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Encyclopedia > Molten salt reactor
Molten salt reactor scheme.

A molten salt reactor (MSR) is a type of nuclear reactor where the primary coolant is a molten salt. Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... Core of a small nuclear reactor used for research. ... Molten salt may refer to: Molten salt battery, a class of primary cell and secondary cell high temperature electric battery that use molten salts as an electrolyte Molten salt reactor, a type of nuclear reactor where the primary coolant is a molten salt Solar thermal energy power plants that use...


There have been many designs put forward for use of this type of reactor as a nuclear power plant and a few prototypes built. The concept is one of those proposed for development as a generation IV reactor. A nuclear power station. ... Generation IV reactors (Gen IV) are a set of theoretical nuclear reactor designs currently being researched. ...


The early concepts and many current ones had the nuclear fuel dissolved in the molten fluoride salt coolant as uranium tetrafluoride (UF4), the fluid would reach criticality by flowing into a graphite core which also served as the moderator. Many current concepts rely on ceramic fuel that is dispersed in a graphite matrix with the molten salt providing low pressure, high temperature cooling. Nuclear Fuel Process A graph comparing nucleon number against binding energy Nuclear fuel is any material that can be consumed to derive nuclear energy, by analogy to chemical fuel that is burned to derive energy. ... Fluoride is the ionic form of fluorine. ... Uranium tetrafluoride (UF4) is a green crystalline solid that melts at about 1,760°F (960°C) and has an insignificant vapor pressure and is very slightly soluble in water. ... A sphere of plutonium surrounded by neutron-reflecting blocks of tungsten carbide. ... For other uses, see Graphite (disambiguation). ... This does not cite any references or sources. ...

Contents

History

The aircraft reactor experiment

Aircraft Reactor Experiment building at ORNL

Extensive research into molten salt reactors started with the US Aircraft Reactor Experiment (ARE). The US Aircraft Reactor Experiment was a 2.5 MWth nuclear reactor experiment designed to attain a high power density for use as an engine in a nuclear powered bomber. The project resulted in several experiments. Three of which resulted in engine tests collectively called the Heat Transfer Reactor Experiments, of which there were three iterations: HTRE-l, HTRE-2, and HTRE-3. One experiment used the molten fluoride salt NaF-ZrF4-UF4 (53-41-6 mol%) as fuel and was moderated by beryllium oxide (BeO), used liquid sodium as a secondary coolant, and had a peak temperature of 860°C, it operated for a 1000 hour cycle in 1954. This experiment used Inconel 600 alloy for the metal structure and piping. Image File history File links Size of this preview: 532 × 599 pixelsFull resolution (618 × 696 pixel, file size: 44 KB, MIME type: image/jpeg) Aircraft Reactor Experiment Building Available: [1] File historyClick on a date/time to view the file as it appeared at that time. ... Image File history File links Size of this preview: 532 × 599 pixelsFull resolution (618 × 696 pixel, file size: 44 KB, MIME type: image/jpeg) Aircraft Reactor Experiment Building Available: [1] File historyClick on a date/time to view the file as it appeared at that time. ... The US Aircraft Reactor Experiment (ARE) was a 2. ... Beryllium oxide (BeO) is a white crystalline oxide. ... Inconel® is a registered trademark of Special Metals Corporation referring to a family of austenitic nickel-based superalloys. ...


The Molten-Salt Reactor Experiment

Molten FLiBe

Oak Ridge National Laboratory took the lead in researching the MSR through 1960s and much of their work culminated with the Molten-Salt Reactor Experiment (MSRE). The MSRE was a 7.4 MWth test reactor simulating the neutronic "kernel" of an inherently safe epithermal Thorium breeder reactor. It tested molten salt fuels of Uranium and Plutonium. The tested 233UF4 fluid fuel has a unique decay path that minimizes waste, with waste isotopes having half-lives under 50 years. The red-hot 650°C temperature of the reactor could power high-efficiency heat engines such as gas turbines. The large, expensive breeding blanket of Thorium salt was omitted in favor of neutron measurements. Image File history File links Size of this preview: 614 × 600 pixelsFull resolution (934 × 912 pixel, file size: 91 KB, MIME type: image/jpeg) Picture of molten FLiBe. ... Image File history File links Size of this preview: 614 × 600 pixelsFull resolution (934 × 912 pixel, file size: 91 KB, MIME type: image/jpeg) Picture of molten FLiBe. ... // Purpose By 1960 a fairly clear picture of a family of molten-salt reactors had emerged. ... A combination of federal, state and private funds is providing $300 million for the construction of 13 facilities on ORNLs new main campus. ... // Purpose By 1960 a fairly clear picture of a family of molten-salt reactors had emerged. ... // The nuclear fuel cycle, also called nuclear fuel chain, consists of front end steps that lead to the preparation of uranium for use as fuel for reactor operation and back end steps that are necessary to safely manage, prepare, and dispose of radioactive waste. ...


