Molten-salt reactor
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 mixture of molten salt with a fissionable material.
Two research MSRs operated in the United States in the mid-20th century. The 1950s Aircraft Reactor Experiment (ARE) was primarily motivated by the technology's compact size, while the 1960s Molten-Salt Reactor Experiment (MSRE) 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. As of May 2023, China had not announced the ignition of its TMSR-LF1 thorium unit following its scheduled date of February 2023.
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 (as in the Fukushima nuclear disaster).
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.
Properties
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:
Disadvantages
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 years
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 (1,560 °F) 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.
Fused salt selection
The salt mixtures are chosen to make the reactor safer and more practical.
Fluorine has only one stable isotope (19F), 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 has two stable isotopes (35Cl and 37Cl), as well as a slow-decaying isotope between them which facilitates neutron absorption by 35Cl.
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, 37Cl, thus reducing production of sulfur tetrachloride that occurs when 35Cl absorbs a neutron to become 36Cl, then degrades by beta decay to 36S.
Lithium must be in the form of purified 7Li, because 6Li effectively captures neutrons and produces tritium. Even if pure 7Li is used, salts containing lithium cause significant tritium production, comparable with heavy water reactors.
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.
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.
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,
In some thorium breeding scenarios, the intermediate product protactinium 233Pa would be removed from the reactor and allowed to decay into highly pure 233U, 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 232U. Because 232U 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
Reprocessing refers to the chemical separation of fissionable uranium and plutonium from spent fuel.
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.
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)
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.
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 °C. 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).
The fluoride salt-cooled high-temperature reactor (FHR), also called advanced high temperature reactor (AHTR),
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.
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.
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.
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.
A prototypical example of a dual fluid reactor is the lead-cooled, salt-fueled reactor.
History
1950s
MSR research started with the U.S. Aircraft Reactor Experiment (ARE) in support of the U.S. Aircraft Nuclear Propulsion program. ARE was a 2.5 MWth nuclear reactor experiment designed to attain a high energy density for use as an engine in a nuclear-powered bomber.
The project included experiments, including high temperature and engine tests collectively called the Heat Transfer Reactor Experiments: HTRE-1, HTRE-2 and HTRE-3 at the National Reactor Test Station (now Idaho National Laboratory) as well as an experimental high-temperature molten-salt reactor at Oak Ridge National Laboratory – the ARE.
ARE used molten fluoride salt NaF/ZrF4/UF4 (53-41-6 mol%) as fuel, moderated by beryllium oxide (BeO). Liquid sodium was a secondary coolant.
The experiment had a peak temperature of 860 °C. It produced 100 MWh over nine days in 1954. This experiment used Inconel 600 alloy for the metal structure and piping.
An MSR was operated at the Critical Experiments Facility of the Oak Ridge National Laboratory in 1957. It was part of the circulating-fuel reactor program of the Pratt & Whitney Aircraft Company (PWAC). This was called Pratt and Whitney Aircraft Reactor-1 (PWAR-1). The experiment was run for a few weeks and at essentially zero power, although it reached criticality. The operating temperature was held constant at approximately 675 °C (1,250 °F). The PWAR-1 used NaF/ZrF4/UF4 as the primary fuel and coolant. It was one of three critical MSRs ever built.
1960s and 1970s
Oak Ridge National Laboratory (ORNL) took the lead in researching MSRs through the 1960s. Much of their work culminated with the Molten-Salt Reactor Experiment (MSRE). MSRE was a 7.4 MWth test reactor simulating the neutronic "kernel" of a type of epithermal thorium molten salt breeder reactor called the liquid fluoride thorium reactor (LFTR). The large (expensive) breeding blanket of thorium salt was omitted in favor of neutron measurements.
