Molten salt reactor

Example of a molten salt reactor scheme

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A molten salt reactor (MSR) is a class of nuclear fission reactor in which the primary nuclear reactor coolant and/or the fuel is a molten salt mixture. Only two MSRs have ever operated, both research reactors in the United States. The 1950's Aircraft Reactor Experiment was primarily motivated by the compact size that the technique offers, while the 1960's Molten-Salt Reactor Experiment aimed to prove the concept of a nuclear power plant which implements a thorium fuel cycle in a breeder reactor. Increased research into Generation IV reactor designs began to renew interest in the technology, with multiple nations having projects and, as of September 2021, China is on the verge of starting its TMSR-LF1 thorium MSR.

MSRs are considered safer than conventional reactors because they operate with fuel already in a molten state, and in event of an emergency, the fuel mixture is designed to drain from the core where it will solidify, preventing the type of nuclear meltdown and associated hydrogen explosions (like what happened in the Fukushima nuclear disaster) that are at risk in conventional (solid-fuel) reactors.

• Nature_2021-09-10 [r]: Add brief definition or description They operate at or close to atmospheric pressure, rather than the 75-150 times atmospheric pressure of a typical light-water reactor (LWR), hence reducing the need for large, expensive reactor pressure vessels used in LWRs. Another characteristic of MSRs is that the radioactive fission gases produced are absorbed into the molten salt, as opposed to conventional reactors where the fuel rod tubes must contain the gas. MSR's can also be refueled while operating (essentially online-nuclear reprocessing) while conventional reactors must be shut down for refueling (Heavy water reactors like the CANDU or the Atucha-class PHWRs being a notable exception).

A further key characteristic of MSRs is operating temperatures of around 700 C ( {{{3}}}), significantly higher than traditional LWRs at around 300 C ( {{{3}}}), providing greater electricity-generation efficiency, the possibility of grid-storage facilities, economical hydrogen production, and, in some cases, 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 in the reactor core.

MSRs offer multiple advantages over conventional nuclear power plants, although for historical reasons they have not been deployed.

Properties

MSRs, especially those with the fuel dissolved in the salt, differ considerably from conventional reactors. Reactor core pressure can be low and the temperature much higher. In this respect an MSR is more similar to a liquid metal cooled reactor than to a conventional light water cooled reactor. MSRs are often planned as breeding reactors with a closed fuel cycle—as opposed to the once-through fuel currently used in U.S. nuclear reactors.

Safety concepts rely on a negative temperature coefficient of reactivity and a large possible temperature rise to limit reactivity excursions. As an additional method for shutdown, a separate, passively cooled container below the reactor can be included. In case of problems, and for regular maintenance, the fuel is drained from the reactor. This stops the nuclear reaction and acts as a second cooling system. Neutron-producing accelerators have been proposed for some super-safe subcritical experimental designs.

The temperatures of some proposed designs are high enough to produce process heat for hydrogen production or other chemical reactions. Because of this, they are included in the GEN-IV roadmap for further study.

Advantages

MSRs offer many potential advantages over current light water reactors:

