Liquid fluoride thorium reactor - AbsoluteAstronomy.com

The liquid fluoride thorium reactor (acronym LFTR; spoken as lifter) is a thermal breeder reactor which uses the thorium fuel cycle

Thorium fuel cycle

The thorium fuel cycle is a nuclear fuel cycle that uses the naturally abundant isotope of thorium, , as the fertile material. In the reactor, is transmuted into the fissile artificial uranium isotope which is the nuclear fuel. Unlike natural uranium, natural thorium contains only trace amounts...

in a fluoride

Fluoride

Fluoride is the anion F−, the reduced form of fluorine when as an ion and when bonded to another element. Both organofluorine compounds and inorganic fluorine containing compounds are called fluorides. Fluoride, like other halides, is a monovalent ion . Its compounds often have properties that are...

-based molten (liquid) salt fuel to achieve high operating temperatures at atmospheric pressure

Atmospheric pressure

Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

.

The LFTR is a type of thorium

Thorium

Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....

molten salt reactor

Molten salt reactor

A molten salt reactor is a type of nuclear fission reactor in which the primary coolant, or even the fuel itself is a molten salt mixture...

(TMSR). Molten-salt-fueled reactors (MSFRs) such as LFTR, where the nuclear fuel

Nuclear fuel

Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...

itself is in the form of liquid molten salt mixture, should not be confused with only molten-salt-cooled but solid-fueled reactors.

This technology was first investigated at the Oak Ridge National Laboratory

Oak Ridge National Laboratory

Oak Ridge National Laboratory is a multiprogram science and technology national laboratory managed for the United States Department of Energy by UT-Battelle. ORNL is the DOE's largest science and energy laboratory. ORNL is located in Oak Ridge, Tennessee, near Knoxville...

Molten-Salt Reactor Experiment

The Molten-Salt Reactor Experiment was an experimental molten-salt reactor at the Oak Ridge National Laboratory researching this technology through the 1960s; constructed by 1964, it went critical in 1965 and was operated until 1969....

in the 1960s. It has been recently investigated by Japan, China, the UK, and private US interests. Flibe Energy

Flibe Energy

Flibe Energy is a company that intends to design, construct and operate small modular reactors based on liquid fluoride thorium reactor technology.-Corporation:...

aims to develop a small modular reactor version using liquid FLiBe

FLiBe

FLiBe is a mixture of lithium fluoride and beryllium fluoride . As a molten salt it is proposed as a nuclear reactor coolant, and two different mixtures were used in the Molten-Salt Reactor Experiment....

salt.

Background

By 1946, only eight years after the discovery of nuclear fission, three fissile isotopes had been publicly identified for use as nuclear fuel

Nuclear fuel

Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...

Uranium-235
Uranium-235
- References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...

, which is already fissile, but occurs as <1% of natural uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
Plutonium-239
Plutonium-239
Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 has also been used and is currently the secondary isotope. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in...

, which can be bred
Fertile material
Fertile material is a term used to describe nuclides which generally themselves do not undergo induced fission but from which fissile material is generated by neutron absorption and subsequent nuclei conversions...

from non-fissile Uranium-238
Uranium-238
Uranium-238 is the most common isotope of uranium found in nature. It is not fissile, but is a fertile material: it can capture a slow neutron and after two beta decays become fissile plutonium-239...

(>99% of natural uranium)
Uranium-233
Uranium-233
Uranium-233 is a fissile isotope of uranium, bred from Thorium as part of the thorium fuel cycle. It has been used in a few nuclear reactors and has been proposed for much wider use as a nuclear fuel. It has a half-life of 160,000 years....

, which can be bred from non-fissile Thorium-232 (~100% of natural thorium
Thorium
Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....

; about four times more common than uranium)

Th-232, U-235 and U-238 are primordial nuclide

Primordial nuclide

In geochemistry and geonuclear physics, primordial nuclides or primordial isotopes are nuclides found on the earth that have existed in their current form since before Earth was formed. Only 288 such nuclides are known...

s, having existed in their current form for over 4.5 billion years

Age of the Earth

The age of the Earth is 4.54 billion years This age is based on evidence from radiometric age dating of meteorite material and is consistent with the ages of the oldest-known terrestrial and lunar samples...

, predating the formation of the Earth; they were forged in the cores of dying stars through the r-process

R-process

The r-process is a nucleosynthesis process, likely occurring in core-collapse supernovae responsible for the creation of approximately half of the neutron-rich atomic nuclei that are heavier than iron. The process entails a succession of rapid neutron captures on seed nuclei, typically Ni-56,...

and scattered across the galaxy by supernova

Supernova

A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...

s. Their radioactive decay

Radioactive decay

Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...

produces about half of the earth's internal heat.

For technical reasons (outlined in a section below), the three are each best suited to different reactor types. U-235 is the world's primary nuclear fuel and is usually used in light water reactor

Light water reactor

The light water reactor is a type of thermal reactor that uses normal water as its coolant and neutron moderator. Thermal reactors are the most common type of nuclear reactor, and light water reactors are the most common type of thermal reactor...

s. U-238/Pu-239 has found the most use in liquid sodium fast breeder reactors. Th-232/U-233 is best suited to molten salt reactor

Molten salt reactor

A molten salt reactor is a type of nuclear fission reactor in which the primary coolant, or even the fuel itself is a molten salt mixture...

s (MSR).

