Heavy water nuclear reactor. Diagram of the workings of a heavy water reactor (HWR), of which the CANDU reactors were a type. In this reactor, the core comprises fissile fuel rods (yellow) surrounded by heavy water (water in which the hydrogen has been replaced by the isotope deuterium). Heavy water is an excellent moderator (which increases the likelihood of fission), but cannot be allowed to boil, so it is kept under very high pressure. The hot heavy water is passed to a steam generator (upper centre), where it boils water (blue) into steam (red). The steam drives a turbine (green), which in turn drives an electricity generator (grey).