Nuclear fission is a reaction wherein a heavy nucleus is bombarded by neutrons and thus becomes unstable, which causes it to decompose into two nuclei with equivalent size and magnitude, with a great detachment of energy and the emission of two or three neutrons.
In a small fraction of a second, the number of fissioned nuclei releases an energy a million times greater than the energy obtained
These neutrons can cause more fissions by interacting with new fissionable nuclei that will then emit new neutrons and so forth. This multiplying effect is known as chain reaction. In a small fraction of a second, the number of fissioned nuclei releases an energy a million times greater than that obtained from burning a block of carbon or exploding a block odf dynamite of the same mass.
Due to the speed of a nuclear reaction, energy is released much faster than in a chemical reaction.
If only one of the liberated neutrons can be made to produce a subsequent fission, the number of fissions per second will be constant and the reaction will be controlled. This is the functioning principle of nuclear reactors, which are controlled sources of nuclear fission energy.