Alvin M. Weinberg

Alvin M. Weinberg

Alvin Martin Weinberg was an American nuclear physicist who was the administrator at Oak Ridge National Laboratory during and after the Manhattan Project period. He came to Oak Ridge, Tennessee in 1945 and remained there until his death in 2006.-Early years in Chicago: Alvin Weinberg was born...

pioneered the use of the MSR

Molten salt reactor

A molten salt reactor is a type of nuclear fission reactor in which the primary coolant, or even the fuel itself is a molten salt mixture...

at Oak Ridge National Laboratory

Oak Ridge National Laboratory

. The Aircraft Reactor Experiment in 1954 and Molten-Salt Reactor Experiment

Molten-Salt Reactor Experiment

from 1965 to 1969 both used liquid fluoride salts; the latter notably demonstrated the use of U-233 as a fuel source. Unfortunately for MSR research, Weinberg was fired and the MSR program closed down in the early 1970s, after which research stagnated in the United States.

Two versus Single fluid

Two concepts were investigated at Oak Ridge, the "two fluid" and "single fluid" thorium thermal breeder molten salt reactors:

Two Fluid Reactor

The "two fluid" reactor has a high-neutron-density core that burns uranium-233

Uranium-233

Uranium-233 is a fissile isotope of uranium, bred from Thorium as part of the thorium fuel cycle. It has been used in a few nuclear reactors and has been proposed for much wider use as a nuclear fuel. It has a half-life of 160,000 years....

from the thorium fuel cycle. A separate blanket of thorium

Thorium

Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....

salt absorbs the neutrons and eventually is transmuted to ²³³U fuel.

The advantage of the-two-fluid design is a simplified chemical system to process the salts. In particular, protactinium-233 is separated from the thorium blanket in a two step process that uses bismuth and fluorination. Protactinium 233 has a 27 day half-life, and decays to the needed fuel, U233. So 10 months after the Protactinium is chemically separated from the salt, it is 99.9% U233. The kernel's salt is also purified, first by fluorination to remove uranium, then vacuum distillation to remove and reuse light-atomic-weight carrier salts. The still bottoms left after the distillation are the fission products of the waste.

The design weakness of the two-fluid design was its complex plumbing. The design used brittle graphite

Graphite

The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...

pipes to hold the fuel salt. The pipes separated the fuel salt and breeding salt, so they were essential. The problem is that graphite expands under intense neutron bombardment. So, graphite pipes would change length, crack and become very leaky. Graphite was the only known substance that combined the needed properties: It is not dissolved by the salt, it must survive the neutron bombardment, it must not absorb a lot of neutrons, it must survive at very high temperatures, and it must be tough enough not to crack. Zirconium alloys would work, except they dissolve in the salt. In modern research, copper-reinforced graphite fiber cloth seems theoretically suitable, but no physical tests have been done. At the time, no solution was known, so this type of reactor was never constructed.

Single Fluid Reactor

The "single fluid" reactor was mechanically much simpler, and was actually prototyped (as the MSRE

Molten-Salt Reactor Experiment

, above.) It was a large tank filled with salt. The moderator was graphite rods immersed in the salt. 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 MSRE provided valuable long-term operating experience.

The disadvantage was that the reprocessing chemistry was much more complex. No simple combination of distillation and fluorination (simple, proven methods) could separate the fission products (the nuclear ashes) from the fuels.

The power reactor design produced by Weinberg's research group was similar to the MSRE

Molten-Salt Reactor Experiment

. This was because the MSRE was designed to test the design of the risky, hot, high-neutron-density "kernel" part of the two fluid "kernel and blanket" thorium breeder (see above).

Safety

inherently safe design: reactor features passive nuclear safety
Passive nuclear safety
Passive nuclear safety is a safety feature of a nuclear reactor that does not require operator actions or electronic feedback in order to shut down safely in the event of a particular type of emergency...

, strong negative temperature coefficient of reactivity and atmospheric operating pressure. (since the core is not pressurised, it cannot explode)
Most MSFRs include a freeze plug at the bottom that has to be actively cooled, usually by a small electric fan. If the cooling fails, say because of a power failure, the fan stops, the fuel in the plug melts, and the fuel drains to a subcritical storage facility, totally stopping the reactor.
Reduced radiotoxicity of reactor wastes. The LFTR uses the Thorium fuel cycle
Thorium fuel cycle
The thorium fuel cycle is a nuclear fuel cycle that uses the naturally abundant isotope of thorium, , as the fertile material. In the reactor, is transmuted into the fissile artificial uranium isotope which is the nuclear fuel. Unlike natural uranium, natural thorium contains only trace amounts...

