Pressurised water nuclear reactor. Diagram of the workings of a pressurised water reactor (PWR). In this reactor, the core comprises fissile fuel rods (lower left) surrounded by water. The water is heated by the nuclear reactions, but is kept under high pressure, so does not boil. The hot pressurised water is passed to a steam generator (upper centre), where it boils water (light grey) into steam (dark grey). The steam drives a turbine, which in turn drives an electricity generator (white). The steam is then condensed back to water by a heat exchanger, using water from a pond (lower right). Control rods (between fuel rods) are used to control the reaction.