AEC
(AEC),= U.S. federal civilian agency established by the Atomic Energy Act, which was signed into law by President Harry S. Truman on Aug. 1, 1946, to control the development and production of nuclear weapons and to direct the research and development of peaceful uses of nuclear energy. On Dec. 31, 1946, the AEC succeeded the Manhattan Engineer District of the U.S. Army Corps of Engineers (which had developed the atomic bomb during World War II) and thus officially took control of the nation's nuclear program.
A nuclear explosion releases energy in a variety of forms, including blast, heat, and radiation (X rays, gamma rays, and neutrons). By varying a weapon's design, these effects could be tailored for a specific military purpose. In an enhanced-radiation weapon, more commonly called a neutron bomb, the objective was to minimize the blast by reducing the fission yield and to enhance the neutron radiation. Such a weapon would prove lethal to invading troops without, it was hoped, destroying the defending country's towns and countryside. It was actually a small (on the order of one kiloton), two-stage thermonuclear weapon that utilized deuterium and tritium, rather than lithium deuteride, to maximize the release of fast neutrons. The first
Though it had virtually created the American nuclear-power industry, the AEC also had to regulate that industry to ensure public health and safety and to safeguard national security. Because these dual roles often conflicted with each other, the U.S. government under the Energy Reorganization Act of 1974 disbanded the AEC and divided its functions between two new agencies: the Nuclear Regulatory Commission (q.v.), which regulates the nuclear-power industry; and the Energy Research and Development Administration, which was disbanded in 1977 when the Department of Energy was created.
"""""autonomous intergovernmental organization dedicated to increasing the contribution of atomic energy to the world's peace and well-being and ensuring that agency assistance is not used for military purposes. The IAEA and its director general, Mohamed ElBaradei, won the Nobel Prize for Peace in 2005.
The agency was established by representatives of more than 80 countries in October 1956, nearly three years after U.S. President Dwight D. Eisenhower's “Atoms for Peace” speech to the United Nations General Assembly, in which Eisenhower called for the creation of an international organization for monitoring the diffusion of nuclear resources and technology. The IAEA's statute officially came into force on July 29, 1957. Its activities include research on the applications of atomic energy to medicine, agriculture, water resources, and industry; the operation of conferences, training programs, fellowships, and publications to promote the exchange of technical information and skills; the provision of technical assistance, especially to less-developed countries; and the establishment and administration of radiation safeguards. As part of the Treaty on the Non-Proliferation of Nuclear Weapons (1968), all non-nuclear powers are required to negotiate a safeguards agreement with the IAEA; as part of that agreement, the IAEA is given authority to monitor nuclear programs and to inspect nuclear facilities.
The General Conference, consisting of all members (in the early 21st century some 135 countries were members), meets annually to approve the budget and programs and to debate the IAEA's general policies; it also is responsible for approving the appointment of a director general and admitting new members. The Board of Governors, which consists of 35 members who meet about five times per year, is charged with carrying out the agency's statutory functions, approving safeguards agreements, and appointing the director general. The day-to-day affairs of the IAEA are run by the Secretariat, which is headed by the director general, who is assisted by six deputies; the Secretariat's departments include nuclear energy, nuclear safety, nuclear sciences and application, safeguards, and technical cooperation. Headquarters are in
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also called atomic weapon , or thermonuclear weapon bomb or other warhead that derives its force from either the fission or the fusion of atomic nuclei and is delivered by an aircraft, missile, Earth satellite, or other strategic delivery system.
Nuclear weapons have enormous explosive force. Their significance may best be appreciated by the coining of the words kiloton (1,000 tons) and megaton (one million tons) to describe their blast effect in equivalent weights of TNT. For example, the first nuclear fission bomb, the one dropped on
The first nuclear weapons were bombs delivered by aircraft; warheads for strategic ballistic missiles, however, have become by far the most important nuclear weapons. There are also smaller tactical nuclear weapons that include artillery projectiles, demolition munitions (land mines), antisubmarine depth bombs, torpedoes, and short-range ballistic and cruise missiles. The
The basic principle of nuclear fission weapons (also called atomic bombs) involves the assembly of a sufficient amount of fissile material (e.g., the uranium isotope uranium-235 or the plutonium isotope plutonium-239) to “go supercritical”—that is, for neutrons (which cause fission and are in turn released during fission) to be produced at a much faster rate than they can escape from the assembly. There are two ways in which a subcritical assembly of fissionable material can be rendered supercritical and made to explode. The subcritical assembly may consist of two parts, each of which is too small to have a positive multiplication rate; the two parts can be shot together by a gun-type device. Alternatively, a subcritical assembly surrounded by a chemical high explosive may be compressed into a supercritical one by detonating the explosive.