, which transmutes Thorium to U233. U233 has two chances to fission as a thermal reactor bombards it with neutrons (as U233 and U235). The fraction of fuel reaching U236, and transmuting into a transuranic element is less than 0.1%. The radiotoxicity of the remaining fission products is dominated by Cesium 137 and Strontium 90. The longer half-life is Cesium, at 30.17 years. So, after 300 years, decay reduces the radioactivity of the cesium to only 0.1%. A related advantage is that the U233 is relatively pure, without other isotopes that are not fuels. In contrast, Uranium fuels are between 97% and 80% U238, which reactors normally transmute to Pu239, a toxic transuranic element. When the two features are combined, the effect of a Thorium fuel cycle is to reduce the production of transuranic wastes by more than a thousand-fold compared to a conventional once-through light-water reactor.
continuous removal of transmutation products insures that only a small amount of them is present in an active reactor at any given time, compared to conventional LWR reactors.
fluoride combines ionically with almost any transmutation product. This is an MSFR's first level of containment. It is especially good at containing biologically active "salt loving" wastes such as Cesium 137.
given an accident beyond the design basis for the multiple levels of containment, dispersion into a biome is difficult. The salts do not burn, explode or degrade in air or water, and the fluoride salts of the radioactive actinides and fission products are generally not soluble in water or air.
the reactor is easy to control at all times. Xenon-135, an important neutron absorber — that makes reactors difficult to control is removed at a predictable, containable place, where the fuel is coolest and most dispersed, namely the pump bowl. In solid-fuel reactors, it remains in the fuel and interferes with control.
the reactor operates at or above 650C, well above the 250C Wigner annealing temperature of graphite. This means that Wigner energy cannot accumulate in the graphite moderator. The continual annealing bleeds it off. So, a sudden release of Wigner energy is not possible, and as such, a Windscale
Windscale fire
The Windscale fire of 10 October 1957 was the worst nuclear accident in Great Britain's history, ranked in severity at level 5 on the 7-point International Nuclear Event Scale. The two piles had been hurriedly built as part of the British atomic bomb project. Windscale Pile No. 1 was operational in...

-style graphite-incited fire cannot be caused by the graphite's nonexistent Wigner energy.
proliferation resistance: LFTR 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 thorium-232 breeding material with chemically inseparable thorium-230. The thorium-230 breeds into uranium-232, which has a powerful gamma-ray emitter in its decay chain (thallium-208) that makes the reactor fuel 233U/232U impractical in a bomb, because it complicates bomb manufacture, harms electronics and reveals the bomb location.

Economy and efficiency

since all natural thorium can be used as a fuel, and the fuel is in the form of a molten salt instead of solid fuel rods, expensive fuel enrichment and solid fuel rods validation procedure and fabricating process is not needed, greatly decreasing LFTR fuel cost
LFTR uses an abundant world supply of thorium to breed uranium-233 fuel. The Earth's crust contains about three times as much thorium as 238U, or 400 times as much as 235U, which makes it about as abundant as lead. Thorium, a byproduct of rare-earth mining normally discarded as waste currently costs US$ 30/kg, while the price of uranium has risen above $100/kg, not including the cost of enrichment, and fuel element fabrication. When used in a LFTR, there is enough economically recoverable thorium on Earth to satisfy the global energy needs for hundreds of thousands of years. In addition to thorium, LFTR can also use nuclear waste from traditional nuclear power plants as a fuel
conventional reactors consume less than one percent of their uranium fuel, leaving the rest as waste. LFTR consumes 99% of its thorium fuel, which results in great increase in fuel efficiency - 1 tonne of natural thorium in a LFTR produces as much energy as 35 t of enriched uranium in conventional reactors (requiring 250 t of natural uranium), or 4 166 000 tonnes of black coal in a coal power plant
the reactor is much cleaner: as a full recycle system, the discharge wastes from the reactor are predominately fission products, most of which have relatively short half lives compared to longer-lived actinide wastes. This results in a significant reduction in the needed waste containment period in a geologic repository to reach safe radiation levels (300 years vs. tens of thousands of years)
it can "burn" problematic radioactive waste with transuranic elements from traditional solid-fuel nuclear reactors, thus solve the High level waste
High level waste
High level waste is a type of nuclear waste created by the reprocessing of spent nuclear fuel. It exists in two main forms:* First and second cycle raffinate and other waste streams created by nuclear reprocessing....

problem by turning liability into an asset
it is highly scalable. Small, 2–8 MW(thermal) or 1–3 MW(electric) versions are possible, enabling submarine or aircraft use
LFTR would have no refueling power outages due to continual refueling
it can react to load changes in less than 60 seconds (unlike "traditional" solid-fuel nuclear power plants), thus it can satisfy both base load and peak load power demands
with a very high temperature reactor such as LFTR, it is possible to use very efficient Brayton cycle
Brayton cycle
The Brayton cycle is a thermodynamic cycle that describes the workings of the gas turbine engine, basis of the airbreathing jet engine and others. It is named after George Brayton , the American engineer who developed it, although it was originally proposed and patented by Englishman John Barber...

