Sodium-cooled fast reactor


A sodium-cooled fast reactor is a fast neutron reactor cooled by liquid sodium.
The acronym SFR particularly refers to two Generation IV reactor proposals, one based on existing liquid metal cooled reactor technology using mixed oxide fuel, the other based on the metal-fueled integral fast reactor.
Several sodium-cooled fast reactors have been built, some still in operation, and others are in planning or under construction.

Fuel cycle

The nuclear fuel cycle employs a full actinide recycle with two major options: One is an intermediate-size sodium-cooled reactor with uranium-plutonium-minor-actinide-zirconium metal alloy fuel, supported by a fuel cycle based on pyrometallurgical reprocessing in facilities integrated with the reactor. The second is a medium to large sodium-cooled reactor with mixed uranium-plutonium oxide fuel, supported by a fuel cycle based upon advanced aqueous processing at a central location serving a number of reactors. The outlet temperature is approximately 510–550 degrees Celsius for both.

Sodium as a coolant

Liquid metallic sodium may be used as the sole coolant, carrying heat from the core. Sodium has only one stable isotope, sodium-23. Sodium-23 is a very weak absorber of neutrons. When it does absorb a neutron it produces sodium-24, which has a half-life of 15 hours and decays to magnesium-24, a stable isotope.

Advantages

The primary advantage of liquid metal coolants, such as liquid sodium, is that metal atoms are weak neutron moderators. Water is a much stronger neutron moderator because the hydrogen atoms found in water are much lighter than metal atoms, and therefore neutrons lose more energy in collisions with hydrogen atoms. This makes it difficult to use water as a coolant for a fast reactor because the water tends to slow the fast neutrons into thermal neutrons.
Another advantage of liquid sodium coolant is that sodium melts at 371K and boils / vaporizes at 1156K, a total temperature range of 785K between solid / frozen and gas / vapor states. By comparison, the liquid temperature range of water is just 100K at normal, sea-level atmospheric pressure conditions. Despite sodium's low specific heat, this enables the absorption of significant heat in the liquid phase, even allowing for safety margins. Moreover, the high thermal conductivity of sodium effectively creates a reservoir of heat capacity which provides thermal inertia against overheating. Sodium also need not be pressurized since its boiling point is much higher than the reactor's operating temperature, and sodium does not corrode steel reactor parts. The high temperatures reached by the coolant permit a higher thermodynamic efficiency than in water cooled reactors. The molten sodium, being electrically conductive, can also be pumped by electromagnetic pumps.

Disadvantages

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 when in contact with air. This was the case at the Monju Nuclear Power Plant in a 1995 accident. In addition, neutrons cause it to become radioactive; however, activated sodium has a half-life of only 15 hours.
Another problem is sodium leaks, regarded by critic of fast reactors M.V. Ramana as "pretty much impossible to prevent".

Design goals

The operating temperature should not exceed the melting temperature of the fuel. Fuel-to-cladding chemical interaction has to be designed against. FCCI is eutectic melting between the fuel and the cladding; uranium, plutonium, and lanthanum 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 could eventually rupture. The amount of transuranic transmutation is limited by the production of plutonium from uranium. A design work-around has been proposed to have an inert matrix. Magnesium oxide has been proposed as the inert matrix. Magnesium oxide has an entire order of magnitude smaller probability of interacting with neutrons than elements like iron.
The SFR is designed for management of high-level wastes and, in particular, management of plutonium and other actinides. Important safety features of the system include a long thermal response time, a large margin to coolant boiling, a primary system that operates near atmospheric pressure, and intermediate sodium system between the radioactive sodium in the primary system and the water and steam in the power plant. With innovations to reduce capital cost, such as making a modular design, removing a primary loop, integrating the pump and intermediate heat exchanger, or simply find better materials for construction, the SFR can be a viable technology for electricity generation.
The SFR's fast spectrum also makes it possible to use available fissile and fertile materials considerably more efficiently than thermal spectrum reactors with once-through fuel cycles.

Reactors

Sodium-cooled reactors have included:
ModelCountryThermal power Electric power Year of commissionYear of decommissionNotes
BN-35013519731999Was used to power a water de-salination plant.
BN-60014706001980OperationalTogether with the BN-800, one of only two commercial fast reactors in the world.
BN-800/21008802015OperationalTogether with the BN-600, one of only two commercial fast reactors in the world.
BN-1200290012202036Not yet constructedIs in the developement. Will be followed by BN-1200M as a model for export.
CEFR65202012Operational
CRBRP1000350Never builtNever built
EBR-11.40.219501964
EBR-262.52019651994
Fermi 12006919631975
Sodium Reactor Experiment206519571964
S1GUnited States naval reactors
S2GUnited States naval reactors
PFR50025019741994
FBTR4013.21985Operational
PFBR5002020Under constructionUnder construction
Monju7142801995/2010Operational/1995Suspended for 15 years. Reactivated in 2010
Jōyō1501971Operational
SNR-30032719851991
Rapsodie402419671983
Phénix59025019732010
Superphénix3000124219861997Largest SFR ever built. Suffered a terrorist attack during its construction.

Most of these were experimental plants, which are no longer operational.
On November 30, 2019, CTV reported that the 3 Canadian provinces of New Brunswick, Ontario and Saskatchewan are planning an announcement about an interprovincial plan to cooperate on small sodium fast modular nuclear reactors from New Brunswick-based ARC Nuclear Canada.
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