A sodium-cooled fast reactor is a fast neutron reactor cooled by liquid sodium. 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. Several sodium-cooled fast reactors have been built and some are in current operation, particularly in Russia. Others are in planning or under construction. 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. Aside from the Russian experience, Japan, India, China, France and the USA are investing in the technology. 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 multiple reactors. The outlet temperature is approximately 510–550 degrees C for both. Sodium coolant 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 produces sodium-24, which has a half-life of 15 hours and decays to stable isotope magnesium-24. Pool or loop type 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 a heat exchanger. 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 loop type, the heat exchangers are outside the reactor tank. The French Rapsodie, British Prototype Fast Reactor and others used this approach. Advantages 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,