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Molten Salt Reactors

(Updated 22 August 2015)

  • Molten Salt Reactors (MSR) operated in the 1960s.
  • They are seen as a promising technology today principally as a thorium fuel cycle prospect or for using spent LWR fuel.
  • A variety of designs is being developed, many as fast neutron types.
  • Global research is currently led by China.
  • Some have solid fuel similar to HTR fuel.

MSRs use molten fluoride salts as primary coolant, at low pressure. This itself is not a radical departure when the fuel is solid and fixed. But extending the concept to dissolving the fissile and fertile fuel in the salt certainly represents a leap in lateral thinking relative to nearly every reactor operated so far. However, the concept is not new, as outlined below. MSRs may operate with epithermal or fast neutron spectrums, and with a variety of fuels. Much of the interest today in reviving the MSR concept relates to using thorium (to breed fissile uranium-233). There are a number of different MSR design concepts, and a number of interesting challenges in the commercialisation of many, especially with thorium.

The salts concerned as primary coolant, mostly lithium-beryllium fluoride and lithium fluoride, remain liquid without pressurization from about 500°C up to about 1400°C, in marked contrast to a PWR which operates at about 315°C under 150 atmospheres pressure. The ultimate MSR concept is to have the fuel dissolved in the coolant as fuel salt. Intermediate designs and the AHTR have fuel particles in solid graphite and have less potential for thorium use.

Background

During the 1960s, the USA developed the molten salt breeder reactor concept at the Oak Ridge National Laboratory, Tennessee (built as part of the wartime Manhattan Project). It was the primary back-up option for the fast breeder reactor (cooled by liquid metal) and a small prototype 8 MWt Molten Salt Reactor Experiment (MSRE) operated at Oak Ridge over four years to 1969 (the MSR programme ran 1957-1976). In the first campaign (1965-68), uranium-235 tetrafluoride (UF4) enriched to 33% was dissolved in molten lithium, beryllium and zirconium fluorides at 600-700°C which flowed through a graphite moderator at ambient pressure. The fuel comprised about one percent of the fluid. 

The coolant salt in a secondary circuit was lithium + beryllium fluoride (FLiBe).* There was no breeding blanket, this being omitted for simplicity in favour of neutron measurements.

* Fuel salt melting point 434°C, coolant salt melting point 455°C. See Wong & Merrill 2004 reference.

The original objectives of the MSRE were achieved by March 1965, and the U-235 campaign concluded. A second campaign (1968-69) used U-233 fuel which was then available, making MSRE the first reactor to use U-233. This program prepared the way for building a MSR breeder utilising thorium.

The R&D program demonstrated the feasibility of this system, including online reprocessing, and highlighted some unique corrosion and safety issues that would need to be addressed if constructing a larger pilot MSR. It also showed that breeding required a different design, with a larger blanket loop and two fluids (heterogeneous). Tritium production was a problem (see below re lithium enrichment).

In the UK a large (2.5 GWe) helium-cooled fast spectrum MSR with the fuel dissolved in a molten chloride salt was designed in the 1970s.

There is now renewed interest in the MSR concept in Japan, Russia, China, France and the USA, and one of the six Generation IV designs selected for further development is the MSR in two distinct variants, the molten salt fast reactor (MSFR) and the Advanced High Temperature Reactor (AHTR) – also known as the fluoride salt-cooled high-temperature reactor (FHR) with solid fuel, or PB-FHR specifically with pebble fuel.

Since the 2002 Generation IV selection process, significant changes in design philosophy have taken place, according to a 2015 report by Energy Process Developments Ltd (EPD). The first is to design simpler, less ambitious, molten salt reactors that do not breed new fuel, do not require online fuel reprocessing and which use the well-established enriched uranium fuel cycle. In this regard, both American researchers and the China Academy of Sciences/SINAP are working on solid fuel, salt-cooled reactor technology as a realistic first step into MSRs. In 2014, as part of an assessment of MSR activity internationally, proposals were made for pilot-scale implementation, where technical readiness was claimed. Six such specific proposals were assessed over 12 months with commissioned expertise from established UK nuclear engineering firms. These proposals were all seen as credible for building a prototype, with one emerging in the EPD report as currently most suitable as a basis for UK MSR development, the Moltex SSR.

