The Stable Salt Reactor (SSR) is a nuclear reactor design under development by Moltex Energy Canada Inc.[1] and its subsidiary Moltex Energy USA LLC, based in Canada, the United States, and the United Kingdom, as well as MoltexFLEX Ltd., based in the United Kingdom.
SSRs, which are protected by robust patents, are being designed so that they will not need expensive containment structures and components to mitigate radioactive releases in accident scenarios. The design would preclude the type of widespread radiological contamination that occurred in the Chernobyl or Fukushima accidents, because any hazardous isotopes that might otherwise become airborne would be chemically bound to the coolant.[2] Additionally, the modular design would allow factory production of components and delivery to site by standard road transportation, reducing costs and construction timescales.
The fuel design is a hybrid between light water reactor fuel assemblies and traditional molten salt reactor approaches, in which the fuel is mixed with the coolant. The liquid salt fuel mixture is contained within fuel assemblies that are very similar to current light water reactor technology. The fuel assemblies are then submerged in a pool of liquid salt coolant.
Moltex Energy Canada Inc. plans to deploy the SSR-W and associated waste recycling facility in New Brunswick, Canada in partnership with NB Power.[3] The company has support and funding from the Canadian federal government,[4] the government of New Brunswick,[5]NB Power, Ontario Power Generation,[6]ARPA-E,[7][8]IDOM,[9]SNC Lavalin.[10]
Technology
Single fuel tube showing 'diving bell' gas vent and an entire assembly.
The basic unit of the reactor core is the fuel assembly. In the SSR-W, each assembly contains nearly 300 fuel tubes of 10 mm diameter, filled to a height of 1.8 m with fuel salt. The tubes have “diving bell” gas vents at the top to allow fission gases to escape. The assemblies are loaded vertically into the core, with fresh assemblies entering through an airlock and inserted into the core through a fuelling machine.
Fuel and materials
The fuel in the SSR is two-thirds sodium chloride (table salt) and one-third mixed lanthanide/actinide trichlorides. Fuel for the initial reactors is planned to come from converted spent nuclear fuel from existing conventional reactors. In the UK, the fuel could come from the stocks of civil plutonium dioxide from PUREX downblended and converted to chloride impurities added to reduce any proliferation concerns.
Trichlorides are more thermodynamically stable than the corresponding fluoride salts, and can therefore be maintained in a strongly reducing state by contact with sacrificial nuclear-grade zirconium metal added as a coating on, or an insert within, the fuel tube of the SSR-W. As a result, using this patented approach, the fuel tube can be made from standard nuclear certified steel without risk of corrosion. Since the reactor operates in the fast spectrum, the tubes will be exposed to very high neutron flux and so will suffer high levels of radiation damage estimated at 100–200 dpa over the tube life. The highly neutron damage tolerant steel, PE16 will therefore be used for the tubes. Other steels with fast-neutron tolerance (such as T9, NF616 and 15-15Ti) could also be used depending on the local supply chain capabilities.
The average power density in the SSR-W fuel salt is 150 kW/L, which allows a large temperature margin below the boiling point of the salt.[11]
Coolants
The coolant salt in the SSR-W reactor tank is a chloride-based coolant salt. The coolant also contains an agent to reduce its redox potential, making it virtually non-corrosive to standard types of steel. The reactor tank, support structures and heat exchangers can therefore be built with standard 316L stainless steel.
The coolant salt is circulated through the reactor core by three pumps attached to the heat exchangers in each module. Flow rates are modest (approximately 1 m/s) with a resulting low requirement for pump power. Redundant engineering would allow operation to continue in the event of a pump failure.
Safety
SSRs are designed with intrinsic safety characteristics being the first line of defence. No operator or active system is required to maintain the reactor in a safe and stable state. The following are primary intrinsic safety features of the SSR.