The MSRE was located at ORNL. Its piping, core vat and structural components were made from Hastelloy-N and its moderator was pyrolytic graphite. It went critical in 1965 and ran for four years. The fuel for the MSRE was LiF-BeF2-ZrF4-UF4 (65-30-5-0.1), the graphite core moderated it, and its secondary coolant was FLiBe (2LiF-BeF2), it operated as hot as 650°C and operated for the equivalent of about 1.5 years of full power operation. (For more information, see the main article) HASTELLOY is the registered trademark name of Haynes International, Inc. ... Pyrolytic carbon is a material similar to graphite, but with some covalent bonding between its graphene sheets. ...


Oak Ridge National Laboratory reactor

The culmination of the Oak Ridge National Laboratory research during the 1970-76 timeframe resulted in a MSR design which would use LiF-BeF2-ThF4-UF4 (72-16-12-0.4) as fuel, was to be moderated by graphite with a 4 year replacement schedule, use NaF-NaBF4 as the secondary coolant, and have a peak operating temperature of 705°C. However, to date the molten salt reactor remains a "paper design", that is, no molten salt reactors have been built other than the experimental MSRE.


Liquid salt very high temperature reactor

Research is currently picking up again for reactors that utilize molten salts for coolant. Both the traditional molten salt reactor and the Very High Temperature Reactor (VHTR) have been picked as potential designs to be studied under the Generation Four Initiative (GEN-IV). A version of the VHTR currently being studied is the Liquid Salt Very High Temperature Reactor (LS-VHTR). It is essentially a standard VHTR design that uses liquid salt as a coolant in the primary loop, rather than a single helium loop. It relies on "TRISO" fuel dispersed in graphite. The fuel graphite would be in the form of graphite rods that would be inserted in hexagonal moderating graphite blocks. The molten salt would pass through holes drilled in the graphite blocks. The LS-VHTR has many attractive features, including: the ability to work at very high temperatures (the boiling point of most molten salts being considered are >1400°C), low pressure cooling that can be used to more easily match hydrogen production facility conditions (most thermo chemical cycles require temperatures in excess of 750°C), better electric conversion efficiency than a helium cooled VHTR operating at similar conditions, passive safety systems, and better retention of fission-products in case an accident occurred. Very high temperature reactor scheme. ... Generation IV reactors (Gen IV) are a set of theoretical nuclear reactor designs currently being researched. ... Nuclear Fuel Process A graph comparing nucleon number against binding energy Nuclear fuel is any material that can be consumed to derive nuclear energy, by analogy to chemical fuel that is burned to derive energy. ... Car safety is the avoidance of car accidents or the minimization of harmful effects of accidents, in particular as pertaining to human life and health. ...


Technological issues

Molten-salt Fueled Reactors

The classic MSFR has been very exciting to many nuclear engineers. Its most prominent champion was Alvin Weinberg, who patented the light-water reactor, and was a director of the U.S.'s Oak Ridge National Laboratory, a prominent nuclear research center. Alvin Martin Weinberg (April 20, 1915 - October 18, 2006) was a nuclear physicist and administrator at Oak Ridge National Laboratory (ORNL). ...


Two concepts were investigated. The "two fluid" reactor had a high-neutron-density core that burned U233 from the Thorium fuel cycle. A blanket of thorium salts absorbed the neutrons and was eventually transmuted to U233 fuel. The engineers discovered that by carefully sculpting the moderator rods (to get neutron densities similar to a core and blanket), and modifying the fuel reprocessing chemistry, both Thorium and Uranium salts could coexist in a simpler, less expensive yet efficient "one fluid" reactor.


The power reactor design produced by Weinberg's research group was similar to the MSRE above, which was designed to validate the risky hot, high-neutron-density "kernel" part of the "kernel and blanket" thorium breeder.