Function

In the normal or basic MSR concept, the fuel is a molten mixture of lithium and beryllium fluoride (FLiBe) salts with dissolved low-enriched uranium (U-235 or U-233) fluorides (UF4). The core consists of unclad graphite moderator arranged to allow the flow of salt at about 700°C and at low pressure. Much higher temperatures are possible but not yet tested. Heat is transferred to a secondary salt circuit and thence to steam or process heat. The basic design is not a fast neutron reactor, but with some moderation by the graphite is epithermal (intermediate neutron speed) and breeding ratio is less than 1.

However, this concept, with fuel dissolved in the salt, is further from commercialisation than solid fuel designs, where the ceramic fuel may be set in prisms, plates, or pebbles, or one design with liquid fuel in static tubes.

Considering liquid-fuel MSR designs, thorium can be dissolved with the uranium in a single fluid MSR, known as a homogeneous design. Two-fluid, or heterogeneous MSRs, would have fertile salt containing thorium in a second loop separate from the fuel salt containing fissile uranium or plutonium and could operate as a breeder reactor (MSBR). Here, the U-233 is progressively removed* and transferred to the primary circuit. However, graphite degradation from neutron flux limits the useful life of the reactor core with the fuel and breeding fluids in close juxtaposition, and in the 1960s it was assumed that the entire reactor vessel in the two-fluid design would be replaced after about eight years.**

* e.g. by bubbling fluorine through the salt so that UF6 is formed and removed as a gas. The UF6 is reduced and added to the fuel stream.

** Graphite is used to slow neutrons in epithermal designs, and deteriorates in a high neutron flux environment. The rate of damage increases with temperature, which is a particular problem with MSRs at 700°C.

The fission products dissolve in the fuel salt and are removed continuously in an adjacent on-line reprocessing loop and replaced with fissile uranium, plutonium and other actinides or, potentially, fertile Th-232 or U-238. Xenon is removed rapidly by outgassing, but protactinium-233 is a problem with thorium as a fuel source. (It is an intermediate product in producing U-233 and is a major neutron absorber.) Constant removal of fission products means that a much higher fuel burn-up can be achieved (> 50%) and the removal of fission products means less decay heat to contend with after reactor shut down. Actinides are fully recycled and remain in the reactor until they fission or are converted to higher actinides which do so. Hence fissile plutonium is largely consumed, and contributes significant energy. The high-level waste comprises fission products only, hence has shorter-lived radioactivity.

Compared with solid-fuelled reactors, MSR systems with circulating fuel salt are claimed to have lower fissile inventories*, no radiation damage constraint on fuel burn-up, no requirement to fabricate and handle solid fuel or solid used fuel, and a homogeneous isotopic composition of fuel in the reactor. Actinides are less-readily formed from U-233 than in fuel with atomic mass greater than 235. These and other characteristics may enable MSRs to have unique capabilities and competitive economics for actinide burning and extending fuel resources. Safety is high due to passive cooling up to any size. Also, several designs have freeze plugs so that the primary salt can be drained by gravity into dump tanks configured to prevent criticality.

* In particular, a small inventory of weapons-fissile material (Pu-242 being the dominant Pu isotope remaining), and low fuel use (the French self-breeding variant claims 50kg of thorium and 50kg U-238 per billion kWh).

MSRs have large negative temperature and void coefficients of reactivity, and are designed to shut down due to expansion of the fuel salt as temperature increases beyond design limits. The negative temperature and void reactivity coefficients passively reduce the rate of power increase in the case of an inadvertent control rod withdrawal (technically known as a ‘reactivity insertion’). When tests were made on the MSRE, a control rod was intentionally withdrawn during normal reactor operations at full power (8 MWt) to observe the dynamic response of core power. Such was the rate of fuel salt thermal expansion that reactor power levelled off at 9 MWt without any operator intervention.

The MSR thus has a significant load-following capability where reduced heat abstraction through the boiler tubes leads to increased coolant temperature, or greater heat removal reduces coolant temperature and increases reactivity. Primary reactivity control is using the secondary coolant salt pump or circulation which changes the temperature of the fuel salt in the core, thus altering reactivity due to its strong negative reactivity coefficient. The MSR works at near atmospheric pressure, eliminating the risk of explosive release of volatile radioactive materials.

