Chain Reaction

A chain reaction is a process in which neutrons released during nuclear fission trigger additional fission reactions in at least one more nucleus. This new fission event, in turn, releases more neutrons, allowing the process to continue in a self-sustaining cycle. Chain reactions can be controlled, as in nuclear power generation, or uncontrolled, as in nuclear weapons. In controlled reactions, the fast-moving neutrons produced by fission need to be slowed down because they are too energetic to efficiently sustain further fission. This is achieved by using a moderator, a substance that reduces the speed of fast neutrons, converting them into thermal neutrons capable of sustaining a nuclear chain reaction in uranium-235 or similar fissile materials. Common moderators include regular water, solid graphite and heavy water. Other materials, such as beryllium, boron or graphite, can also be used as moderators.