Nuclear Weapons

I

Introduction

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, near 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.

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 tennis ball produced an explosion equal to 20,000 tons of TNT (see Trinitrotoluene). Nuclear weapons were the first true weapons of mass destruction and their first use in warfare, at the end of World War II, and their subsequent deployment, changed the nature of international relations for all time. This article focuses on how the weapons work, and their effects.

The first nuclear weapons were 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 scientists, including the physicists Enrico Fermi, Richard Feynman, Isador Rabi, and Edward Teller, and the chemist Harold Urey, as well as scientists from Britain, were associated with what was, to date, the world’s biggest scientific project, whose military head was US Army engineer Major General Leslie Groves. The scientific director of the project—which was based at Los Alamos, New Mexico—was physicist J. Robert Oppenheimer.

Work had already started during the Manhattan Project on even more powerful bombs, chiefly 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 (see Thermonuclear, or Fusion, Weapons below).

This fusion research, begun largely by Edmund Teller, resulted in the production of bombs that range in power from a fraction of a kiloton (1,000 tons of TNT equivalent) to many megatons (1 million tons of TNT equivalent). Furthermore, the physical size of nuclear bombs was drastically reduced, permitting the development of battlefield nuclear weapons, such as nuclear artillery shells and small missiles that can be fired from portable launchers in the field. This allowed strategists to consider the possibility of waging a limited nuclear war.

After the war, the US Atomic Energy Commission became responsible for the supervision of all nuclear matters in the United States, including weapons research. Although nuclear bombs were originally developed as strategic weapons to be carried by large bombers, such as the B-29 and B-52, from the 1950s nuclear weapons were produced for both strategic and tactical applications. Not only can they be delivered by different types of military aircraft, but rockets and guided missiles of many sizes were built to carry nuclear warheads, to be launched from the ground, the air, or under the sea. Intercontinental ballistic missiles are large rockets capable of carrying multiple warheads for delivery to separate targets thousands of kilometres from launch.

Research in nuclear weapons continues in the United States at Los Alamos; at Lawrence Livermore Laboratory, California; in Britain, at Aldermaston; and in Russia, France, and China. Reduction in superpower nuclear forces, as a result of treaties such as the Strategic Arms Reduction Treaties (START) I and II, has produced a need for labs to apply research to the decommissioning and refitting of weapons.

II

Fission Weapons

In 1905 the renowned physicist 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, in a series of experiments, the German chemists Otto Hahn and Fritz Strassmann split the uranium atom into two roughly equal parts by bombardment with neutrons. As a result, the Austrian physicist Lise Meitner, with her nephew, Austrian physicist Otto Frisch, identified the process of nuclear fission, which placed the release of atomic energy within reach.

III

The Chain Reaction

When the uranium 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 a continuous release of energy.

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.

IV

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 tennis ball, 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 quickly, 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 (1s 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 per cent. In the first A-bomb tests, this kind of efficiency was not approached. Moreover, a 0.5-kg (1s-lb) mass is too small for a critical assembly.

V

Detonation of Atomic Bombs