The process of breaking up the heavier nucleus into two or more smaller nucleus having approx. same mass is called nuclear fission. There exists a mass defect during nuclear fission process. The total mass of products of fission is always less than the reactants of nuclear fission process.
This loss in mass can be determined by Einstein’s mass energy relationship.
The fission of U235 is as shown:
92U235 + 0n1——> 56Ba141 + 36Kr92 + 3 0n1
The energy released in one fission reaction is different from another fission reaction.