Nuclear fission is either a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into smaller parts. The fission process produces free neutrons and photons. It also releases a large amount of energy.
Fission is a form of nuclear transmutation due to the resulting fragments which are not the same element as the original atom. The nuclei produced are often comparable but slightly different sizes. Typically the mass ratio of the two nuclei’s is about 3 to 2.
Fission is most commonly encountered when it is man-made in a nuclear reaction induced by a neutron bombardment. In nature it is only encountered as a natural form of spontaneous radioactive decay, which does not require a neutron. This occurs especially in high-mass-number isotopes.
Nuclear fission produces energy for nuclear power and to drive the explosion of nuclear weapons. Both uses are possibly because certain substances, nuclear fuels, undergo fission when struck by fission neutrons and emits neutrons when they break apart. This makes a self-sustaining nuclear chain reaction that releases energy at a controlled rate in a nuclear reactor or at a rapid uncontrolled rate in a nuclear weapon.
The amount of energy contained in nuclear fuel is millions of times the amount of energy contained in a similar mass of chemical fuel, like gasoline. The nuclear fission gas is a very dense source of energy. The products of nuclear fission however are extremely radioactive which causes nuclear waste problems.