The basic principle of the fusion weapon (also called the thermonuclear or hydrogen bomb) is to produce ignition conditions in a thermonuclear fuel such as deuterium, an isotope of hydrogen with double the weight of normal hydrogen, or lithium deuteride. The Sun may be considered a thermonuclear device; its main fuel is deuterium, which it consumes in its core at temperatures of 18,000,000° to 36,000,000° F (10,000,000° to 20,000,000° C). To achieve comparable temperatures in a weapon, a fission triggering device is used.
Following the discovery of artificial radioactivity in the 1930s, the Italian physicist Enrico Fermi performed a series of experiments in which he exposed many elements to low-velocity neutrons. When he exposed thorium and uranium, chemically different radioactive products resulted, indicating that new elements had been formed, rather than merely isotopes of the original elements. Fermi concluded that he had produced elements beyond uranium (element 92), then the last element in the periodic table; he called them transuranic elements and named two of them ausenium (element 93) and hesperium (element 94). During the autumn of 1938, however, when Fermi was receiving the Nobel Prize for his work, Otto Hahn and Fritz Strassmann of Germany discovered that one of the “new” elements was actually barium (element 56).
The Danish scientist Niels Bohr visited the United States in January 1939, carrying with him an explanation, devised by the Austrian refugee scientist Lise Meitner and her nephew Otto Frisch, of the process behind Hahn's surprising data. Low-velocity neutrons caused the uranium nucleus to fission, or break apart, into two smaller pieces; the combined atomic numbers of the two pieces—for example, barium and krypton—equalled that of the uranium nucleus. Much energy was released in the process. This news set off experiments at many laboratories. Bohr worked with John Wheeler at Princeton; they postulated that the uranium isotope uranium-235 was the one undergoing fission; the other isotope, uranium-238, merely absorbed the neutrons. It was discovered that neutrons were produced during the fission process; on the average, each fissioning atom produced more than two neutrons. If the proper amount of material were assembled, these free neutrons might create a chain reaction. Under special conditions, a very fast chain reaction might produce a very large release of energy; in short, a weapon of fantastic power might be feasible.
The possibility that such a weapon might first be developed by Nazi Germany alarmed many scientists and was drawn to the attention of President Franklin D. Roosevelt by Albert Einstein, then living in the
During the summer of 1940, Edwin McMillan and Philip Abelson of the University of California at Berkeley discovered element 93, named neptunium; they inferred that this element would decay into element 94. The Bohr and Wheeler fission theory suggested that one of the isotopes, mass number 239, of this new element might also fission under low-velocity neutron bombardment. The cyclotron at the University of California at Berkeley was put to work to make enough element 94 for experiments; by mid-1941, element 94 had been firmly identified and named plutonium, and its fission characteristics had been established. Low-velocity neutrons did indeed cause it to undergo fission, and at a rate much higher than that of uranium-235. The
In May 1941 a review committee reported that a nuclear explosive probably could not be available before 1945, that a chain reaction in natural uranium was probably 18 months off, and that it would take at least an additional year to produce enough plutonium for a bomb and three to five years to separate enough uranium-235. Further, it was held that all of these estimates were optimistic. In late June 1941 President Roosevelt established the Office of Scientific Research and Development under the direction of the scientist Vannevar Bush.
