Nuclear Weapons

V

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 made of fissile uranium. It had the energy of anywhere between 12.5 and 15 kilotons of TNT. Three days later the United States dropped a second atomic bomb made of plutonium over Nagasaki, Japan, with the energy equivalent of about 20 kilotons of TNT.

A more complex method, known as implosion, is used 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 United States 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.

VI

Production of Fissile Material

Much experimentation was necessary to make the production of fissile material practical.

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A

Separation of Uranium Isotopes

The fissile isotope uranium-235 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. 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 (see Diffusion). 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.

B

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 (see Neptunium). 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. See Nuclear Chemistry; Nuclear Energy.

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.

VII

Thermonuclear, or Fusion, Weapons

Even before the first atomic bomb was developed, scientists realized that a type of nuclear reaction different from the fission process was theoretically possible as a source of nuclear energy. Instead of using the energy released as a result of a chain reaction in fissile material, nuclear weapons could use the energy liberated in the fusion of light elements. This process is the opposite of fission, since it involves the fusing together of the nuclei of isotopes of light atoms such as hydrogen. It is for this reason that the weapons based on nuclear-fusion reactions are often called hydrogen bombs, or H-bombs. Of the three isotopes of hydrogen the two heaviest species, deuterium and tritium, combine most readily to form helium. Although the energy release in the fusion process is less per nuclear reaction than in fission, 0.5 kg (1.1 lb) of the lighter material contains many more atoms; thus, the energy liberated from 0.5 kg (1.1 lb) of hydrogen-isotope fuel is equivalent to that of about 29 kilotons of TNT, or almost three times as much as from uranium. This estimate, however, is based on complete fusion of all hydrogen atoms. Fusion reactions occur only at temperatures of several millions of degrees, the rate increasing enormously with increasing temperature; such reactions consequently are known as thermonuclear (heat-induced) reactions. Strictly speaking, the term thermonuclear implies that the nuclei have a range (or distribution) of energies characteristic of the temperature. This plays an important role in making rapid fusion reactions possible by an increase in temperature.

Development of the hydrogen bomb was impossible before the perfection of A-bombs, for only the latter could yield that tremendous heat necessary to achieve fusion of hydrogen atoms. Atomic scientists regarded the A-bomb as the trigger of the projected thermonuclear device.

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