Atomic bomb explosion. Mushroom cloud produced by the detonation of XX-12 Grable, a 15-kiloton nuclear shell, on 25 May 1953. This was part of Operation Upshot-Knothole at the Nevada Proving Grounds, USA. XX-12 Grable was fired from a 280mm artillery gun, the only time a nuclear artillery shell was ever fired. Atomic or nuclear bombs work by releasing energy from the nucleus of atoms. Fission bombs contain enriched uranium or plutonium. When an atom absorbs a neutron, it splits into smaller atoms, releasing energy (in the form of heat and radiation) and more neutrons, which propagate a chain reaction.