The Effects of Nuclear War

GENERAL DESCRIPTION OF EFFECTS

Blast

Most damage to cities from large weapons comes from the explosive blast. The blast drives air away from the site of the explosion, producing sudden changes in air pressure (called static overpressure) that can crush objects, and high winds (called dynamic pressure) that can move them suddenly or knock them down. In general, large buildings are destroyed by the overpressure, while people and objects such as trees and utility poles are destroyed by the wind.

For example, consider the effects of a 1-megaton (Mt) air burst on things 4 miles [6 km] away. The overpressure will be in excess of 5 pounds per square inch (psi), which will exert a force of more than 180 tons on the wall of a typical two-story house. At the same place, there would be a wind of 160 mph [255 km]; while 5 psi is not enough to crush a man, a wind of 180 mph would create fatal collisions is ions between people and nearby objects.

The magnitude of the blast effect (generally measured in pounds per square inch) diminishes with distance from the center of the explosion. It is related in a more complicated way to the height of the burst above ground level. For any given distance from the center of the explosion, there is an optimum burst height that will produce the greatest overpressure, and the greater the distance the greater the optimum burst height. As a result, a burst on the surface produces the greatest overpressure at very close ranges (which is why surface bursts are used to attack very hard, very small targets such as missile silos), but less overpressure than an air burst at somewhat longer ranges. Raising the height of the burst reduces the overpressure directly under the bomb, but widens the area at which a given smaller overpressure is produced. Thus, an attack on factories with a 1-Mt weapon might use an air burst at an altitude of 8,000 feet [2,400 m], which would maximize the area (about 28 mi [7,200 hectares]) that would receive 10 psi or more of overpressure.

Table 3 shows the ranges of overpressures and effects from such a blast.

When a nuclear weapon is detonated on or near the surface of the Earth, the blast digs out a large crater. Some of the material that used to be in the crater is deposited on the rim of the crater; the rest is carried up into the air and returns to Earth as fallout. An explosion that is farther above the Earth’s surface than the radius of the fireball does not dig a crater and produces negligible immediate fallout.

Table 3. – Blast Effects of a 1-Mt Explosion 8,000 ft Above the Earth’s Surface

Distance from ground zero
(stat. miles)	(kilometers)	Peak overpressure	Peak wind velocity (mph)	Typical blast effects
.8	1.3	20 psi	470	Reinforced concrete structures are leveled.
3.0	4.8	10 psi	290	Most factories and commercial buildings are collapsed. Small wood-frame and brick residences destroyed and distributed as debris,
4.4	7.0	5 psi	160	Lightly constructed commercial buildings and typical residences are destroyed, heavier construction IS severely damaged
5.9	9.5	3 psi	95	Walls of typical steel-frame buildings are blown away: severe damage to residences. Winds sufficient to kill people in the open.
11.6	18.6	1 psi	35	Damage to structures, people endangered by flying glass and debris

For the most part, blast kills people by indirect means rather than by direct pressure. While a human body can withstand up to 30 psi of simple overpressure, the winds associated with as little as 2 to 3 psi could be expected to blow people out of typical modern office buildings. Most blast deaths result from the collapse of occupied buildings, from people being blown into objects, or from buildings or smaller objects being blown onto or into people. Clearly, then, it is impossible to calculate with any precision how many people would be killed by a given blast—the effects would vary from building to building.

In order to estimate the number of casualties from any given explosion, it is necessary to make assumptions about the proportion of people who will be killed or injured at any given overpressure. The assumptions used in this chapter are shown in figure 1. They are relatively conservative. For example, weapons tests suggest that a typical residence will be collapsed by an overpressure of about 5 psi. People standing in such a residence have a 50-percent chance of being killed by an overpressure of 3.5 psi, but people who are lying down at the moment the blast wave hits have a 50-percent chance of surviving a 7-psi overpressure. The calculations used here assume a mean lethal overpressure of 5 to 6 psi for people in residences, meaning that more than half of those whose houses are blown down on top of them will nevertheless survive. Some studies use a simpler technique: they assume that the number of people who survive in areas receiving more than 5 psi equal the number of people killed in areas receiving less than 5 psi, and hence that fatalities are equal to the number of people inside a 5-psi ring.

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