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 LMFR technology using MOX 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 (150–600 MWe) 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 (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 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.
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 (moderate) the fast neutrons into thermal neutrons (though concepts for reduced moderation water reactors exist). 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 (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, 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.[1] 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.[1] The high temperatures reached by the coolant (the Phénix reactor outlet temperature was 560 C) permit a higher thermodynamic efficiency than in water cooled reactors.[2][2]
A disadvantage of sodium is its chemical reactivity, which requires special precautions to prevent and suppress fires.
Another problem is sodium leaks, regarded by critic of fast reactors M.V. Ramana as "pretty much impossible to prevent".[3]
Design goals
The operating temperature should not exceed the melting temperature of the fuel.
The SFR is designed for management of high-level wastes and, in particular, management of plutonium and other actinides.
The SFR's fast spectrum also makes it possible to use available fissile and fertile materials (including depleted uranium) considerably more efficiently than thermal spectrum reactors with once-through fuel cycles.
Reactors
Sodium-cooled reactors have included:
Most of these were experimental plants, which are no longer operational
Related:
- Fast Flux Test Facility, United States, a sodium-cooled fast neutron reactor
See also
- Fast breeder reactor
- Fast neutron reactor
- Integral Fast Reactor
- Lead-cooled fast reactor
- Gas-cooled fast reactor
- Generation IV reactor