what is the cause of release of unusually large energies in nuclear fission reactions? How is the energy per fission calculated?

During the nuclear fission process a heavy nucleus splits to form particles. As we know that the subatomic particles are held together by strong nuclear forces and when the particle breaks it releases a tremendous amount of energy. The energy released during the fission reaction is calculated by using the famous relation:

E = mc2


E = Energy released

m = mass defect = Total Mass before the reaction – Total mass after the reaction

c = speed of light