generating turbines. The thermal efficiency from the high temperature of its operation reduces fuel use, waste emission and the cost of auxiliary equipment (major capital expenses) by 50% or more
since the core is not pressurised, it does not need the most expensive item in a light water reactor, a high-pressure reactor vessel for the core. Instead, there is a low-pressure vessel and pipes (for molten salt) constructed of relatively thin materials. Although the metal is an exotic nickel alloy that resists heat and corrosion, Hastelloy-N, the amount needed is relatively small and the thin metal is less expensive to form and weld.
by using liquid salt as the coolant instead of presurised water a containment structure only slightly bigger than the reactor vessel can be used. Light water reactors use pressurised water which flashes to steam and expands a thousandfold in the case of a leak, necessitating a containment building a thousandfold bigger in volume than the reactor vessel. This gives the LFTR a substantial theoretical advantage in terms of lower construction cost.
it can be air-cooled, which is critical for use in many regions where water is scarce
fission products of a LFTR include stable rare elements such as rhodium, ruthenium, technetium, cesium and xenon, which are relied heavily on in modern electronics and industrial processes. These can be extracted from the waste. Medically valuable isotopes are also among LFTR fission products

Kirk Sorensen expects that with these advantages, LFTR technology will produce energy significantly cheaper than coal; he comments that this would make moot both carbon pricing

Carbon pricing

Carbon pricing is the generic term for placing a price on carbon through either subsidies, a carbon tax, or an emissions trading system....

schemes and more expensive alternative energy

Alternative energy

Alternative energy is an umbrella term that refers to any source of usable energy intended to replace fuel sources without the undesired consequences of the replaced fuels....

solutions In remarks prepared for the Low-Carbon Energy Summit on 20 October 2011, Sorensen stated that "The most important thing that we can do to fight climate change is to replace coal

Coal

Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...

as our primary source of electricity" and advocated the LFTR as an "even less expensive" replacement. The ultimate goal is to "provide electricity for less cost than any other competing solution" which Sorensen thinks will "eventually get to 1 cent per kilowatt hour using this technology"

Ease of reprocessing

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 1 GW power plant: All the salt has to be reprocessed, but only every ten days. The reactor's total inventory of expensive, poisonous radioactive materials is therefore much smaller than in a conventional light-water-reactor's fuel cycle, which has to store spent fuel rod assemblies. Also, everything except fuel and waste stays inside the plant. The reprocessing cycle is:

A sparge
Sparging (chemistry)
In chemistry, sparging, also known as gas flushing in metallurgy, is a technique which involves bubbling a chemically inert gas, such as nitrogen, argon, or helium, through a liquid. This can be used to remove dissolved gases In chemistry, sparging, also known as gas flushing in metallurgy, is a...

of fluorine
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...

removes volatile high-valence
Valence (chemistry)
In chemistry, valence, also known as valency or valence number, is a measure of the number of bonds formed by an atom of a given element. "Valence" can be defined as the number of valence bonds...

fluorides as gas, including uranium hexafluoride
Uranium hexafluoride
Uranium hexafluoride , referred to as "hex" in the nuclear industry, is a compound used in the uranium enrichment process that produces fuel for nuclear reactors and nuclear weapons. It forms solid grey crystals at standard temperature and pressure , is highly toxic, reacts violently with water...

containing the uranium-233 fuel as well as other isotopes of uranium
Isotopes of uranium
Uranium is a naturally occurring radioactive element that has no stable isotopes but two primordial isotopes that have long half-life and are found in appreciable quantity in the Earth's crust, along with the decay product uranium-234. The average atomic mass of natural uranium is 238.02891 u...

; neptunium
Neptunium
Neptunium is a chemical element with the symbol Np and atomic number 93. A radioactive metal, neptunium is the first transuranic element and belongs to the actinide series. Its most stable isotope, 237Np, is a by-product of nuclear reactors and plutonium production and it can be used as a...

hexafluoride
Hexafluoride
A hexafluoride is a chemical compound with the general formula XF6. Sixteen elements are known to form stable hexafluorides. Nine of these elements are transition metals, three are actinides, and four are nonmetals or metalloids.- Physical properties :...

; technetium hexafluoride and selenium hexafluoride
Selenium hexafluoride
Selenium hexafluoride is the inorganic compound with the formula SeF6. It is a colourless gas described as having a "repulsive" odor. It is not widely encountered and has no commercial applications.-Structure, preparation, and reactions:...

containing the long-lived fission product
Long-lived fission product
Long-lived fission products are radioactive materials with a long half-life produced by nuclear fission.-Evolution of radioactivity in nuclear waste:...

s technetium-99
Technetium-99
Technetium-99 is an isotope of technetium which decays with a half-life of 211,000 years to stable ruthenium-99, emitting soft beta rays, but no gamma rays....

and selenium-79
Selenium-79
Selenium-79 is a radioisotope of selenium present in spent nuclear fuel and the wastes resulting from reprocessing this fuel. It is one of only 7 long-lived fission products. Its yield is low as it is near the lower end of the mass range for fission products...

, as well as fluorides of various strongly radioactive short-lived fission product
Fission product
Nuclear fission products are the atomic fragments left after a large atomic nucleus fissions. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons and a large release of energy in the form of heat , gamma rays and neutrinos. The...

s such as iodine-131
Iodine-131
Iodine-131 , also called radioiodine , is an important radioisotope of iodine. It has a radioactive decay half-life of about eight days. Its uses are mostly medical and pharmaceutical...

