It is is a nuclear reaction that splits the nucleus of an atom into smaller, subatomic particles. It often produces free neutrons and photons. Fission of heavy elements can release large amounts of energy. For fission to produce energy, the total binding energy of the resulting element has to be lower than that of the original element. Fission is a form of transmutation because the resulting fragments are not same as the reactants.

Nuclear fission can occur without neutron bombardment(radioactive decay). This type of fission only occurs in a few heavy isotopes. In nuclear devices, all nuclear fission occurs from a neutron bombardment process that results from the collision of two subatomic particles. The most common nuclear fuels are 235uranium and 230plutonium.

Typical fission events release about two hundred million eV(electron volts) of energy for each fission event. Most chemical fuels only release a few eV. The energy of nuclear fission is released as kinetic energy, fragments, gamma rays, along with a huge amount of heat. Neutron bombs release a larger fraction of their energy as ionizing radiation, but these are all thermonuclear devices which rely on the fusion stage to produce the extra radiation.

Nuclear fission could be used to create a nuclear chain reaction. If the number of secondary neutrons produced by each fission nucleus was greater than one, then each fission reaction could, in theory, trigger two more reactions. Such a system of exponential growth held out the possibility of using uranium fission as a means to generate large amounts of energy, either for civilian or military purposes. The atomic bomb was born.

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