Sodium cooled fast reactor

The sodium-cooled fast reactor (SFR) is a Generation IV reactor project to design an advanced fast neutron reactor.

It builds on two closely related existing projects, the fast breeder reactor and the integral fast reactor, with the objective of producing a fast-spectrum, sodium-cooled reactor.

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.

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.

An advantage of liquid metal coolants is that despite low specific heat, sodium melts at 371K and boils / vaporizes at 1156K, allowing a total "temperature outlier" range of 785K of heat variation between solid / frozen and gas / vapor states allowing the absorption of significant heat, less safety margins, in liquid phase. The high thermal conductivity properties effectively create a reservoir of heat capacity which provides thermal inertia against overheating.Water is difficult to use as a coolant for a fast reactor because water acts as a neutron moderator that slows the fast neutrons into thermal neutrons. Unlike liquid sodium, water has a higher specific heat, with a smaller liquid range of just 100K between ice and gas at normal, sea-level atmospheric pressure conditions. While it may be possible to use supercritical water as a coolant in a fast reactor, this would require a very high pressure. In contrast, sodium atoms are much heavier than both the oxygen and hydrogen atoms found in water, and therefore the neutrons lose less energy in collisions with sodium atoms. 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 (up to 1156K for pure molten sodium, less all Generation IV margins of alarm call safety) permit a higher thermodynamic efficiency than in water cooled reactors. The molten sodium, being electrically conductive, can be pumped by electromagnetic pumps.

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