, molybdenum-99, and tellurium-132. See fluoride volatility
Fluoride volatility
Fluoride volatility is jargon that describes the volatility of fluorides, which is relevant to the separation of radionuclides. Hexafluorides and pentafluorides have much lower boiling points than the lower-valence fluorides. Most difluorides and trifluorides have high boiling points, while most...

for boiling points. The volatile fluorides are condensed from the sparge fluorine, reduced back to less volatile lower-valence fluorides, and returned to the reactor.
A molten bismuth
Bismuth
Bismuth is a chemical element with symbol Bi and atomic number 83. Bismuth, a trivalent poor metal, chemically resembles arsenic and antimony. Elemental bismuth may occur naturally uncombined, although its sulfide and oxide form important commercial ores. The free element is 86% as dense as lead...

column separates protactinium from the fuel salt, which is stored to allow protactinium-233 to decay to uranium-233 without being destroyed by neutron capture in the reactor. With a half-life of 27 days, ten months of storage assures that 99.9% of ²³³Pa decays to ²³³U fuel, while any ²³¹Pa remains in the bismuth.
The remaining fuel salt is then distilled at increased temperature and lowered pressure. The distillate is condensed and returned to the reactor.
- The light carrier salts beryllium fluoride
  Beryllium fluoride
  Beryllium fluoride is the inorganic compound with the formula BeF2. This white solid is the principal precursor for the manufacture of beryllium metal.-Structure and bonding:...
  
  and lithium fluoride
  Lithium fluoride
  Lithium fluoride is an inorganic compound with the formula LiF. It is the lithium salt of hydrofluoric acid. This white solid is a simple ionic compound. Its structure is analogous to that of sodium chloride, but it is much less soluble in water. It is mainly used as a component of molten...
  
  form the bulk of the salt; individually at atmospheric pressure they evaporate at 1169 °C and 1676 °C, while the 2:1 FLiBe
  FLiBe
  FLiBe is a mixture of lithium fluoride and beryllium fluoride . As a molten salt it is proposed as a nuclear reactor coolant, and two different mixtures were used in the Molten-Salt Reactor Experiment....
  
  mixture with proportions Li₂BeF₄ evaporates at 1430 °C. Boiling points under vacuum are lower.
- Cesium fluoride, containing the most radioactive medium-lived fission product cesium-137, boils at 1251 °C under standard conditions, and will evaporate with the carrier salt fluorides.
- Thorium(IV) fluoride
  Thorium(IV) fluoride
  Thorium fluoride is an inorganic chemical compound. It is a white, hygroscopic powder which can be produced by reacting thorium with fluorine gas. At temperatures above 500 °C, it reacts with atmospheric moisture to produce ThOF2.-Uses:...
  
  evaporates at temperatures about 1680 °C or less under vacuum.
- Lanthanide
  Lanthanide
  The lanthanide or lanthanoid series comprises the fifteen metallic chemical elements with atomic numbers 57 through 71, from lanthanum through lutetium...
  
  trifluorides and alkaline earth fluorides have boiling points higher than 2200 °C under standard conditions, and would remain after thorium fluoride evaporates; the lanthanides include the worst long-term neutron poisons, while strontium fluoride
  Strontium fluoride
  Strontium fluoride, SrF2, also called strontium difluoride and strontium fluoride, is a fluoride of strontium. It is a stable brittle white crystalline solid with melting point of 1477°C and boiling point 2460°C.-Preparation:...
  
  contains the other major medium-lived fission product strontium-90
  Strontium-90
  Strontium-90 is a radioactive isotope of strontium, with a half-life of 28.8 years.-Radioactivity:Natural strontium is nonradioactive and nontoxic, but 90Sr is a radioactivity hazard...
  
  .

The amount of waste involved is about 800 kg per gigawatt-year generated (1.5 grams/minute for a 1 GW reactor), so the equipment is very small. Salts of long-lived transuranic metals go back into the reactor as fuel. With salt distillation, an MSFR can burn plutonium

Plutonium

Plutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...

, or even fluorinated nuclear waste from light water reactors

Spent nuclear fuel

Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor...

Theoretically, a "two-fluid" reactor design could separate the fertile
Fertile material
Fertile material is a term used to describe nuclides which generally themselves do not undergo induced fission but from which fissile material is generated by neutron absorption and subsequent nuclei conversions...

thorium from the fissile
Fissile
In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission. By definition, fissile materials can sustain a chain reaction with neutrons of any energy. The predominant neutron energy may be typified by either slow neutrons or fast neutrons...

fuel salts. This would eliminate the technologically challenging separation of thorium fluoride (boiling point 1680 °C) and lanthanide
Lanthanide
The lanthanide or lanthanoid series comprises the fifteen metallic chemical elements with atomic numbers 57 through 71, from lanthanum through lutetium...

fission product
Fission product
Nuclear fission products are the atomic fragments left after a large atomic nucleus fissions. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons and a large release of energy in the form of heat , gamma rays and neutrinos. The...

fluoride
Fluoride
Fluoride is the anion F−, the reduced form of fluorine when as an ion and when bonded to another element. Both organofluorine compounds and inorganic fluorine containing compounds are called fluorides. Fluoride, like other halides, is a monovalent ion . Its compounds often have properties that are...

s via high-temperature distillation
Fluoride volatility
Fluoride volatility is jargon that describes the volatility of fluorides, which is relevant to the separation of radionuclides. Hexafluorides and pentafluorides have much lower boiling points than the lower-valence fluorides. Most difluorides and trifluorides have high boiling points, while most...

