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Asodium-cooled fast reactor (SFR) is afast neutron reactor cooled by liquidsodium.

The initialsSFR in particular refer to twoGeneration IV reactor proposals, one based on existingliquid metal cooled reactor (LMFR) technology usingmixed oxide fuel (MOX), and one based on the metal-fueledintegral fast reactor.

CurrentlyChina,Russia andIndia have operational sodium-cooled fast reactors (see the list of reactors).^[1]

In 2020, Natrium received an $80M grant from theUS Department of Energy for development of its SFR. The program plans to useHigh-Assay, Low Enriched Uranium fuel containing 5-20% uranium. The reactor was expected to be sited underground and have gravity-inserted control rods. Because it operates at atmospheric pressure, a large containment shield is not necessary. Because of its large heat storage capacity, it was expected to be able to produce surge power of 500 MWe for 5+ hours, beyond its continuous power of 345 MWe.^[2]

In the United States,TerraPower (using its Traveling Wave technology) is building its own reactor along with molten salt energy storage in partnership with GEHitachi's PRISM integral fast reactor design, under theNatrium appellation inKemmerer, Wyoming.^[3][4][5][6]

Non-nuclear construction began in 2024, while the work on the nuclear island is expected to begin in 2026 (after the application is approved by the US Nuclear Regulatory Commission).^[7][8]

In 2023,ARC Clean Technology Canada signed amemorandum of understanding with theGovernment of Alberta according to which Invest Alberta entity will support ARC'sARC-100 sodium-cooled 100 MWe reactor (based onExperimental Breeder Reactor II). ARC said thatARC-100 could become operational in 2029.ARC-100 project is a pool type reactor.^[9]

Thenuclear fuel cycle employs a fullactinide recycle with two major options: One is an intermediate-size (150–600 MWe) sodium-cooled reactor withuranium-plutonium-minor-actinide-zirconium metal alloy fuel, supported by a fuel cycle based onpyrometallurgical reprocessing in facilities integrated with the reactor. The second is a medium to large (500–1,500 MWe) sodium-cooled reactor with mixed uranium-plutonium oxide fuel, supported by a fuel cycle based upon advanced aqueous processing at a central location serving multiple reactors. The outlet temperature is approximately 510–550 degrees C for both.

Liquid metallic sodium may be used to carry heat from the core.Sodium has only one stable isotope,sodium-23, which is a weak neutron absorber. When it does absorb a neutron it producessodium-24, which has a half-life of 15 hours and decays to stable isotopemagnesium-24.

The two main design approaches to sodium-cooled reactors are pool type and loop type.

In the pool type, the primary coolant is contained in the main reactor vessel, which therefore includes the reactor core and aheat exchanger. The USEBR-2, FrenchPhénix and others used this approach, and it is used by India'sPrototype Fast Breeder Reactor and China'sCFR-600.

In the loop type, the heat exchangers are outside the reactor tank. The FrenchRapsodie, BritishPrototype Fast Reactor and others used this approach.

All fast reactors have several advantages over the current fleet of water based reactors in that the waste streams are significantly reduced. Crucially, when a reactor runs on fast neutrons, the plutonium isotopes are far more likely to fission upon absorbing a neutron. Thus, fast neutrons have a smaller chance of being captured by the uranium and plutonium, but when they are captured, have a much bigger chance of causing a fission. This means that the inventory oftransuranic waste is non existent from fast reactors.

The primary advantage of liquid metal coolants, such as liquid sodium, is that metal atoms are weakneutron moderators. Water is a much strongerneutron moderator because the hydrogen atoms found inwater are much lighter than metal atoms, and therefore neutrons lose more energy incollisions with hydrogen atoms. This makes it difficult to use water as a coolant for a fast reactor because the water tends to slow (moderate) the fast neutrons into thermal neutrons (although concepts forreduced moderation water reactors exist).

Another advantage of liquid sodium coolant is that sodium melts at 371K (98°C) and boils / vaporizes at 1156K (883°C), a difference of 785K (785°C) between solid / frozen and gas / vapor states. By comparison, the liquid temperature range of water (between ice and gas) is just 100K at normal, sea-level atmospheric pressure conditions. Despite sodium's low specific heat (as compared to water), this enables the absorption of significant heat in the liquid phase, while maintaining large safety margins. Moreover, the high thermal conductivity of sodium effectively creates a reservoir ofheat capacity that provides thermal inertia against overheating.^[10] Sodium need not be pressurized since itsboiling point is much higher than the reactor'soperating temperature, and sodium does not corrode steel reactor parts, and in fact, protects metals from corrosion.^[10] The high temperatures reached by the coolant (the Phénix reactor outlet temperature was 833K (560°C)) permit a higherthermodynamic efficiency than in water cooled reactors.^[11] The electrically conductive molten sodium can be moved byelectromagnetic pumps.^[11]

The fact that the sodium is not pressurized implies that a much thinner reactor vessel can be used (e.g. 2 cm thick). Combined with the much higher temperatures achieved in the reactor, this means that the reactor in shutdown mode can be passively cooled. For example, air ducts can be engineered so that all thedecay heat after shutdown is removed by natural convection, and no pumping action is required. Reactors of this type are self-controlling. If the temperature of the core increases, the core will expand slightly, which means that more neutrons will escape the core, slowing down the reaction.