Reactivity control
As the SSR is self-controlling, no mechanical control is required. This is made possible by the combination of a high negative temperature coefficient of reactivity and the ability to continually extract heat from the fuel tubes. As heat is taken out of the system the temperature drops, causing the reactivity to go up. Conversely, when the reactor heats up, reactivity decreases. This provides security against all overpower scenarios, such as a reactivity insertion accident. For the SSR-W, diverse and redundant safety is also provided by an array of gravitationally driven boron carbide control rods.[12]
Non-volatile radioactive material
Use of molten salt fuel with the appropriate chemistry eliminates the hazardous volatile iodine and caesium, making multi-layered containment unnecessary to prevent airborne radioactive plumes in severe accident scenarios. For the SSR-W, the noble gasesxenon and krypton would leave the reactor core in normal operation, but would be trapped until their radioactive isotopes decay, so there would be very little that could be released in an accident.[2]
No high pressures
In a water-cooled reactor, high internal pressures provide a driving force for dispersion of radioactive materials in the event of an accident. In contrast, molten salt fuels and coolants have boiling points far above the SSR's operating temperature. So, its core runs at atmospheric pressure. Physical separation of the steam-generating system from the radioactive core, by means of a secondary coolant loop, eliminates high pressure within the reactor. High pressures within fuel tubes are also avoided by venting off fission gases into the surrounding coolant salt.
Immediately after a nuclear reactor shuts down, almost 7% of its previous operating power continues to be generated, from the decay of short half-life fission products. In conventional reactors, removing this decay heat passively is challenging because of the reactors’ low temperatures. An SSR operates at much higher temperatures; so, this heat can be rapidly transferred away from the core. In the event of a reactor shutdown and failure of all active heat-removal systems in the SSR, decay heat from the core would dissipate into air-cooling ducts around the perimeter of the tank that operate continually. This is known as the Emergency Heat Removal System. The main heat-transfer mechanism is radiative. Heat transfer goes up substantially with temperature; so, it is negligible at operating temperatures but sufficient at higher-temperature accident conditions. The reactor components are not damaged during this process and the plant can be restarted afterwards.
Consumption of nuclear waste
Most countries that use nuclear power plan to store spent nuclear fuel deep underground until its radioactivity has reduced to levels similar to that of natural uranium. As the SSR-W consumes nuclear waste, the countries could use them to reduce the volume of waste that ends up in long-term storage.
Operating in the fast spectrum, the SSR-W is effective at transmuting long-lived actinides into more stable isotopes. Today’s reactors that are fuelled by reprocessed spent fuel need very high-purity plutonium to form a stable pellet. The SSR-W can have any level of lanthanide and actinide contamination in its fuel, so long as it can still go critical. This low level of purity greatly simplifies the recycling method for existing waste.
The well established recycling method is based on pyroprocessing. A 2016 report by the Canadian Nuclear Laboratories on recycling of CANDU fuel estimates that pyroprocessing would cost about half as much as more conventional reprocessing. Pyroprocessing for the SSR-W uses only one third of the steps of conventional pyroprocessing, which will make it even cheaper. It is potentially competitive with the cost of manufacturing fresh fuel from mined uranium.
Waste from the SSR-W will take the form of solid salt in tubes. This can be vitrified and stored underground for over 100,000 years, as is planned today, or it can be recycled. In that case, fission products would be separated out and safely stored at ground level for the several hundred years needed for them to decay to radioactivity levels similar to that of uranium ore. The troublesome long-lived actinides and the remaining fuel would go back into the reactor, where they could be burned and transmuted into more-stable isotopes.
Other stable salt reactor designs
Stable salt reactor technology is highly flexible and can be adapted to several different reactor designs. The use of molten salt fuel in standard fuel assemblies allows stable salt versions of many of the large variety of nuclear reactors considered for development worldwide. The industry’s focus today however is to allow rapid development and roll out of low-cost reactors.
Another design now in development, by MoltexFLEX Ltd., is the FLEX reactor, a thermal spectrum SSR fuelled by low-enriched uranium (around 6%). The FLEX reactor may be more suited to nations without an existing nuclear fleet and concerns about waste. It is moderated with graphite as part of the fuel assembly and has significant peaking plant capabilities.
Moltex Energy Canada Inc., Moltex Energy USA LLC and MoltexFLEX Ltd. have also conceptualized a thorium breeding version of the SSR (SSR-Th). This reactor would contain thorium in the coolant salt, which could breed new fuel. Thorium is an abundant fuel source that can provide energy security to nations that do not have their own uranium reserves.
With this range of reactor options and the large global reserves of uranium and thorium available, SSRs could fuel the planet for several thousands of years.
Economics
The capital cost of the SSR-W was estimated at $1,950/kW USD by an independent UK nuclear engineering firm.[13] For comparison, the capital cost of a modern pulverized coal power station in the United States is $3,250/kW and the cost of large-scale nuclear is $5,500/kW.[14] Further reductions to this cost are expected for modular factory-based construction.