In the fall of 1941 the
The U.S. entry into World War II in December 1941 was decisive in providing funds for a massive research and production effort for obtaining fissionable materials, and in May 1942 the momentous decision was made to proceed simultaneously on all promising production methods. Bush decided that the army should be brought into the production plant construction activities. The Corps of Engineers opened an office in
Meantime, as part of the June 1942 reorganization, J. Robert Oppenheimer became, in October, the director of Project Y, the group that was to design the actual weapon. This effort was spread over several locations. On November 16
The emphasis during the summer and fall of 1943 was on the gun method of assembly, in which the projectile, a subcritical piece of uranium-235 (or plutonium-239), would be placed in a gun barrel and fired into the target, another subcritical piece of uranium-235. After the mass was joined (and now supercritical), a neutron source would be used to start the chain reaction. A problem developed with applying the gun method to plutonium, however. In manufacturing plutonium-239 from uranium-238 in a reactor, some of the plutonium-239 absorbs a neutron and becomes plutonium-240. This material undergoes spontaneous fission, producing neutrons. Some neutrons will always be present in a plutonium assembly and cause it to begin multiplying as soon as it goes critical, before it reaches supercriticality; it will then explode prematurely and produce comparatively little energy. The gun designers tried to beat this problem by achieving higher projectile speeds, but they lost out in the end to a better idea—the implosion method.
In April 1943 a Project Y physicist, Seth Neddermeyer, proposed to assemble a supercritical mass from many directions, instead of just two as in the gun. In particular, a number of shaped charges placed on the surface of a sphere would fire many subcritical pieces into one common ball at the centre of the sphere. John von Neumann, a mathematician who had had experience in shaped-charge, armour-piercing work, supported the implosion method enthusiastically and pointed out that the greater speed of assembly might solve the plutonium-240 problem. The physicist Edward Teller suggested that the converging material might also become compressed, offering the possibility that less material would be needed. By late 1943 the implosion method was being given an increasingly higher priority; by July 1944 it had become clear that the plutonium gun could not be built. The only way to use plutonium in a weapon was by the implosion method.
By 1944 the Manhattan Project was spending money at a rate of more than $1 billion per year. The situation was likened to a nightmarish horse race; no one could say which of the horses (the calutron plant, the diffusion plant, or the plutonium reactors) was likely to win or whether any of them would even finish the race. In July 1944 the first Y-12 calutrons had been running for three months but were operating at less than 50 percent efficiency; the main problem was in recovering the large amounts of material that reached neither the uranium-235 nor uranium-238 boxes and, thus, had to be rerun through the system. The gaseous diffusion plant was far from completion, the production of satisfactory barriers remaining the major problem. And the first plutonium reactor at
Within 24 hours of Roosevelt's death on April 12, 1945, President Harry S. Truman was told briefly about the atomic bomb by Secretary of War Henry Stimson. On April 25 Stimson, with
The test of the plutonium weapon was named Trinity; it was fired at 5:29:45 AM (local time) on July 16, 1945, at the
A single B-29 bomber, named the Enola Gay, flew over Hiroshima,
Scientists in several countries performed experiments in connection with nuclear reactors and fission weapons during World War II, but no country other than the
By the time the war began on Sept. 1, 1939, Germany had a special office for the military application of nuclear fission; chain-reaction experiments with uranium and carbon were being planned, and ways of separating the uranium isotopes were under study. Some measurements on carbon, later shown to be in error, led the physicist Werner Heisenberg to recommend that heavy water be used, instead, for the moderator. This dependence on scarce heavy water was a major reason the German experiments never reached a successful conclusion. The isotope separation studies were oriented toward low enrichments (about 1 percent uranium-235) for the chain reaction experiments; they never got past the laboratory apparatus stage, and several times these prototypes were destroyed in bombing attacks. As for the fission weapon itself, it was a rather distant goal, and practically nothing but “back-of-the-envelope” studies were done on it.
