The above curve shows that
(i) when a heavy nucleus breaks into two medium sized nuclei (in nuclear fission), the BE / nucleon increases resulting in the release of energy.
(ii) when two small nuclei combine to form a relatively bigger nucleus in nuclear fusion, BE i nucleon increases,resulting in the release of energy.aa
The constancy of the binding energy in the range 30 < A < 170 is a consequence of the fact that the nuclear force is short-ranged. If a nucleon can have a maximum of p neighbours within the range of nuclear force, its binding energy would be proportional to p. If we increase A by adding nucleons, they will not change the binding energv of a nucleon inside. Since most of the nucleons in a large nucleus reside inside it and not on the surface, the change in binding energy per nucleon would be small. Hence, the binding energy per nucleon is a constant, [saturation property of nuclear force.]