This low capital cost results in a levelised cost of electricity (LCOE) of $44.64/MWh USD with substantial potential for further reductions, because of the greater simplicity and intrinsic safety of the SSR.[15]
Given the pre-commercial nature of the technology, the figures for capital cost and LCOE are estimates, and may increase or decrease during completion of the development and licensing processes.
The International Energy Agency predicts that nuclear power will maintain a constant small role in the global energy supply, with a market opportunity of 219 GWe up to 2040. With the improved economics of the SSR, Moltex Energy predicts that it has the potential to access a market of over 1,300 GWe by 2040.
Development
The fundamental patent on the use of unpumped molten salt fuel was granted to Moltex Energy Ltd in 2014,[16] and further implementation-related patents have been applied for and granted since.
The SSR-W has completed Vendor Design Review Phase 1[17] review with the Canadian Nuclear Safety Commission. Both the US[18][19] and Canadian[20][21] governments are supporting development of elements of the SSR technology.
Moltex Energy Canada Inc. plans to build, by the early 2030s, a demonstration SSR-W at the Point Lepreau nuclear power plant site in Canada under an agreement signed with NB Power.[22]
Recognition
As well as the selection for development support by the US and Canadian governments noted above, the SSR has been identified as a leading SMR technology by a 2020 Tractebel analysis,[23] and the SSR-W was selected as one of two SMR candidates for further progression by NB Power, out of a field of 90 candidates.[24] It was also selected as part of the UK government's Phase 1 Advanced Modular Reactor competition but was not selected for the Phase 2 part of the funding.[25]
Related Research Articles
A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat from nuclear fission is passed to a working fluid, which in turn runs through steam turbines. These either drive a ship's propellers or turn electrical generators' shafts. Nuclear generated steam in principle can be used for industrial process heat or for district heating. Some reactors are used to produce isotopes for medical and industrial use, or for production of weapons-grade plutonium. As of 2022, the International Atomic Energy Agency reports there are 422 nuclear power reactors and 223 nuclear research reactors in operation around the world.
A pressurized water reactor (PWR) is a type of light-water nuclear reactor. PWRs constitute the large majority of the world's nuclear power plants. In a PWR, the primary coolant (water) is pumped under high pressure to the reactor core where it is heated by the energy released by the fission of atoms. The heated, high pressure water then flows to a steam generator, where it transfers its thermal energy to lower pressure water of a secondary system where steam is generated. The steam then drives turbines, which spin an electric generator. In contrast to a boiling water reactor (BWR), pressure in the primary coolant loop prevents the water from boiling within the reactor. All light-water reactors use ordinary water as both coolant and neutron moderator. Most use anywhere from two to four vertically mounted steam generators; VVER reactors use horizontal steam generators.
The nuclear fuel cycle, also called nuclear fuel chain, is the progression of nuclear fuel through a series of differing stages. It consists of steps in the front end, which are the preparation of the fuel, steps in the service period in which the fuel is used during reactor operation, and steps in the back end, which are necessary to safely manage, contain, and either reprocess or dispose of spent nuclear fuel. If spent fuel is not reprocessed, the fuel cycle is referred to as an open fuel cycle ; if the spent fuel is reprocessed, it is referred to as a closed fuel cycle.
Nuclear reprocessing is the chemical separation of fission products and actinides from spent nuclear fuel. Originally, reprocessing was used solely to extract plutonium for producing nuclear weapons. With commercialization of nuclear power, the reprocessed plutonium was recycled back into MOX nuclear fuel for thermal reactors. The reprocessed uranium, also known as the spent fuel material, can in principle also be re-used as fuel, but that is only economical when uranium supply is low and prices are high. Nuclear reprocessing may extend beyond fuel and include the reprocessing of other nuclear reactor material, such as Zircaloy cladding.
A breeder reactor is a nuclear reactor that generates more fissile material than it consumes. These reactors can be fuelled with more commonly available isotopes of uranium and thorium, such as uranium-238 or thorium-232, as opposed to the rare uranium-235 which is used in conventional reactors. These materials are called fertile materials since they can be bred into fuel by these breeder reactors.