MSRE's piping, core vat and structural components were made from Hastelloy-N, moderated by pyrolytic graphite. It went critical in 1965 and ran for four years. Its fuel was LiF/BeF2/ZrF4/UF4 (65-29-5-1)mol%. The graphite core moderated it. Its secondary coolant was FLiBe (2LiF·BeF2). It reached temperatures as high as 650 °C (1,202 °F) and achieved the equivalent of about 1.5 years of full power operation.
From 1970 to 1976 ORNL researched during the 1970–1976 a molten salt breeder reactor (MSBR) design. Fuel was to be LiF/BeF2/ThF4/UF4 (72-16-12-0.4) mol% with graphite moderator. The secondary coolant was to be NaF/Na
The MSBR project received funding from 1968 to 1976 of (in 2023 dollars
Officially, the program was cancelled because:
The denatured molten-salt reactor (DMSR) was an Oak Ridge theoretical design that was never built.
Engel et al. 1980 said the project "examine
Other goals of the DMSR were to minimize research and development and to maximize feasibility. The Generation IV international Forum (GIF) includes "salt processing" as a technology gap for molten-salt reactors.
The UK's Atomic Energy Research Establishment (AERE) was developing an alternative MSR design across its National Laboratories at Harwell, Culham, Risley and Winfrith. AERE opted to focus on a lead-cooled 2.5 GWe Molten Salt Fast Reactor (MSFR) concept using a chloride.
The UK MSFR would have been fuelled by plutonium, a fuel considered to be 'free' by the program's research scientists, because of the UK's plutonium stockpile.
Despite their different designs, ORNL and AERE maintained contact during this period with information exchange and expert visits. Theoretical work on the concept was conducted between 1964 and 1966, while experimental work was ongoing between 1968 and 1973. The program received annual government funding of around £100,000–£200,000 (equivalent to £2m–£3m in 2005). This funding came to an end in 1974, partly due to the success of the Prototype Fast Reactor at Dounreay which was considered a priority for funding as it went critical in the same year.
In the USSR, a molten-salt reactor research program was started in the second half of the 1970s at the Kurchatov Institute. It included theoretical and experimental studies, particularly the investigation of mechanical, corrosion and radiation properties of the molten salt container materials. The main findings supported the conclusion that no physical nor technological obstacles prevented the practical implementation of MSRs.
Twenty-first century
MSR interest resumed in the new millennium due to continuing delays in fusion power and other nuclear power programs and increasing demand for energy sources that would incur minimal greenhouse gas (GHG) emissions.
Commercial/national/international projects
Canada
Terrestrial Energy, a Canadian-based company, is developing a DMSR design called the Integral Molten-Salt Reactor (IMSR). The IMSR is designed to be deployable as a small modular reactor (SMR). Their design currently undergoing licensing is 400MW thermal (190MW electrical). With high operating temperatures, the IMSR has applications in industrial heat markets as well as traditional power markets. The main design features include neutron moderation from graphite, fueling with low-enriched uranium and a compact and replaceable Core-unit. Decay heat is removed passively using nitrogen (with air as an emergency alternative). The latter feature permits the operational simplicity necessary for industrial deployment.
Terrestrial completed the first phase of a prelicensing review by the Canadian Nuclear Safety Commission in 2017, which provided a regulatory opinion that the design features are generally safe enough to eventually obtain a license to construct the reactor.
Moltex Energy Canada, a subsidiary of UK-based Moltex Energy Ltd, has obtained support from New Brunswick Power for the development of a pilot plant in Point Lepreau, Canada,
China
China initiated a thorium research project in January 2011, and spent about 3 billion yuan (US$500 million) on it by 2021.
In 2021, China stated that Wuwei prototype operation could start power generation from thorium in September,
Further work on commercial reactors was announced with the target completion date of 2030.
In 2022, Shanghai Institute of Applied Physics (SINAP) was given approval by the Ministry of Ecology and Environment to commission an experimental thorium-powered MSR.