In the MSBR, the reactor-grade U-233 bred in the secondary circuit needs to be removed, or it will fission there and contaminate that circuit with ‘hot’ fission products. Therefore in practice the protactinium (Pa-233) formed from the thorium needs to be removed before it decays to U-233*, but this process is unproven at any scale. It is relatively easy to remove the U-233 from the Pa-233 by fluorination to UF6 before reducing it to UF4 for adding to the primary fuel salt circuit. However, the U-233 is contaminated with up to 400 ppm U-232 which complicates processing, due to its highly gamma-active decay progeny.

* Th-232 gains a neutron to form Th-233, which soon beta decays (half-life 22 minutes) to protactinium-233. The Pa-233 (half-life of 27 days) decays into U-233. Some U-232 is also formed via Pa-232 along with Th-233, and a decay product of this is very gamma active.

MSRs would normally operate at much higher temperatures than LWRs – up to at least 700°C, and hence have potential for process heat.

Primary and secondary cooling, the fluoride salts

Fluoride salts have very low vapour pressure even at red heat, carry more heat than the same volume of water, have reasonably good heat transfer properties, are not damaged by radiation, do not react violently with air or water, and are inert to some common structural metals. However having the fuel in solution also means that the primary coolant salt becomes radioactive, complicating maintenance procedures, and the chemistry of the salt must be monitored closely to maintain a chemically reduced state to minimise corrosion. Also the beryllium in the salt is toxic, which leads to at least one design avoiding it, though this requires higher temperatures to keep LiF liquid. LiF however can carry a higher concentration of uranium than FLiBe, allowing less enrichment.

Lithium used in the salt must be fairly pure Li-7, since Li-6 produces tritium when fissioned by neutrons. Li-7 has a very small neutron cross-section (0.045 barns). This means that lithium must be enriched beyond its natural 92.5% Li-7 level. Lithium-7 is being produced at least in Russia and possibly China today as a by-product of enriching lithium-6 to produce tritium for thermonuclear weapons. See also Lithium paper.

LiF is exceptionally stable chemically, and the LiF-BeF2 mix ('FLiBe')* is eutectic (at 459°C it has a lower melting point than either ingredient – LiF is about 500°C). It boils at 1430°C. It is favoured in MSR and AHTR primary cooling and when uncontaminated has a low corrosion effect. The three nuclides (Li-7, Be, F) are among the few to have low enough thermal neutron capture cross-sections not to interfere with fission reactions. 

* Approx. 2:1 molar, hence sometimes represented as Li2BeF4

LiF without the toxic beryllium solidifies at about 500°C and boils at about 1200°C. FLiNaK (LiF-NaF-KF) is also eutectic and solidifies at 454°C and boils at 1570°C. It has a higher neutron cross-section than FLiBe or LiF but can be used in intermediate cooling loops. Sodium-beryllium fluoride (BeF2-NaF) solidifying at 385°C is used as fuel salt in one design for cost reasons.

The hot molten salt in the primary circuit can be used with secondary salt circuit or secondary helium coolant generating power via the Brayton cycle as with HTR designs, with potential thermal efficiencies of 48% at 750°C to 59% at 1000°C, or simply with steam generators. In industrial applications molten fluoride salts (possibly simply cryolite – Na-Al fluoride) are a preferred interface fluid in a secondary circuit between the nuclear heat source and any chemical plant. The aluminium smelting industry provides substantial experience in managing them safely.

Most secondary coolant salts do not use lithium, for cost reasons. ZrF4-NaF-KF, ZrF4-KF, NaF-BeF2 eutectic mixes are usual, as well as LiF-NaF-KF.

In the secondary cooling circuit of the AHTR concept, air is compressed, heated, flows through gas turbines producing electricity, enters a steam recovery boiler producing steam that produces additional electricity, and exits to the atmosphere. Added peak power can be produced by injecting natural gas (or hydrogen in the future) after nuclear heating of the compressed air to raise gas temperatures and plant output, giving it rapidly variable output (of great value in grid stability and for peak load demand where renewables have significant input). This is described as an air Brayton combined-cycle (ABCC) system in secondary circuit.

In the 1960s MSRE, an alternative secondary coolant salt considered was 8% NaF + 92% NaF-BeF2 with melting point 385°C, though this would be more corrosive.

A British design contains the fuel salt in vertical tubes and relies on convection to circulate the secondary salt coolant.