Like their counterparts elsewhere, Japanese scientists initiated research on an atomic bomb. In December 1940,
The British weapon project started informally, as in the
The formal postwar decision to manufacture a British atomic bomb was made by Prime Minister Clement Attlee's government during a meeting of the Defence Subcommittee of the Cabinet in early January 1947. The construction of a first reactor to produce fissile material and associated facilities had gotten under way the year before. William Penney, a member of the British team at Los Alamos during the war, was placed in charge of fabricating and testing the bomb, which was to be of a plutonium type similar to the one dropped on
In the decade before the war, Soviet physicists were actively engaged in nuclear and atomic research. By 1939 they had established that, once uranium has been fissioned, each nucleus emits neutrons and can therefore, at least in theory, begin a chain reaction. The following year, physicists concluded that such a chain reaction could be ignited in either natural uranium or its isotope, uranium-235, and that this reaction could be sustained and controlled with a moderator such as heavy water. In June 1940 the Soviet Academy of Sciences established the Uranium Commission to study the “uranium problem.”
In February 1939, news had reached Soviet physicists of the discovery of nuclear fission in the West. The military implications of such a discovery were immediately apparent, but Soviet research was brought to a halt by the German invasion in June 1941. In early 1942 the physicist Georgy N. Flerov noticed that articles on nuclear fission were no longer appearing in western journals; this indicated that research on the subject had become secret. In response, Flerov wrote to, among others, Premier Joseph Stalin, insisting that “we must build the uranium bomb without delay.” In 1943 Stalin ordered the commencement of a research project under the supervision of Igor V. Kurchatov, who had been director of the nuclear physics laboratory at the Physico-Technical Institute in
By the end of 1944, 100 scientists were working under Kurchatov, and by the time of the Potsdam Conference, which brought the Allied leaders together the day after the Trinity test, the project on the atomic bomb was seriously under way. During one session at the conference, Truman remarked to Stalin that the
Upon his return from
French scientists, such as Henri Becquerel, Marie and Pierre Curie, and Frédéric and Irène Joliot-Curie, made important contributions to 20th-century atomic physics. During World War II several French scientists participated in an Anglo-Canadian project in
On Oct. 18, 1945, the Atomic Energy Commission (Commissariat à l'Énergie Atomique; CEA) was established by General Charles de Gaulle with the objective of exploiting the scientific, industrial, and military potential of atomic energy. The military application of atomic energy did not begin until 1951. In July 1952 the National Assembly adopted a five-year plan, a primary goal of which was to build plutonium production reactors. Work began on a reactor at Marcoule in the summer of 1954 and on a plutonium separating plant the following year.
On Dec. 26, 1954, the issue of proceeding with a French bomb was raised at Cabinet level. The outcome was that Prime Minister Pierre Mendès-France launched a secret program to develop a bomb. On Nov. 30, 1956, a protocol was signed specifying tasks the CEA and the Defense Ministry would perform. These included providing the plutonium, assembling a device, and preparing a test site. On July 22, 1958, de Gaulle, who had resumed power as prime minister, set the date for the first atomic explosion to occur within the first three months of 1960. On Feb. 13, 1960, the French detonated their first atomic bomb from a 330-foot tower in the
On Jan. 15, 1955, Mao Zedong (Mao Tse-tung) and the Chinese leadership decided to obtain their own nuclear arsenal. From 1955 to 1958 the Chinese were partially dependent upon the
Unlike the initial
On May 18, 1974, India detonated a nuclear device in the Rājasthān desert near Pokaran with a reported yield of 15 kilotons.
Several other countries were believed to have built nuclear weapons or to have acquired the capability of assembling them on short notice. Israel was believed to have built an arsenal of more than 200 weapons, including thermonuclear bombs. In August 1988 the South African foreign minister said that South Africa had “the capability to [produce a nuclear bomb] should we want to.” Argentina, Brazil, South Korea, and Taiwan also had the scientific and industrial base to develop and produce nuclear weapons, but they did not seem to have active programs.