• As in all low-pressure reactor designs, passive decay heat removal is achieved in MSRs. In some designs, the fuel and the coolant are the same fluid, so a loss of coolant removes the reactor's fuel, similar to how loss of coolant also removes the moderator in LWRs. Unlike steam, fluoride salts dissolve poorly in water, and do not form burnable hydrogen. Unlike steel and solid uranium oxide, molten salts are not damaged by the core's neutron bombardment, though the reactor vessel still is.
• A low-pressure MSR lacks a BWR's high-pressure radioactive steam and therefore do not experience leaks of radioactive steam and cooling water, and the expensive containment, steel core vessel, piping and safety equipment needed to contain radioactive steam. However, most MSR designs require radioactive fission product-containing fluid in direct contact with pumps and heat exchangers.
• MSRs may make closed nuclear fuel cycles cheaper because they can operate with slow neutrons. If fully implemented, any reactor that closes the nuclear fuel cycle reduces environmental impacts: Chemical separation turns long-lived actinides back into reactor fuel. The discharged wastes are mostly fission products (nuclear ashes) with shorter half-lives. This reduces the needed geologic containment to 300 years rather than the tens of thousands of years needed by a light-water reactor's spent nuclear fuel. It also permits the use of alternate nuclear fuels, such as thorium.
• The fuel's liquid phase might be pyroprocessed to separate fission products (nuclear ashes) from actinide fuels. This may have advantages over conventional reprocessing, though much development is still needed.
• Fuel rod fabrication is not required (replaced with fuel salt synthesis).
• Some designs are compatible with the fast neutron spectrum, which can "burn" problematic transuranic elements like Pu240, Pu241 and up (reactor grade plutonium) from traditional light-water nuclear reactors.
• An MSR can react to load changes in under 60 seconds (unlike "traditional" solid-fuel nuclear power plants that suffer from xenon poisoning).
• 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," that is high power at a low mass as demonstrated by ARE.
• A possibly good neutron economy makes the MSR attractive for the neutron-poor thorium fuel cycle

Disadvantages

• Little development compared to most Gen IV designs
• In circulating-fuel-salt designs, radionuclides dissolved in fuel come in contact with major equipment such as pumps and heat exchangers, likely requiring fully remote and possibly expensive maintenance.
• Required onsite chemical plant to manage core mixture and remove fission products
• Required regulatory changes to deal with radically different design features
• MSR designs rely on nickel-based alloys to hold the molten salt. Alloys based on nickel and iron are prone to embrittlement under high neutron flux.[[]]
• Corrosion risk. Molten salts require careful management of their redox state to handle corrosion risks. This is particularly challenging for circulating-fuel-salt designs, in which a complex mix of fissile/fertile isotopes and their transmutation/fission/decay products is being circulated through the reactor. Static fuel salt designs profit from modularising the problem: the fuel salt is contained within fuel pins whose regular replacement, primarily due to neutron irradiation damage, is part of the operating concept; while the coolant salt has a simpler chemical composition and, under appropriate redox state management, does not pose a corrosion risk either to the fuel pins or to the reactor vessel. (Regarding redox state management, see the descriptions for the stable salt reactor's fuel and coolant salts). The MSRs developed at ORNL in the 60's were only safe to operate for a few years, and operated at only about 650 °C. Potential corrosion risks include dissolution of chromium by liquid fluoride thorium salts at >700 °C, hence endangering stainless steel components. Neutron radiation can also transmute other common alloying agents such as Co and Ni, shortening lifespan. If using lithium salts (e.g. FLiBe), it is preferable, if expensive, to use 7Li to reduce tritium generation (tritium can permeate stainless steels, cause embrittlement, and escape into the environment). ORNL developed Hastelloy N to help address these issues, and there is an effort to certify other structural steels for use in reactors (316H, 800H, inco 617).
• As a breeder reactor, a modified MSR might be able to produce weapons-grade nuclear material
• The MSRE and aircraft nuclear reactors used enrichment levels so high that they approach the levels of nuclear weapons. These levels would be illegal in most modern regulatory regimes for power plants. Some modern designs avoid this issue.
• Neutron damage to solid moderator materials can limit the core lifetime of an MSR that uses moderated thermal neutrons. For example, the MSRE was designed so that its graphite moderator sticks had very loose tolerances, so neutron damage could change their size without damage. "Two fluid" MSR designs are unable to use graphite piping because graphite changes size when it is bombarded with neutrons, and graphite pipes would crack and leak. MSR using fast neutrons cannot use graphite anyway to avoid moderation.
• 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 (MSCR), advanced high-temperature reactors (AHTRs), or fluoride high-temperature reactors (FHR, preferred DOE designation).

