Nuclear energy is the energy stored in the nucleus of an atom and released through fission, fusion, or radioactivity. The most pressing problems concerning nuclear energy are the possibility of an accident at a nuclear reactor or fuel plant, such as those which occurred at Three Mile Island (1979), Chernobyl (1986), and Takaimura, Japan (1999), and the potential threat posed by nuclear weapons.
The process of nuclear fission was discovered in 1938 by Otto Hahn and Fritz Strassmann and was explained in early 1939 by Lise Meitner and Otto Frisch. The fissionable isotope of uranium, U-235, can be split by bombarding it with a slow neutron.
Since this reaction also releases an average of 2.5 neutrons, a chain reaction is possible. In an atomic bomb, the number of neutrons released is greater than 1 and the reaction increases rapidly to an explosion. In a nuclear reactor, where the chain reaction is controlled, the number of neutrons released must be exactly 1.0 in order to maintain a steady flow of energy.
Nuclear fusion was not achieved by scientists until the 1950s. The temperatures required to initiate fusion reactions are more than 1,000,000°C. In the hydrogen bomb, such temperatures are provided by the detonation of a fission bomb. For sustained, controlled fusion reactions, a fission bomb obviously cannot be used to trigger the reaction. There are other difficulties too. If practical controlled fusion is achieved, it could have great advantages over fission as a source of energy. The energy released during fusion is much greater than that released during fission. Moreover, the fuel for fusion reactions, isotopes of hydrogen, is readily available in large amounts, and there is no release of radioactive byproducts.