The advantages cited by Weinberg and his associates at Oak Ridge National Laboratory include:

  • It's safe to operate and maintain: Molten fluoride salts are mechanically and chemically stable at sea-level pressures at intense heats and radioactivity. Fluoride combines ionically with almost any transmutation product, keeping it out of circulation. Even radioactive noble gases come out in a predictable, containable place, where the fuel is coolest and most dispersed, the pump bowl.
  • There's no high pressure steam in the core, just low-pressure molten salt. This means that the MSR's core cannot have a steam explosion, and does not need the most expensive item in a light water reactor, a high-pressure steam vessel for the core. Instead, there is a vat and low-pressure pipes (for molten salt) constructed of thick sheet metal. The metal is an exotic nickel alloy that resists heat and corrosion, Hastelloy-N, but there is much less of it, and the thin metal is less expensive to form and weld.
  • With fuel reprocessing, the Thorium fuel cycle, so impractical in other types of reactors, produces 0.1% of the long-term high-level radioactive waste of a light-water reactor without reprocessing (all modern reactors in the U.S.). As Thorium captures neutrons, it first becomes Th233, which quickly decays to Protactinium (Pa233). Pa233 in turn decays to U233 with a half-life of 27 days. U233 is an excellent reactor fuel. As U233 is bombarded by neutrons with a thermal spectrum of speeds, each absorbed neutron either splits the Uranium or produces a heavier isotope of Uranium, which fission to elements similar to those from U233. These fission products almost all have half-lives less than 30 years. The only source of high-radioactivity long-lived transuranic elements is that a tiny bit of Neptunium is produced from the tiny fraction of U236 produced at the tail-end of this process. The Neptunium can be separated by the fuel-salt reprocessing, or it is such a tiny fraction that it can be left in the salt and fissioned by excess neutrons. If the transuranics are left in, the separated wastes are pure Uranium fission wastes. All have half-lives less than 30 years. Reprocessed waste from Thorium is therefore less radioactive than natural ores in 300 years.
  • The Thorium breeder reactor uses low-energy "thermal" neutrons very similar to light water reactors. It is therefore much safer than the touchy fast-neutron breeder reactors that the uranium-to-plutonium fuel cycle requires. The Thorium fuel cycle therefore combines safe reactors, a long-term source of abundant fuel, and no need for expensive fuel-enrichment facilities.
  • A molten salt reactor's fuel can be continuously reprocessed with a small adjacent chemical plant. Weinberg's groups at Oak Ridge National Laboratory found that a very small reprocessing facility can service a large 1Gw power plant: All the salt has to be reprocessed, but only every ten days. Society's total inventory of expensive, poisonous radioactives is therefore much less than in a conventional light-water-reactor's fuel cycle, which moves entire cores to recycling plants. Also, everything except fuel and waste stays inside the plant. The reprocessing cycle is:
    • A sparge of fluorine to remove U233 fuel from the salt. This has to be done before the next step.
    • A 4-meter-tall molten Bismuth column separates Protactinium from the fuel salt.
    • A small storage facility to let the Protactinium from the Bismuth column decay to U233. With a 27 day half life, ten months of storage assures that 99.9% decays to U233 fuel.
    • A small vapor-phase fluoride-salt distillation system distills the salts. Each salt has a distinct temperature of vaporization. The light carrier salts evaporate at low temperatures, and form the bulk of the salt. The thorium salts must be separated from the fission wastes at higher temperatures. The amounts involved are about 80kg of waste per year per GW generated, so the equipment is very small. Salts of long-lived transuranic metals go back into the reactor as fuel.
  • With continuous reprocessing, a molten-salt-fueled reactor has more than 97% burn-up of fuel. This is very efficient, compared to any system, anywhere. Light water reactors burn up about 2% of fuel on a once-through fuel cycle (current practice, 2007).
  • With salt distillation, an MSFR can burn Plutonium, or even fluoridated nuclear waste from light water reactors.
  • The molten-salt-fueled reactor operates much hotter than LWR reactors, from 650C on conservative designs, to as hot as 950C on aggressive designs. So very efficient Brayton cycle (gas turbine) generators are possible. This is also very efficient, a goal of "generation IV reactors" that has already been achieved by MSRs. This reduces fuel use and auxiliary equipment (major capital expenses) by 50% or more.
  • MSRs work in small sizes, as well as large, so a utility could easily build several small reactors (say 100Mwe) from income, reducing interest expense and business risks.
  • Molten salt fuel reactors are not experimental. Several have been constructed and operated at 650C temperatures for extended times, with simple, practical validated designs. There's no need for new science at all, and very little risk in engineering new, larger or modular designs.
  • The reactor, like all nuclear plants, has little effect on biomes. In particular, it uses only small amounts of land, relatively small amounts of construction, and the waste is separated from the biome, unlike both fossil and renewable energy projects.

