The effects of nuclear weapons were carefully observed, both after the bombings of Hiroshima and Nagasakiand after many test explosions in the 1950s and early 1960s.
As is the case with explosions caused by conventional weapons, most of the damage to buildings and other structures from a nuclear explosion results, directly or indirectly, from the effects of blast. The very rapid expansion of the bomb materials produces a high-pressure pulse, or shock wave, that moves rapidly outward from the exploding bomb. In air, this shock wave is called a blast wave because it is equivalent to and is accompanied by powerful winds of much greater than hurricane force. Damage is caused both by the high excess (or overpressure) of air at the front of the blast wave and by the extremely strong winds that remain after the wave front has passed. The degree of blast damage suffered on the ground depends on the TNT equivalent of the explosion; the altitude at which the bomb is exploded, referred to as the height of burst; and the distance of the structure from ground zero, that is, the point directly under the bomb. For the 20-kiloton A-bombs exploded over Japan, the height of burst was about 580 m ( about 1,900 ft), because it was estimated that this height would produce a maximum area of damage. If the TNT equivalent had been larger, a greater height of burst would have been chosen. Assuming a height of burst that will maximize the damage area, a 10-kiloton bomb will cause severe damage to wood-frame houses, such as are common in the United States, to a distance of more than 1.6 km (more than 1 mi) from ground zero and moderate damage as far as 2.4 km (1.5 mi). (A severely damaged house probably would be beyond repair.) The damage radius increases with the power of the bomb, approximately in proportion to its cube root. If exploded at the maximum height, therefore, a 10-megaton weapon, which is 1,000 times as powerful as a 10-kiloton weapon, will increase the distance tenfold, that is, out to 17.7 km (11 mi) for severe damage and 24 km (15 mi) for moderate damage a frame house.
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 increased -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 bring radioactivity in materials, especially earth and water, within a relatively small area around the explosion. Thus the neutron bomb is considered a skillful 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.
Besides the blast and radiation damage from individual bombs, a large-scale nuclear exchange between nations could conceivably have a disastrous 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 large 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, allowing further damage as a result of the sun’s ultraviolet radiation. Were the results sufficiently extended, they could tell 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.
The very high temperatures attained in a nuclear explosion result in the formation of an extremely hot incandescent mass of gas called a fireball. For a 10-kiloton explosion in the air, the fireball will attain a maximum diameter of about 300 m (about 1,000 ft); for a 10-megaton weapon the fireball may be 4.8 km (3 mi) across. A flash of thermal (or heat) radiation is emitted from the fireball and spreads out over a large area, but with steadily decreasing intensity. The amount of heat energy received a certain distance from the nuclear explosion depends on the power of the weapon and the state of the atmosphere. If the visibility is poor or the explosion takes place above clouds, the effect of the heat flash is decreased. The thermal radiation falling on exposed skin can cause flash burns. A 10-kiloton explosion in the air can produce moderate (second-degree) flash burns, which require some medical attention, as far as 2.4 km (1.5 mi) from ground zero; for a 10-megaton bomb, the corresponding distance would be more than 32 km (more than 20 mi). Milder burns of bare skin would be experienced even farther out. Most ordinary clothing provides protection from the heat radiation, as does almost any opaque object. Flash burns occur only when the bare skin is directly exposed, or if the clothing is too thin to absorb the thermal radiation. The heat radiation can initiate fires in dry, flammable materials, for example, paper and some fabrics, and such fires may spread if conditions are suitable. The evidence from the A-bomb explosions over Japan indicates that many fires, especially in the area near ground zero, originated from secondary causes, such as electrical short circuits, broken gas lines, and upset furnaces and boilers in industrial plants. The blast damage produced debris that helped to maintain the fires and denied access to fire-fighting equipment. Thus, much of the fire damage in Japan was a secondary effect of the blast wave. Under some conditions, such as existed at Hiroshima but not at Nagasaki, many individual fires can combine to produce a fire storm similar to those that accompany some large forest fires. The heat of the fire causes a strong updraft, which produces strong winds drawn in toward the center of the burning area. These winds increases the flame and convert the area into a holocaust in which everything flammable is destroyed. In as much as the flames are drawn inward, however, the area over which such a fire spreads may be limited.
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 gave way 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 similar in effect to X rays (see X Ray). Both neutrons and gamma rays have the ability to penetrate solid matter, so that considerable thicknesses of defensive 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 come down 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 come down 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 death causing effects of nuclear weapons.