, at the cost of a more complex reactor. Oak Ridge researchers abandoned two-fluid designs because no good pipe materials were known to operate in the high-temperature, high-neutron, corrosive environment of a MSR core.

Disadvantages

The proposed salt mixture FLiBe
FLiBe
FLiBe is a mixture of lithium fluoride and beryllium fluoride . As a molten salt it is proposed as a nuclear reactor coolant, and two different mixtures were used in the Molten-Salt Reactor Experiment....

, has large amounts of beryllium
Beryllium
Beryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...

, a poisonous element. The salt in the primary and secondary cooling loops must be isolated from workers and the environment to protect from beryllium poisoning.
Hot fluoride salts naturally produce hydrofluoric acid
Hydrofluoric acid
Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....

when in contact with water. When cool, fluoride salts are nearly insoluble in water. Although HF generation would be taken into consideration in the reactor's design and shutdown/decommission
Nuclear decommissioning
Nuclear decommissioning is the dismantling of a nuclear power plant and decontamination of the site to a state no longer requiring protection from radiation for the general public...

processes, this hazard needs to be addressed in emergency situations that damage all five levels of the reactor's containment while the salt is hot.

Design challenges

High neutron fluxes and temperatures in a compact MSR core can change the shape of a graphite moderator element, causing it to require refurbishing in as little as four years of operation. Eliminating graphite from sealed piping was a major incentive to switch to a single-fluid design. 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.
Corrosion is significant if the reactor is exposed to any isotope of hydrogen, which forms corrosive, chemically reactive, radioactive hydrogen fluoride
Hydrogen fluoride
Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the...

(HF) gas. The high neutron density in the core rapidly transmutes lithium-6 to tritium
Tritium
Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...

, a radioactive isotope of hydrogen, which is nearly identical, chemically speaking. In hot fluoride salts, the tritium forms tritium fluoride. Because of this, if a MSR design uses a lithium salt, it uses the lithium-7 isotope in order to prevent tritium formation. In the MSRE, Tritium formation was prevented by the removal of lithium-6 from the fuel salt via isotopic enrichment. Since lithium-7 is at least 16% heavier than lithium-6, and is the most common isotope of lithium, the lithium-6 is comparatively easy and inexpensive to extract from naturally occurring lithium. Vacuum distillation of lithium achieves efficiencies of up to 8% per stage and only requires heating of raw lithium in a vacuum chamber. The aforementioned method worked in preventing the hydrogen corrosion in the MSRE.. Practical MSRs also operate the salt under a blanket of dry inert gas, usually helium.

The reactor makes small amounts of Tellurium as a fission product. In the MSRE, this caused some small amounts of corrosion at the grain boundaries of the special Nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...

alloy, Hastelloy
Hastelloy
Hastelloy is the registered trademark name of Haynes International, Inc. The trademark is applied as the prefix name of a range of twenty two different highly corrosion-resistant metal alloys loosely grouped by the metallurgical industry under the material term “superalloys” or “high-performance...

-N used for the reactor. Metallurgical studies showed that adding 1 to 2% Niobium
Niobium
Niobium or columbium , is a chemical element with the symbol Nb and atomic number 41. It's a soft, grey, ductile transition metal, which is often found in the pyrochlore mineral, the main commercial source for niobium, and columbite...

to the Hastelloy
Hastelloy
Hastelloy is the registered trademark name of Haynes International, Inc. The trademark is applied as the prefix name of a range of twenty two different highly corrosion-resistant metal alloys loosely grouped by the metallurgical industry under the material term “superalloys” or “high-performance...

-N alloy was found to offer improved resistance to corrosion by Tellurium. One additional strategy against corrosion was to keep the fuel salt slightly reducing by maintaining the ratio of / to less than 60.

If the Fluoride fuel salts are stored in solid form over multi-decades, radiation can cause the release of corrosive Fluorine
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...

gas, and Uranium hexafluoride
Uranium hexafluoride
Uranium hexafluoride , referred to as "hex" in the nuclear industry, is a compound used in the uranium enrichment process that produces fuel for nuclear reactors and nuclear weapons. It forms solid grey crystals at standard temperature and pressure , is highly toxic, reacts violently with water...

. The salts should be defueled and wastes removed before extended shutdowns. Fluoride containing wastes could go through a vitrification process to be encased in borosilicate glass suitable for long-term disposal.

The Fuji MSR

The FUJI MSR is a 100 to 200 MWe

MWE

MWE may refer to:*Manufacturer's Weight Empty*McDermott Will & Emery*Midwest Express, an airline*Merowe Airport - IATA code*Multiword expressionMWe may refer to:*Megawatt electrical...

molten-salt-fueled thorium fuel cycle

Thorium fuel cycle

thermal breeder reactor

Breeder reactor

A breeder reactor is a nuclear reactor capable of generating more fissile material than it consumes because its neutron economy is high enough to breed fissile from fertile material like uranium-238 or thorium-232. Breeders were at first considered superior because of their superior fuel economy...

design, using technology similar to the Oak Ridge National Laboratory Reactor. It is being developed by a consortium including members from Japan, the U.S. and Russia. As a breeder reactor, it converts thorium into nuclear fuels. As a thermal-spectrum reactor, its neutron regulation is inherently safe. Like all molten salt reactors, its core is chemically inert, under low pressures to prevent explosions and toxic releases. It would likely take 20 years to develop a full size reactor but the project seems to lack funding.

