Nuclear energy is produced when an atom undergoes nuclear reaction. Those nuclear reaction can be of two types either nuclear fusion, where two atoms of same or different elements(atom of less atomic weight) combine to form single atom (atom with heavy atomic weight), or nuclear fission where single atom of one element (atom with heavy atomic weight) is broken into two or more atom of different elements(atom of less atomic weight). In earth nuclear power plant produce nuclear energy using controlled nuclear fission. We have not been able to use nuclear fusion to create energy on earth. Nuclear fusion is operating only in stars or to build bombs.