Diagram of the workings of a gas-cooled nuclear reactor (GCR). In this reactor, the core comprises fissile fuel rods (red, left) surrounded by gas (usually carbon dioxide or helium). The fuel rods heat the gas, which is then passed over heat exchangers (coil). These contain water (blue), which is converted to steam (red). The steam drives a turbine (centre) which drives the electricity generator (right of turbine). The steam is then condensed back to water. Control rods (pale green) between the fuel rods are used to moderate the rate of the reaction. The electricity generated is stepped up to high voltages by substation transformers for distribution.