Liquid fluoride thorium reactor

"LFTR" redirects here. For the American indie rock band "LFTR PLLR", see Lifter Puller.

See also: Thorium-based nuclear power

Liquid FLiBe salt

The liquid fluoride thorium reactor (acronym LFTR; spoken as lifter) is a thermal breeder reactor that uses the thorium fuel cycle in a fluoride-based molten (liquid) salt fuel to achieve high operating temperatures at atmospheric pressure.

The LFTR is a type of thorium molten salt reactor (TMSR). Molten-salt-fueled reactors (MSRs) such as LFTR, where the nuclear fuel itself is in the liquid form of molten salt mixture, should not be confused with solid-fueled molten salt-cooled high temperature reactors (fluoride high-temperature reactors, FHRs).

Molten salt reactors, as a class, can include both burners and breeders in fast or thermal spectra, using fluoride or chloride salt-based fuels and a range of fissile or fertile consumables. LFTR refers to a particular design subset of MSRs defined by use of fluoride fuel salts and breeding of thorium into uranium-233 in the thermal spectrum.

In a LFTR, thorium and uranium-233 are dissolved in carrier salts, forming a liquid fuel. In a typical operation, the liquid fuel salt is pumped between a critical core and an external heat exchanger where the heat is transferred to a nonradioactive secondary salt. The secondary salt then transfers its heat again to a steam turbine or closed-cycle gas turbine.

This technology was first investigated at the Oak Ridge National Laboratory Molten-Salt Reactor Experiment in the 1960s. It has recently been the subject of a renewed interest worldwide. Japan, China, the UK, as well as private US, Czech and Australian companies have expressed intent to develop and commercialize the technology.

LFTRs are a particular class of molten salt reactor that includes several design variations. LFTRs differ from other power reactors in almost every aspect: thorium fuel cycle, low operating pressure, liquid fuel, salt coolant, higher operating temperature, and online refueling and reprocessing. The unique characteristics of a LFTR give rise to many potential advantages and also to a number of design challenges. This article addresses some LFTR reactor design variations and their advantages and disadvantages. Power conversion cycles that could be used to generate electricity with a LFTR are discussed, as well as online removal of fission products which is an important aspect of LFTRs.

Background

Thorium is relatively abundant in the earth's crust.

Tiny crystals of Thorite, a thorium mineral, under magnification.

Molten Salt Reactor at Oak Ridge

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

• Uranium-235, which is already fissile, and occurs as 0.72% of natural uranium
• Plutonium-239, which can be bred from non-fissile uranium-238 (>99% of natural uranium)
• Uranium-233, which can be bred from non-fissile thorium-232 (~100% of natural thorium; which has about four times greater abundance in the earth's crust than uranium)

Th-232, U-235 and U-238 are primordial nuclides, having existed in their current form for over 4.5 billion years, predating the formation of the Earth; they were forged in the cores of dying stars through the r-process and scattered across the galaxy by supernovas. Their radioactive decay produces about half of the earth's internal heat.

For technical and historical reasons, the three are each associated with different reactor types. U-235 is the world's primary nuclear fuel and is usually used in light water reactors. U-238/Pu-239 has found the most use in liquid sodium fast breeder reactors and CANDU Reactors. Th-232/U-233 is best suited to molten salt reactors (MSR).

Alvin M. Weinberg pioneered the use of the MSR at Oak Ridge National Laboratory. At ORNL, two prototype molten salt reactors were successfully designed, constructed and operated. These were the Aircraft Reactor Experiment in 1954 and Molten-Salt Reactor Experiment from 1965 to 1969. Both test reactors used liquid fluoride fuel salts. The MSRE notably demonstrated fueling with U-233 and U-235 during separate test runs. Weinberg was removed from his post and the MSR program closed down in the early 1970s, after which research stagnated in the United States. Today, the ARE and the MSRE remain the only molten salt reactors ever operated.

Breeding basics

In a nuclear power reactor, there are two types of fuel. The first is a small amount of fissile, which splits ("burns") when hit by neutrons, releasing a large amount of energy and also releasing two or three new neutrons. These can split more fissile material, resulting in a continued chain reaction. Examples of fissile fuels are U-233, U-235 and Pu-239. The second type of fuel is called fertile. Examples of fertile fuel are Th-232 (mined thorium) and U-238 (mined uranium). The amount of fertile fuel in the reactor is far bigger than the amount of fissile, but it cannot be fissioned directly. It must first absorb one of the 2 or 3 neutrons produced in the fission process, which is called neutron capture, then it becomes a fissile isotope by radioactive decay. This process is called breeding. All reactors breed some fuel this way, but today's solid fuelled thermal reactors don't breed enough new fuel from the fertile to make up for the amount of fissile they consume. This is because today's reactors use the mined uranium-plutonium cycle in a moderated neutron spectrum. Such a fuel cycle, using slowed down neutrons, gives back less than 2 new neutrons from fissioning the bred plutonium. Since 1 neutron is required to sustain the fission reaction, this leaves a budget of less than 1 neutron per fission to breed new fuel. In addition, the materials in the core such as metals, moderators, and the ashes from the fission reaction, the fission products, absorb ("steal") some neutrons, leaving too few neutrons to breed enough fuel to continue operating the reactor. As a consequence they must add new fissile fuel periodically and swap out some of the old fuel to make room for the new fuel.

In a reactor that breeds at least as much new fuel as it consumes, it is not necessary to add new fissile fuel. Only new fertile fuel is added, which breeds to fissile inside the reactor. This type of reactor is called a breeder reactor. A LFTR is usually designed as a breeder reactor. If it breeds just as much new fissile from fertile to keep operating indefinitely, it is called a break-even breeder or isobreeder. Thorium goes in, fission products come out.

Reactors that use the uranium-plutonium fuel cycle require fast reactors to sustain breeding, because only with fast moving neutrons, the fission process provides more than 2 neutrons per fission. With thorium, it is possible to breed using a thermal reactor. This was proven to work in the Shippingport Atomic Power Station, whose final fuel load bred slightly more fissile from thorium than it consumed, despite being a very neutron inefficient reactor type (Shippingport was a fairly standard light water reactor). Such thermal reactors require much less of the expensive fissile fuel to start and have a slow, gentle response in power changes.

There are two ways to configure a breeder reactor to do the required breeding. One can place the fertile and fissile fuel together, so breeding and splitting occurs in the same place. Alternatively, fissile and fertile can be separated. The latter is known as core-and-blanket, because a fissile core produces the heat and neutrons while a separate blanket does all the breeding.

Reactor primary system design variations

Oak Ridge investigated both ways to make a breeder for their Molten Salt Breeder Reactor. Because the fuel is liquid, they are called the "single fluid" and "two fluid" thorium thermal breeder molten salt reactors:

Single Fluid Reactor

Simplified schematic of a single fluid reactor.

The one-fluid design includes a large reactor vessel filled with fluoride salt containing thorium and uranium. Graphite rods immersed in the salt function as a moderator and to guide the flow of salt. In the ORNL MSBR design a reduced amount of graphite near the edge of the reactor core would make the outer region under-moderated, and increased the capture of neutrons there by the thorium. With this arrangement, most of the neutrons were generated at some distance from the reactor boundary, and reduced the neutron leakage to an acceptable level. Still, a single fluid design needs a considerable size to permit breeding.

