Nuclear energy defines the energy in the core or the nucleus of the atom.
Nuclear energy can be produced by nuclear fission, nuclear fusion, and nuclear decay.
Nuclear fission
Nuclear fission is a type of nuclear reaction in which a heavy nucleus with the release of energy splits spontaneously or on impact with another particle.
Nuclear fusion
Nuclear fusion is a type of reaction in which two or more light nuclei form a heavier nucleus on collision.
It is basically a kind of reaction that released energy because of the presence of high-powered atomic bonds between particles present in a nucleus.
So, nuclear fusion is when two or smaller atoms fuse together to create a larger heavier atom.
The nuclei of an atom as a result of nuclear instability exhibit the phenomenon of Radioactivity or nuclear decay.
Nuclear decay
Nuclear decay occurs when the nucleus of an atom is unstable and spontaneously emits energy in the form of radiation.
The result is that the nucleus changes into the nucleus of one or more other elements.
These daughter nuclei have a lower mass and are more stable (lower in energy) than the parent nucleus.
Hence, nuclear fusion, nuclear fission, and nuclear decay are the three sources of nuclear energy.