Energy released during a nuclear reaction in accordance with the mass energy equation is called nuclear energy. All matter is composed of atoms and each atom has a nucleus composed of neutrons and protons.
The nucleus of every atom remains intact with binding energy. It is the binding energy that when released slowly and under control produces heat that can power stream-driven electricity generators in nuclear power plants and it is also the binding energy that when released all at once, produces the destructive impact in a nuclear bomb.
Theoretically, we can obtain energy from almost any substance but this is not practical because the energy needed for triggering the release of binding energy via breaking the nucleus apart it would be more than the energy released. The substance would be less significant for power generation.
Thus, practically nuclear energy can be obtained only from some elements which are easily broken apart releasing energy in the form of heat. These elements are called fissile or radioactive elements.
Nuclear energy is produced by two types of nuclear reactions:
Nuclear Fission and Nuclear Fusion:
It is a nuclear reaction in which a heavy atomic nucleus is split into two approximately equal nuclei, releasing very large amount of binding energy, mainly in the form of heat. This split also objects several neutrons each of which in turn can strike other atomic nuclei to trigger further splits and further release of energy.
This fission reaction involves a series of nuclear changes following major heat yielding split. At each step in the series energy given off in the form of radioactive particles, rays and some highly dangerous radioactive waste which must be safely stored.
Nuclear Fusion: It is a nuclear reaction in which fight atomic nuclei fuse together to form a single heavy nucleus with the release of large quantity of energy. In fusion hydrogen isotopes are fused to form helium with the release of enormous quantities of energy. A hydrogen bomb is literally a one-short nuclear fusion reactor.