In a breeder configuration, extensive fuel processing was specified to remove fission products from the fuel salt. In a converter configuration fuel processing requirement was simplified to reduce plant cost. The trade-off was the requirement of periodic Uranium refueling.

The MSRE was a core region only prototype reactor. The MSRE provided valuable long-term operating experience.

According to estimates of Japanese scientists, a single fluid LFTR program could be achieved through a relatively modest investment of roughly 300–400 million dollars over 5–10 years to fund research to fill minor technical gaps and build a small reactor prototype comparable to the MSRE.

Two Fluid Reactor

The two-fluid design is mechanically more complicated compared to the "single fluid" reactor design. The "two fluid" reactor has a high-neutron-density core that burns uranium-233 from the thorium fuel cycle. A separate blanket of thorium salt absorbs the neutrons and its thorium is converted to protactinium-233. Protactinium-233 can be left in the blanket region where neutron flux is lower, so that it slowly decays to U-233 fissile fuel, rather than capture neutrons. This bred fissile U-233 can be recovered by simple fluorination, and placed in the core to fission. The core'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 waste of a LFTR.

The advantages of separating the core and blanket fluid include:

1. Simplified fuel processing. Thorium is chemically similar to several fission products, called lanthanides. With thorium in a separate blanket, thorium is kept isolated from the lanthanides. If thorium is in the core fluid along with the fissile fuel, it is hard to recover thorium without removing lanthanide fission products.
2. Low fissile inventory. Because the fissile fuel is concentrated in a small core fluid, the actual reactor core is more compact. There is no fissile material in the outer blanket that contains the fertile fuel for breeding. Because of this, the 1968 ORNL design required just 315 kilograms of fissile materials to start up a 250 MW(e) two fluid MSBR reactor. This reduces the cost of the initial fissile startup charge, and allows more reactors to be started up on any given amount of fissile material.
3. More efficient breeding. The thorium blanket can effectively capture leaked neutrons from the core region. There is nearly zero fission occurring in the blanket, so the blanket itself does not leak significant numbers of neutrons. This results in a high efficiency of neutron use (neutron economy), and a higher breeding ratio, especially with small reactors.

One design weakness of the two-fluid design was the necessity for a barrier wall between the core and the blanket region, a wall that would have to be replaced periodically because of fast neutron damage. Graphite was the chosen material by ORNL because of its low neutron absorption, compatibility with the molten salts, high temperature resistance, and sufficient strength and integrity to separate the fuel and blanket salts. The effect of neutron radiation on graphite is to slowly shrink and then swell the graphite to cause an increase in porosity and a deterioration in physical properties. Graphite pipes would change length, and may crack and leak. ORNL chose not to pursue the two-fluid design, and no examples of the two-fluid reactor were ever constructed.

One additional design weakness of the two-fluid design was its complex plumbing. ORNL thought it necessary to use complex interleaving of the core and blanket piping in order to get a high reactor power level with acceptably low power density. More recent research has put into question the need for complex interleaving graphite tubing, suggesting a simple elongated tube-in-shell reactor would allow high total reactor power without complex tubing.

The recovery of high-purity uranium-233 has been raised as a potential nuclear proliferation concern. A design with no Protactinium separation would ensure that any U-233 is contaminated with U-232 whose decay chain emits 2 MeV gamma rays too hazardous for weapons workers.

Hybrid "one and a half fluid" reactor

A two fluid reactor that has thorium in the fuel salt is sometimes called a "one and a half fluid" reactor, or 1.5 fluid reactor. This is a hybrid, with some of the advantages and disadvantages of both 1 fluid and 2 fluid reactors. Like the 1 fluid reactor, it has thorium in the fuel salt, which complicates the fuel processing. And yet, like the 2 fluid reactor, it can use a highly effective separate blanket to absorb neutrons that leak from the core. The added disadvantage of keeping the fluids separate using a barrier remains, but with thorium present in the fuel salt there are fewer neutrons that must pass through this barrier into the blanket fluid. This results in less damage to the barrier. Any leak in the barrier would also be of lower consequence, as the processing system must already deal with thorium in the core.

The main design question when deciding between a 1/1.5 fluid or two fluid LFTR is whether a more complicated reprocessing or a more demanding structural barrier will be easier to solve.

Calculated nuclear performance of 1000-MW(e) MSBR design concepts
Design concept	Breeding ratio	Fissile inventory
Single-fluid, 30 year graphite life, fuel processing	1.06	2300 kg
Single-fluid, 4 year graphite life, fuel processing	1.06	1500 kg
1.5 fluid, replaceable core, fuel processing	1.07	900 kg
Two-fluid, replaceable core, fuel processing	1.07	700 kg

Power Generation

The LFTR with a high operating temperature of 700 degrees Celsius can operate at a thermal efficiency to electrical of 45%. This is higher than today's light water reactors (LWRs) that are at 32–36% thermal to electrical efficiency.

Rankine Cycle

Rankine steam cycle

Main article: Rankine Cycle

The Rankine cycle is the most basic thermodynamic power cycle. The simplest cycle consists of a steam generators, a turbine, a condenser, and a pump. The working fluid is usually water. A Rankine power conversion system coupled to a LFTR could take advantage of increased steam temperature to improve its thermal efficiency. The subcritical Rankine steam cycle is currently used in commercial power plants, with the newest plants utilizing the higher temperature, higher pressure, supercritical Rankine steam cycles. The work of ORNL from the 1960s and 1970s on the MSBR assumed the use of a standard supercritical steam turbine with an efficiency of 44%, and had done considerable design work on developing molten fluoride salt – steam generators.

Brayton Cycle

Closed-cycle gas turbine schematic

Main article: Brayton Cycle

The working gas of a Brayton cycle can be Helium, Nitrogen, or Carbon Dioxide. The high-pressure working gas is expanded in a turbine to produce power. The low-pressure warm gas is cooled in an ambient cooler. The low-pressure cold gas is compressed to the high-pressure of the system. Often the turbine and the compressor are mechanically connected through a single shaft. High pressure Brayton cycles are expected to have a smaller generator footprint compared to lower pressure Rankine cycles. A Brayton cycle heat engine can operate at lower pressure with wider diameter piping. The world's first commercial Brayton cycle solar power module (100 kW) was built and demonstrated in the Israel's Arava desert in 2009.

Other applications

In addition to electricity generation, concentrated thermal energy from LFTR can enable other applications:

• Industrial process heat for many uses
• Desalination of water
• Hydrogen production by water splitting
• Combined heat and power
• Nuclear marine propulsion

Removal of fission products

Molten salt reactors can benefit greatly from a mechanism to remove the fission products from the fuel salt. Some fission products in the salt absorb neutrons and reduce the production of new fissile fuel. Especially the concentration of some of the rare earth elements needs to be kept low, as they have a large cross section for neutron capture. Some other elements with a small cross section like Cs or Zr can be tolerated in much higher concentrations, so they may accumulate over years of operation.

Removal of fission products is similar to reprocessing of solid fuel elements, without the need to remove and rebuild the fuel cladding. As the fuel of a LFTR is a molten salt mixture, it is attractive to use pyroprocessing, high temperature methods working directly from the hot molten salt. Pyroprocessing does not use radiation sensitive solvents and is not easily disturbed by decay heat. It can be used on the highly radioactive fuel directly from the reactor. Having the chemical separation on site, close to the reactor avoids transport and keeps the total inventory of the fuel cycle low. Ideally everything except new fuel (thorium) and waste (fission products) stays inside the plant.