FHRs cannot reprocess fuel easily and have fuel rods that need to be fabricated and validated, requiring up to twenty yearsTemplate:Citation needed 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 (as is present in BWRs), and no large, expensive steel pressure vessel (as required for PWRs). 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 850 °C thermochemical production of hydrogen becomes possible. Materials for this temperature range have not been validated, though carbon composites, molybdenum alloys (e.g. TZM), carbides, and refractory metal based or ODS alloys might be feasible.

A workaround suggested by a private researcher is to use the new beta-titanium Au alloys as this would also allow extreme temperature operation as well as increasing the safety margin.Template:Citation needed

Fused salt selection

Molten FLiBe

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

Fluorine

Fluorine has only one stable isotope (F-19), and does not easily become radioactive under neutron bombardment. Compared to chlorine and other halides, fluorine also absorbs fewer neutrons and slows ("moderates") 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 ( Template:Chem/link

and 

Template:Chem/link ), as well as a slow-decaying isotope between them which facilitates neutron absorption by Template:Chem/link .

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, chlorine-37, thus reducing production of sulfur tetrachloride that occurs when chlorine-35 absorbs a neutron to become chlorine-36, then degrades by beta decay to sulfur-36.

Lithium

Lithium must be in the form of purified Template:Chem/link , because Template:Chem/link

effectively captures neutrons and produces tritium. Even if pure 7Li is used, salts containing lithium cause significant tritium production, comparable with heavy water reactors.

Mixtures

Reactor salts are usually close to eutectic mixtures to reduce their melting point. A low melting point simplifies melting the salt at startup and reduces the risk of the salt freezing as it is cooled in the heat exchanger.

Due to the high "redox window" of fused fluoride salts, the redox potential of the fused salt system can be changed. Fluorine-lithium-beryllium ("FLiBe") can be used with beryllium additions to lower the redox potential and nearly eliminate corrosion. However, since beryllium is extremely toxic, special precautions must be engineered into the design to prevent its release into the environment. Many other salts can cause plumbing corrosion, especially if the reactor is hot enough to make highly reactive hydrogen.

To date, most research has focused on FLiBe, because lithium and beryllium are reasonably effective moderators and form a eutectic salt mixture with a lower melting point than each of the constituent salts. Beryllium also performs neutron doubling, improving the neutron economy. This process occurs when the beryllium nucleus emits two neutrons after absorbing a single neutron. For the fuel carrying salts, generally 1% or 2% (by mole) of UF₄ is added. Thorium and plutonium fluorides have also been used.

Fused salt purification

Techniques for preparing and handling molten salt were first developed at ORNL. The purpose of salt purification is to eliminate oxides, sulfur and metal impurities. Oxides could result in the deposition of solid particles in reactor operation. Sulfur must be removed because of its corrosive attack on nickel-based alloys at operational temperature. Structural metal such as chromium, nickel, and iron must be removed for corrosion control.

A water content reduction purification stage using HF and helium sweep gas was specified to run at 400 °C. Oxide and sulfur contamination in the salt mixtures were removed using gas sparging of HF – H₂ mixture, with the salt heated to 600 °C.[[]] Structural metal contamination in the salt mixtures were removed using hydrogen gas sparging, at 700 °C.[[]] Solid ammonium hydrofluoride was proposed as a safer alternative for oxide removal.

Fused salt processing

The possibility of online processing can be an MSR advantage. Continuous processing would reduce the inventory of fission products, control corrosion and improve neutron economy by removing fission products with high neutron absorption cross-section, especially xenon. This makes the MSR particularly suited to the neutron-poor thorium fuel cycle. Online fuel processing can introduce risks of fuel processing accidents,[[]] which can trigger release of radio isotopes.

In some thorium breeding scenarios, the intermediate product protactinium Template:Chem/link

would be removed from the reactor and allowed to decay into highly pure 

Template:Chem/link , an attractive bomb-making material. More modern designs propose to use a lower specific power or a separate thorium breeding blanket. This dilutes the protactinium to such an extent that few protactinium atoms absorb a second neutron or, via a (n, 2n) reaction (in which an incident neutron is not absorbed but instead knocks a neutron out of the nucleus), generate Template:Chem/link . Because Template:Chem/link

has a short half-life and its decay chain contains hard gamma emitters, it makes the isotopic mix of uranium less attractive for bomb-making. This benefit would come with the added expense of a larger fissile inventory or a 2-fluid design with a large quantity of blanket salt.