A disadvantage of sodium is its chemical reactivity, which requires special precautions to prevent and suppress fires. If sodium comes into contact with water it reacts to produce sodium hydroxide and hydrogen, and the hydrogen burns in contact with air. This was the case at theMonju Nuclear Power Plant in a 1995 accident. In addition, neutron capture causes it to become radioactive; albeit with a half-life of only 15 hours.^[10]

Another problem is leaks. Sodium at high temperatures ignites in contact with oxygen. Such sodium fires can be extinguished by powder, or by replacing the air withnitrogen. A Russian breeder reactor, the BN-600, reported 27 sodium leaks in a 17-year period, 14 of which led to sodium fires.^[12]

The operating temperature must not exceed the fuel's boiling temperature. Fuel-to-cladding chemical interaction (FCCI) has to be accommodated. FCCI iseutectic melting between the fuel and the cladding; uranium, plutonium, andlanthanum (afission product) inter-diffuse with the iron of the cladding. The alloy that forms has a low eutectic melting temperature. FCCI causes the cladding to reduce in strength and even rupture. The amount of transuranic transmutation is limited by the production of plutonium from uranium. One work-around is to have an inert matrix, using, e.g.,magnesium oxide. Magnesium oxide has an order of magnitude lower probability of interacting with neutrons (thermal and fast) than elements such as iron.^[18]

High-level wastes and, in particular, management of plutonium and other actinides must be handled. Safety features include a long thermal response time, a large margin to coolant boiling, a primary cooling system that operates near atmospheric pressure, and an intermediate sodium system between the radioactive sodium in the primary system and the water and steam in the power plant. Innovations can reduce capital cost, such as modular designs, removing a primary loop, integrating the pump and intermediate heat exchanger, and better materials.^[19]

The SFR's fast spectrum makes it possible to use available fissile and fertile materials (includingdepleted uranium) considerably more efficiently than thermal spectrum reactors with once-through fuel cycles.

Model	Country	Thermal power (MW)	Electric power (MW)	Year of commission	Year of decommission	Notes
BN-350	Soviet Union		350	1973	1999	BN-350 used to power a water de-salination plant.
BN-600	Soviet Union		600	1980	Operational	Expected to operate until 2040[20][21]
BN-800	Russia	2100	880	2015	Operational
BN-1200M	Russia	2900	1220			Preparation stage for construction
CEFR	China	65	20	2012	Operational
CFR-600	China	1500	600	2023	Under construction	Two reactors being constructed on Changbiao Island inXiapu County. The second CFR-600 reactor will open in 2026.[22]
CFR-1000	China		1200	After 2030 (est.)		Awaiting approval for construction[23][24]
CRBRP	United States	1000	350	Never built
EBR-1	United States	1.4	0.2	1950	1964
EBR-2	United States	62.5	20	1965	1994
Fermi 1	United States	200	69	1963	1975
Fast Flux Test Facility	United States	400		1978	1993	Not for power generation
PFR	United Kingdom	500	250	1974	1994
FBTR	India	40	13.2	1985	Operational
PFBR	India		500	2025 (est.)		Under commissioning
FBR-600	India		600	2025 (est.)		Under commissioning
Monju	Japan	714	280	1995/2010	2010	Suspended for 15 years. Reactivated in 2010, then permanently closed
Jōyō	Japan	150		1971	Under repair	Expected to be restarted at the end of 2026[25][26]
SNR-300	Germany		327	1985	1991	Never critical/operational
Rapsodie	France	40	24	1967	1983
Phénix	France	590	250	1973	2010
Superphénix	France	3000	1242	1986	1997	Largest SFR ever built.
ASTRID	France		600	Never built		2012–2019 €735 million spent

• Idaho National Laboratory Sodium-cooled Fast Reactor Fact Sheet
• Generation IV International Forum SFR website
• INL SFR workshop summary
• ALMR/PRISM
• ASME
• Richardson JH (November 17, 2009)."Meet the Man Who Could End Global Warming".Esquire. Archived fromthe original on November 21, 2009.... Eric Loewen is the evangelist of the sodium fast reactor, which burns nuclear waste, emits no CO₂, ...

• Liquid metal fast reactors
• Radioactive waste

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