On site processing is planned to work continuously, cleaning a small fraction of the salt every day and sending it back to the reactor. There is no need to make the fuel salt very clean, the purpose is to keep the concentration of fission products and other impurities (e.g. oxygen) low enough.

The more "noble" metals (Pd, Ru, Ag, Mo, Nb, Sb, Tc... ) do not form fluorides in the normal salt, but form fine metallic particles in the salt. They can plate out at metal surfaces like the heat exchanger or some kind of high surface area filters that are easier to remove. Still there is some uncertainty where these noble elements end up, as the MSRE only provided a relatively short operating experience and independent laboratory experiments are difficult.

Some elements like Xe and Kr come out easily as gas, assisted by a sparge of helium. In addition a part of the "noble" metals are removed together with the gas as a fine mist. Especially the fast removal of Xe-135 is important, as this a very strong neutron poison and makes reactor control more difficult if left in the reactor. Removal of Xe also improves neutron economy. The gas (mainly He, Xe and Kr) is held up for about 2 days until a large fraction of the Xe-135 and other short lived isotopes have decayed. Most of the gas can then be recycled. After an additional hold up of several months, radioactivity is low enough to separate the gas at low temperatures into helium (for reuse), xenon (for sale) and krypton. The krypton needs storage (e.g. in compressed form) for an extended time (several decades) to wait for the decay of Kr-85.

For cleaning the salt mixture several methods of chemical separation were proposed. Compared to classical Purex reprocessing, pyroprocessing can be more compact and produce less secondary waste. The pyroprocesses of the LFTR salt already starts with a suitable liquid form, so it may be less expensive than using solid oxide fuels. However, because no complete molten salt reprocessing plant has been built, all testing has been limited to the laboratory, and with only a few elements. There is still more research and development needed to improve separation and make reprocessing more economically viable.

Uranium and some other elements can be removed from the salt by a process called fluorine volatility: A sparge of fluorine removes volatile high-valence fluorides as a gas. This is mainly uranium hexafluoride, containing the uranium-233 fuel, but also neptunium hexafluoride, technetium hexafluoride and selenium hexafluoride, as well as fluorides of various highly radioactive short-lived fission products such as iodine-131, ⁹⁹molybdenum, and ¹³²tellurium. The volatile fluorides can be further separated by adsorption and distillation. Handling uranium hexafluoride is well established in enrichment. The higher valence fluorides are quite corrosive at high temperatures and require more resistant materials than Hastelloy. One suggestion in the MSBR program at ORNL was using solidified salt as a protective layer. At the MSRE reactor fluorine volatility was used to remove uranium from the fuel salt. Also for use with solid fuel elements fluorine volatility is quite well developed and tested.

Another simple method, tested during the MSRE program, is high temperature vacuum distillation. The lower boiling point fluorides like uranium tetrafluoride and the LiF and BeF carrier salt can be removed by distillation. Under vacuum the temperature can be lower than the ambient pressure boiling point. So a temperature of about 1000 °C is sufficient to recover most of the FLiBe carrier salt. However, while possible in principle, separation of thorium fluoride from the even higher boiling point lanthanide fluorides would require very high temperatures and new materials. The chemical separation for the 2-fluid designs, using uranium as a fissile fuel can work with these two relatively simple processes: Uranium from the blanket salt can be removed by fluorine volatility, and transferred to the core salt. To remove the fissile products from the core salt, first the uranium is removed via fluorine volatility. Then the carrier salt can be recovered by high temperature distillation. The fluorides with a high boiling point, including the lanthanides stay behind as waste.

The early Oak Ridge's chemistry designs were not concerned with proliferation and aimed for fast breeding. They planned to separate and store protactinium-233, so it could decay to uranium-233 without being destroyed by neutron capture in the reactor. With a half-life of 27 days, 2 months of storage would assure that 75% of the ²³³Pa decays to ²³³U fuel. The protactinium removal step is not required per se for a LFTR. Alternate solutions are operating at a lower power density and thus a larger fissile inventory (for 1 or 1.5 fluid) or a larger blanket (for 2 fluid). Also a harder neutron spectrum helps to achieve acceptable breeding without protactinium isolation.

If Pa separation is specified, this must be done quite often (for example, every 10 days) to be effective. For a 1 GW, 1-fluid plant this means about 10% of the fuel or about 15 t of fuel salt need to go through reprocessing every day. This is only feasible if the costs are much lower than current costs for reprocessing solid fuel.

Newer designs usually avoid the Pa removal and send less salt to reprocessing, which reduces the required size and costs for the chemical separation. It also avoids proliferation concerns due to high purity U-233 that might be available from the decay of the chemical separated Pa.

Separation is more difficult if the fission products are mixed with thorium, because thorium, plutonium and the lanthanides (rare earth elements) are chemically similar. One process suggested for both separation of protactinium and the removal of the lanthanides is the contact with molten bismuth. In a redox-reaction some metals can be transferred to the bismuth melt in exchange for lithium added to the bismuth melt. At low lithium concentrations U, Pu and Pa move to the bismuth melt. At more reducing conditions (more lithium in the bismuth melt) the lanthanides and thorium transfer to the bismuth melt too. The fission products are then removed from the bismuth alloy in a separate step, e.g. by contact to a LiCl melt. However this method is far less developed. A similar method may also possible be with other liquid metals like aluminum.

Advantages

Thorium-fueled molten salt reactor offers many potential advantages compared to conventional solid uranium fueled light water reactors:

Safety

• Inherent safety. LFTR designs use a strong negative temperature coefficient of reactivity to achieve passive inherently safe against excursions of reactivity. The temperature dependence comes from 3 sources. The first is that thorium absorbs more neutrons if it overheats, the so-called Doppler effect. This leaves fewer neutrons to continue the chain reaction, reducing power. The second part is heating the graphite moderator, that usually causes a positive contribution to the temperature coefficient. The third effect has to do with thermal expansion of the fuel. If the fuel overheats, it expands considerably, which, due to the liquid nature of the fuel, will push fuel out of the active core region. In a small (e.g. the MSRE test reactor) or well moderated core this reduces the reactivity. However in a large under-moderated core (e.g. the ORNL MSBR design) less fuel salt means better moderation and thus more reactivity and an undesirable positive temperature coefficient.
• Stable coolant. Molten fluorides are chemically stable and impervious to radiation. The salts do not burn, explode, or decompose, even under high temperature and radiation. There are no rapid violent reactions with water and air that sodium coolant has. There is no combustible hydrogen production that water coolants have. However the salt is not stable to radiation at low (less than 100 C) temperatures due to radiolysis.
• Low pressure operation. Because the coolant salts remain liquid at high temperatures, LFTR cores are designed to operate at low pressures, like 0.6 MPa (comparable to the pressure in the drinking water system) from the pump and hydrostatic pressure. Even if the core fails, there is little increase in volume. Thus the containment building cannot blow up. LFTR coolant salts are chosen to have very high boiling points. Even a several hundred degree heatup during a transient or accident does not cause a meaningful pressure increase. There is no water or hydrogen in the reactor that can cause a large pressure rise or explosion as happened during the Fukushima Daiichi nuclear accident.
• Leak Resistance. Due to the low pressure operation and low pressure differences through the primary heat exchangers, the potential for large leaks is also greatly reduced.
• No pressure buildup from fission. LFTRs prevent pressure buildup due to gaseous and volatile fission products. The liquid fuel allows for online removal of gaseous fission products, such as Xenon, for processing, thereby these decay products would not be spread in a disaster. Further, fissile products are chemically bonded to the fluoride-salt, including iodine, cesium, and strontium, capturing the radiation and preventing the spread of radioactive material to the environment.
• Easier to control. A molten fuel reactor has the advantage of removal of Xenon-135. Xenon-135, an important neutron absorber, makes solid fueled reactors difficult to control. In a molten fueled reactor, Xenon-135 can be removed. In solid-fuel reactors, Xenon-135 remains in the fuel and interferes with reactor control.
• Slow heatup. Coolant and fuel are inseparable, so any leak or movement of fuel will be intrinsically accompanied by a large amount of coolant. Molten fluorides have high volumetric heat capacity, some such as FLiBe, even higher than water. This allows them to absorb large amounts of heat during transients or accidents.
• Passive decay heat cooling. LFTRs operate at high temperature, at least 650 degrees Celsius, which makes passive decay heat cooling after shutdown much more effective; hot objects lose heat much faster than warm objects (see Newton's law of cooling). The hot cell/containment building could be simply designed to constantly lose some heat to the environment, ensuring effective decay heat removal after shutdown. Similarly, fully passively cooled heat exchangers (no pumps or moving parts) can take advantage of the high temperatures to passively transfer heat to the environment.
• Fail safe core. LFTRs can 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 plug melts, and the fuel drains to a subcritical passively cooled storage facility. This not only stops the reactor, also the storage tank can more easily shed the decay heat from the short-lived radioactive decay of irradiated nuclear fuels. Even in the event of a major leak from the core such as a pipe breaking, the salt will spill onto the kitchen-sink-shaped room where the reactor is in, which will drain the fuel salt by gravity into the passively cooled dump tank.
• Less activated waste^{[citation needed]}. LFTRs have very little structural material inside the core. Only the fuel salt, graphite, and small amounts of metals or composites are inside the actual reactor core. This reduces the amount of neutrons lost to structural components, improving the neutron economy, and reducing the amount of activated structural waste. Fluorine, lithium and beryllium do not have significant long term neutron activation.
• Less long lived waste. LFTRs can dramatically reduce the long-term radiotoxicity of their reactor wastes. Light water reactors with uranium fuel have fuel that is more than 95% U-238. These reactors normally transmute part of the U-238 to Pu-239, a toxic transuranic isotope. Almost all of the fuel is therefore only one step away from becoming a transuranic long lived element. Plutonium-239 has a half life of 24,000 years, and is the most common transuranic in spent nuclear fuel from light water reactors. Transuranics like Pu-239 cause the perception that reactor wastes are an eternal problem. In contrast, the LFTR uses the thorium fuel cycle, which transmutes thorium to U-233. Because thorium is a lighter element, more neutron captures are required to produce the transuranic elements. U-233 has two chances to fission in a LFTR. First as U-233 (90% will fission) and then the remaining 10% has another chance as it transmutes to U-235 (80% will fission). The fraction of fuel reaching Neptunium-237, the most likely transuranic element, is therefore only 2%, about 15 kg per GWe-year. This is a transuranic production 20x smaller than light water reactors, which produce 300 kg of transuranics per GWe-year. Importantly, because of this much smaller transuranic production, it is much easier to recycle the transuranics. That is, they are sent back to the core to eventually fission. Reactors operating on the U238-plutonium fuel cycle produce far more transuranics, making full recycle difficult on both reactor neutronics and the recycling system. In the LFTR, only a fraction of a percent, as reprocessing losses, goes to the final waste. When these two benefits of lower transuranic production, and recycling, are combined, a thorium fuel cycle reduces the production of transuranic wastes by more than a thousand-fold compared to a conventional once-through uranium-fueled light water reactor. The only significant long lived waste is the uranium fuel itself, but this can be used indefinitely by recycling, always generating electricity. If the thorium age ever has to be shut down, part of the reactors can be shut down and their uranium fuel inventory burned out in the remaining reactors, allowing a burndown of even this final waste to as small a level as society demands. The LFTR does still produce radioactive fission products in its waste, but they don't last very long – the radiotoxicity of these fission products is dominated by cesium-137 and strontium-90. The longer half-life is cesium: 30.17 years. So, after 30.17 years, decay reduces the radioactivity by a half. Ten half-lives will reduce the radioactivity to two raised to a power of ten, a factor of 1,024. Fission products at that point, in about 300 years, are less radioactive than natural uranium. What's more, the fact liquid state of the fuel material allows separation of the fission products not only from the fuel, but from each other as well, which enables them to be sorted by the length of each fission product's half-life, so that the ones with shorter half-lives can be brought out of storage sooner than those with longer half-lives.
• Destruction of existing long lived wastes. LFTRs can use existing transuranic wastes for their initial fissile startup charge better than any solid fueled reactor for various technical and physical reasons. Because the fuel is a liquid homogeneous solution, it is always perfectly mixed, impervious to radiation damage and can accept any composition of plutonium, neptunium, americium and curium up to the solubility limit. Solid fueled reactors, such as solid fueled fast reactors, while theoretically outperforming the LFTR in burning of these higher actinides, can only accept limited amounts of these higher actinides (neptunium, americium and curium are often called minor actinides). This is because the fuel is not perfectly mixed, as it is confined in solid fuel elements, and also because the coolant void coefficient (coolant overheating) can become positive for too high levels of minor actinides. In addition, manufacturing solid fuels with high amounts of americium and curium is also difficult due to decay heat generation and helium production rates. As a result, solid fueled reactors usually only use reprocessed plutonium but do not use the americium and curium, which constitute a large portion of the radiotoxicity of the long lived waste.
• Proliferation resistance. The LFTR resists diversion of its fuel to nuclear weapons. There are four ways: First, the thorium-232 breeds by converting first to protactinium-233, which then decays to uranium-233. If the protactinium remains in the reactor, small amounts of U-232 are also produced. U-232 has a decay chain product (thallium-208) that emits powerful, dangerous gamma rays. These are not a problem inside a reactor, but in a bomb, they complicate bomb manufacture, harm electronics and reveal the bomb's location. The second proliferation resistant feature comes from the fact that LFTRs produce very little plutonium, around 15 kg per gigawatt-year of electricity (this is the output of a single large reactor over a year). This plutonium is also mostly Pu-238, which makes it unsuitable for fission bomb building, due to the high heat and spontaneous neutrons emitted. The third track, a LFTR doesn't make much spare fuel. It produces at most 9% more fuel than it burns each year, and it's even easier to design a reactor that makes 1% more fuel. With this kind of reactor, building bombs quickly will take power plants out of operation, and this is an easy indication of national intentions. And finally, use of thorium can reduce or even eliminate the need to enrich uranium. Uranium enrichment is one of the two primary methods by which states have obtained bomb making materials.

Economy and efficiency

Comparison of annual fuel requirements and waste products of a 1GW uranium-fueled LWR and 1GW thorium-fueled LFTR power plant.