Combining the above, some form of molten-salt thorium breeder could be the most efficient well-developed energy source known, whether measured by cost per kW, capital cost or social costs. HASTELLOY is the registered trademark name of Haynes International, Inc. ... // The nuclear fuel cycle, also called nuclear fuel chain, consists of front end steps that lead to the preparation of uranium for use as fuel for reactor operation and back end steps that are necessary to safely manage, prepare, and dispose of radioactive waste. ... General Name, Symbol, Number plutonium, Pu, 94 Chemical series actinides Group, Period, Block n/a, 7, f Appearance silvery white Standard atomic weight (244) g·mol−1 Electron configuration [Rn] 5f6 7s2 Electrons per shell 2, 8, 18, 32, 24, 8, 2 Physical properties Phase solid Density (near r. ... Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant) to the point where it is no longer useful in sustaining a nuclear reaction. ... The Brayton cycle is a constant-pressure cycle named after George Brayton (1830–1892), the American engineer who developed it. ... This machine has a single-stage centrifugal compressor and turbine, a recuperator, and foil bearings. ...


There are some design and social advantages:

  • Thorium's fuel cycle resist proliferation in two ways.
    • It's verifiable because the epithermal thorium breeder produces only at most 9% more fuel than it burns in each year. Building bombs quickly will take power plants out of operation.
    • Also, an easy variation of the thorium fuel cycle would contaminate the Th232 breeding material with chemically inseparable Th230. The Th230 breeds into U232, which has a powerful gamma emitter in its decay chain (Tl-208) that makes the reactor fuel U233/U232 impractical in a bomb, because it harms electronics.
  • Thorium is more abundant than Uranium. The Earth's crust has about three times as much.
  • Thorium is cheap. Currently, it's US$ 30/kg.
  • Control of the salt's corrosivity is easy. The Uranium buffers the salt, forming more UF4 from UF3 as more F is present. UF3 can be regenerated by adding small amounts of metallic Beryllium to absorb F. In the MSRE, a beryllium rod was inserted into the salt until the Uf3 was the correct concentration. [1]
  • Extensive validation (fuel rod design validation normally takes years and prevents effective deployment of new nuclear technologies)is not needed. The fuel is molten, chemical reprocessing eliminates reaction products, and there are tested fuel mixtures, notably FLi7BeU.
  • There's no need for fuel fabrication. This greatly reduces the MSR's fuel expenses. It poses a business challenge, because reactor manufacturers customarily get their long-term profits from fuel fabrication. A government agency could, however, type-license a design, which utilities could replicate.
  • Molten-fuel reactors can be made inherently safe: Tested fuel-salt mixtures have negative reactivity coefficients, so that they decrease power generation as they get too hot. Most fuel-salt reactor vessels also have a freeze-plug at the bottom that has to be actively cooled. If the cooling fails, the fuel drains to a subcritical storage facility.
  • Continuous reprocessing simplifies numerous reactor design and operating issues. For example, the poisoning effects from Xenon-135 are not present. Neutron poisoning from fission products is continuously mitigated. Transuranics, the frighteningly-long lived "wastes" of light water reactors, are burned as fuel.
  • A fuel-salt reactor is mechanically and neutronically simpler than light-water reactors. There are only two items in the core: fuel salts and moderators. This reduces concerns with moderating interactions with positive void coefficients as water boils, chemical interactions, etc.
  • Coolant and piping need never enter the high-neutron-flux zone, because the fuel is used to cool the core. The fuel is cooled in low-neutron-flux heat-exchangers outside the core. This reduces worries about neutron effects on pipes, testing, development issues, etc.
  • The salt distillation process means that chemical separation and recycling of fission products, say for nuclear batteries, is actually cheap. Xenon and other valuable transmuted noble gases separate out of the molten fuel in the pump-bowl. Any transuranics go right back into the fuel for burn-up.

Molten salt reactors, nevertheless, present a number of design challenges. Known issues include: A radioisotope thermoelectric generator (RTG) is a simple electrical generator which obtains its power from radioactive decay. ... General Name, Symbol, Number xenon, Xe, 54 Chemical series noble gases Group, Period, Block 18, 5, p Appearance colorless Standard atomic weight 131. ... Neon, like all noble gases, has a full valence (outermost) electron shell. ... In chemistry, transuranium elements (also known as transuranic elements) are the chemical elements with atomic numbers greater than 92, the atomic number of Uranium. ...

