Nuclear Fusion and Fission When a heavy nucleus  undergoes nuclear fission , it splits to two smaller, more stable nuclei and produces heat, similarly, when two light nuclei fuse, to make a heavier, more stable nucleus and produces energy.

The produced energy, is due to the mass defect between the reactants and the products, which is transformed to energy, according the mass-energy equivalence.


  But if the products of each process(fission, and fusion) are more stable, that means that they have  more binding energy, thus, shouldn't the mass defect be transformed to nucleus binding energy, rather than emitted as heat or another sort of energy ?
Note that binding energy of nucleons does not mean contained energy, but the energy needed to break it. 

Therefore kernels with lower binding energy per nucleon contain more nuclear energy per nucleon and have higher mass per nucleon.

It means forming kernels with higher binding energy per nucleon releases energy and vice versa.

It is the same as for chemical bonds, just some 6-7 orders higher.

E.g. nitroglycerine has lower binding energy per atom than reaction products formed by its explosion.