Chain Reaction

A chain reaction is a process in which neutrons released in fission produce additional fission in at least one further nucleus. This nucleus, in turn, produces neutrons, and the process repeats. The process may be controlled (nuclear power) or uncontrolled (nuclear weapons). In controlled reactions, neutrons from fission must be slow down because they have very high speeds. For this slowing down a moderator is used. A moderator changes fast neutrons into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235 or a similar nuclide. As moderators are used regular water, solid graphite and heavy water. It could be also Beryllium, Boron or Graphite.