Molten salt reactors – nuclear power reactors that use liquid salt as primary coolant or a molten salt mixture as fuel – have many favourable characteristics for nuclear safety and sustainability. The concept was developed in the 1960s, but put aside in favour of what has become mainstream nuclear technology since. In recent years, however, technological advances have led to growing interest in molten salt technology and to the launch of new initiatives. The technology needs at least a decade of further intensive research, validation and qualification before commercialization.
“It is the first time a comprehensive IAEA international meeting on molten salt reactors has ever taken place,” said Stefano Monti, Head of the Nuclear Power Development Section at the IAEA. “Given the interest of Member States, the IAEA could provide a platform for international cooperation and information exchange on the development of these advanced nuclear systems.”
Molten salt reactors operate at higher temperatures, making them more efficient in generating electricity. In addition, their low operating pressure can reduce the risk of coolant loss, which could otherwise result in an accident. Molten salt reactors can run on various types of nuclear fuel and use different fuel cycles. This conserves fuel resources and reduces the volume, radiotoxicity and lifetime of high-level radioactive waste.
Molten salt reactor technology has attracted private funding over the last few years, and several reactor concepts are under development. One area under research is the compatibility between the salt coolant and the structural materials and, for some designs, the chemical processes related to the associated fuel cycle, Monti said.
Molten Salt reactor projects and designs
The IAEA Advanced reactor database lists two MSR at the engineering design phase but there are many other projects listed below.
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 ZrF4-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.
American researchers and the China Academy of Sciences/ SINAP are working primarily on solid fuel MSR technology.
A 5 MWt prototype is under construction at Shanghai Institute of Nuclear Applied Physics (SINAP, under the China Academy of Sciences) with 2020 target for operation.
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
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.
Moltex Energy LLP’s Stable Salt Reactor is a conceptual UK reactor design with no pumps (only small impellers in the secondary salt bath) and relies on convection from static vertical fuel tubes in the core to convey heat to the steam generators. Core temperature is 500-600°C, at atmospheric pressure. Decay heat is removed by natural air convection. 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 and use similar structural materials. The individual fuel tubes are vented so that fission product gases escape into the coolant salt, which is a NaF-KF-ZrF4 mix (Li-7 fluoride is avoided for cost reasons). The assemblies can be moved laterally without removing them. Refuelling is thus continuous online, and after five years depleted assemblies are stored at one side of the pool pending reprocessing.
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.
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 program. 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.
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.
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.
Southern Company Services in the USA is developing a molten chloride fast reactor (MCFR) with TerraPower, Oak Ridge National Laboratory (ORNL) – which hosts the work, the Electric Power Research Institute (EPRI) and Vanderbilt University. No details are available, and it is not certain that it is a single-fluid type. However, fuel is in the chloride salt (see section above) and as a fast reactor it can burn U-238, actinides and thorium as well as used light water reactor fuel, requiring no enrichment apart from initial fuel load (these details from TerraPower, not Southern). In January 2016 the US DOE awarded a Gateway for Accelerated Innovation in Nuclear (GAIN) grant to the project, worth up to $40 million. In August 2016 Southern Nuclear Operating Company signed an agreement to work with X-energy to collaborate on development and commercialization of their respective small reactor designs. With TerraPower and ORNL, X-energy is designing the Xe-100 pebble-bed HTR of 48 MWe.
Russia's Molten Salt Actinide Recycler and Transmuter (MOSART) is a fast reactor fuelled only by transuranic (TRU) 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.
The challenges are not only technical. Nuclear regulators will need to review existing safety regulations to see how these can be modified, if necessary, to fit molten salt reactors, since they differ significantly from reactors in use today, said Stewart Magruder, senior nuclear safety officer at the IAEA.
Participants, including researchers, designers and industry representatives, emphasized the need for an international platform for information exchange.
“While the United States is actively developing both technology and safety regulations for molten salt reactors, the meeting is an important platform to exchange knowledge and information with Member States not engaged in the existing forums,” said David Holcomb from the Oak Ridge National Laboratory one of the 35 participants at the meeting last week. The development of molten salt reactors began with an experiment conducted by the Oak Ridge National Laboratory in the 1960s.
From bilateral to multilateral cooperation
To help speed up research, it is essential to move from bilateral to multilateral cooperation, said Chen Kun from the Shanghai Institute of Applied Physics of the Chinese Academy of Sciences. “It is the first time China has the opportunity to share knowledge with India, Indonesia and Turkey on this technology.”
Indonesia is considering building its first nuclear power plant with molten salt reactor design, said Bob Soelaiman Effendi from Indonesia Thorium Energy Community. “For a developing country like Indonesia, a molten salt reactor’s higher efficiency in electricity generation makes it more economical and affordable than fossil-fuel power plants.”
Molten salt reactors and other advanced nuclear reactors have received increased attention over the last few years as the world is looking for alternative technologies for energy production. Advanced reactors, which could increase the sustainability of nuclear power, are at various stages of development. Some advanced reactors, such as the sodium-cooled fast reactor BN-800 in Russia and the High Temperature Reactor Prototype Module in China are already connected to the grid or are in an advanced stage of construction. Others, such as molten salt reactors, are in the design phase.
SOURCES- IAEA, World Nuclear Association