  • Since it uses unfabricated fuel, basically just a mixture of chemicals, current reactor vendors don't want to develop it. They derive their long-term profits from sales of fabricated fuel assemblies.
  • Uncooled graphite moderators can cause some geometries of this reactor to increase in reactivity with higher temperatures, making such designs unsafe. Careful design may fix this, however.
  • High neutron fluxes and temperatures in a compact MSR core can rapidly change the shape of a graphite moderator element, to require refurbishing in as little as four years. Eliminating graphite from sealed piping was a major incentive to switch to a single-fluid design.[1] Most MSR designs do not use graphite as a structural material, and arrange for it to be easy to replace. At least one design used graphite balls floating in salt, which could be removed and inspected continuously without shutting down the reactor. [2]
  • A safe thorium breeder reactor using slow thermal-energy neutrons also has a low breeding rate. Each year it can only breed thorium into about 109% of the U233 fuel it consumes. This means that obtaining enough U233 for a new reactor can take eight years or more, which would slow deployment of this type of reactor. Most practical, fast deployment plans would start the new Thorium reactors with Plutonium from existing light-water reactor wastes or decommissioned nuclear weapons. This scheme also decreases society's stock of high-level wastes.
  • The high neutron density in the core rapidly transmutes most isotopes of Lithium to Tritium, a radioactive isotope of Hydrogen. In an MSR, the Tritium forms Hydrogen Fluoride (HF). Tritium HF is a corrosive, chemically poisonous, radiotoxic gas. All MSR designs used very expensive isotopically purified Lithium 7 for their carrier salts in order to reduce Tritium formation as far as possible. The MSRE proved that this worked.
  • Some slow corrosion occurs even in the exotic nickel alloy, Hastelloy-N used for the reactor. The corrosion is more extreme if the reactor is exposed to Hydrogen which forms corrosive HF gas. Mere exposure to water-vapor causes uptake of corrosive amounts of Hydrogen, so practical MSRs operate the salt under a blanket of dry inert gas, usually Helium.
  • When cold, the fuel salts radiogenically produce poisonous Fluorine gas. The salts should be defueled and wastes removed before extended shutdowns. Unfortunately, this was discovered the unpleasant way, while the MSRE was shut-down over a 20-year period.
  • The salt mixture is toxic, and water-soluble. The reactor design must therefore isolate the salt from the biome. This is a normal reactor safety requirement anyway.

An MSR based on chloride salts has many of the same advantages. However, the larger, less-dense atoms of Chlorine causes the reactor to be a fast breeder. Theoretically, it wastes even fewer neutrons and breeds more efficiently, though it may be less safe. It would require a salt with an isotopically-separated Chlorine, Cl37, to prevent neutronic activation of the Chlorine into sulfur which would form corrosive sulfur chloride. HASTELLOY is the registered trademark name of Haynes International, Inc. ...


Molten-salt cooled reactors

Molten-salt-fueled reactors (MSFR) are quite different from molten-salt-cooled reactors (MSCR), a Gen IV proposal. The MSCR can't reprocess fuel easily and has fuel rods that need to be fabricated and validated, delaying deployment by up to twenty years from project inception. However, since it uses fabricated fuel, reactor manufacturers can still proft by selling fuel assemblies. Also, the reactor's core retains many safety and cost advantages. Notably, there's no steam in the core to cause an explosion, and no large, expensive steel pressure vessel. Since it can operate at high temperatures, the conversion of the heat to electricity can also use an efficient, light weight Brayton cycle gas turbine. The Brayton cycle is a constant-pressure cycle named after George Brayton (1830–1892), the American engineer who developed it. ...


Much of the current research on MSCRs is focused on small compact heat exchangers. By using smaller heat exchangers, less molten salt needs to be used and therefore significant cost savings could be achieved. A heat exchanger is a device built for efficient heat transfer from one fluid to another, whether the fluids are separated by a solid wall so that they never mix, or the fluids are directly contacted. ...


Molten salts can be highly corrosive, more so as temperatures rise. For the primary cooling loop of the MSR, a material is needed that can withstand corrosion at high temperatures and intense radiation. Experiments show that Hastelloy-N and similar alloys are quite suited to the tasks at operating temperatures up to about 700°C. However, long-term experience with a production scale reactor has yet to be gained. Higher operating temperatures would be desirable, especially since at 850°C thermo chemical production of hydrogen becomes possible. Materials for this temperature range have not yet been found, though carbon composites, carbides, and refractory metal based or ODS alloys might be feasible. For the hazard, see corrosive. ... Radiation as used in physics, is energy in the form of waves or moving subatomic particles. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... For other uses, see Carbon (disambiguation). ... Calcium carbide. ...


Fused salt selection

The types of fused salts that are chosen come from an optimization of salt characteristics. Fused fluorides are generally chosen over other salts because of the usefulness of the elements without isotope separation, better neutron economy and moderating efficiency, lower vapor pressure and better chemical stability. Chlorides have also been considered for molten salt reactors, but not nearly as much work has been done on reactor designs that utilize them. Additionally, whenever lithium fluoride is used as part of the salt composition, the lithium must be enriched to a very high purity (99.999%?) in lithium-7 to get tritium production under control. Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes, for example separating natural uranium into enriched uranium and depleted uranium. ... Vapor pressure is the pressure of a vapor in equilibrium with its non-vapor phases. ... The chloride ion is formed when the element chlorine picks up one electron to form the negatively charged ion Cl−. The salts of hydrochloric acid HCl are also called chlorides. ... This article is about the chemical element named Lithium. ... Tritium (symbol T or 3H) is a radioactive isotope of hydrogen. ...


