Themes > Science > Chemistry > Nuclear Chemistry > Nuclear Weapons > The First Nuclear Chain Reaction > Nuclear Fission Bombs

Nuclear Weapons, explosive devices, designed to release nuclear energy on a large scale, used primarily in military applications. The first atomic bomb (or A-bomb), which was tested on July 16, 1945, at Alamogordo, New Mexico, represented a completely new type of artificial explosive. All explosives prior to that time derived their power from the rapid burning or decomposition of some chemical compound. Such chemical processes release only the energy of the outermost electrons in the atom. Deadly Mushroom Cloud over Hiroshima On August 6, 1945, during World War II, the United States dropped an atomic bomb on Hiroshima, a Japanese city and military center. About 130,000 people were reported killed, injured, or missing. Another 177,000 were made homeless. It was the first atomic bomb used against an enemy. Nuclear explosives, on the other hand, involve energy sources within the core, or nucleus, of the atom. The A-bomb gained its power from the splitting, or fission, of all the atomic nuclei in several kilograms of plutonium. A sphere about the size of a baseball produced an explosion equal to 20,000 tons of TNT.  The A-bomb was developed, constructed, and tested by the Manhattan Project, a massive United States enterprise that was established in August 1942, during World War II. Many prominent American scientists including the physicists Enrico Fermi and J. Robert Oppenheimer, and the chemist Harold Urey, were associated with the project, which was headed by a U.S. Army engineer, Major General Leslie Groves. After the war, the U.S. Atomic Energy Commission became responsible for the oversight of all nuclear matters, including weapons research. Other types of bombs were developed to tap the energy of light elements, such as hydrogen. In these bombs the source of energy is the fusion process, in which nuclei of the isotopes of hydrogen combine to form a heavier helium nucleus. This weapons research has resulted in the production of bombs that range in power from a fraction of a kiloton (1000 tons of TNT equivalent) to many megatons (1 million tons of TNT equivalent). Furthermore, the physical size of the bomb has been drastically reduced, permitting the development of nuclear artillery shells and small missiles that can be fired from portable launchers in the field. Although nuclear bombs were originally developed as strategic weapons to be carried by large bombers, nuclear weapons are now available for a variety of both strategic and tactical applications. Not only can they be delivered by different types of aircraft, but rockets and guided missiles of many sizes can now carry nuclear warheads and can be launched from the ground, the air, or underwater. Large rockets can carry multiple warheads for delivery to separate targets. Fission Weapons In 1905 Albert Einstein published his special theory of relativity. According to this theory, the relation between mass and energy is expressed by the equation E = mc2, which states that a given mass (m) is associated with an amount of energy (E) equal to this mass multiplied by the square of the speed of light (c). A very small amount of matter is equivalent to a vast amount of energy. For example, 1 kg (2.2 lb) of matter converted completely into energy would be equivalent to the energy released by exploding 22 megatons of TNT. In 1939, as a result of experiments by the German chemists Otto Hahn and Fritz Strassmann (1902-80), who split the uranium atom into two roughly equal parts by bombardment with neutrons See Neutron, the Austrian physicist Lise Meitner, with her nephew, the British physicist Otto Robert Frisch (1904-79), explained the process of nuclear fission, which placed the release of atomic energy within reach. The Chain Reaction When the uranium or other suitable nucleus fissions, it breaks up into a pair of nuclear fragments and releases energy. At the same time, the nucleus emits very quickly a number of fast neutrons, the same type of particle that initiated the fission of the uranium nucleus. This makes it possible to achieve a self-sustaining series of nuclear fissions; the neutrons that are emitted in fission produce a chain reaction, with continuous release of energy.

