An example of an induced nuclear fission event — A neutron is absorbed by the nucleus of a uranium-235 atom, which in turn splits into fast-moving lighter elements (fission products) and free neutrons. Though both reactors and nuclear weapons rely on nuclear chain reactions, the rate of reactions in a reactor is much slower than in a bomb.
Photo Credit: Public Domain, https://commons.wikimedia.org/w/index.php?curid=486924