|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.
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
Nuclear Fission and Nuclear
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.
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
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
Separation of Uranium
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.
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.