From the notes for Class 10 Science chapter 14 , Nuclear energy is a reaction in which the nucleus of an atom undergoes a transition to generate a new atom and releases a massive quantity of energy. Nuclear energy can be obtained in two ways:

(a) through nuclear fission and (b) through nuclear fusion.
Nuclear energy is the energy released when changes occur in the nucleus of a substance's atom.
It is used to generate heat and as fuel for marine vessels. The main advantage of nuclear energy is that it needs a very small amount of fuel but produces a very large amount of energy. It is a very expensive plant and also pollutes the environment.