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 (yellow) surrounded by gas (usually carbon dioxide or helium, green). The fuel rods heat the gas, which is then passed over heat exchangers. These contain water (blue), which is converted to steam (red). The steam drives a turbine (green) which drives the electricity generator (grey). The steam is then condensed back to water. Control rods (blue) between the fuel rods are used to moderate the rate of the reaction. Magnox reactors are no longer built, but modified forms are still used.