Nuclear energy refers to the energy consumed or produced in modifying the composition of the atomic nucleus. The force that arms the \Tatomic bomb\t and \Thydrogen bomb\t and other \Lnuclear weapon\ls, nuclear energy also powers electricity-generating plants in countries throughout the world. It is seen by many as the source of inexpensive, clean power; but, because of the hazardous radiation emitted in producing that power and the \Tradioactivity\t of the materials used, others feel that it may not be a viable energy alternative to the use of fossil fuels or solar energy. This article discusses the science involved in the release of nuclear energy, and the use of that science by the industries that produce electric power. The process by which nuclear-based electricity is produced is examined in \Tnuclear reactor\t, as are some of the safety issues involved. The unwanted byproducts of nuclear energy production are described in \Tnuclear waste\t. BASIC SCIENTIFIC DEFINITIONS The processes that change the state or composition of matter are inevitably accompanied by the consumption or production of energy. Common processes such as combustion produce energy by the chemical rearrangement of atoms or molecules. For example, the combustion of methane (natural gas) is represented by the chemical reaction CH(4) + 2O(2) = CO(2) + 2H(2)O + energy For this example the energy release is 8 electron volts (ev). The electron volt is a unit of energy used by nuclear physicists and represents the gain in kinetic energy when an electron is accelerated through a potential drop of one volt. The most well-known nuclear reaction is fission, in which a heavy nucleus combines with a neutron and separates into two other, lighter nuclei. A typical fission reaction involving uranium-235 is 92 U235 + 1 neutron = 38 Sr96 + 54 XE138 + 2 neutrons+energy where the energy release is about 200 million electron volts (meV), a factor of 25 million greater than the combustion reaction of methane (see \Tfission, nuclear\t). Another important nuclear reaction is fusion, in which two light elements combine to form a heavier atom. An important fusion reaction is 1 H(2) + 1 H(3) = 2 He(4) + 1 neutron + energy where the energy release of the reaction is 18 million eV (see \Tfusion, nuclear\t). Nuclear power plants harness the enormous energy releases from nuclear reactions for large-scale energy production. In a modern coal plant the combustion of one pound of coal produces about 1 kilowatt hour (kWh) of electric energy. The fissioning of one pound of uranium in a modern nuclear power plant produces about 3 million kWh of electric energy. It is the incredible energy density (energy per unit mass) that makes nuclear energy sources of such interest. At present, only the fission process is used in the commercial production of energy, usually to make electricity, but also occasionally to produce steam for district heating or industrial applications. Fusion research has not yet produced a feasible power production technology (see \Tfusion energy\t). DEVELOPMENT OF FISSION TECHNOLOGY The discovery of the fission process occurred in the late 1930s, the result of a long sequence of nuclear physics studies. The German scientists Otto \THahn\t and Fritz Strassmann reported on an experiment involving neutron irradiation of uranium in early 1939. Subsequently, Otto Frisch and Lise \TMeitner\t interpreted the experiment as the fissioning of uranium into lighter elements. The possibility of a self-sustaining chain reaction was apparent, and provided added impetus for accelerated research. Secret government research into the military applications of nuclear fission began with World War II (see \TManhattan Project\t). The development of a weapon required that a self-sustaining fission reaction could be created and, further, that an adequate amount of fissionable material could be produced for use in a weapon. On December 2, 1942, at the University of Chicago, a team under the direction of Enrico \TFermi\t successfully brought the world's first reactor to a self-sustaining, or "critical," state. The reactor was fueled with natural uranium imbedded in graphite blocks. The fissioning occurred in the isotope of uranium, U-235. Natural uranium contains only 0.7% of U-235, while the remaining 99.3% of the uranium is U-238, which does not fission except with very high energy neutrons not available from the fission process. In making a bomb, it was necessary to provide much higher concentrations of U-235, or "enriched uranium," and ultimately, a form of gaseous diffusion was used to separate U-235 from U-238. A significant part of the Manhattan Project was devoted to developing a processing technology to separate U-235 from U-238. Ultimately, the solution was found in a form of gaseous diffusion that was used to separate the two materials. An alternative pathway to obtaining weapons material is to use a different