Nuclear Fission

Nuclear fission is the process whereby an atomic nucleus breaks up into two or more major fragments with the emission of two or three neutrons. It is accompanied by the release of energy in the form of gamma radiation and the kinetic energy of the emitted particles.
Fission occurs spontaneously in nuclei of uranium-235, the main fuel used in nuclear reactors. However, the process can also be induced by bombarding nuclei with neutrons because a nucleus that has absorbed a neutron becomes unstable and soon splits.

The mass defect is large and appears mostly as k.e. of the fission fragments. These fly apart at great speed, colliding with surrounding atoms and raising their average k.e., that is, their temperature. Heat is therefore produced.
If the fission neutrons split other uranium-235 nuclei, a chain reaction is set up. In practice, some fission neutrons are lost by escaping from the surface of the uranium before this happens. The ratio of those escaping to those causing fission decreases as the mass of uranium-235 increases.
This must exceed a certain critical mass for a chain reaction to start. Critical mass is thus the minimum mass of fissile material that can undergo a continuous chain reaction. Above the critical mass, the reaction may accelerate into a nuclear explosion if uncontrolled.
The U-238 isotope would make an ideal nuclear reactor fuel because it is abundant in nature. But U-238 nuclei usually absorb free neutrons without fissioning. An absorbed neutron simply becomes part of the nucleus. The scarce uranium isotope U-235 is the only natural material that nuclear reactors can use to produce a chain reaction. Uranium with an abundant amount of U-235 is called enriched uranium.