The reactions that are used for the production of a tremendous amount of energy is called nuclear fission and fusion reaction. Fission and fusion reactions are different since for a fission reaction to proceed it requires a very small amount of energy and with that energy, heavier nuclei are split into lighter nuclei with the release of energy. A very common example of nuclear fission is the splitting of Uranium-235 for the formation of barium.  While a nuclear fusion reaction is a reaction where the required a large amount of energy to fuse the small nuclei and thereby forming a large nucleus with a tremendous release of energy. A very common example of a nuclear fusion reaction is the combination of deuterium and tritium for the formation of helium with the mass number 4.