The necessary fuel salt reprocessing technology has been demonstrated, but only at laboratory scale. A prerequisite to full-scale commercial reactor design is the R&D to engineer an economically competitive fuel salt cleaning system.

Fuel reprocessing

Changes in the composition of a MSR fast neutron (kg/GW)

Reprocessing refers to the chemical separation of fissionable uranium and plutonium from spent fuel. Such recovery could increase the risk of nuclear proliferation. In the United States the regulatory regime has varied dramatically across administrations.

Costs and economics

A systematic literature review from 2020 concludes that there is very limited information on economics and finance of MSRs, with low quality of the information and that cost estimations are uncertain.

In the specific case of the stable salt reactor (SSR) where the radioactive fuel is contained as a molten salt within fuel pins and the primary circuit is not radioactive, operating costs are likely to be lower.Template:Verify sourceTemplate:Additional citation needed

Types of molten salt reactors

While many design variants have been proposed, there are three main categories regarding the role of molten salt:

The use of molten salt as fuel and as coolant are independent design choices - the original circulating-fuel-salt MSRE and the more recent static-fuel-salt SSR use salt as fuel and salt as coolant; the DFR uses salt as fuel but metal as coolant; and the FHR has solid fuel but salt as coolant.

Designs

MSRs can be burners or breeders. They can be fast or thermal or epithermal. Thermal reactors typically employ a moderator (usually graphite) to slow the neutrons down and moderate temperature. They can accept a variety of fuels (low-enriched uranium, thorium, depleted uranium, waste products) and coolants (fluoride, chloride, lithium, beryllium, mixed). Fuel cycle can be either closed or once-through. They can be monolithic or modular, large or small. The reactor can adopt a loop, modular or integral configuration. Variations include:

Molten salt fast reactor

The molten salt fast reactor (MSFR) is a proposed design with the fuel dissolved in a fluoride salt coolant. The MSFR is one of the two variants of MSRs selected by the Generation IV International Forum (GIF) for further development, the other being the FHR or AHTR. The MSFR is based on a fast neutron spectrum and is believed to be a long-term substitute to solid-fueled fast reactors. They have been studied for almost a decade, mainly by calculations and determination of basic physical and chemical properties in the European Union and Russian Federation. A MSFR is regarded sustainable because there are no fuel shortages. Operation of a MSFR does in theory not generate or require large amounts of transuranic (TRU) elements. When steady state is achieved in a MSFR, there is no longer a need for uranium enrichment facilities.

MSFRs may be breeder reactors. They operate without a moderator in the core such as graphite, so graphite life-span is no longer a problem. This results in a breeder reactor with a fast neutron spectrum that operates in the Thorium fuel cycle. MSFRs contain relatively small initial inventories of ²³³U. MSFRs run on liquid fuel with no solid matter inside the core. This leads to the possibility of reaching specific power that is much higher than reactors using solid fuel. The heat produced goes directly into the heat transfer fluid. In the MSFR, a small amount of molten salt is set aside to be processed for fission product removal and then returned to the reactor. This gives MSFRs the capability of reprocessing the fuel without stopping the reactor. This is very different compared to solid-fueled reactors because they have separate facilities to produce the solid fuel and process spent nuclear fuel. The MSFR can operate using a large variety of fuel compositions due to its on-line fuel control and flexible fuel processing.