• Thorium abundance. A LFTR breeds thorium into uranium-233 fuel. The Earth's crust contains about three to four times as much thorium as U-238 (thorium is about as abundant as lead). It is a byproduct of rare-earth mining, normally discarded as waste. Using LFTRs, there is enough affordable thorium to satisfy the global energy needs for hundreds of thousands of years.
• No shortage of natural resources. Sufficient other natural resources such as beryllium, lithium, nickel and molybdenum are available to build thousands of LFTRs.
• Reactor efficiency. Conventional reactors consume less than one percent of the mined uranium, leaving the rest as waste. With well working reprocessing LFTR may consume about 99% of its thorium fuel. The improved fuel efficiency means that 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.
• Thermodynamic efficiency. LFTRs operating with modern supercritical steam turbines would operate at 45% thermal to electrical efficiency. With future closed gas Brayton cycles, which could be used in a LFTR power plant due to its high temperature operation, the efficiency could be up to 54%. This is 20 to 40% higher than today's light water reactors (33%), resulting in the same 20 to 40% reduction in fissile and fertile fuel consumption, fission products produced, waste heat rejection for cooling, and reactor thermal power.
• No enrichment and fuel element fabrication. Since 100% of 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 procedures and fabricating processes are not needed. This greatly decreases LFTR fuel costs. Even if the LFTR is started up on enriched uranium, it only needs this enrichment once just to get started. After startup, no further enrichment is required.
• Lower fuel cost. The salts are fairly inexpensive compared to solid fuel production. For example, while beryllium is quite expensive per kg, the amount of beryllium required for a large 1 GWe reactor is quite small. ORNL's MSBR required 5.1 tonnes of beryllium metal, as 26 tonnes of BeF₂. At a price of $147/kg BeF₂, this inventory would cost less than $4 million, a modest cost for a multi billion dollar power plant. Consequently a beryllium price increase over the level assumed here has little effect in the total cost of the power plant. The cost of enriched lithium-7 is less certain, at $120–800/kg LiF. and an inventory (again based on the MSBR system) of 17.9 tonnes lithium-7 as 66.5 tonnes LiF makes between $8 million and $53 million for the LiF. Adding the 99.1 tonnes of thorium @ $30/kg adds only $3 million. Fissile material is more expensive, especially if expensively reprocessed plutonium is used, at a cost of $100 per gram fissile plutonium. With an startup fissile charge of only 1.5 tonnes, made possible through the soft neutron spectrum this makes $150 million. Adding everything up brings the total cost of the one time fuel charge at $165 to $210 million. This is similar to the cost of a first core for a light water reactor. Depending on the details of reprocessing the salt inventory once can last for decades, whereas the LWR needs a completely new core every 4 to 6 years (1/3 is replaced every 12 to 24 months). ORNL's own estimate for the total salt cost of even the more expensive 3 loop system, was much lower, only around $30 million, which is less than 100 million in today's money.
• LFTRs are cleaner: as a fully recycling system, the discharge wastes from a LFTR are predominantly 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. After 300 years the radiotoxicity of the thorium fuel cycle waste is 10,000 times less than that of the uranium/plutonium fuel cycle waste.
• Low corrosion, long lasting materials. ORNL developed a special alloy, Hastelloy N, for the MSRE. They later modified the alloy for improved resistance to radiation damage and tellurium embrittlement, by adding some titanium and niobium, respectively. This resulted in very low corrosion rates compared to light water reactors,. Graphite is completely inert in redox controlled fluoride melts, and while it needs to be replaced every 4–30 years (depending on core power density) due to fast neutron radiation damage, the estimated cost of replacement graphite is very low, 0.01 cent per kWh (0.1 mill/kWh, or $0.1/MWh) in 1969 dollars. Taking into account inflation, in today's dollars this is 0.03 cents per kWh.
• Destruction of existing long lived nuclear wastes. The LFTR can "burn" problematic radioactive waste with transuranic elements from traditional solid-fuel nuclear reactors without producing new transuranic waste in the process, thus solving the long term high level waste problem by turning the liability into an asset.
• Less fissile fuel needed. Because LFTRs are thermal spectrum reactors, they need much less fissile fuel to get started. Only 1–2 tonnes of fissile are required to start up a single fluid LFTR, and potentially as low as 0.4 ton for a two fluid design. In comparison, solid fueled fast breeder reactors need at least 8 tonnes of fissile fuel to start the reactor. While fast reactors can theoretically start up very well on the transuranic waste, their high fissile fuel startup makes this very expensive.^{[citation needed]}
• No downtime for refueling. LFTRs have liquid fuels, and therefore there is no need to shutdown and take apart the reactor just to refuel it. LFTRs can thus refuel without causing a power outage (online refueling).
• Load following. As the LFTR does not have xenon poisoning, there is no problem reducing the power in times of low demand for electricity and turn back on at any time.
• No high pressure vessel. Since the core is not pressurized, 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. Thicker components can be made of a lower cost, high strength high temperature steel, such as Incolloy 800H, that is then clad with a corrosion resistant layer on the salt contacting side, one such metal is the more expensive but highly fluoride resistant Hastelloy N. This reduces cost of the metals, and is commonly used in light water reactors, that use high strength steel and a stainless steel liner for corrosion protection against the hot pressurized water.
• Excellent heat transfer. Liquid fluoride salts, especially LiF based salts, have good heat transfer properties. Fuel salt such as LiF-ThF₄ has a volumetric heat capacity that is around 22% higher than water, FLiBe has around 12% higher heat capacity than water. In addition, the LiF based salts have a thermal conductivity around twice that of the hot pressurized water in a pressurized water reactor. This results in efficient heat transfer and a compact primary loop. Compared to helium, a competing high temperature reactor coolant, the difference is even bigger. The fuel salt has over 200 times higher volumetric heat capacity as hot pressurized helium and over 3 times the thermal conductivity. A molten salt loop will use piping of 1/5 the diameter, and pumps 1/20 the power, of those required for high-pressure helium, while staying at atmospheric pressure
• Smaller, low pressure containment. By using liquid salt as the coolant instead of pressurized water, a containment structure only slightly bigger than the reactor vessel can be used. Light water reactors use pressurized 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. The LFTR containment can not only be smaller in physical size, its containment is also inherently low pressure. There are no sources of stored energy that could cause a rapid pressure rise (such as hydrogen or steam) in the containment. This gives the LFTR a substantial theoretical advantage not only in terms of inherent safety, but also in terms of smaller size, lower materials use, and lower construction cost.
• Air cooling. A high temperature power cycle can be air-cooled at little loss in efficiency, which is critical for use in many regions where water is scarce. No need for large water cooling towers used in conventional steam-powered systems would also decrease power plant construction costs.
• LFTRs scale well. Given the diminished scale of LFTRs, no need for big containment, pressure vessel and cooling towers for the air-cooled designs, LFTR small modular reactors lend themselves well to factory mass production. According to American Scientist, a reasonable cost projection is that 100 MW reactors could be factory produced for around $200 million.
• From waste to resource. There are suggestions that it might be possible to reuse some of the fission products. However, compared to the produced energy, the value of the fission products is low, and chemical purification is expensive.

