Nuclear energy is the energy released during a nuclear fission reaction or a nuclear fusion reaction. During a nuclear fission reaction, a heavy nucleus is bombarded which releases energy and this energy is used for producing electricity.Complete answer:
The atomic energy released during a nuclear fission reaction and a nuclear fusion reaction releases nuclear energy. The diagram below shows nuclear fission reactions and nuclear fusion reactions.
During a nuclear fission reaction, a heavy nucleus atom breaks into two nearly equal mass lighter nuclei via excitation by particles like a neutron as shown in the above figure. The daughter products of a $U-235$ when excited by a neutron are $Ba-114$, $Kr-89$and three neutrons, reaction takes place with the release of a large amount of heat. The neutrons released react with other heavy nuclei and this sequence goes on.
During a nuclear fusion reaction, light nuclei merge to form heavy nuclei as shown in the above figure. A deuterium nucleus and a tritium nucleus fuse together and thus in the process form a helium nucleus and a neutron, with the release of energy.
Nuclear fusion is used in many countries to produce electricity. For example, the fusion reaction in which a uranium atom breaks and releases a huge amount of energy and thus this heat is used to produce steam to turn turbines to generate electricity. Uranium is the most widely used fuel in nuclear power plants for nuclear fusion reactions to produce electricity.Note:
Nuclear power is the cheapest method for producing large scale electricity. Some states generate half of the electricity by using energy produced in nuclear power plants. Nuclear energy is also used in many other applications like seawater distillation, hydrogen production, etc.