Release of Energy:

When the atom splits, the masses of the fragments and the neutrons produced do not add up to the mass of the original. A tiny amount of matter disappears. This lost matter turns  into energy. The amount of energy  'E'  generated by the.  lost matter of mass 'm'  is given by  the famous equation due to Einstein: 

E = mc², where c is the speed of light. 

c is large (about 300 million metres/sec) and c² is enormous (about 90,000 trillion m²/sec²). Thus, a small amount of lost matter would get converted into very very large amounts of energy.