Chinese Thorium MSR project

The People’s Republic of China has initiated a research and development project in thorium molten-salt reactor technology. It was formally announced at the Chinese Academy of Sciences

Chinese Academy of Sciences

The Chinese Academy of Sciences , formerly known as Academia Sinica, is the national academy for the natural sciences of the People's Republic of China. It is an institution of the State Council of China. It is headquartered in Beijing, with institutes all over the People's Republic of China...

(CAS) annual conference in January 2011. Its ultimate target is to investigate and develop a thorium based molten salt nuclear system in about 20 years.

Flibe Energy

Kirk Sorensen, former NASA scientist and Chief Nuclear Technologist at Teledyne Brown Engineering, has been a long time promoter of thorium fuel cycle

Thorium fuel cycle

and particularly liquid fluoride thorium reactors. He first researched thorium reactors while working at NASA

NASA

The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

, while evaluating power plant designs suitable for lunar colonies. Material about this fuel cycle was surprisingly hard to find, so in 2006 Sorensen started "energyfromthorium.com", a document repository, forum, and blog to promote this technology. In 2011, Sorensen founded Flibe Energy

Flibe Energy

Flibe Energy is a company that intends to design, construct and operate small modular reactors based on liquid fluoride thorium reactor technology.-Corporation:...

, a company aimed to develop 20-50 MW LFTR reactor designs to power military bases. (it is easier to approve novel military designs than civilian power station designs in today's US nuclear regulatory environment).

Small modular design

Kirk Sorensen of Flibe Energy, presenting at the 2011 Thorium Energy Conference, described how various factors influence design for small modular reactors.

Neutron temperature

Neutron temperature

The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term temperature is used, since hot, thermal and cold neutrons are moderated in a medium with a certain temperature. The neutron energy distribution is...

requirements matter on two fronts. Primarily is fuel choice:

U-235 and Th-232/U-233 work most efficiently with thermal spectrum neutrons (<1 eV)
U-238/Pu-239 requires fast spectrum neutrons (>100,000 eV) to sustain breeding

Second is reactor size. Fast spectrum neutrons deal with a much smaller nuclear cross section, meaning that for any given distance, they are less likely to be absorbed by a fissile or breedable nucleus than thermal spectrum neutrons. This drives up the minimum reactor size for a given power level.

Operating Temperature has two basic ranges. "Moderate" is defined as 250-350 C, and is comparable to conventional Light Water Reactor

Light water reactor

and fossil plant

Fossil fuel

Fossil fuels are fuels formed by natural processes such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years...

temperature ranges. "High" is defined as 700-1000 C, which provides greater efficiency due to the higher temperature gradient with ambient, but provides challenges for material selection.

Operating Pressure can be anywhere between "Atmospheric

Atmospheric pressure

" and "High" pressure (15.5 MPa (153 atm) for a Pressurized water reactor

Pressurized water reactor

Pressurized water reactors constitute a large majority of all western nuclear power plants and are one of three types of light water reactor , the other types being boiling water reactors and supercritical water reactors...

is considered high). These ranges are related to coolant type.

Here are four examples among the proposed small modular reactor designs, one for each temperature/pressure combination:

Water
Pressurized water reactor
Pressurized water reactors constitute a large majority of all western nuclear power plants and are one of three types of light water reactor , the other types being boiling water reactors and supercritical water reactors...

: Moderate Temperature, High Pressure (e.g. B&W mPower
B&W mPower
The B&W mPower is a proposed 125 MW modular, advanced light water nuclear reactor. The reactor is to be built by Babcock & Wilcox Co. in North America, and shipped by rail to generating sites. The reactor's power output is approximately 125 MWe, or approximately 10% of a typical reactor...

, NuScale
NuScale
NuScale Power LLC is a company formed to construct and sell dedicated design of relatively small nuclear reactors, which they claim will be modular, inexpensive, inherently safe, and proliferation-resistant.-History:The basic design is based on the MASLWR developed at Oregon...

, Westinghouse, IRIS
International Reactor Innovative and Secure
International Reactor Innovative and Secure is a Generation IV reactor design made by an international team of companies, laboratories, and universities and coordinated by Westinghouse. IRIS is hoped to open up new markets for nuclear power and make a bridge from Generation III reactor to...

, KLT-40)
Gas
Gas-cooled reactor
A gas-cooled reactor is a nuclear reactor that uses graphite as a neutron moderator and carbon dioxide as coolant...

: High Temperature, High Pressure (e.g. Pebble bed modular reactor
Pebble bed modular reactor
The Pebble Bed Modular Reactor is a particular design of pebble bed reactor under development by South African company PBMR Ltd since 1994...