U.S. research on thermonuclear weapons started from a conversation in September 1941 between Fermi and Teller. Fermi wondered if the explosion of a fission weapon could ignite a mass of deuterium sufficiently to begin thermonuclear fusion. (Deuterium, an isotope of hydrogen with one proton and one neutron in the nucleus—i.e., twice the normal weight—makes up 0.015 percent of natural hydrogen and can be separated in quantity by electrolysis and distillation. It exists in liquid form only below about −417° F, or −250° C.) Teller undertook to analyze the thermonuclear processes in some detail and presented his findings to a group of theoretical physicists convened by Oppenheimer in
As a result of these discussions the participants concluded that a weapon based on thermonuclear fusion was possible. When the
In the fall of 1945, after the success of the atomic bomb and the end of World War II, the future of the Manhattan Project, including
From April 18 to 20, 1946, a conference led by Teller at
One of the two central design problems was how to ignite the thermonuclear fuel. It was recognized early on that a mixture of deuterium and tritium theoretically could be ignited at lower temperatures and would have a faster reaction time than deuterium alone, but the question of how to achieve ignition remained unresolved. The other problem, equally difficult, was whether and under what conditions burning might proceed in thermonuclear fuel once ignition had taken place. An exploding thermonuclear weapon involves many extremely complicated, interacting physical and nuclear processes. The speeds of the exploding materials can be up to millions of feet per second, temperatures and pressures are greater than those at the centre of the Sun, and time scales are billionths of a second. To resolve whether the “classical Super” or any other design would work required accurate numerical models of these processes—a formidable task, since the computers that would be needed to perform the calculations were still under development. Also, the requisite fission triggers were not yet ready, and the limited resources of
On Sept. 23, 1949, Truman announced that “we have evidence that within recent weeks an atomic explosion occurred in the U.S.S.R.” This first Soviet test stimulated an intense, four-month, secret debate about whether to proceed with the hydrogen bomb project. One of the strongest statements of opposition against proceeding with a hydrogen bomb program came from the General Advisory Committee (GAC) of the AEC, chaired by Oppenheimer. In their report of Oct. 30, 1949, the majority recommended “strongly against” initiating an all-out effort, believing “that extreme dangers to mankind inherent in the proposal wholly outweigh any military advantages that could come from this development.” “A super bomb,” they went on to say, “might become a weapon of genocide.” They believed that “a super bomb should never be produced.” Nevertheless, the Joint Chiefs of Staff, the State and Defense departments, the Joint Committee on Atomic Energy, and a special subcommittee of the National Security Council all recommended proceeding with the hydrogen bomb. Truman announced on Jan. 31, 1950, that he had directed the AEC to continue its work on all forms of atomic weapons, including hydrogen bombs. In March,
In the months that followed Truman's decision, the prospect of actually being able to build a hydrogen bomb became less and less likely. The mathematician Stanislaw M. Ulam, with the assistance of Cornelius J. Everett, had undertaken calculations of the amount of tritium that would be needed for ignition of the classical Super. Their results were spectacular and, to Teller, discouraging: the amount needed was estimated to be enormous. In the summer of 1950 more detailed and thorough calculations by other members of the Los Alamos Theoretical Division confirmed Ulam's estimates. This meant that the cost of the Super program would be prohibitive.
Also in the summer of 1950, Fermi and Ulam calculated that liquid deuterium probably would not burn—that is, there would probably be no self-sustaining and propagating reaction. Barring surprises, therefore, the theoretical work to 1950 indicated that every important assumption regarding the viability of the classical Super was wrong. If success was to come, it would have to be accomplished by other means.
The other means became apparent between February and April 1951, following breakthroughs achieved at
The major figures in these breakthroughs were Ulam and Teller. In December 1950 Ulam had proposed a new fission weapon design, using the mechanical shock of an ordinary fission bomb to compress to a very high density a second fissile core. (This two-stage fission device was conceived entirely independently of the thermonuclear program, its aim being to use fissionable materials more economically.) Early in 1951 Ulam went to see Teller and proposed that the two-stage approach be used to compress and ignite a thermonuclear secondary. Teller suggested radiation implosion, rather than mechanical shock, as the mechanism for compressing the thermonuclear fuel in the second stage. On March 9, 1951, Teller and Ulam presented a report containing both alternatives, entitled “On Heterocatalytic Detonations I. Hydrodynamic Lenses and Radiation Mirrors.” A second report, dated April 4, by Teller, included some extensive calculations by Frederic de Hoffmann and elaborated on how a thermonuclear bomb could be constructed. The two-stage radiation implosion design proposed by these reports, which led to the modern concept of thermonuclear weapons, became known as the Teller–Ulam configuration.
