Gas-cooled nuclear reactor. Diagram of the workings of a gas-cooled reactor (GCR), of which the Magnox reactors were a type. In this reactor, the core comprises fissile fuel rods (grey columns) surrounded by gas (usually carbon dioxide or helium). The fuel rods heat the gas, which is then passed over heat exchangers. These contain water (dark grey), which is converted to steam (light grey). The steam drives a turbine (black) which drives the electricity generator (white). The steam is then condensed back to water. Control rods (white, between fuel rods) are used to moderate the reaction rate. Magnox reactors are no longer built, but modified forms are still used.