Nuclei can produce energy via two different types of reactions, namely, fission and fusion reactions. Fission is a break up of a nucleus in two or more pieces (smaller nuclei). Fusion is the opposite process: Formation of a bigger nucleus from two small nuclei.
A question may arise: How two opposite processes can both produce energy? Can we make an inexhaustible souce of energy by breaking up and then fusing the same nuclei? Of course not! The energy conservation law cannot be circumvented in no way. When speaking about fusion and fission, we speak about different ranges of nuclei. Energy can only be released when either light nuclei fuse or heavy nuclei fission.
To understand why this is so, let us recollect that for releasing energy the mass of initial nuclei must be greater than the mass of the products of a nuclear reaction. The mass difference is transformed into the released energy. And why the product nuclei can loose some mass as compared to the initial nuclei? Because they are more tightly bound, i.e. their binding energies are lager.
Fig. 15.3 shows the dependence of the binding energy per nucleon on the number of nucleons constituting a nucleus. As you see, the curve reaches the maximum value of MeV per nucleon at around . The nuclei with such number of nucleons cannot produce energy neither through fusion nor through fission. They are kind of ashes and cannot serve as a fuel. In contrast to them, very light nuclei, when fused with each other, make more tightly bound products as well as very heavy nuclei do when split up in lighter fragments.
In fission processes, which were discovered and used first, a heavy nucleus like, for example, uranium or plutonium, splits up in two fragments which are both positively charged. These fragments repel each other by an electric force and move apart at a high speed, distributing their kinetic energy in the surrounding material.
In fusion reactions everything goes in the opposite direction. Very light nuclei, like hydrogen or helium isotopes, when approaching each other to a distance of a few fm ( cm), experience strong attraction which overpowers their Coulomb (that is electric) repulsion. As a result the two nuclei fuse into a single nucleus. They collapse with extremely high speeds towards each other. To form a stable nucleus they must get rid of the excessive energy. This energy is emitted by ejecting a neutron or a photon.