fissionable nucleus. Such a material is the synthetic isotope of plutonium, Pu-239, formed when U-238 reacts with neutrons to produce U-239. U-239 is radioactive and decays in two steps to produce Pu-239. In order to produce plutonium, however, a large reactor is needed to irradiate the U-238. Once Fermi had demonstrated that a critical reactor was feasible, a major effort was undertaken to build reactors to produce plutonium. The first such reactor, in Oak Ridge, Tennessee, was followed by large-scale plants in Hanford, Washington. The technology required to design, build, and operate these plants was developed in a remarkably short time--less than three years--and the knowledge created was the driving force behind the realization of the potential of nuclear energy in the commercial world. The concept of an energy source that promised extended naval voyages without refueling was obviously worth investigation. Under the direction of Hyman \TRickover\t, a naval reactor program began in the late 1940s and the first nuclear submarine, the \TNautilus\t, was launched in 1954. An outstanding success, the Nautilus proved the merits of nuclear propulsion for naval vessels. Its reactor was the prototype for the first commercial nuclear power plant, built in Shippingport, Pennsylvania, in 1957. The decision to declassify much nuclear-related information in order to foster peaceful applications was made by President Dwight D. Eisenhower and announced in his "Atoms for Peace" speech at the United Nations in December 1953. Other nations joined in the search for peaceful uses for the atom, and the first international conference on nuclear energy was held in Geneva in 1955. In the United States the Atomic Energy Commission--founded in 1946 to oversee civilian uses of nuclear power--sponsored research on a host of reactor concepts that led to the birth of the civilian industry. Britain entered into the production of nuclear-fueled electricity in 1956. The first Soviet nuclear power plant came on line in 1954, and the French began construction of their first commercial plants in 1957. By the early 1960s nuclear power had been established as a viable commercial energy source. NUCLEAR ENERGY TODAY In the 50 years since the discovery of fission, nuclear power has become a major source of the world's electric energy. At the end of 1989 there were 416 nuclear plants operating worldwide, generating about 17% of the world's electricity, with another 130 in the design or construction stages. Nuclear plants operate in 27 nations, and 5 additional nations have them under construction. The nuclear energy program in the United States is the world's largest: 108 operating plants (1989) have a capacity of about 100,000 MW and provide nearly 20% of U.S. power generation. Nuclear power is now the second largest source of U.S. electricity, exceeded only by coal, which provides about 55% of the country's electricity. Other contributors to electric generation include natural gas (9%), oil (6%), and hydropower (9%). The nuclear fraction is expected to reach about 25% during the 1990s. In general, nuclear plants are more complex and costly to build than plants using fossil fuels--although the cost of fuel for nuclear plants is significantly lower. On balance, the fuel cost difference is such that nuclear electricity is cheaper than fossil electricity for most nations. For the industrialized countries of Europe and Asia the difference in cost may be as large as a factor of two. The French Nuclear Program The French nuclear program was begun in the 1940s in order to create a nuclear weapons capability. As in the U.S. program, the first French reactors were built for plutonium production. The first French commercial units, which used air as a coolant, were in operation by 1957. Their operation was a technical, but not an economic, success. As a result, in 1970, the French adopted the U.S. light-water technology. Subsequently, the French have built 54 domestic reactors with 9 more under construction. The French standardized their designs to improve the efficiency of construction and operation. They have also built units for Belgium, South Africa, South Korea, and China. The Japanese Nuclear Program The Japanese also have a vigorous and successful nuclear program. Lacking any significant indigenous energy resources, in 1955 the Japanese government selected nuclear power as its major electric-supply technology. The program has carefully nurtured the internal capability to manufacture equipment and construct nuclear plants, to operate a high-quality power system, and to provide complete technology for the entire fuel cycle. The utilities in Japan have become leaders in plant operation; and by 2020 the nuclear-fueled portion of Japan's electric supply is expected to exceed 50 percent. In the future the Japanese plan to exploit the potential of \Lbreeder reactor\ls, which convert nonfissionable