Nuclear fission is either a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into smaller parts (lighter nuclei). The fission process often produces free neutrons and photons (in the form of gamma rays), and releases a very large amount of energy.
Nuclear fission is an exothermic reaction. The heat of this process is much greater than the energy of chemical reactions. Therefore, the division is used in nuclear power and military affairs to create an atomic bomb.