Heavy water reactor
The heavy water reactor and the pressurized heavy water reactor are thermal neutron reactors using heavy water as a moderator. Heavy water refers to D2O, which is water in which deuterium, an isotope of hydrogen, is included in the hydrogen structure of water. In the natural state, natural uranium can not reach its critical state by itself because of the 235U, which is too small. 235U inside the reactor, on the other hand, leads to a sustained chain reaction. For this reason, a material that can slow down neutrons is needed. This material is called a moderator. Generally, moderator water is used, and since hard water absorbs neutrons, 235U should be concentrated. However, since heavy water absorbs less neutrons, normal natural uranium can be used. Most pressurized water reactors using heavy water are CANDU. Heavy water coolant is kept under pressure, so it can be heated at high temperature without boiling, so it can be used for pressurized water reactor.