Fission reactions are induced in nuclear power plants because they produce great amounts of energy. Often, a nucleus must be forced to undergo fission by being hit by another particle. A U-235 nucleus being hit with some other particle causes it to undergo fission and emit more particles. Those particles eventually hit other U-235 nuclei, which propagates the reaction. What is this type of reaction called

A chain reaction is a reaction that sustains itself. It has the ability to continue for a very long time without adding any more materials to the reaction system. It may be succinctly described as a self propagating reaction.

In a nuclear fission, uranium-235 is bombarded with neutrons to produce unstable uranium-236 which disintegrates to form daughter nuclei and produce more neutrons that bombard more uranium-235 and the reaction continues indefinitely.