Due to the high "redox window" available for fused fluoride salts, allowing for the chemical potential of the fused salt system to be manipulated, the following types of salts are the most promising. FLiBe can be used in conjunction with beryllium additions to drive down the electrochemical potential and virtually eliminate corrosion issues. However, beryllium is extremely toxic to humans. Many other salts have potential corrosion issues, especially at the elevated temperatures being talked about for future hydrogen production facilities. Illustration of a redox reaction Redox (shorthand for oxidation/reduction reaction) describes all chemical reactions in which atoms have their oxidation number (oxidation state) changed. ... In thermodynamics and chemistry, chemical potential, symbolized by μ, is a term introduced in 1876 by the American mathematical physicist Willard Gibbs, which he defined as follows: Gibbs noted also that for the purposes of this definition, any chemical element or combination of elements in given proportions may be considered a... General Name, symbol, number beryllium, Be, 4 Chemical series alkaline earth metals Group, period, block 2, 2, s Appearance white-gray metallic Standard atomic weight 9. ... Electrochemical potential is a thermodynamic measure that reflects energy from entropy and electrostatics and is typically invoked in molecular processes that involve diffusion. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ...


To date, most research has focused on FLiBe for the nuclear heat transport system, for the obvious reasons that 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 has a measureable benefit to the neutron economy of the reactor, due to neutron doubling. This process occurs when the Beryllium nucleus re-emit two neutrons after absorbing a single neutron. For the fuel carrying salts, generally 1% or 2% by mole fraction of UF4 is added, however thorium and plutonium fluorides have also been used. The MSFR is the only system that has run a single reactor, the MSRE, from all three known nuclear fuels. General Name, Symbol, Number thorium, Th, 90 Chemical series Actinides Group, Period, Block n/a, 7, f Appearance silvery white Standard atomic weight 232. ... General Name, Symbol, Number plutonium, Pu, 94 Chemical series actinides Group, Period, Block n/a, 7, f Appearance silvery white Standard atomic weight (244) g·mol−1 Electron configuration [Rn] 5f6 7s2 Electrons per shell 2, 8, 18, 32, 24, 8, 2 Physical properties Phase solid Density (near r. ...

Material Total Neutron Capture Relative to Graphite (per unit volume) Moderating Ratio (Avg. 0.1 to 10 eV)
Heavy Water 0.2 11449
Light Water 75 246
Graphite 1 863
Sodium 47 2
UCO 285 2
UO2 3583 0.1
2LiF-BeF2 8 60
LiF-BeF2-ZrF4 (64.5-30.5-5) 8 54
NaF-BeF2 (57-43) 28 15
LiF-NaF-BeF2 (31-31-38) 20 22
LiF-ZrF4 (51-49) 9 29
NaF-ZrF4 (59.5-40.5) 24 10
LiF-NaF-ZrF4 (26-37-37) 20 13
KF-ZrF4 (58-42) 67 3
RbF-ZrF4 (58-42) 14 13
LiF-KF (50-50) 97 2
LiF-RbF (44-56) 19 9
LiF-NaF-KF (46.5-11.5-42) 90 2
LiF-NaF-RbF (42-6-52) 20 8

Above is a table comparing the neutron capture and moderating efficiency of several materials. Red are Be bearing salts, blue are ZrF4 bearing salts, and green are LiF bearing salts. (Source: ORNL/TM-2005/218, Status of Physics and Safety Analyses for the Liquid-Salt-Cooled Very High-Temperature Reactor (LS-VHTR), December 2005, D. T. Ingersoll)


Fused salt purification and reprocessing

Salts must be extremely pure initially, and would most likely be continuously cleaned in a large-scale molten salt reactor. Any water vapor in the salt will form hydrofluoric acid (HF) which is extremely corrosive. Other impurities can cause non-beneficial chemical reactions and would most likely have to be cleansed from the system. It should be noted that most power plants have to ensure that the primary coolant they are using is extremely pure; otherwise, they would encounter corrosion issues as well. R-phrases , S-phrases , , , , Flash point nonflammable Related Compounds Other anions Hydrochloric acid Hydrobromic acid Hydroiodic acid Related compounds Hydrogen fluoride fluorosilicic acid Supplementary data page Structure and properties n, εr, etc. ...


