In simple terms you can write thus as the energy released during nuclear fission or fusion, especially when used to generate electricity.
But if you need a detailed solution you can use the below solution.
Everything around you is made up of tiny objects called atoms. Most of the mass of each atom is concentrated in the center (which is called the nucleus), and the rest of the mass is in the cloud of electrons surrounding the nucleus. Protons and neutrons are subatomic particles that comprise the nucleus.
Under certain circumstances, the nucleus of a very large atom can split in two. In this process, a certain amount of the large atom’s mass is converted to pure energy following Einstein’s famous formula E = MC2, where M is the small amount of mass and C is the speed of light (a very large number).
This energy is called nuclear energy.