What is the purpose of a moderator?

Answer 1

The purpose of a moderator in a nuclear reactor is to slow down (moderate) the neutrons produced when uranium (or other nuclear fuel) is fissioned (split). Uranium is naturally radioactive, meaning it spontaneously (without help) breaks apart (decays) into smaller nuclear particles (other elements) and in the process releases radiation (alpha, beta and gamma), heat, and very high speed neutrons. It is these neutrons that are needed to sustain a chain reaction of uranium fissioning, thus producing the heat needed to boil water and produce steam to turn turbines connected to electric generators. Ordinarily the nuclear fuel (e.g. uranium oxide) does not fission often enough to produce a sustained chain reaction, mainly because the two or three neutrons released during fissioning are travelling too fast to split the nuclei of other atoms Therefore, the fuel is suspended in a moderator (Canada uses "heavy water"), which slows down the neutrons enough to make the chain reaction sustainable for enough heat to be generated at a constant rate.