Nuclear Weapons

Besides heat and blast, an exploding nuclear bomb has a unique effect—it releases penetrating nuclear radiation, which is quite different from thermal (or heat) radiation (see Radioactivity). When absorbed by the body, nuclear radiation can cause serious injury. For an explosion high in the air, the injury range for these radiations is less than for blast and fire damage or flash burns. In Japan, however, many individuals who were protected from blast and burns succumbed later to radiation injury.

Nuclear radiation from an explosion may be divided into two categories, namely, prompt radiation and residual radiation. The prompt radiation consists of an instantaneous burst of neutrons and gamma rays, which travel over an area of several square miles. Gamma rays are identical in effect to X rays (see X Ray). Both neutrons and gamma rays have the ability to penetrate solid matter, so that substantial thicknesses of shielding materials are required.

The residual nuclear radiation, generally known as fallout, can be a hazard over very large areas that are completely free from other effects of a nuclear explosion. In bombs that gain their energy from fission of uranium-235 or plutonium-239, two radioactive nuclei are produced for every fissile nucleus split. These fission products account for the persistent radioactivity in bomb debris, because many of the atoms have half-lives measured in days, months, or years.

Two distinct categories of fallout, namely, early and delayed, are known. If a nuclear explosion occurs near the surface, earth or water is taken up into a mushroom-shaped cloud and becomes contaminated with the radioactive weapon residues. The contaminated material begins to descend within a few minutes and may continue to fall for about 24 hours, covering an area of thousands of square miles downwind from the explosion. This constitutes the early fallout, which is an immediate hazard to human beings. No early fallout is associated with high-altitude explosions. If a nuclear bomb is exploded well above the ground, the radioactive residues rise to a great height in the mushroom cloud and descend gradually over a large area.

Human experience with radioactive fallout has been minimal. The principal known case histories have been derived from the accidental exposure of fishermen and local residents to the fallout from the 15-megaton explosion that occurred on March 1, 1954. The nature of radioactivity, however, and the immense areas contaminable by a single bomb undoubtedly make radioactive fallout potentially one of the most lethal effects of nuclear weapons.

Besides the blast and radiation damage from individual bombs, a large-scale nuclear exchange between nations could conceivably have a catastrophic global effect on climate. This possibility, proposed in a paper published by an international group of scientists in December 1983, has come to be known as the “nuclear winter” theory. According to these scientists, the explosion of not even one-half of the combined number of warheads in the United States and Russia would throw enormous quantities of dust and smoke into the atmosphere. The amount could be sufficient to block off sunlight for several months, particularly in the northern hemisphere, destroying plant life and creating a subfreezing climate until the dust dispersed. The ozone layer might also be affected, permitting further damage as a result of the sun’s ultraviolet radiation. Were the results sufficiently prolonged, they could spell the virtual end of human civilization. The nuclear winter theory has since become the subject of enormous controversy. It found support in a study released in December 1984 by the U.S. National Research Council, and other groups have undertaken similar research. In 1985, however, the U.S. Department of Defense released a report acknowledging the validity of the concept but saying that it would not affect defense policies.

On the average, about 50 percent of the power of an H-bomb results from thermonuclear-fusion reactions and the other 50 percent from fission that occurs in the A-bomb trigger and in the uranium jacket. A clean H-bomb is defined as one in which a significantly smaller proportion than 50 percent of the energy arises from fission. Because fusion does not produce any radioactive products directly, the fallout from a clean weapon is less than that from a normal or average H-bomb of the same total power. If an H-bomb were made with no uranium jacket but with a fission trigger, it would be relatively clean. Perhaps as little as 5 percent of the total explosive force might result from fission; the weapon would thus be 95 percent clean. The enhanced-radiation fusion bomb, also called the neutron bomb, which has been tested by the United States and other nuclear powers, does not release long-lasting radioactive fission products. However, the large number of neutrons released in thermonuclear reactions is known to induce radioactivity in materials, especially earth and water, within a relatively small area around the explosion. Thus the neutron bomb is considered a tactical weapon because it can do serious damage on the battlefield, penetrating tanks and other armored vehicles and causing death or serious injury to exposed individuals, without producing the radioactive fallout that endangers people or structures miles away. See also Arms Control; Guided Missiles; Warfare.