Disadvantages

LFTR's deviate strongly from today's operating commercial power reactors. A different fuel cycle (thorium rather than uranium), low pressure operation (rather than high pressure), liquid fuel (versus solid fuel), use of molten salts (rather than water or gasses), online refueling and reprocessing using pyroprocesses (opposed to off-site processing using water solvents), LFTRs are different in almost every aspect. This gives rise to a uniquely different set of design challenges and trade-offs with varying levels of design, political and inherent difficulties:

• Mothballed technology. Only a few MSRs have actually been built; those experimental reactors having been constructed more than 40 years ago. This leads some technologists to say that it is difficult to critically assess the concept.
• Startup fuel. Unlike mined uranium, mined thorium does not have a fissile isotope. Thorium reactors breed fissile uranium-233 from thorium, but require a considerable amount of U-233 for the initial start up. Currently there is very little of this material available. This raises the problem of how to start up the reactors in a reasonable time frame. There are a number of ways to start up the reactors. One option is that U-233 could be produced in today's solid fuelled reactors, then reprocessing the U-233 out of the solid fuel to start up a LFTR. A LFTR can also be started up by different fissile isotopes. The two alternative options for LFTR startup are enriched uranium and plutonium from reactors or decommissioned bombs. For enriched uranium startup, a quite high enrichment is needed. Decommissioned uranium bombs have a high enough enrichment, but not enough is available to start up a large number of LFTRs. For plutonium startup, it is more difficult to separate plutonium fluoride from lanthanide fission products. One option for a two fluid reactor is to operate with plutonium or enriched uranium in the fuel salt, breeding U-233 in the blanket, but storing it instead of sending it back to the core. Instead, add makeup plutonium or enriched uranium to continue the chain reaction, similar to today's solid fuel reactors. When enough U-233 is bred, replace the fuel salt with new fuel salt, holding the U-233 as new startup fuel. A similar option exists for a single fluid reactor operating as a converter. Such a reactor would not reprocess fuel online, instead would startup on plutonium with thorium as the fertile, and add makeup plutonium. After many years the plutonium burns out and U-233 is produced in place. At the end of the reactor fuel life, the spent fuel salt can be reprocessed to recover the bred U-233 to start up new LFTRs.
• Salts freezing. The fluoride salt mixtures have high melting points, depending on the mixture it ranges from 300 to over 600 degrees Celsius. The salts, especially those with beryllium fluoride, are very viscous close to their freezing point. This requires careful design and freeze protection in the containment and heat exchangers. Freezing must be prevented in normal operation, during transients, and during extended station blackouts. The primary loop salt contains the decay heat generating fission products, so these help to keep the salt hot and liquid. For the MSBR, ORNL planned on keeping the entire reactor room (the hot cell) at high temperature, like an oven. This avoided the need for individual electric heater lines on all piping and provided more even heating of the primary loop components.. One "liquid oven" concept developed for molten salt cooled, solid fueled reactors, employs a separate buffer salt pool where the entire primary loop is suspended in. Because of the high heat capacity and considerable density of the buffer salt, the buffer salt not only prevents fuel salt freezing, but also participates in the passive decay heat cooling system, provides radiation shielding, and reduces deadweight stresses on primary loop components. This design could also be adopted for LFTRs.
• Beryllium toxicity. The proposed salt mixture FLiBe, contains large amounts of beryllium, a poisonous element. The salt in the primary cooling loops must be isolated from workers and the environment to protect them from beryllium poisoning. This is routinely done very effectively in various industries that handle beryllium. Based on this industrial experience with beryllium, the added cost due to beryllium safety is expected to cost 0.012 cent per kWh ($0.12/MWh), which is very small. Furthermore, after start up, the fission process in the primary fuel salt will produce highly radioactive fission products that produce a high gamma and neutron radiation field. A highly effective containment will therefore be a primary requirement of any molten salt reactor. It is also possible to operate without beryllium fluoride in the salt. It is possible to operate on lithium fluoride-thorium fluoride eutectic without beryllium, as the French LFTR design, the "TMSR", has chosen. This comes at the cost of a somewhat higher melting point, but has the additional advantages of simplicity (avoid BeF₂ in the reprocessing systems), increased solubility for plutonium-trifluoride, reduced tritium production (Beryllium produces lithium-6, which in turn produces tritium) and improved heat transfer (BeF₂ increases the viscosity of the salt mixture). Alternatively various additional solvents are available such as the fluorides of sodium, rubidium and zirconium that allow lower melting points at a tradeoff in breeding.
• Loss of delayed neutrons. The production of the abovementioned delayed neutrons in the heat exchanger and other external piping introduces another design constriction. In order to be gently, slowly and predictably controlled, nuclear reactors rely critically on the delayed neutrons. In other words, they require the additional slowly evolving neutrons from fission product decay to continue the chain reaction normally. Because the delayed neutrons evolve slowly, this makes the reactor very gently controllable. In a LFTR, the presence of the fission products outside of the core in the heat exchanger and piping means a portion of these delayed neutrons are also lost. They do not participate in the critical chain reaction of the core, which in turn means the reactor will be less gently behaving during changes of flow, power etc. Approximately up to half of the delayed neutrons can be lost to retain the advantage of easy reactor behavior. In practice, it means that the heat exchanger must be compact so that the volume outside the core is as low as possible. The more compact (higher power density) the core is, the more important this issue becomes. Having more fuel outside the core in the heat exchangers also means more fissile fuel is needed to start up the reactor, which is expensive. This makes a fairly compact heat exchanger an important design requirement for a LFTR.
• Waste management. There is also a need to manage the waste, which is still very radioactive, even though it is hazardous for a shorter period. Because some fission products, in their fluoride form, are highly water soluble, fluorides are a less suited long term storage form. For example, cesium fluoride has a very high solubility in water. For long term storage, conversion to an insoluble form such as a glass, could be desirable.
• Decommissioning costs are uncertain. Cleanup of the Molten-Salt Reactor Experiment was about $130 Million, for a small 8 MW(th) unit. Much of the high cost was caused by the unpleasant surprise of fluorine and uranium hexafluoride evolution from cold fuel salt in storage that ORNL did not defuel and store correctly, but this has now been taken into consideration in MSR design. In addition, decommissioning costs don't scale strongly with plant size based on previous experience, and costs are incurred at the end of plant life, so a small per kWh fee on the electricity produced is sufficient to pay for it. For example, a GWe reactor plant produces over 300 billion kWh of electricity over a 40 year lifetime, so a 0.2 cent per kWh decommissioning fee delivers $600 million at the end of the plant lifetime.
• Noble metal buildup. Some of the radioactive fission products, such as noble metals, don't form salts, but deposit somewhere in the piping. This includes most of the reprocessing part, that is not yet well tested. Equipment, such as nickel wool sponge cartridges, will have to be developed to filter and trap the noble metals to prevent them from building up excessively in the piping, reprocessing plant, and heat exchanger over time.
• Limited graphite lifetime. Compact designs have a limited lifetime for the graphite moderator and fuel / breeding loop separator. Under the influence of fast neutrons, the graphite first shrinks, then expands indefinitely until it becomes very weak and can crack, leading to mechanical problems and causing the graphite to absorb more fission products that poison the nuclear reaction. A replacement of this central part can be challenging and needs to be done using remote equipment. However this must be compared to today's solid fuelled reactors, which must typically replace 1/3 of the entire core, including all of the highly radioactive fission products therein, every 12 to 24 months. The 1960 two-fluid design had an estimated graphite replacement period of four years. Eliminating graphite from sealed piping was a major incentive to switch to a single-fluid design. Most MSR designs arrange for it to be easy to replace. In a molten salt reactor, virtually all of the fuel and fission products can be drained out of the core to a holdup tank, when the graphite must be replaced. Only a fraction of a percent of the fission products end up in the graphite, primarily due to fissions occurring close to the bare graphite and the resulting fission products slamming into the graphite. However, this does make the used graphite surface itself radioactive, and without recycling/removal of at least the shallow surface layer, this will create a fairly bulky waste stream. Removing the thin surface layer of the graphite that contains the embedded fission products, and recycling the remainder of the bulk graphite, would solve this issue. Several techniques exist to recycle or dispose of nuclear moderator graphite. Graphite is inert and immobile at low temperatures, so it can also be readily stored or buried if required. At least one design used graphite balls (pebbles) floating in salt, which could be removed and inspected continuously without shutting down the reactor. Reducing the power density of the reactor design increases graphite lifetime.
• Graphite causes positive reactivity feedback. When graphite heats up, it increases the amount of fission of U233, thus causing an undesirable positive feedback. The LFTR-Design must avoid certain combinations of graphite and salt, and certain core geometries. This problem can be solved by employing either a well thermalized spectrum with lots of graphite, or using little or no graphite at all resulting in a faster spectrum. However this comes at the price of reduced breeding or increasing fissile inventory.
• The solubility for plutonium is limited. The fluorides of plutonium, americium, and curium, occur as trifluorides, which means they have three fluorine atoms attached (PuF₃, AmF₃, CmF₃). Such trifluorides have a limited solubility in the FLiBe carrier salt. This makes startup on these transuranic wastes more difficult especially for a compact design that uses a smaller primary salt inventory. This solubility can be increased by operating with less or no beryllium fluoride (which has no solubility for trifluorides) or by operating at a higher temperature (as with most other liquids, the solubility goes up with temperature). A thermal spectrum, lower power density core does not have issues with plutonium solubility.

