NUCLEAR FUSION

process by which nuclear reactions between light elements form heavier ones (up to iron). Substantial amounts of energy are released in cases where the interacting nuclei belong to elements with low atomic numbers. In 1939 the physicist Hans A. Bethe suggested that much of the energy output of the Sun and other stars results from energy-releasing fusion reactions in which four hydrogen nuclei unite and form one helium nucleus. During the early 1950s American researchers produced the hydrogen bomb by inducing fusion reactions in a mixture of the heavy hydrogen isotopes deuterium and tritium, the reactions being ignited by the extremely high temperatures created in the fission reaction of an atomic bomb. More recently, scientists have sought to devise a practical method of controlled nuclear fusion, in which deuterium and tritium nuclei would combine to form helium nuclei under controlled high temperatures rather than in the uncontrollable heat of a detonating atomic bomb. Controlled nuclear fusion would provide a relatively inexpensive alternative energy source for electric-power generation and thereby help conserve the world's dwindling supply of oil, natural gas, and coal. Fusion also would be more advantageous than nuclear fission (q.v.), another kind of energy-producing nuclear reaction that occurs when a heavy nucleus such as the isotope uranium-235 absorbs a neutron, becomes unstable, and splits into two lighter nuclei. Deuterium, the primary fuel for a fusion-power system, is far more abundant and cheaper than any of the materials required for fission reactions, since it can be extracted from ordinary water. (Eight gallons of water contain about one gram of deuterium, which has an energy content equivalent to roughly 9,500 litres [2,500 gallons] of gasoline.) Many experts, however, believe that controlled fusion will not be achieved for some years because of various technical difficulties. A fusion reaction can occur only if two nuclei approach each other within a distance on the order of 10-13 centimetre. At such a short distance it is possible for the nuclear forces of attraction to overcome the electrostatic forces of repulsion that result from the presence of positive electric charges on both nuclei. Because the forces of repulsion are so effective in keeping nuclei apart, reactions useful for controlled fusion seem largely limited to deuterium and tritium, nuclei with the lowest possible charge. Controlled energy-releasing reactions can be produced by heating a plasma (gas consisting of unbound electrons and an equal number of positively charged nuclei) of a deuterium-tritium mixture to many millions of degrees kelvin. Such high temperatures will not only induce but also sustain thermonuclear reactions so as to produce enough energy for electric-power generation. However, it has proved extremely difficult to contain plasmas at the high temperature levels required to achieve self-sustaining fusion reactions because the hot gases tend to expand and escape from the enclosing structure. Large-scale fusion reactor experiments have been conducted in various countries, including the United States and Russia, in an effort to develop a means of overcoming this problem. See also fusion reactor. process by which nuclear reactions between light elements form heavier ones (up to iron). Substantial amounts of energy are released in cases where the interacting nuclei belong to elements with low atomic numbers. Fusion reactions constitute the fundamental energy source of stars, including the Sun. Stellar evolution can be viewed as the passing of a star through various stages as thermonuclear reactions and nucleosynthesis cause compositional changes over long time periods. Hydrogen burning initiates the fusion energy source of stars and leads to the formation of helium. Generation of fusion energy for practical use also relies on fusion reactions between the lightest elements that burn to form helium. In fact, the heavy isotopes of hydrogen, deuterium (2/1H, or D) and tritium (3/1H, or T), react more efficiently with each other and yield more energy per reaction than do two hydrogen nuclei (protons) when they undergo fusion. The deuterium nucleus has one proton and one neutron, while that of tritium consists of a proton bound together with two neutrons. Fusion reactions between light elements, like fission reactions that split heavy elements, release energy due to a key feature of nuclear matter. A parameter called the binding energy of the nucleus is a measure of the efficiency with which its constituent nucleons are bound together. Take, for example, an element with Z protons and N neutrons in its nucleus. The element's atomic mass number or atomic weight, A, is Z + N and its atomic number is Z. The binding energy is the energy associated with the mass difference between the Z protons and N neutrons considered separately and the nucleons bound together (Z + N) in a nucleus of mass, M. The formula is where mp and mn are the proton and neutron masses, and c is the speed of light. It has been determined experimentally that the binding energy per nucleon is a maximum of about 8.8 MeV at an atomic mass number of approximately 60. Accordingly, the fusion of lighter elements or the splitting of heavier ones generally leads to a net release of energy. Additional reading Donald D. Clayton, Principles of Stellar Evolution and Nucleosynthesis (1968, reprinted 1983), a description of nuclear astrophysics, covering energy generation and transport in stars, thermonuclear fusion reactions, and star burning; Francis F. Chen, Introduction to Plasma Physics and Controlled Fusion, vol. 1, Plasma Physics, 2nd ed. (1984), a basic introduction; V.E. Golant, A.P. Zhilinsky, and I.E. Sakharov, Fundamentals of Plasma Physics (1980; originally published in Russian, 1977), an advanced text; J. Raeder et al., Controlled Nuclear Fusion: Fundamentals of Its Utilization for Energy Supply (1986; originally published in German, 1981), an introduction to fusion energy, its technology, and the engineering aspects of conceptual fusion power reactors; Robert W. Conn, The Engineering of Magnetic Fusion Reactors, Scientific American, 249(4):6071 (October 1983), a descriptive article on the technology of fusion machines and future fusion-energy reactors; R. Stephen Craxton, Robert L. McRory, and John M. Soures, Progress in Laser Fusion, Scientific American, 255(2):6879 (August 1986), a description of inertial confinement fusion and specifically laser fusion; and Edward Teller (ed.), Fusion, vol. 1, Magnetic Confinement, 2 vol., pt. A and B (1981), a series of technical articles on confinement approaches to fusion, such as the tokamak, stellarator, and magnetic mirror, and on the technology of fusion energy. Robert W. Conn