When an atom of U-235 is bomborded with a neutron, it absorbs that neutron and converts into U-236. U-236 atom is extremely unstable and instantly disintegrates into lighter atoms of Barium (A = 141) and Krypton (A = 92). In this process, 200 MeV energy is released alongwith three more neutrons. These neutrons can bring about the fission of three more U-235 atoms. This starts a chain reaction in which extremely large amount of energy is released.

This is the principle used in the atom bomb.