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Nuclear energy

Nuclear energy, energy released through the fission or fusion of atomic nuclei. In fission, the nucleus of a heavy atom absorbs an extra neutron, which causes it to become unstable and split apart into 2 lighter nuclei plus other subatomic particles, including other neutrons. Fission can occur only in a few of the heaviest, least stable nuclei. The energy released by a fission reaction consists mainly of the transformation of nuclear forces (the forces holding the nucleus together) into heat. Fusion is the opposite process: here the lightest nuclei (usually isotopes of hydrogen) are squeezed together under conditions of extreme heat and pressure until they merge, forming a new nucleus whose mass is very slightly smaller than the total masses of the nuclei that were fused. The extra mass is converted into energy (mostly heat) according to Einstein's formula E=mc2, where E is energy, m is mass, and c is the speed of light. Since c2 is a very large number, the energy yielded by this reaction is large even when the mass, m, is very small. Fission reactions were first observed in 1938. When the nucleus of Uranium-235 is bombarded with neutrons, it splits apart, releasing an average of 2.5 free neutrons. If these released neutrons collide with other nuclei of U-235, a chain reaction ensues. If this reaction is uncontrolled, the result is an atomic explosion like the one caused by the atomic bombs dropped on the Japanese cities of Hiroshima and Nagasaki in 1945. This is what happens in a nuclear reactor. Fusion, unlike fission, requires very high temperatures. In a hydrogen bomb, these temperature are brought about by a fission explosion, which triggers uncontrolled fusion reactions in the hydrogen that is packed around the fission bomb. So far, attempts to create controlled fusion reactions are only at the research stage. One line of experimentation has been to use magnetic fields to contain hydrogen fuel in the form of a plasma, a fully ionized gas containing equal numbers of positive and negative ions. Another has been to bombard pellets of frozen deuterium (a hydrogen isotope) with high-powered laser beams. The successful control of fusion would be an epoch-making achievement in providing energy. Unlike fissionable fuel and fossil fuels like oil, hydrogen is virtually limitless, and fusion produces far fewer radioactive by-products than fission. This means that fusion reactors, if they are possible, would produce much less radioactive waste than today's fission reactors do.

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