Nuclear fission is the process in which a heavy nucleus splits into two lighter nuclei of nearly the same size, when bombarded with slow neutrons. In each fission reaction, a tremendous amount of energy is released.
Uranium is one substance that can be used for nuclear fission.
Example of fission reaction is —
92235U(uranium)++01n(neutron)3689Kr(krypton)⟶+(92236U)3 01n(neutrons)⟶+56144Ba(barium)energy