, Gas turbine modular helium reactor
Gas turbine modular helium reactor
The Gas Turbine Modular Helium Reactor is a nuclear fission power reactor design under development by General Atomics. It is a helium cooled, graphite moderated reactor and uses TRISO fuel compacts in a prismatic core design.-Construction:...

, Energy Multiplier Module
Energy Multiplier Module
The Energy Multiplier Module is a nuclear fission power reactor under development by General Atomics. It is a modified version of the Gas Turbine Modular Helium Reactor and is capable of converting spent nuclear fuel into electricity and industrial process heat, without separative or conventional...

)
Liquid Metal
Liquid metal cooled reactor
A liquid metal cooled nuclear reactor, liquid metal fast reactor or LMFR is an advanced type of nuclear reactor where the primary coolant is a liquid metal. Liquid metal cooled reactors were first adapted for nuclear submarine use but have also been extensively studied for power generation...

: Moderate Temperature, Atmospheric Pressure (e.g. Hyperion
Hyperion Power Generation
Hyperion Power Generation, Inc. is a privately held corporation formed to construct and sell several designs of relatively small nuclear reactors, which they claim will be modular, inexpensive, inherently safe, and proliferation-resistant...

, Toshiba 4S
Toshiba 4S
- General description :The plant design is offered by a partnership that includes Toshiba and the Central Research Institute of Electric Power Industry of Japan.The technical specifications of the 4S reactor are unique in the nuclear industry...

, GE PRISM
S-PRISM
S-PRISM, also called PRISM , is the name of a nuclear power plant design by GE Hitachi Nuclear Energy based on a sodium-cooled fast breeder reactor. The design utilizes reactor modules, each having a power output of 311 MWe, to enable factory fabrication at low cost. The design is based on the...

)
Molten salt reactor
Molten salt reactor
A molten salt reactor is a type of nuclear fission reactor in which the primary coolant, or even the fuel itself is a molten salt mixture...

: High Temperature, Atmospheric Pressure (e.g. LFTR)

Various conclusions about the three fuels and possible reactor types are then drawn:

Higher temperature reactors can operate at higher thermal efficiency

Thermal efficiency

In thermodynamics, the thermal efficiency is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, a boiler, a furnace, or a refrigerator for example.-Overview:...

(e.g. with Brayton cycle

Brayton cycle

The Brayton cycle is a thermodynamic cycle that describes the workings of the gas turbine engine, basis of the airbreathing jet engine and others. It is named after George Brayton , the American engineer who developed it, although it was originally proposed and patented by Englishman John Barber...

turbines), which is desirable. High turbine pressure is a safety concern, as the proposed turbines - using Supercritical carbon dioxide

Supercritical carbon dioxide

Supercritical carbon dioxide is a fluid state of carbon dioxide where it is held at or above its critical temperature and critical pressure.Carbon dioxide usually behaves as a gas in air at STP or as a solid called dry ice when frozen...

- would need to operate at over 20 MPa (195 atm). The safety concern is more industrial than radiological, however, as turbine systems are generally not built close enough to their heat generators to be a risk to them.

The main drawback of U-235 is its scarcity. Even so, most currently operating reactors use it in water-cooled reactors. Gas-based concepts (e.g. PBMR, VHTR, GT-MHR) are also feasible.

The liquid metal coolants used are poor neutron moderator

Neutron moderator

In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....

s, thus such systems strongly favor U-238/Pu-239 usage; adding moderators to enable use with U-235 or Th-232/U-233 would be "feasible but unattractive". Conversely, water is a good moderator and this rules out exclusive plutonium breeding in such systems. Gas-cooled systems with U-238/Pu-239 (Gas Cooled Fast Breeder Reactor (GCFR) and EM2 concepts) are described as feasible but with difficult fuel processing, while molten salt systems with U-238/Pu-239 (e.g. MSFR) are only "somewhat feasible."

Sorensen notes that while Th-232/U-233 was used in a water-cooled reactor at the Shippingport Atomic Power Station and a gas-cooled reactor at the Fort St. Vrain Generating Station

Fort St. Vrain Generating Station

Fort Saint Vrain Generating Station is a natural gas powered electricity generating facility located near the town of Platteville in northern Colorado in the United States. It currently has a capacity of just under 1000MW and is owned and operated by Xcel Energy, the successor to the plant's...

, thorium dioxide fuel is "very difficult to process," making Th-232/U-233 unattractive for all systems except liquid salt, e.g. where thorium and uranium fluorides are used instead.

In Sorenson's opinion, the LFTR design combines the desirable characteristics of abundant fuel supply, high operating temperature, atmospheric operating pressure and simple fuel processing.

The Weinberg Foundation

The Weinberg Foundation is a British non-profit organisation founded in 2011 dedicated to promotion and development of a liquid fluoride thorium reactor. It was formally launched at the House of Lords on 8 September 2011.

External links

Videos:

Media articles:

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Background

Two versus Single fluid

Two Fluid Reactor

Single Fluid Reactor

Safety

Economy and efficiency

Ease of reprocessing

Disadvantages

Design challenges

The Fuji MSR

Chinese Thorium MSR project

Flibe Energy

Small modular design

The Weinberg Foundation

See also

External links