Nuclear energy, also known as atomic energy, is the energy released in large quantities by operations that influence atomic nuclei, the dense centres of atoms. It differs from the energy of other atomic phenomena like typical chemical reactions, which solely involve atoms' orbital electrons. Controlled nuclear fission in reactors, which are now used to generate electricity in many parts of the world, is one means of releasing nuclear energy. Controlled nuclear fusion, another way for getting nuclear energy, has shown promise but will not be perfected by 2020. Nuclear fusion and nuclear fission have both unleashed massive amounts of nuclear energy.

The nucleus of an atom, such as uranium or plutonium, split up into two lighter nuclei of about equal mass in nuclear fission. Excitation of the nucleus by a number of particles (e.g., neutrons, protons, deuterons, or alpha particles) or electromagnetic radiation in the form of gamma rays might cause the process to occur spontaneously in some situations. A substantial amount of energy is produced, radioactive products are created, and numerous neutrons are discharged during the fission process.These neutrons can cause fission in a neighbouring nucleus of fissionable material, releasing more neutrons that can repeat the process, resulting in a chain reaction in which a significant number of nuclei fission and a huge quantity of energy is produced. A chain reaction like this can offer power to civilization if it is managed in a nuclear reactor. If left unchecked, as in the case of the so-called atomic bomb, it can result in a massive devastating explosion.

Nuclear fusion is the process by which light elements react with each other to generate heavier elements. Significant amounts of energy are released when the interacting nuclei are from elements with low atomic numbers (for example, hydrogen [atomic number 1] and its isotopes deuterium and tritium). The tremendous energy potential of nuclear fusion was initially harnessed in thermonuclear weapons, often known as hydrogen bombs, created in the decade following WWII. The potential benign applications of nuclear fusion have sparked a massive effort to harness this process for the generation of power, especially given the nearly endless availability of fusion fuel on Earth.Although real fusion reactors have yet to be built, the essential plasma temperature and heat insulation conditions have been attained to a great extent, implying that fusion energy for electric power generation is now a genuine prospect. Commercial fusion reactors promise an endless supply of electricity for countries all over the world.