Nuclear energy is the energy in the nucleus of an atom. Nuclear energy can be used to produce electricity. This energy can be obtained in two ways: nuclear fusion and nuclear fission. In nuclear fusion, energy is released when atoms are combined or fused together to form a larger atom. The sun produces energy like this. In nuclear fission, atoms are split into smaller atoms, releasing energy. Actually, nuclear power plants can only use nuclear fission to produce electricity.
I hope I helped! =D