The standard MSFR would be a 3000 MWth reactor that has a total fuel salt volume of 18 m³ with a mean fuel temperature of 750 ^oC. The core's shape is a compact cylinder with a height to diameter ratio of 1 where liquid fluoride fuel salt flows from the bottom to the top. The return circulation of the salt, from top to bottom, is broken up into 16 groups of pumps and heat exchangers located around the core. The fuel salt takes approximately 3 to 4 seconds to complete a full cycle. At any given time during operation, half of the total fuel salt volume is in the core and the rest is in the external fuel circuit (salt collectors, salt-bubble separators, fuel heat exchangers, pumps, salt injectors and pipes). MSFRs contain an emergency draining system that is triggered and achieved by redundant and reliable devices such as detection and opening technology. During operation, the fuel salt circulation speed can be adjusted by controlling the power of the pumps in each sector. The intermediate fluid circulation speed can be adjusted by controlling the power of the intermediate circuit pumps. The temperature of the intermediate fluid in the intermediate exchangers can be managed through the use of a double bypass. This allows the temperature of the intermediate fluid at the conversion exchanger inlet to be held constant while its temperature is increased in a controlled way at the inlet of the intermediate exchangers. The temperature of the core can be adjusted by varying the proportion of bubbles injected in the core since it reduces the salt density. As a result, it reduces the mean temperature of the fuel salt. Usually the fuel salt temperature can be brought down by 100 ^oC using a 3% proportion of bubbles. MSFRs have two draining modes, controlled routine draining and emergency draining. During controlled routine draining, fuel salt is transferred to actively cooled storage tanks. The fuel temperature can be lowered before draining, this may slow down the process. This type of draining could be done every 1 to 5 years when the sectors are replaced. Emergency draining is done when an irregularity occurs during operation. The fuel salt can be drained directly into the emergency draining tank either by active devices or by passive means. The draining must be fast to limit the fuel salt heating in a loss of heat removal event.

Fluoride salt-cooled high-temperature reactor

The fluoride salt-cooled high-temperature reactor (FHR), also called advanced high temperature reactor (AHTR), is also a proposed Generation IV molten salt reactor variant regarded promising for the long-term future. The FHR/AHTR reactor uses a solid-fuel system along with a molten fluoride salt as coolant.

One version of the Very-high-temperature reactor (VHTR) under study was the liquid-salt very-high-temperature reactor (LS-VHTR). It uses liquid salt as a coolant in the primary loop, rather than a single helium loop. It relies on "TRISO" fuel dispersed in graphite. Early AHTR research focused on graphite in the form of graphite rods that would be inserted in hexagonal moderating graphite blocks, but current studies focus primarily on pebble-type fuel.Template:Citation needed The LS-VHTR can work at very high temperatures (the boiling point of most molten salt candidates is >1400 °C); low-pressure cooling that can be used to match hydrogen production facility conditions (most thermochemical cycles require temperatures in excess of 750 °C); better electric conversion efficiency than a helium-cooled VHTR operating in similar conditions; passive safety systems and better retention of fission products in the event of an accident.Template:Citation needed

Liquid fluoride thorium reactor

Reactors containing molten thorium salt, called liquid fluoride thorium reactors (LFTR), would tap the thorium fuel cycle. Private companies from Japan, Russia, Australia and the United States, and the Chinese government, have expressed interest in developing this technology.

Advocates estimate that five hundred metric tons of thorium could supply U.S. energy needs for one year. The U.S. Geological Survey estimates that the largest-known U.S. thorium deposit, the Lemhi Pass district on the Montana-Idaho border, contains thorium reserves of 64,000 metric tons.

Traditionally, these reactors were known as molten salt breeder reactors (MSBRs) or thorium molten salt reactors (TMSRs), but the name LFTR was promoted as a rebrand in the early 2000s by Kirk Sorensen.

Stable salt reactor

The stable salt reactor is a relatively recent concept which holds the molten salt fuel statically in traditional LWR fuel pins. Pumping of the fuel salt, and all the corrosion/deposition/maintenance/containment issues arising from circulating a highly radioactive, hot and chemically complex fluid, are no longer required. The fuel pins are immersed in a separate, non-fissionable fluoride salt which acts as primary coolant.

Dual-fluid molten salt reactors

A prototypical example of a dual fluid reactor is the lead-cooled, salt-fueled reactor.