Denmark
Copenhagen Atomics is a Danish molten salt technology company developing mass manufacturable molten salt reactors. The Copenhagen Atomics Waste Burner is a single-fluid, heavy water moderated, fluoride-based, thermal spectrum and autonomously controlled molten-salt reactor. This is designed to fit inside of a leak-tight, 40-foot, stainless steel shipping container. The heavy water moderator is thermally insulated from the salt and continuously drained and cooled to below 50 °C (122 °F). A molten lithium-7 deuteroxide (7LiOD) moderator version is also being researched. The reactor utilizes the thorium fuel cycle using separated plutonium from spent nuclear fuel as the initial fissile load for the first generation of reactors, eventually transitioning to a thorium breeder.
Seaborg Technologies is developing the core for a compact molten-salt reactor (CMSR). The CMSR is a high temperature, single salt, thermal MSR designed to go critical on commercially available low enriched uranium. The CMSR design is modular, and uses proprietary NaOH moderator.
France
The CNRS project EVOL (Evaluation and viability of liquid fuel fast reactor system) project, with the objective of proposing a design of the molten salt fast reactor (MSFR),
The EVOL project will be continued by the EU-funded Safety Assessment of the Molten Salt Fast Reactor (SAMOFAR) project, in which several European research institutes and universities collaborate.
Germany
The German Institute for Solid State Nuclear Physics in Berlin has proposed the dual fluid reactor as a concept for a fast breeder lead-cooled MSR. The original MSR concept used the fluid salt to provide the fission materials and also to remove the heat. Thus it had problems with the needed flow speed. Using 2 different fluids in separate circles is thought to solve the problem.
India
In 2015, Indian researchers published a MSR design,
Indonesia
Thorcon is developing the TMSR-500 molten-salt reactor for the Indonesian market.
Japan
The Fuji Molten-Salt Reactor is a 100 to 200 MWe LFTR, using technology similar to the Oak Ridge project. A consortium including members from Japan, the U.S. and Russia are developing the project. The project would likely take 20 years to develop a full size reactor,
Russia
In 2020, Rosatom announced plans to build a 10 MWth FLiBe burner MSR. It would be fueled by plutonium from reprocessed VVER spent nuclear fuel and fluorides of minor actinides. It is expected to launch in 2031 at Mining and Chemical Combine.
United Kingdom
The Alvin Weinberg Foundation is a British non-profit organization founded in 2011, dedicated to raising awareness about the potential of thorium energy and LFTR. It was formally launched at the House of Lords on 8 September 2011.
Moltex Energy's stable-salt reactor design was selected as the most suitable of six MSR designs for UK implementation in a 2015 study commissioned by the UK's innovation agency, Innovate UK.
United States
Idaho National Laboratory designed
Kirk Sorensen, former NASA scientist and chief nuclear technologist at Teledyne Brown Engineering, is a long-time promoter of the thorium fuel cycle, coining the term liquid fluoride thorium reactor. In 2011, Sorensen founded Flibe Energy,
Transatomic Power pursued what it termed a waste-annihilating molten-salt reactor (WAMSR), intended to consume existing spent nuclear fuel,
In January 2016, the United States Department of Energy announced a $80m award fund to develop Generation IV reactor designs.
In 2021, Tennessee Valley Authority (TVA) and Kairos Power announced a TRISO-fueled, low-pressure fluoride salt-cooled 140 MWe test reactor to be built in Oak Ridge, Tennessee. A construction permit for the project was issued by the US Nuclear Regulatory Commission (NCR) in 2023. The design is expected to operate at 45% efficiency. The outlet temperature is 650 °C (1,202 °F). The main steam pressure is 19 MPa. The reactor structure is 316 stainless steel. The fuel is enriched to 19.75%. Loss-of-power cooling is passive.
Also in 2021, Southern Company, in collaboration with TerraPower and the U.S. Department of Energy announced plans to build the Molten Chloride Reactor Experiment, the first fast-spectrum salt reactor at the Idaho National Laboratory.