Supercritical water-cooled reactor. Diagram of the workings of a supercritical water-cooled reactor (SCWR), a category of experimental nuclear reactor. The core (green) comprises fissile fuel rods cooled by supercritical water (yellow). This is water at very high temperature and pressure where it is reaches a state neither liquid nor gas. The fuel rods heat the supercritical water, which drives a turbine (centre). This drives the electricity generator (grey, right) producing power (red arrow). The water is then returned to the core. Control rods (top) between the fuel rods are used to moderate the rate of the fission reaction.