• Potential proliferation risk from reprocessing. If reprocessing of the salt mixture works well, this technology like any advance in reprocessing technology can pose a proliferation-risk. As an alternative method of reprocessing this could be used to separate plutonium from other reactors as well. However, as stated above, plutonium is chemically difficult to separate from thorium, and plutonium cannot be used in bombs if diluted in large amounts of thorium. In addition, the plutonium produced by the thorium fuel cycle is mostly Pu-238, which creates high levels of spontaneous neutrons and decay heat that greatly complicate fission bomb design and reduces the bomb yield drastically by predetonation due to Pu-238 starting the chain reaction before the fissile material in the bomb is fully assembled (causing a "fizzle" rather than a fission bomb explosion). Finally, all of the reprocessing steps involve automated, non-human handling in a fully closed and contained hot cell, this is due to the high temperatures and radiation levels, which makes diversion of bomb material very difficult. Compared to today's water solvent extraction methods such as PUREX, the pyroprocesses are inaccessible and produce impure fissile materials, often with large amounts of fission products contamination. This is not a problem for an automated reactor-hot cell system, but poses severe difficulties for would-be bomb makers.
• Proliferation risk from protactinium separation for some specific designs. Compact designs can breed only using rapid separation of protactinium, a potential proliferation-risk, since this gives possible access to high purity 233-U without the U-232 contamination that is in the core. This is not easy, but still a possible new path to weapons-grade material. Because of this possibility, commercial power reactors may have to be designed without separation of protactinium. In practice, this means either not breeding or operating at a lower power density. For a two fluid design, this could simply operate with a bigger blanket, and keep the high power density core (which has no thorium and therefore no protactinium).
• Proliferation of Neptunium-237. In designs utilizing a fluorinator, Np-237 comes out with uranium as gaseous hexafluoride and can be easily separated using solid fluoride pellet absorption beds. Theoretically, it should be possible to use Np-237 as fission bomb material. No one has ever successfully produced a bomb with this material, but it should be theoretically possible to use it, because of its considerable fast fission cross section and low critical mass. When the Np-237 is kept in the reactor, it will transmute to Pu-238, an extremely high value fuel for space radioisotope thermal generators. A single gram is worth thousands of dollars. Pu-238 is itself an excellent proliferation deterrent, as explained earlier. Because of this, the Np-237 will likely be sent back to the reactor to be transmuted to Pu-238, which also is a highly sought-after fuel for use in radioisotope thermoelectric generators to power deep-space probes. In addition, it is possible to use vacuum distillation instead of fluorination, which does not separate neptunium at all. It should be noted, that all reactors, not just thorium reactors, produce considerable amounts of neptunium, which is always present in high (mono)isotopic quality, and it is just as easily extracted chemically. This is therefore not a distinguishing issue for LFTRs in particular. In fact, americium could also be theoretically used for nuclear weapons, and LFTRs do not produce meaningful quantities of americium, indeed they are one of the few reactor types that can burn existing stockpiles of americium and neptunium with high efficiency.
• Neutron poisoning and tritium production from lithium-6. Lithium-6 is a strong neutron poison; using LiF with natural lithium, with its 7.5% Lithium-6 content, will not even allow the reactor to start up. The high neutron density in the core rapidly transmutes lithium-6 to tritium, losing precious neutrons that are required to sustain at least break-even breeding, and also producing tritium in the process. Tritium is a radioactive isotope of hydrogen, which is nearly identical, chemically, to ordinary hydrogen. In hot fluoride salts, hydrogen will be present as elemental tritium. In the MSR the tritium is quite mobile because, in its elemental form, it rapidly diffuses through metals at high temperature. So some effort is needed to keep the emissions low. If the lithium is isotopically enriched in lithium-7, and the isotopic separation level is high enough (99.995% lithium-7), the amount of tritium produced is only a few hundred grams per year for a 1 GWe reactor. This much smaller amount of tritium comes mostly from the lithium-7 – tritium reaction reaction and from beryllium, which can produce tritium indirectly by first transmuting to lithium-6, thus regenerating the tritium producing lithium-6. Because of these reasons, if a LFTR design uses a lithium salt, it uses the lithium-7 isotope, via enrichment of natural lithium, to reduce tritium formation. In the MSRE, tritium formation was also reduced by this 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 methods of chemistry control and lithium isotopic separation worked in preventing hydrogen corrosion and excessive tritium generation in the MSRE. Practical MSRs also operate the salt under a blanket of dry inert gas, usually helium. Fortunately, in a LFTR, there is also a good chance to recover the tritium, since it is not diluted in a large amount of water as it is in CANDU reactors. Various methods exist to trap tritium, such as hydriding it to a stable metal hydride using titanium, oxidizing it to less mobile (but still volatile) forms such as by using sodium fluoroborate or molten nitrate salt as coolant loops, or trapping it in the turbine power cycle gas and offgas using copper oxide pellets . For future systems, ORNL developed a secondary loop coolant system that would chemically trap the few hundred grams of residual tritium to a less mobile form, so that it could be trapped and removed from the secondary coolant rather than diffusing into the turbine power cycle. This technique by itself would reduce tritium emissions to the actual environment to acceptable levels, ORNL calculated.
• Corrosion from tellurium. The reactor makes small amounts of tellurium as a fission product. In the MSRE, this caused small amounts of corrosion at the grain boundaries of the special nickel alloy, Hastelloy-N used for the reactor. Metallurgical studies showed that adding 1 to 2% niobium to the Hastelloy-N alloy improves resistance to corrosion by tellurium. One additional strategy against corrosion was to keep the fuel salt slightly reducing by maintaining the ratio of UF
  4/UF
  3 to less than 60. This was done in the MSRE by continually contacting the flowing fuel salt with a beryllium metal rod submersed in a cage inside the pump bowl. This causes a fluorine shortage in the salt, reducing tellurium to a less aggressive (elemental) form. This method is also effective in reducing corrosion in general from the fluoride salt, because the fission process produces more fluorine atoms freed from the fissioned uranium that would otherwise attack the structural metals.
• Radiation damage to nickel alloys. The standard Hastelloy N alloy, a high nickel alloy used for constructing the primary fuel salt loop, was found to be embrittled by the neutron radiation from the reactor core. This was caused by neutrons reacting with nickel to form helium. This helium, being a gas, concentrated at specific points inside the alloy. There, it caused stresses to build up. ORNL addressed this problem by adding 1–2% titanium or niobium to the Hastelloy N. This small titanium addition changed the internal structure of the alloy, so that the helium produced inside it would not concentrate in specific areas but would be finely divided in it. This relieved the stress and allowed the new modified Hastelloy N to withstand considerable neutron flux. However the maximum temperature is limited to about 650 °C. A number of other alloys also showed promise. The outer vessel wall that contains the salt can have neutronic shielding, such as boron carbide, to effectively protect it from neutron damage.
• Long term fuel salt storage issues. If the fluoride fuel salts are stored in solid form over many decades, radiation can cause the release of corrosive fluorine gas, and uranium hexafluoride. This was due to radiolysis of the salt from remaining fission products, when colder than 100 degrees Celsius. The salts should be defueled and wastes removed before extended shutdowns. The fluorine and uranium hexafluoride evolution can be prevented by storing the salts at a temperature above 100 degrees Celsius. Because some of the fission product fluorides have high solubility in water, fluorides are less suitable for long term storage anyway. For longer term storage, fluoride containing wastes could go through a vitrification process to be encased in insoluble borosilicate glass suitable for long-term disposal.
• Business model. Today's solid fuelled reactor vendors make long term revenues by making profit on the fuel fabrication. Without any fuel to fabricate and sell, a LFTR would have to adopt a different business model.
• Development of the power cycle. Developing a large helium or supercritical carbon dioxide turbine is needed for the highest efficiency designs. These gas cycles offer numerous potential advantages for use with molten salt fuelled or molten salt cooled reactors. These closed gas cycles are not yet fully developed though, and face some design challenges and engineering upscaling work for a commercial turbine-generator set. A standard supercritical steam turbine could be used at least in initial designs, at a small penalty in efficiency (the net efficiency of the MSBR was designed to be approximately 44%, using an old 1970s steam turbine). However, even in that case, a molten salt to steam generator would still have to be developed. Currently, molten nitrate salt steam generators are being used in concentrated solar thermal power plants such as Andasol in Spain. Such a nitrate salt loop and steam generator could also be used for a molten salt reactor, as an additional, third circulating loop, where it would allow the use of existing molten nitrate salt steam generator equipment, and as an added bonus effectively trap any tritium that diffuses through the primary and secondary heat exchanger