MSR research emphasis

American researchers and the China Academy of Sciences/ SINAP are working primarily on solid fuel MSR technology. The main reason is that this is a realistic first step. In China this is focused on thorium-fueled versions (see LFTR/ TMSR below). The technical difficulty of using molten salts is significantly lower when they do not have the very high activity levels associated with them bearing the dissolved fuels and wastes. The experience gained with component design, operation, and maintenance with clean salts makes it much easier then to move on and consider the use of liquid fuels, while gaining several key advantages from the ability to operate reactors at low pressure and deliver higher temperatures.

Russia's Molten Salt Actinide Recycler and Transmuter (MOSART) is a fast reactor fuelled only by transuranic fluorides from uranium and MOX LWR used fuel. It is part of the MARS project (minor actinide recycling in molten salt) involving RIAR, Kurchatov and other research organisations. The 2400 MWt design has a homogeneous core of Li-Na-Be or Li-Be fluorides without graphite moderator and has reduced reprocessing compared with original US design. Thorium may also be used, though it is described as a burner-converter rather than a breeder.

In the Generation IV program for the MSR, collaborative R&D is pursued by interested members under the auspices of a provisional steering committee. There will be a long lead time to prototypes, and the R&D orientation has changed since the project was set up, due to increased interest. It now has two baseline concepts:

  • The Molten Salt Fast Neutron Reactor (MSFR), which will take in thorium fuel cycle, recycling of actinides, closed Th/U fuel cycle with no U enrichment, with enhanced safety and minimal wastes. it is a liquid-fuel design.
  • The Advanced High-Temperature Reactor (AHTR) – also known as the fluoride salt-cooled high-temperature reactor (FHR) – with the same graphite and solid fuel core structures as the VHTR and molten salt as coolant instead of helium, enabling power densities 4 to 6 times greater than HTRs and power levels up to 4000 MWt with passive safety systems. A 5 MWt prototype is under construction at Shanghai Institute of Nuclear Applied Physics (SINAP, under the China Academy of Sciences) with 2015 target for operation.

The GIF 2014 Roadmap said that a lot of work needed to be done on salts before demonstration reactors were operational, and suggested 2025 as the end of the viability R&D phase.

China's dual programme

The China Academy of Sciences in January 2011 launched an R&D program on LFTR, known there as the thorium-breeding molten-salt reactor (Th-MSR or TMSR), and claimed to have the world's largest national effort on it, hoping to obtain full intellectual property rights on the technology. The TMSR Research Centre has a 5 MWe MSR prototype under construction at Shanghai Institute of Applied Physics (SINAP, under the Academy) originally with 2015 target for operation, now 2020.

SINAP has two streams of TMSR development – solid fuel (TRISO in pebbles or prisms/blocks) with once-through fuel cycle, and liquid fuel (dissolved in FLiBe coolant) with reprocessing and recycle. A third stream of fast reactors to consume actinides from LWRs is planned.

  • The TMSR-SF stream has only partial utilization of thorium, relying on some breeding as with U-238, and needing fissile uranium input as well. SINAP aims at a 2 MW pilot plant (TMSR-SF1) initially, and a 100 MWt experimental pebble bed plant (TMSR-SF2) with open fuel cycle by about 2025, then a 1 GW demonstration plant (TMSR-SF3) by 2030. TRISO particles will be with both low-enriched uranium and thorium, separately.
  • The TMSR-LF stream claims full closed Th-U fuel cycle with breeding of U-233 and much better sustainability with thorium but greater technical difficulty. SINAP aims for a 2 MWt pilot plant (TMSR-LF1) by 2018, a 10 MWt experimental reactor (TMSR-LF2) by 2025 and a 100 MWt demonstration plant (TMSR-LF3) with full electrometallurgical reprocessing by 2035, followed by 1 a GW demonstration plant.
  • A TMSFR-LF fast reactor optimized for burning minor actinides is to follow.

SINAP sees molten salt fuel being superior to the TRISO fuel in effectively unlimited burn-up, less waste, and lower fabricating cost, but achieving lower temperatures (600°C+) than the TRISO fuel reactors (1200°C+). Near-term goals include preparing nuclear-grade ThF4 and ThO2 and testing them in a MSR.

The US Department of Energy is collaborating with the China Academy of Sciences on the program, which had a start-up budget of $350 million. The target date for TMSR commercial deployment is 2032.

