![SSR-W-cutaway-no-labels-1464x2048](https://upload.wikimedia.org/wikipedia/commons/2/2d/SSR-W-cutaway-no-labels-1464x2048.png)
The Stable Salt Reactor (SSR) is a nuclear reactor design under development by Moltex Energy Canada Inc. and its subsidiary Moltex Energy USA LLC, based in
Canada
Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...
, the
United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...
, and the
United Kingdom
The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and North ...
, as well as MoltexFLEX Ltd., based in the United Kingdom.
The SSR design being developed by Moltex Energy Canada Inc. is the Stable Salt Reactor - Wasteburner (SSR-W), which incorporates elements of the
molten salt reactor
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 Ai ...
, and aims to have improved safety characteristics (
intrinsically safe
Intrinsic safety (IS) is a protection technique for safe operation of electrical equipment in hazardous areas by limiting the energy, electrical and thermal, available for ignition. In signal and control circuits that can operate with low curre ...
) and economics (
LCOE of $45/MWh USD or less) over traditional
light water reactors
The light-water reactor (LWR) is a type of thermal-neutron reactor that uses normal water, as opposed to heavy water, as both its coolant and neutron moderator; furthermore a solid form of fissile elements is used as fuel. Thermal-neutron reac ...
.
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
Chernobyl ( , ; russian: Чернобыль, ) or Chornobyl ( uk, Чорнобиль, ) is a partially abandoned city in the Chernobyl Exclusion Zone, situated in the Vyshhorod Raion of northern Kyiv Oblast, Ukraine. Chernobyl is about no ...
or
Fukushima accidents, because any hazardous isotopes that might otherwise become airborne would be chemically bound to the coolant.
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. The company has support and funding from the Canadian federal government, the government of New Brunswick,
NB Power
New Brunswick Power Corporation (french: Société d’énergie du Nouveau-Brunswick), operating as NB Power (french: Énergie NB), is the primary electric utility in the Canadian province of New Brunswick. NB Power is a vertically-integrated C ...
,
Ontario Power Generation
Ontario Power Generation Inc. (OPG) is a Crown corporations of Canada, Crown corporation and "government business enterprise" that is responsible for approximately half of the electricity generation in the province of Ontario, Canada. It is w ...
,
ARPA-E
ARPA-E, or Advanced Research Projects Agency–Energy is a United States government agency tasked with promoting and funding research and development of advanced energy technologies. It is modeled after the Defense Advanced Research Projects Agenc ...
,
IDOM,
SNC Lavalin
SNC-Lavalin Group Inc. is a Canadian company based in Montreal that provides engineering, procurement, and construction (EPC) services to various industries, including mining and metallurgy, oil and gas, environment and water, infrastructure, an ...
.
Technology
![RSS - crayon & assemblage](https://upload.wikimedia.org/wikipedia/commons/7/7d/RSS_-_crayon_%26_assemblage.jpg)
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
Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g ...
(table salt) and one-third mixed
lanthanide
The lanthanide () or lanthanoid () series of chemical elements comprises the 15 metallic chemical elements with atomic numbers 57–71, from lanthanum through lutetium. These elements, along with the chemically similar elements scandium and yttr ...
/
actinide
The actinide () or actinoid () series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinide series derives its name from the first element in the series, actinium. The inform ...
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
PUREX (plutonium uranium reduction extraction) is a chemical method used to purify fuel for nuclear reactors or nuclear weapons. PUREX is the ''de facto'' standard aqueous nuclear reprocessing method for the recovery of uranium and plutonium fr ...
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
Nuclear may refer to:
Physics
Relating to the nucleus of the atom:
* Nuclear engineering
*Nuclear physics
*Nuclear power
*Nuclear reactor
*Nuclear weapon
*Nuclear medicine
*Radiation therapy
*Nuclear warfare
Mathematics
*Nuclear space
*Nuclear ...
without risk of corrosion. Since the reactor operates in the fast spectrum, the tubes will be exposed to very high
neutron flux
The neutron flux, φ, is a scalar quantity used in nuclear physics and nuclear reactor physics. It is the total length travelled by all free neutrons per unit time and volume. Equivalently, it can be defined as the number of neutrons travelling ...
and so will suffer high levels of
radiation damage
Radiation damage is the effect of ionizing radiation on physical objects including non-living structural materials. It can be either detrimental or beneficial for materials.