Dr. Charles Forsberg of ORNL discussed the technological challenges for a commercial MSR in a 2006 paper.

Dr. David Leblanc from Carleton University discussed the basics, trade-offs, problem areas and possible design remedies in a presentation.

Recent developments

The Fuji MSR

The FUJI MSR was a design for a 100 to 200 MWe molten-salt-fueled thorium fuel cycle thermal breeder reactor, using technology similar to the Oak Ridge National Laboratory Reactor Experiment. It was being developed by a consortium including members from Japan, the United States, and Russia. As a breeder reactor, it converts thorium into nuclear fuels. Like all molten salt reactors, its core is chemically inert, under low pressures to prevent explosions and toxic releases. An industry group presented updated plans about FUJI MSR in July 2010.

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 (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. An expected intermediate outcome of the TMSR research program is to build a 2 MW pebble bed fluoride salt cooled research reactor in 2015, and a 2 MW molten salt fueled research reactor in 2017. This would be followed by a 10 MW demonstrator reactor and a 100 MW pilot reactors. The project is spearheaded by Jiang Mianheng, with a start-up budget of $350 million, and has already recruited 140 PhD scientists, working full-time on thorium molten salt reactor research at the Shanghai Institute of Applied Physics. An expansion to 750 staff is planned by 2015.

Flibe Energy

Main article: 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 and particularly liquid fluoride thorium reactors. He first researched thorium reactors while working at NASA, 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 2006, Kirk Sorensen also coined the Liquid Fluoride Thorium Reactor and LFTR nomenclature to describe a subset of molten salt reactor designs based on liquid fluoride-salt fuels with breeding of thorium into uranium-233 in the thermal spectrum. In 2011, Sorensen founded Flibe Energy, a company that initially intends to develop 20–50 MW LFTR small modular 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).

Thorium Energy Generation Pty. Limited (TEG)

Thorium Energy Generation Pty. Limited (TEG) is an Australian Research and Development company dedicated to the worldwide commercial development of LFTR reactors, as well as thorium accelerator-driven systems. In November 2011, TEG announced the formation of a joint venture with Czech Republic scientists intended to develop a 60MW pilot plant in Prague, with preparatory work on the prototype to be finalized in 2012.

The Weinberg Foundation

The Weinberg Foundation is a British non-profit organisation founded in 2011, dedicated to act as a communications, debate and lobbying hub to raise awareness about the potential of thorium energy and LFTR. It was formally launched at the House of Lords on 8 September 2011. It is named after American nuclear physicist Alvin M. Weinberg, who pioneered the thorium molten salt reactor research.

See also

• Generation IV reactor
• List of small nuclear reactor designs
• Small modular reactor
• Thorium Energy Alliance

Further reading

• Hargraves, Robert (2009). Aim High!: Thorium energy cheaper than from coal solves more than just global warming. BookSurge Publishing. ISBN 1-4392-2538-9. 
• Martin, Richard (2012). SuperFuel: Thorium, the Green Energy Source for the Future. Palgrave Macmillan. ISBN 0-230-11647-7. 
• Cooper, N.; Minakata, D.; Begovic, M.; Crittenden, J. (2011). "Should We Consider Using Liquid Fluoride Thorium Reactors for Power Generation?". Environmental Science & Technology 45 (15): 6237. doi:10.1021/es2021318. 

External links

• Uranium Is So Last Century — Enter Thorium, the New Green Nuke Wired Magazine article
• Is Thorium the Biggest Energy Breakthrough Since Fire? Possibly. Forbes article

Advocacy

• Energy from Thorium — Web site about LFTR with a blog, ORNL molten salt reactor program reports, research papers repository and discussion forum
• Thorium Energy Alliance — advocacy and educational organisation dedicated to thorium energy
• International Thorium Energy Organisation
• International Thorium Molten-Salt Forum
• ThoriumMSR — comprehensive website and blog about thorium molten salt reactor technology
• The Weinberg Foundation website

Videos

• TEDxYYC — Kirk Sorensen — Thorium Sorensen's presentation about LFTR at TEDxYYC 2011
• Liquid Fluoride Thorium Reactor: What Fusion Wanted To Be Google TechTalk by Dr. Joe Bonometti NASA / Naval Postgraduate School
• The Thorium Molten-Salt Reactor: Why Didn't This Happen Google TechTalk by Kirk Sorensen examining the history of thorium molten salt reactor development at Oak Ridge, political climate and reasons responsible for the cancellation of the program
• Kirk Sorensen — A Global Alternative @ TEAC4 Kirk Sorensen's presentation at Thorium Energy Alliance Conference #4 in Chicago.