Other solid- or fixed-fuel types

AHTR / FHR

Research on molten salt coolant has been revived at Oak Ridge National Laboratory (ORNL) in the USA with the Advanced High Temperature Reactor (AHTR). This is a larger reactor using a coated-particle graphite-matrix fuel like that in the GT-MHR (see Small Reactors paper) and with molten fluoride salt as primary coolant. It is also known as the Fluoride High-temperature Reactor (FHR). While similar to the gas-cooled HTR it operates at low pressure (less than 1 atmosphere) and higher temperature, and gives better heat transfer than helium. The FLiBe salt is used solely as coolant, and achieves temperatures of 750-1000°C or more while at low pressure. This could be used in thermochemical hydrogen manufacture.

This could be used in thermochemical hydrogen manufacture. As noted above, a 5 MW thorium-fueled prototype is under construction at Shanghai Institute of Nuclear Applied Physics (SINAP, under the China Academy of Sciences) originally with 2015 target for operation. A 100 MWt demonstration pebble bed plant with open fuel cycle is planned by about 2025. SINAP sees this design having potential for higher temperatures than MSRs with fuel salt.

In the USA a consortium including UC Berkeley, ORNL and Westinghouse is designing a 100 MWe pebble bed FHR, with annular core. It is designed for modular construction, and from 100 MWe base-load is able to deliver 242 MWe with gas co-firing for meeting peak loads. Fuel pebbles are 30 mm diameter, much less than gas-cooled HTRs. A 410 MWe/900 MWt pebble bed version was also being designed with UC-Berkeley.

AHTR reactor sizes of 1500 MWe/3600 MWt are envisaged, with capital costs estimated at less than $1000/kW.

In the secondary cooling circuit, air is compressed, heated, flows through gas turbines producing electricity, enters a steam recovery boiler producing steam that produces additional electricity, and exits to the atmosphere. Added peak power can be produced by injecting natural gas (or hydrogen in the future) after nuclear heating of the compressed air to raise gas temperatures and plant output, giving it rapidly variable output (of great value in grid stability and for peak load demand where renewables have significant input). This is described as an air Brayton combined-cycle (ABCC) system in secondary circuit.

Moltex SSR

Moltex Energy LLP’s Stable Salt Reactor is a conceptual UK design of fast reactor with no pumps (only small impellers in the secondary salt bath) and relies on convection from vertical fuel tubes in the core to convey heat to the integral steam generators. Core temperature is 500-600°C, at atmospheric pressure. Fuel tubes of nickel-chromium alloy three-quarters filled with the molten fuel salt (60% NaCl, 40% Pu, U & lanthanide trichlorides) are grouped into fuel assemblies which are similar to those used in standard reactors. The individual fuel tubes are vented so that fission product gases escape into the coolant salt, which is NaF-KF-ZrF mix (Li-7 fluoride is avoided for cost reasons). The assemblies can be moved laterally without removing them. Refuelling is thus continuous on line, and after five years depleted assemblies are stored at one side of the pool pending reprocessing. The primary fissile fuel is plutonium-239 chloride recovered from LWR fuel, with 12% Pu quoted in one report. Thorium can potentially be used. The SSR can be run with thermal or fast neutron spectrums, though a 150 MWt pilot plant will be fast. Commercial plants of 1000 MWe are envisaged. Overnight capital cost is estimated at about £1400 per kW.

The fuel assemblies are arranged at the centre of a circular tank half filled with the coolant salt which transfers heat away from the fuel assemblies to the peripheral but integral steam generators, essentially by convection. The fuel assemblies are held in place by gravity in fittings set in the base of the tank, which is at atmospheric pressure.

Other liquid-fuel types: two-fluid breeders

Liquid Fluoride Thorium Reactor (LFTR)

The Liquid Fluoride Thorium Reactor (LFTR) is a heterogeneous MSR design which breeds its U-233 fuel from a fertile blanket of lithium-beryllium fluoride (FLiBe) salts with thorium fluoride. The thorium-232 captures neutrons from the reactor core to become protactinium-233, which decays (27-day half-life) to U-233. It may be possible to separate Pa-233 on-line and let it decay to U-233. Otherwise, newly-formed U-233 forms soluble uranium tetrafluoride (UF4), which is converted to gaseous uranium hexafluoride (UF6) by bubbling fluorine gas through the salt (which does not chemically affect the less-reactive thorium tetrafluoride). The volatile uranium hexafluoride is captured, reduced back to soluble UF4 by hydrogen gas, and finally is added to the FLiBe core to serve as fissile fuel. A complication is that traces of U-232 are formed, reporting with the U-233, and having highly gamma-active decay progeny.