Nuclear Fission and Nuclear Fusion Both nuclear fission and nuclear fusion reactions can be used to generate large amounts of energy for destructive purposes. When an atom of 235U is bombarded by a neutron, it splits into atoms of cesium and rubidium, releasing a large amount of energy and three additional neutrons. These neutrons, if not controlled, can then cause more 235U atoms to split, leading rapidly to a nuclear explosion (A-bomb). Fusion reactions release energy when two light nuclei combine to make a heavier atom. Fission Bomb The first atomic bomb used in warfare was dropped by the United States on August 6, 1945. Called Little Boy, it produced an explosion that devastated the city of Hiroshima in Japan and killed tens of thousands of people in less than one minute. In this bomb, a mass of uranium about the size of a baseball produced an explosion as powerful as 20 kilotons of TNT. Little Boy, which is schematized here, was a gun-type fission bomb. A small wedge of uranium was fired at a larger target piece of uranium and, upon impact, the two pieces fused together briefly, forming what is called a supercritical mass (a mass slightly greater than that necessary to sustain a chain reaction). The light isotope of uranium, uranium-235, is easily split by the fission neutrons and, upon fission, emits an average of about 2.5 neutrons. One neutron per generation of nuclear fissions is necessary to sustain the chain reactions. Others may be lost by escape from the mass of chain-reacting material, or they may be absorbed in impurities or in the heavy uranium isotope, uranium-238, if it is present. Any substance capable of sustaining a fission chain reaction is known as a fissile material. Critical Mass  A small sphere of pure fissile material, such as uranium-235, about the size of a golf ball, would not sustain a chain reaction. Too many neutrons escape through the surface area, which is relatively large compared with its volume, and thus are lost to the chain reaction. In a mass of uranium-235 about the size of a baseball, however, the number of neutrons lost through the surface is compensated for by the neutrons generated in additional fissions taking place within the sphere. The minimum amount of fissile material (of a given shape) required to maintain the chain reaction is known as the critical mass. Increasing the size of the sphere produces a supercritical assembly, in which the successive generations of fissions increase very rapidly, leading to a possible explosion as a result of the extremely rapid release of a large amount of energy. In an atomic bomb, therefore, a mass of fissile material greater than the critical size must be assembled instantaneously and held together for about a millionth of a second to permit the chain reaction to propagate before the bomb explodes. A heavy material, called a tamper, surrounds the fissile mass and prevents its premature disruption. The tamper also reduces the number of neutrons that escape. If every atom in 0.5 kg (1.1 lb) of uranium were to split, the energy produced would equal the explosive power of 9.9 kilotons of TNT. In this hypothetical case, the efficiency of the process would be 100 percent. In the first A-bomb tests, this kind of efficiency was not approached. Moreover, a 0.5-kg (1.1-lb) mass is too small for a critical assembly. Detonation of Atomic Bombs Various systems have been devised to detonate the atomic bomb. The simplest system is the gun-type weapon, in which a projectile made of fissile material is fired at a target of the same material so that the two weld together into a supercritical assembly. The atomic bomb exploded by the United States over Hiroshima, Japan, on August 6, 1945, was a gun-type weapon. It had the energy of anywhere between 12.5 to 15 kilotons of TNT. Three days later the United States dropped a second atomic bomb over Nagasaki, Japan, with the energy equivalent of close to 22 kilotons of TNT.

First Atomic Bomb Developed by American scientists, the first atomic bomb was detonated at the Trinity test site near Alamogordo, New Mexico, on July 16, 1945. The test convinced the United States government that such weapons were viable in warfare. A more complex method, known as implosion, is utilized in a spherically shaped weapon. The outer part of the sphere consists of a layer of closely fitted and specially shaped devices, called lenses, consisting of high explosive and designed to concentrate the blast toward the center of the bomb. Each segment of the high explosive is equipped with a detonator, which in turn is wired to all other segments. An electrical impulse explodes all the chunks of high explosive simultaneously, resulting in a detonation wave that converges toward the core of the weapon. At the core is a sphere of fissile material, which is compressed by the powerful, inwardly directed pressure, or implosion. The density of the metal is increased, and a supercritical assembly is produced. The Alamogordo test bomb, as well as the one dropped by the U.S. on Nagasaki, Japan, on August 9, 1945, were of the implosion type. Each was equivalent to about 20 kilotons of TNT.  Regardless of the method used to attain a supercritical assembly, the chain reaction proceeds for about a millionth of a second, liberating vast amounts of heat energy. The extremely fast release of a very large amount of energy in a relatively small volume causes the temperature to rise to tens of millions of degrees. The resulting rapid expansion and vaporization of the bomb material causes a powerful explosion. Production of Fissile Material Much experimentation was necessary to make the production of fissile material practical. Separation of Uranium Isotopes The fissile uranium-235 isotope accounts for only 0.7 percent of natural uranium; the remainder is composed of the heavier uranium-238. No chemical methods suffice to separate uranium-235 from ordinary uranium, because both uranium isotopes are chemically identical. A number of techniques were devised to separate the two, all of which depend in principle on the slight difference in weight between the two types of uranium atoms. A huge gaseous-diffusion plant was built during World War II in Oak Ridge, Tennessee. This plant was enlarged after the war, and two similar plants were built near Paducah, Kentucky, and Portsmouth, Ohio. The feed material for this type of plant consists of extremely corrosive uranium hexafluoride gas, UF. The gas is pumped against barriers that have many millions of tiny holes, through which the lighter molecules, which contain uranium-235 atoms, diffuse at a slightly greater rate than the heavier molecules, containing uranium-238. After the gas has been cycled through thousands of barriers, known as stages, it is highly enriched in the lighter isotope of uranium. The final product is weapon-grade uranium containing more than 90 percent uranium-235. Producing Plutonium Although the heavy uranium isotope uranium-238 will not sustain a chain reaction, it can be converted into a fissile material by bombarding it with neutrons and transforming it into a new species of element. When the uranium-238 atom captures a neutron in its nucleus, it is transformed into the heavier isotope uranium-239. This nuclear species quickly disintegrates to form neptunium-239, an isotope of element 93. Another disintegration transmutes this isotope into an isotope of element 94, called plutonium-239. Plutonium-239, like uranium-235, undergoes fission after the absorption of a neutron and can be used as a bomb material. Producing plutonium-239 in large quantities requires an intense source of neutrons; the source is provided by the controlled chain reaction in a nuclear reactor. During World War II nuclear reactors were designed to provide neutrons to produce plutonium. Reactors capable of manufacturing large quantities of plutonium were established in Hanford, Washington, and near Aiken, South Carolina.