According to the International Atomic Energy Agency, nuclear fission is the process by which large, heavy atoms split into lighter ones, releasing substantial amounts of energy in the process. This can occur due to natural radioactive decay, or it can be triggered by an atomic collision. Nuclear fission can power reactors or create massive explosions if uncontrolled.
Induced nuclear fission requires specific isotopes of radioactive material. Isotopes like Uranium-235 and Plutonium-239 are unstable, and when struck by a relatively low-energy neutron, the atoms split. When one of these atoms breaks apart, it releases energy, heat, radiation and particles that can trigger other nuclei to split as well.
In a nuclear power plant, the heat given off by nuclear fission is used to boil water, and the resulting steam turns electric turbines to generate electricity. Nuclear fission is only self-sustaining when the fuel contains enough unstable atoms to cause a chain reaction. Reactors use neutron-absorbing materials to shut down fission reactions, allowing operators to control how much energy is produced and prevent runaway reactions that could lead to explosions or meltdowns.
In weapons, a runaway reaction is the desired outcome. Nuclear weapons contain large amounts of nuclear fuel packed with the unstable isotopes to ensure that once the reaction starts, it releases enormous amounts of energy and heat.