LFTRs can rapidly change their power output, and hence be used for load-following. Because they are expected to be inexpensive to build and operate, 100 MWe LFTRs could be used as peak and back-up reserve power units.

Flibe LFTR

Flibe Energy in the USA is studying a 40 MW two-fluid graphite-moderated thermal reactor concept based on the 1970s MSRE. It uses lithium fluoride/beryllium fluoride (FLiBe) salt as its primary coolant in both circuits. This is based on earlier US work on the molten salt reactor programme. Fuel is uranium-233 bred from thorium in FLiBe blanket salt. Fuel salt circulates through graphite logs. Secondary loop coolant salt is sodium-beryllium fluoride (BeF2-NaF). A 2 MWt pilot plant is envisaged, and eventually 2225 MWt commercial plants.

Other liquid-fuel types: single-fluid

Integral MSR

Canada-based Terrestrial Energy Inc (TEI) has designed the Integral MSR. This simplified MSR integrates the primary reactor components, including primary heat exchangers to secondary clean salt circuit, in a sealed and replaceable core vessel that has a projected life of seven years. The IMSR will operate at 600-700°C, which can support many industrial process heat applications. It operates in the thermal neutron spectrum with a hexagonal arrangement of graphite elements forming the moderator. The fuel-salt is a eutectic of low-enriched (2-4%) uranium-235 fuel (as UF4) and a fluoride carrier salt – likely sodium rubidium fluoride with potential to change to FLiBe – at atmospheric pressure. Secondary loop coolant salt is ZrF-KF. Multiple pumps and six heat exchangers allow for redundancy. Emergency cooling and residual heat removal are passive. When the sealed core is replaced after seven years, it is then left for fission products to decay. Each plant would have space for two reactors, allowing seven-year changeover, with the used unit removed for off-site reprocessing when it has cooled.

The IMSR is designed in three sizes: 80 MWth (32.5 MWe), 300 MWth, and 600 MWth. The total levelized cost of electricity from the largest is projected to be competitive with natural gas. The smallest is designed for off-grid, remote power applications, and as a prototype. The company expects to complete CNSC pre-licence review by the end of 2016, and hopes to commission its first commercial reactor by the early 2020s. In January 2015 the company announced a collaborative agreement with US Oak Ridge National Laboratory (ORNL) to advance the design.

Transatomic TAP

Transatomic Power Corp is a new US company partly funded by Founders Fund and aiming to develop a single-fluid MSR using very low-enriched uranium fuel (1.8%) or the entire actinide component of used LWR fuel. The TAP reactor has an efficient zirconium hydride* moderator and a LiF-based fuel salt bearing the UF4 and actinides, hence a very compact core. The secondary coolant is FLiNaK salt (LiF-KF-NaF) to a steam generator.

* as used in TRIGA research reactors and TOPAZ and SNAP reactors for space programme.

Owing to the ZrH moderator, there are significantly more neutrons in the thermal region (less than 1 eV) compared with a graphite moderator, thereby enabling the reactor to generate power from very low-enriched uranium or used LWR fuel. The epithermal (1 eV - 1 MeV) spectrum is lower than that with graphite, but in the fast spectrum (over 1 MeV) the neutron flux is greater than with graphite moderator, and therefore contributes strongly to actinide burning. It would give up to 96% actinide burn-up. Fission products are mostly removed batch-wise and fresh fuel added. In addition to negative void and thermal coefficients, the moderator starts to fail at higher temperatures due to hydrogen loss. Decay heat removal can be by convection.

After a 20 MWt demonstration reactor, the envisaged first commercial plant will be 1250 MWt/550 MWe running at 44% thermal efficiency with 650°C in primary loop, using steam cycle. The overnight cost for an nth-of-a-kind 550 MWe plant, including lithium-7 inventory and on-line fission product removal and storage, is estimated at $2 billion with a three-year construction schedule. A version of the reactor may utilize thorium fuel.

