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. Sodium 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, and in fact, protects metals from corrosion. Moreover, the high thermal conductivity of sodium effectively creates a reservoir of heat capacity that provides thermal inertia against overheating. 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. By comparison, the liquid temperature range of water (between ice and gas) is just 100K at normal, sea-level atmospheric pressure conditions. 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 for reduced moderation water reactors exist).Īnother advantage of liquid sodium coolant is that sodium melts at 371K and boils / vaporizes at 1156K, a difference of 785K between solid / frozen and gas / vapor states. 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. The primary advantage of liquid metal coolants, such as liquid sodium, is that metal atoms are weak neutron moderators. This means that the inventory of transuranic waste is non existent from fast reactors. 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. Crucially, when a reactor runs on fast neutrons, the plutonium isotopes are far more likely to fission upon absorbing a neutron. The French Rapsodie, British Prototype Fast Reactor and others used this approach.Īll fast reactors have several advantages over the current fleet of water based reactors in that the waste streams are significantly reduced. In the loop type, the heat exchangers are outside the reactor tank. The US EBR-2, French Phénix and others used this approach, and it is used by India's Prototype Fast Breeder Reactor and China's CFR-600. In the pool type, the primary coolant is contained in the main reactor vessel, which therefore includes the reactor core and a heat exchanger. The two main design approaches to sodium-cooled reactors are pool type and loop type.
Schematic diagram showing the difference between the Pool and Loop designs of a liquid metal fast breeder reactor When it does absorb a neutron it produces sodium-24, which has a half-life of 15 hours and decays to stable isotope magnesium-24. Sodium has only one stable isotope, sodium-23, which is a weak neutron absorber. Liquid metallic sodium may be used to carry heat from the core. The outlet temperature is approximately 510–550 degrees C for both. 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.
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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. Īside from the Russian experience, Japan, India, China, France and the USA are investing in the technology. For example in 2022, in the USA, TerraPower (using its Traveling Wave technology ) is planning to build its own reactors along with molten salt energy storage in partnership with GEHitachi's PRISM integral fast reactor design, under the Natrium appellation in Kemmerer, Wyoming. Others are in planning or under construction. Several sodium-cooled fast reactors have been built and some are in current operation, particularly in Russia. The initials SFR in particular refer to two Generation IV reactor proposals, one based on existing liquid metal cooled reactor (LMFR) technology using mixed oxide fuel (MOX), and one based on the metal-fueled integral fast reactor.
Pool type sodium-cooled fast reactor (SFR)Ī sodium-cooled fast reactor is a fast neutron reactor cooled by liquid sodium.
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