U-238 into fissionable plutonium-239. A successful breeder reactor program could eliminate Japan's need to import any fuels for the production of electricity. To date, however, the cost of electricity from breeders exceeds the cost from conventional light-water reactors. The Japanese long-range policy assumes that uranium fuel will ultimately become scarce, making the breeder technology economical. The Slowdown in Other National Programs Nuclear power programs in most other countries have come to a virtual standstill. (In the United States there has not been an order for a new plant since the mid-1970s.). A major cause has been the move toward increased efficiency in the consumption of oil, and a drop in energy demand. Equally significant have been concerns about the safety of nuclear reactors and in increasing awareness of the problems created by nuclear waste. Public opinion remained largely favorable toward nuclear energy until the Three Mile Island (TMI) reactor accident in the spring of 1979. The accident began with the failure of some of the plant hardware. By itself, the failure would not have caused serious damage to the reactor, but a series of mistakes in interpreting the condition of the reactor led to more mistakes, which removed reactor coolant and caused a sizable portion of the fuel to melt. Although there was extensive damage to the reactor, the containment system functioned, preventing the release of much radioactivity to the environment. Nevertheless, there was widespread apprehension for several days among the nearby population. The events at TMI captured the attention of the world, dominated the media for days, and caused a historic shift in attitudes toward nuclear power. The accident also had serious impacts on the licensing of new plants. Regulations were drastically modified to prevent a recurrence of the events of TMI. The modifications complicated the construction of new plants as well as the operation of existing plants. Construction times expanded from about 6 years to more than 12 years, and plant costs accelerated rapidly because of the new requirements. Another factor contributing to the stagnation of new construction was the intervention by anti-nuclear groups in licensing proceedings for new plants. Such intervention has proven to be time consuming and costly to the industry, particularly for those plants in the late stages of construction, when interest costs mount on the billions that have been borrowed. The Shoreham (New York) and Seabrook (New Hampshire) plants are notable examples of cost overruns, caused in part by completion delays. Although few other countries permit the extent of public intervention in licensing hearings that is allowed in the United States, all the major nuclear nations impose strict regulations on their nuclear energy plants. Nevertheless, studies indicate that, for the most part, the U.S. industry performs far less efficiently than do those in Switzerland, Germany, France, and Japan. A key factor in their superior performance may be the cooperation that exists between the industries and their suppliers and regulators--a cooperation that, until recently, was not apparent in the United States. In its early years, nuclear power was cost competitive with coal. Some of the cheapest sources of electricity in the United States today are nuclear plants built in the period before TMI. The current environment, however, has made nuclear power an uneconomical choice for U.S. utilities. Chernobyl The accident in April 1986 at the \TChernobyl\t plant in the \TUSSR\t was as devastating as a nuclear accident can be. A very large amount of radioactive material--between 30% and 50% of the total material in the reactor--was released. Radioactive fallout from the event spread, forcing the long-term evacuation of over 100,000 local people and causing the pollution of foods in large portions of Europe. The Chernobyl reactor design uses water as a coolant and graphite as the moderator. This type of reactor is known to be hazardous and is used only in the \TUSSR\t. (Such a design would not be licensed in the Western nations.) Nevertheless, the accident has profoundly influenced worldwide public acceptance of nuclear power. It is too early to know whether or not the Chernobyl accident has permanently crippled the future of nuclear power in industrialized countries. Kent F. Hansen Bibliography: Kneif, Ronald A., Nuclear Energy Technology (1981); Landsberg, Hans H., et al., Energy: The Next Twenty Years (1979); Leclercq, Jacques, The Nuclear Age (1986); Morone, Joseph G., and Woodhouse, Edward J., The Demise of Nuclear Energy (1989); Rhodes, Richard, The Making of the Atomic Bomb (1986); Schmidt, Fred H., and Bodansky, David, The Fight over Nuclear Power (1976); Smyth, Henry D., Atomic Energy for Military Purposes, new, enlarged ed. (1989); Spurgeon, M. Keeny, Jr., et al., Nuclear Power: Issues and Choices (1977).