The possibility of online reprocessing can be an advantage of the MSR design. Continuous reprocessing ensures a low inventory of fission products at all times, which improves neutron economy. This makes the MSR particularly suited to the neutron-poor thorium fuel cycle. To allow breeding from thorium, the intermediate product protactinium has to be removed from the reactor and stored for some months while it decays into uranium 233. Left in the fuel it would absorb too many neutrons to make breeding with a graphite moderator and thermal spectrum possible (though some heavy water moderated reactor designs could overcome this, albeit at a lower thermal efficiency ). The necessary reprocessing technology, which has to process the complete fuel every 10 days, has only been demonstrated at laboratory scale. For a power reactor such a large reprocessing facility is currently deemed uneconomic. // The nuclear fuel cycle, also called nuclear fuel chain, consists of front end steps that lead to the preparation of uranium for use as fuel for reactor operation and back end steps that are necessary to safely manage, prepare, and dispose of radioactive waste. ... General Name, Symbol, Number thorium, Th, 90 Chemical series Actinides Group, Period, Block n/a, 7, f Appearance silvery white Standard atomic weight 232. ... General Name, Symbol, Number protactinium, Pa, 91 Chemical series actinides Group, Period, Block n/a, 7, f Appearance bright, silvery metallic luster Standard atomic weight 231. ... General Name, symbol, number uranium, U, 92 Chemical series actinides Group, period, block n/a, 7, f Appearance silvery gray metallic; corrodes to a spalling black oxide coat in air Standard atomic weight 238. ...


Political issues

To exploit the molten salt reactor's breeding potential to the fullest, the reactor must be co-located with a reprocessing facility. Any kind of nuclear reprocessing is still illegal in many countries. Some people fear that operating an MSR could pave the way to the plutonium economy with its associated proliferation dangers. (A similar argument lead to the shutdown of the Integral Fast Reactor project in 1994.) The fast breeder or fast breeder reactor (FBR) is a type of fast neutron reactor that produces more fissile material than it consumes. ... The Integral Fast Reactor or Advanced Liquid-Metal Reactor is a design for a nuclear fast reactor with a specialized nuclear fuel cycle. ... Year 1994 (MCMXCIV) The year 1994 was designated as the International Year of the Family and the International Year of the Sport and the Olympic Ideal by the United Nations. ...


Because of the recent lack of orders for new nuclear reactors, the nuclear industry business involved the selling fuel bundles to reload reactors and providing services for the reactor operators to keep the reactors running.[verification needed] The fuel fabrication and servicing business is highly competitive, and only a few vendors who have the depth of experience have survived. The business model for molten-salt fueled reactors would be similar to the model for light water reactors, although it would not involve fabricating fuel assemblies. The nuclear industry would need to develop experience and confidence in the viability of molten salt reactors before it is commercialized. This would involve building demonstration plants and developing decades of operating experience.[verification needed]


Comparison to ordinary light water reactors

Molten salt reactors are an immature technology. No large-scale reactor has been built and operated for a long period, and unexpected problems are likely. Whether an MSR will be economically and technologically viable is unknown. Small, experimental MSRs have operated as long as several years, and the problems were fixed. A light water reactor or LWR is a thermal nuclear reactor that uses ordinary water, also called light water, as its neutron moderator. ...


MSRs may be safer. Molten salts trap fission products chemically, and react slowly or not at all in air. Also, the fuel salt does not burn in air or water. The core and primary cooling loop is operated at atmospheric pressure, and has no steam, so a pressure explosion is impossible. Even in the unlikely case of an accident, most radioactive fission products would stay in the salt instead dispersing into the atmosphere. A molten core is meltdown-proof, so the worst possible accident would be a leak. In this case, the fuel salt can be drained into passively cooled storage, managing the accident. Neutron-producing accelerators have even been proposed for some super-safe subcritical experimental designs. Fission products are the residues of fission processes. ...


Some types of molten salt reactor are very efficient. Since the core and primary coolant loop are low pressure, it can be constructed of thin, relatively inexpensive weldments. So, it can be far less expensive than the massive pressure vessel required by the core of a light water reactor. Also, some form of fluid-fueled thorium breeder could use less fissile material per megawatt than any other reactor. Molten salt reactors can run at extremely high temperatures, with exremely high efficiencies when producing electricity. The tempearture are high enough to produce process heat for hydrogen production or other chemical reactions. Because of this, they have been included in the GEN-IV roadmap for further study. This article or section should include material from Fissile material In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission. ... The megawatt (symbol: MW) is a unit for measuring power corresponding to one million (106) watts. ...


Molten-salt-fueled thorium breeders close the nuclear fuel cycle and potentially eliminate the need for both fuel enrichment and fuel fabrication, both major expenses. The nuclear fuel cycle, also called nuclear fuel chain, is the progression of nuclear fuel through a series of differing stages. ...