It was immediately clear to all scientists concerned that these new ideas—achieving a high density in the thermonuclear fuel by compression using a fission primary—provided for the first time a firm basis for a fusion weapon. Without hesitation,
Just prior to the conference, on May 8 at Enewetak atoll in the western Pacific, a test explosion called George had successfully used a fission bomb to ignite a small quantity of deuterium and tritium. The original purpose of George had been to confirm the burning of these thermonuclear fuels (about which there had never been any doubt), but with the new conceptual understanding contributed by Teller and Ulam, the test provided the bonus of successfully demonstrating radiation implosion.
In September 1951,
With the Teller–Ulam configuration proved, deliverable thermonuclear weapons were designed and initially tested during
With completion of Castle, the feasibility of lightweight, solid-fuel thermonuclear weapons was proved. Vast quantities of tritium would not be needed after all. New possibilities for adaptation of thermonuclear weapons to various kinds of missiles began to be explored.
In 1948 Kurchatov organized a theoretical group, under the supervision of physicist Igor Y. Tamm, to begin work on a fusion bomb. (This group included Andrey Sakharov, who, after contributing several important ideas to the effort, later became known as the “father of the Soviet H-bomb.”) In general, the Soviet program was two to three years behind that of the
Minister of Defence Harold Macmillan announced in his Statement of Defence, on Feb. 17, 1955, that the
It remained unclear exactly when the first British thermonuclear test occurred. Three high-yield tests in May and June 1957 near
Well before their first atomic test, the French assumed they would eventually have to become a thermonuclear power as well. The first French thermonuclear test was conducted on Aug. 24, 1968.
Plans to proceed toward a Chinese hydrogen bomb were begun in 1960, with the formation of a group by the
From the late 1940s,
The first advances came through the test series Operation Sandstone, conducted in the spring of 1948. These three tests used implosion designs of a second generation, which incorporated composite and levitated cores. A composite core consisted of concentric shells of both uranium-235 and plutonium-239, permitting more efficient use of these fissile materials. Higher compression of the fissile material was achieved by levitating the core—that is, introducing an air gap into the weapon to obtain a higher yield for the same amount of fissile material.
Tests during Operation Ranger in early 1951 included implosion devices with cores containing a fraction of a critical mass—a concept originated in 1944 during the Manhattan Project. Unlike the original Fat Man design, these “fractional crit” weapons relied on compressing the fissile core to a higher density in order to achieve a supercritical mass. These designs could achieve appreciable yields with less material.
One technique for enhancing the yield of a fission explosion was called “boosting.” Boosting referred to a process whereby thermonuclear reactions were used as a source of neutrons for inducing fissions at a much higher rate than could be achieved with neutrons from fission chain reactions alone. The concept was invented by Teller by the middle of 1943. By incorporating deuterium and tritium into the core of the fissile material, a higher yield could be obtained from a given quantity of fissile material—or, alternatively, the same yield could be achieved with a smaller amount. The fourth test of Operation Greenhouse, on May 24, 1951, was the first proof test of a booster design. In subsequent decades approximately 90 percent of nuclear weapons in the
Refinements of the basic two-stage Teller–Ulam configuration resulted in thermonuclear weapons with a wide variety of characteristics and applications. Some high-yield deliverable weapons incorporated additional thermonuclear fuel (lithium deuteride) and fissionable material (uranium-235 and uranium-238) in a third stage. While there was no theoretical limit to the yield that could be achieved from a thermonuclear bomb (for example, by adding more stages), there were practical limits on the size and weight of weapons that could be carried by aircraft or missiles. The largest
The AEC was headed by a five-member board of commissioners, one of whom served as chairman. During the late 1940s and early '50s, the AEC devoted most of its resources to developing and producing nuclear weapons, though it also built several small-scale nuclear-power plants for research purposes. In 1954 the Atomic Energy Act was revised to permit private industry to build nuclear reactors (for electric power), and in 1956 the AEC authorized construction of the world's first two large, privately owned atomic-power plants. Under the chairmanship (1961–71) of Glenn T. Seaborg, the AEC worked with private industry to develop nuclear fission reactors that were economically competitive with thermal generating plants, and the 1970s witnessed an ever-increasing commercial utilization of nuclear power in the United States.