Neutron

When the element uranium is bombarded by neutrons, a unique reaction called fission takes place. The uranium nucleus breaks into two pieces, which fly apart with a large release of energy. In addition, several extra neutrons are emitted. These cause more uranium nuclei to split apart, which creates more energy and more neutrons in a so-called "chain reaction" process. In an atomic bomb, the chain reaction becomes an uncontrolled explosion. In a nuclear power plant, the chain reaction is maintained in a steady state by control rods which absorb extra neutrons.