The MSR also has far better neutron economy and, depending on the design, a harder neutron spectrum. So, it can operate with less reactive fuels. Some designs (such as the MSRE) can operate a single design from all three common nuclear fuels. For example, it can breed from uranium-238, thorium or even burn the transuranic spent nuclear fuel from light water reactors. In contrast, a water-cooled reactor cannot completely consume the plutonium it produces, because the increasing impurities from the fission wastes capture too many neutrons, "poisoning" the reaction. In chemistry, transuranium elements (also known as transuranic elements) are the chemical elements with atomic numbers greater than 92, the atomic number of Uranium. ... Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant) to the point where it is no longer useful in sustaining a nuclear reaction. ... A light water reactor or LWR is a thermal nuclear reactor that uses ordinary water, also called light water, as its neutron moderator. ...


Molten salt-fueled thorium breeders can operate for extended periods, possibly decades, without refueling, by chemically precipitating neutronic poisons.


MSRs scale over a wide range of powers. Reactors as small as several megawatts have been constructed and operated. Theoretical designs up to several gigawatts have been proposed[3].


Because of their low structures and compact cores, MSRs weigh less per watt (that is, they have a greater "specific power") than other proven reactor designs. So, in small sizes, with long refueling intervals, they are an excellent choice to power vehicles, including ships, aircraft and spacecraft.


References

  1. ^ [http://energyfromthorium.com/forum/viewtopic.php?t=34 Energy from Thorium blog->Reactor Design->Graphite and Two-Fluid vs. One-Fluid LFRs ] Viewed 6/2007
  2. ^ ORNL-4548: Molten-Salt Reactor Program: Semiannual Progress Report for Period Ending February 28, 1970, pg. 57
  3. ^ Weinberg et al. WASH 1080, ORNL
  • Energy from Thorium's Document Repository Contains scanned versions of many of the U.S. government engineering reports. This repository is the main reference for the molten-salt fueled reactor's technical discussion.
  • Moir, R.W; Cost of Electricity from MSRs; Lawrence Livermore National Laboratory, U.S.
  • J.H. Devan et al. (unknown date). Material Considerations for Molten Salt Accelerator-based Plutonium Conversion Systems, pg. 475-486
  • W.D. Manely et al. (1960). Metallurgical Problems in Molten Fluoride Systems. Progress in Nuclear Energy, VOl. 2, pg. 164-179
  • http://energy.inel.gov/gen-iv/msr.shtml
  • "The First Nuclear Era : The Life and Times of a Technological Fixer", by Alvin Martin Weinberg (1994). Book by a former director of the Oak Ridge National Laboratory, and a promoter of nuclear power and molten salt reactors.
  • Generation IV International Forum MSR website
  • INL MSR workshop summary

Alvin Martin Weinberg (April 20, 1915 - October 18, 2006) was a nuclear physicist and administrator at Oak Ridge National Laboratory (ORNL). ...

See also

Generation IV reactors (Gen IV) are a set of theoretical nuclear reactor designs currently being researched. ... This article is about applications of nuclear fission reactors as power sources. ... Core of a small nuclear reactor used for research. ... Nuclear Fuel Process A graph comparing nucleon number against binding energy Nuclear fuel is any material that can be consumed to derive nuclear energy, by analogy to chemical fuel that is burned to derive energy. ... Nuclear material consists of materials used in nuclear systems, such as nuclear reactors and nuclear weapons. ... Atomic physics (or atom physics) is the field of physics that studies atoms as isolated systems comprised of electrons and an atomic nucleus. ... A liquid-fluoride reactor (a specific example of a Molten salt reactor) is a nuclear reactor wherein the nuclear materials are fluoride salts dissolved in a solution of other fluoride salts. ...

External links

  • Energy from Thorium Blog
  • Nuclear Engineering
  • Wikibooks

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The fuel of the Molten Salt Reactor at Oak Ridge National Laboratory (ORNL) consisted of enriched uranium tetrafluoride that was mixed in a molten form with lithium and beryllium salts.
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The first use of molten salt fuel at ORNL was in the experiments of the early 1950s aimed at developing nuclear reactors for aircraft.
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Molten salt reactors were first proposed by Ed Bettis and Ray Briant of ORNL during the post-World War II attempt to design a nuclear-powered aircraft.
The fuel salt is heated from 1075° F to 1225° F in the core and is circulated from the reactor vessel to four primary heat exchangers by four fuel pumps.
The Molten Salt Group concluded in 1971 that the existing technology was sufficient to justify construction of a molten salt demonstration plant.
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