Nuclear Weapons

A

Gun Method

Various systems were devised to detonate the prototype atomic bombs. 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 equivalent of about 13 kilotons of TNT.

B

Implosion Method

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 lenses, which are composed of high explosive and designed to concentrate the blast towards the centre 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 towards 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 Trinity test bomb exploded near Alamogordo, as well as the one dropped by the United States on Nagasaki, Japan, on August 9, 1945, was 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 the explosion.

VI

Production of Fissile Material

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

A

Separation of Uranium Isotopes

The fissile uranium-235 isotope accounts for only 0.7 per cent 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, such as gaseous diffusion, 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 at 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 consisted 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, pass by diffusion 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 weapons-grade uranium containing more than 90 per cent 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 being bombarded with neutrons. This transforms 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, which is plutonium-239. Plutonium-239, like uranium-235, undergoes fission after the absorption of a neutron and can be used as 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 Physics.

During World War II nuclear reactors built in the United States, and later in Britain, were designed to provide neutrons to produce plutonium. Reactors capable of manufacturing large quantities of plutonium were established at a vast site in Hanford, Washington, which is now in the process of a large-scale environmental clear-up.