up to about 700 °C. However, operating experience is limited. Still higher operating temperatures are desirable—at thermochemical production of hydrogen becomes possible. Materials for this temperature range have not been validated, though carbon composites, molybdenum alloys, carbides, and refractory metal based or ODS alloys might be feasible.

Fused salt selection

The salt mixtures are chosen to make the reactor safer and more practical.

Fluorine

Fluorine has only one stable isotope, and does not easily become radioactive under neutron bombardment. Compared to chlorine and other halides, fluorine also absorbs fewer neutrons and slows neutrons better. Low-valence fluorides boil at high temperatures, though many pentafluorides and hexafluorides boil at low temperatures. They must be very hot before they break down into their constituent elements. Such molten salts are "chemically stable" when maintained well below their boiling points. Fluoride salts dissolve poorly in water, and do not form burnable hydrogen.

Chlorine

Chlorine has two stable isotopes, as well as a slow-decaying isotope between them which facilitates neutron absorption by.
Chlorides permit fast breeder reactors to be constructed. Much less research has been done on reactor designs using chloride salts. Chlorine, unlike fluorine, must be purified to isolate the heavier stable isotope,, thus reducing production of sulfur tetrachloride that occurs when absorbs a neutron to become, then degrades by beta decay to.