Sodium-cooled fast reactor. Diagram of the workings of a sodium-cooled fast reactor (SFR), a category of experimental nuclear reactor in which the fission chain reaction is sustained by fast neutrons from uranium and plutonium fuel. The reactor core (left) comprises fissile fuel rods surrounded by liquid sodium coolant. The fuel rods heat the sodium, and the heat is transmitted via heat exchange systems to a steam generator. The steam drives a turbine which drives the electricity generator (right) producing power (red arrow). The steam is then condensed back to water. Control rods between the fuel rods are used to moderate the rate of the reaction.