If the splitting releases energy and the fusion release the energy, so where is the breaking point? For understanding this issue, it is better to relate the binding energy to one nucleon to obtain a nuclear binding curve. The binding energy per one nucleon is not linear. There is a peak in the binding energy curve in the region of stability near iron. This means that either the breakup of heavier nuclei than iron or the combining of lighter nuclei than iron will yield energy.

The reason the trend reverses after iron peak is the growing positive charge of the nuclei. The electric force has greater range than strong nuclear force. While the strong nuclear force binds only close neighbors the electric force of each proton repels the other protons.

See previous:

Nuclear Binding Energy

See above:

Nuclear Binding Curve