Nuclear energy is obtained from two types of nuclear reactions:

(i) Nuclear fission

(ii) Nuclear fusion

Nuclear Fission: The process in which heavy nucleus of radioactive atoms (such as uranium, thorium or plutonium) splits up into smaller nuclei when bombarded with low energy neutrons, is called nuclear fission.

Nuclear Fusion: The process in which two nuclei of light elements (such as Hydrogen) combine to form a heavy nucleus (such as Helium) is called nuclear fusion.

In both the cases, the tremendous amount of nuclear energy is released due to the small loss of mass.

Nuclear Fission Reaction:

Nuclear Fusion Reaction:

Nuclear energy is used to generate electricity. Nuclear power plants use controlled fission reaction to generate it.