A fast-neutron reactor (FNR) or fast-spectrum reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons, as opposed to slow thermal neutrons used in thermal-neutron reactors. Such a fast reactor needs no neutron moderator, but requires fuel that is relatively rich in fissile material when compared to that required for a thermal-neutron reactor. Around 20 land based fast reactors have been built, accumulating over 400 reactor years of operation globally. The largest of this was the Superphénix Sodium cooled fast reactor in France that was designed to deliver 1,242 MWe. Fast reactors have been intensely studied since the 1950s, as they provide certain decisive advantages over the existing fleet of water cooled and water moderated reactors. These are:
The integral fast reactor is a design for a nuclear reactor using fast neutrons and no neutron moderator. IFR would breed more fuel and is distinguished by a nuclear fuel cycle that uses reprocessing via electrorefining at the reactor site.
A molten salt reactor (MSR) is a class of nuclear fission reactor in which the primary nuclear reactor coolant and/or the fuel is a molten salt mixture. Only two MSRs have ever operated, both research reactors in the United States. The 1950's Aircraft Reactor Experiment was primarily motivated by the compact size that the technique offers, while the 1960's Molten-Salt Reactor Experiment aimed to prove the concept of a nuclear power plant which implements a thorium fuel cycle in a breeder reactor. Increased research into Generation IV reactor designs began to renew interest in the technology, with multiple nations having projects, and as of September 2021, China is on the verge of starting its TMSR-LF1 thorium MSR.
Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission.
The supercritical water reactor (SCWR) is a concept Generation IV reactor, designed as a light water reactor (LWR) that operates at supercritical pressure. The term critical in this context refers to the critical point of water, and must not be confused with the concept of criticality of the nuclear reactor.
Generation IV reactors are nuclear reactor designs that are envisioned as successors of generation III reactors. The Generation IV International Forum (GIF) is an international organization that coordinates the development of generation IV reactors.
A high-temperature gas-cooled reactor (HTGR), is a nuclear reactor that uses a graphite moderator with a once-through uranium fuel cycle. The HTGR is a type of high-temperature reactor (HTR) that can conceptually have an outlet temperature of 750 °C (1,380 °F). The reactor core can be either a "prismatic block" or a "pebble-bed" core. The high temperatures enable applications such as process heat or hydrogen production via the thermochemical sulfur–iodine cycle.
A sodium-cooled fast reactor is a fast neutron reactor cooled by liquid sodium.
The Molten-Salt Reactor Experiment (MSRE) was an experimental molten salt reactor research reactor at the Oak Ridge National Laboratory (ORNL). This technology was researched through the 1960s, the reactor was constructed by 1964, it went critical in 1965, and was operated until 1969. The costs of a cleanup project were estimated at about $130 million.
The liquid fluoride thorium reactor is a type of molten salt reactor. LFTRs use the thorium fuel cycle with a fluoride-based molten (liquid) salt for fuel. In a typical design, the liquid 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 to a steam turbine or closed-cycle gas turbine.
TerraPower is an American nuclear reactor design and development engineering company headquartered in Bellevue, Washington. TerraPower is developing a class of nuclear fast reactors termed traveling wave reactors (TWR).
Small modular reactors (SMRs) are a proposed class of nuclear fission reactors, smaller than conventional nuclear reactors, which can be built in one location, then shipped, commissioned, and operated at a separate site. The term SMR refers to the size, capacity and modular construction only, not to the reactor type and the nuclear process which is applied. Designs range from scaled down versions of existing designs to generation IV designs. Both thermal-neutron reactors and fast-neutron reactors have been proposed, along with molten salt and gas cooled reactor models.
The Integral Molten Salt Reactor (IMSR) is a nuclear power plant design targeted at developing a commercial product for the small modular reactor (SMR) market. It employs molten salt reactor technology which is being developed by the Canadian company Terrestrial Energy. It is based closely on the denatured molten salt reactor (DMSR), a reactor design from Oak Ridge National Laboratory. It also incorporates elements found in the SmAHTR, a later design from the same laboratory. The IMSR belongs to the DMSR class of molten salt reactors (MSR) and hence is a "burner" reactor that employs a liquid fuel rather than a conventional solid fuel; this liquid contains the nuclear fuel and also serves as primary coolant.
Moltex Energy Limited is a 4th generation nuclear reactor technology company. Started in Stratford-upon-avon in 2014 they have received funding for a molten salt nuclear reactor called the Stable salt reactor which is being developed by their Canadian sister company Moltex Energy Canada Inc.
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