Nuclear Weapons

A

Blast Effects

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 known as 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 persist 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 13- and 20-kiloton A-bombs detonated over Japan, the height of burst was about 550 m (1,800 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 lighter structures such as wood-frame houses (such as are common in Japan and the United States) to a distance of more than 1.6 km (1 mi) from ground zero. It will cause moderate damage as far as 2.4 km (1y 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 optimum 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, to 17.7 km (11 mi) for severe damage and 24 km (15 mi) for moderate damage of a frame house.

B

Thermal Effects

The very high temperatures attained in a nuclear explosion result in the formation of a fireball—an extremely hot incandescent mass of gas. For a 10-kiloton explosion in the air, the fireball will attain a maximum diameter of about 300 m (1,000 ft); for a 10-megaton weapon the fireball may be 4.8 km (3 mi) across. A flash of thermal (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 effectiveness of the heat flash is decreased. The thermal radiation falling on exposed skin will cause flash burns. A 10-kiloton explosion in the air will produce moderate (second-degree) flash burns, which require medical attention, as far as 2.4 km (1y mi) from ground zero; for a 10-megaton bomb, the corresponding distance would be more than 32 km (20 mi). Milder burns of bare skin would be experienced even further out. Most ordinary clothing provides protection from the heat radiation, as does almost any opaque object. Flash burns occur when the bare skin is directly exposed, or if the clothing is too thin to absorb the thermal radiation.

The heat radiation can start fires in dry, flammable materials such as paper and some fabrics; these fires spread in typically urban conditions. In the A-bomb explosions over Japan 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.

Under some conditions, such as existed at Hiroshima but not at Nagasaki, many individual fires can combine to produce a firestorm similar to those that accompany some large forest fires. The heat of the fire causes a strong updraught, which produces strong winds drawn in towards the centre of the burning area. These winds fan the flames and convert the area into a holocaust in which everything flammable is destroyed. Inasmuch as the flames are drawn inward, however, the area over which such a fire spreads may be limited.

C

Penetrating Radiation

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. The nature of radioactivity and the immense areas contaminable by a single bomb undoubtedly make radioactive fallout potentially one of the most lethal effects of nuclear weapons. When absorbed by the body, nuclear radiation causes 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 sickness.

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

Residual nuclear radiation, generally known as fallout, is a hazard that affects 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 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 from nuclear weapons is based on case histories resulting from the Hiroshima and Nagasaki bombs and from the exposure of the Marshall islanders and the Lucky Dragon fishermen to the fallout from the March 1954 15-megaton H-bomb test.

D

Electromagnetic Pulse

A nuclear explosion also produces a very powerful surge of electromagnetic energy capable of overloading power supply systems and burning out transistors and capacitors. A single 10-megaton nuclear bomb exploded high above ground would destroy or disrupt telecommunications and all electronic and electrical systems on a nationwide scale.

E

Climatic Effects

Besides the widespread blast, radiation, and fire damage from individual bombs, a large-scale nuclear exchange between nations could conceivably have a catastrophic global effect on climate. This possibility, initially proposed in a paper published by an international group of scientists in December 1983, became known as the nuclear winter theory. According to this theory, 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, sufficient to block out sunlight for several months, particularly in the Northern hemisphere. This would destroy plant life and create a sub-freezing climate until the dust dispersed. The ozone layer would 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.