When a nucleus is separated into its individual constituents, the sum of the masses of its individual constituents, is always greater than the total mass of the nucleus.
The nuclear binding energy of a nucleus is defined as the work done on the nucleus to separate it into its constituent neutrons and protons.
mnucleusc2 + Binding Energy = mindividual particles (neutrons and protons)c2
Binding Energy = Mass defect X c2
Energy is released when a nucleus is formed from its constituent nucleons.