Fuji MSR

The Fuji MSR is a 100-200 MWe graphite-moderated design to operate as a near-breeder and was being developed internationally by a Japanese, Russian and US consortium: the International Thorium Molten Salt Forum (ITMSF). It is based on the Oak Ridge MSBR, and several variants have been designed, including a 10 MWe mini Fuji. Thorium Tech Solutions Inc (TTS) plan to commercialise the Fuji concept, and is working on it with the Halden test reactor in Norway.

Thorcon

Martingale in the USA is designing the ThorCon MSR, which is a 250 MWe scaled-up Oak Ridge MSRE. It is a single-fluid thorium converter reactor in the thermal spectrum, graphite moderated. It uses a combination of U-233 from thorium and U-235 enriched from mined uranium. Fuel salt is sodium-beryllium fluoride (BeF2-NaF) with dissolved uranium and thorium tetrafluorides (Li-7 fluoride is avoided for cost reasons). Secondary loop coolant salt is also sodium-beryllium fluoride. It operates at 700°C. There is no on-line processing – this takes place in a centralized plant at the end of the core life, with off-gassing of some fission products meanwhile. A pilot plant would be similar to the mini Fuji.

Martingale aims for an operating prototype by 2020, with modular construction. Several 550 MWt units would comprise a power station, and a 1000 MWe Thorcon plant would comprise about 200 factory- or shipyard-build modules installed below grade (30 m down). All components are deigned to be easily and frequently replaced. For instance, every four years the entire primary loop would be changed out, returned to a centralized recycling facility, decontaminated, disassembled, inspected, and refurbished. Incipient problems would be rectified and major upgrades could be introduced without significantly disrupting power generation. The company claims generation costs of 3 to 5 c/kWh depending on scale, and is "targeting its first installations in forward-looking countries that support technology-neutral nuclear regulations and see the benefits of the license-by-test process."

Seaborg Waste Burner – SWaB

Seaborg Technologies in Denmark has a thermal-epithermal single fluid reactor design for 50 MWt pilot unit with a view to 250 MWt commercial modular units fuelled by spent LWR fuel and thorium. Fuel salt is Li-7 fluoride with thorium, plutonium and minor actinides as fluorides. This is pumped through the graphite column core and heat exchanger. Fission products are extracted on-line. Secondary coolant salt is FLiNaK, at 700°C. Spent LWR fuel would have the uranium extracted for recycle, leaving Pu and minor actinides to become part of the MSR fuel, with thorium.

References

Ho M.K.M., Yeoh G.H., & Braoudakis G., 2013, Molten Salt Reactors, in Materials and processes for energy: communicating current research and technological developments, ed A.Mendez-Vilas, Formatex Research Centre

Merle-Lucotte, E. et al 2009, Minimising the fissile inventory of the Molten Salt Fast Reactor, Advances in Nuclear Fuel Management IV (ANFM 2009), American Nuclear Soc.

Merle-Lucotte, E. et al 2007, The Thorium molten salt reactor: launching the thorium cycle while closing the current fuel cycle, ENC 2007

Forsberg, C.W., Peterson, P.F., Zhao, H.H. 2004, An advanced molten salt reactor using high-temperature reactor technology, American Nuclear Soc.

LeBlanc, D, 2009, Molten Salt Reactors: a new beginning for an old idea, Nuclear Engineering & Design 2010, Elsevier

Transatomic Power Corp., technical white paper, March 2014

Ignatiev, V & Feynberg, O, Kurchatov Inst, Molten Salt Reactor: overview and perspectives, OECD 2012

Appendix 6.0 Molten Salt Reactor, Generation IV Nuclear Energy Systems Ten-Year Program Plan – Fiscal Year 2007, Department of Energy Office of Nuclear Energy (September 2007)

Hargraves, R & Moir, R, 2010, Liquid Fluoride Thorium Reactors, American Scientist 98

Fluoride-Salt-Cooled High-Temperature Reactors (FHRs) for Base-Load and Peak Electricity, Grid Stabilization, and Process Heat, Forsberg, Hu, Peterson, Sridharan, 2013, MIT

Wong, C & Merrill, B, 2004, Relevant MSRE and MSR Experience, ITER TBM Project Meeting at UCLA, 23-25 February 2004

Energy Process Developments Ltd, July 2015, MSR Review: Feasibility of Developing a Pilot Scale Molten Salt Reactor in the UK, July 2015