Radiobiology is the study of the action of ionizing radiation on living ...
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.
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
Redox potential (also known as oxidation / reduction potential, ''ORP'', ''pe'', ''E_'', or E_) is a measure of the tendency of a chemical species to acquire electrons from or lose electrons to an electrode and thereby be reduced or oxidised respe ...
, 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 Fuel temperature coefficient of reactivity is the change in reactivity of the nuclear fuel per degree change in the fuel temperature. The coefficient quantifies the amount of neutrons that the nuclear fuel (such as uranium-238) absorbs from the fi ...
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.
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 gas
The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low chemi ...
es
xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
and
krypton
Krypton (from grc, κρυπτός, translit=kryptos 'the hidden one') is a chemical element with the symbol Kr and atomic number 36. It is a colorless, odorless, tasteless noble gas that occurs in trace amounts in the atmosphere and is often ...
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.
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.
Low chemical reactivity
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'', ...
in
pressurized water reactor
A pressurized water reactor (PWR) is a type of light-water reactor, light-water nuclear reactor. PWRs constitute the large majority of the world's nuclear power plants (with notable exceptions being the UK, Japan and Canada). In a PWR, the primary ...
s and
sodium
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable iso ...
in
fast reactor
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 (carrying energies above 1 MeV or greater, on average), as opposed t ...
s both create the potential for severe explosion and fire risks. No chemically reactive materials are used in the SSR.
Decay heat removal
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
Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release ...
. In conventional reactors, removing this
decay heat
Decay heat is the heat released as a result of radioactive decay. This heat is produced as an effect of radiation on materials: the energy of the alpha, beta or gamma radiation is converted into the thermal movement of atoms.
Decay heat occurs na ...
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
Pyroprocessing (from Greek Πυρος = ''fire'') is a process in which materials are subjected to high temperatures (typically over 800 °C) in order to bring about a chemical or physical change. Pyroprocessing includes such terms as ore-ro ...
. 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
Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release ...
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
actinide
The actinide () or actinoid () series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinide series derives its name from the first element in the series, actinium. The inform ...
s 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
Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written 235U) has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238 (238U ...
(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
Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large ...
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
Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately soft and malleable and has a high me ...
in the coolant salt, which could breed new fuel. Thorium is an abundant fuel source that can provide
energy security
Energy security is the association between national security and the availability of natural resources for energy consumption. Access to (relatively) cheap energy has become essential to the functioning of modern economies. However, the uneven ...
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
Capital costs are fixed, one-time expenses incurred on the purchase of land, buildings, construction, and equipment used in the production of goods or in the rendering of services. In other words, it is the total cost needed to bring a project to a ...
of the SSR-W was estimated at $1,950/kW USD by an independent UK nuclear engineering firm. 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. Further reductions to this cost are expected for modular factory-based construction.
This low capital cost results in a
levelised cost of electricity
The levelized cost of electricity (LCOE), or levelized cost of energy, is a measure of the average net present cost of electricity generation for a generator over its lifetime. It is used for investment planning and to compare different methods ...
(LCOE) of $44.64/MWh USD with substantial potential for further reductions, because of the greater simplicity and intrinsic safety of the SSR.
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
The International Energy Agency (IEA) is a Paris-based autonomous intergovernmental organisation, established in 1974, that provides policy recommendations, analysis and data on the entire global energy sector, with a recent focus on curbing carb ...
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, and further implementation-related patents have been applied for and granted since.
The SSR-W has completed Vendor Design Review Phase 1 review with the
Canadian Nuclear Safety Commission
The Canadian Nuclear Safety Commission (CNSC; french: Commission Canadienne de sûreté nucléaire) is the federal regulator of nuclear power and materials in Canada.
Mandate and history
Canadian Nuclear Safety Commission was established under t ...
. Both the US and Canadian 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.
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
Tractebel is a Belgian subsidiary which provides consultancy and engineering services in the energy, water, nuclear and infrastructure sectors for the ENGIE group, as well as for national and international institutions and customers in the publ ...
analysis, 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. 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.
References
External links
{{Commonscatinline
Stable Salt Reactor Technology Introduction YouTube video
Modular Stable Salt Reactors– a simpler way to use molten salt fuel – Ian Scott Moltex Energy
Nuclear power reactor types
Nuclear power
Molten salt reactors