energy that is released in significant amounts in processes that affect atomic nuclei, the dense cores of atoms. It is distinct from the energy of other atomic phenomena such as ordinary chemical reactions, which involve only the orbital electrons of atoms. One method of releasing nuclear energy is by controlled nuclear fission in devices called reactors, which now operate in many parts of the world. (See nuclear reactor.) Another method for obtaining nuclear energy, controlled nuclear fusion, had not been perfected by the late 1980s. Nuclear energy has been released explosively by both nuclear fusion and nuclear fission (qq.v.).