Basic Effects of Nuclear Weapons

Nuclear explosions produce both immediate and delayed destructive effects. Blast, thermal radiation, and prompt ionizing radiation cause significant destruction within seconds or minutes of a nuclear detonation. The delayed effects, such as radioactive fallout and other possible environmental effects, inflict damage over an extended period ranging from hours to years. Each of these effects are calculated from the point of detonation.

Ground Zero

The term “ground zero” refers to the point on the earth’s surface immediately below (or above) the point of detonation. For a burst over (or under) water, the corresponding point is generally called “surface zero”. The term “surface zero” or “surface ground zero” is also commonly used for ground surface and underground explosions. In some publications, ground (or surface) zero is called the “hypocenter” of the explosion.

Blast Effects

Most damage comes from the explosive blast. The shock wave of air radiates outward, producing sudden changes in air pressure that can crush objects, and high winds that can knock objects down. In general, large buildings are destroyed by the change in air pressure, while people and objects such as trees and utility poles are destroyed by the wind.

The magnitude of the blast effect is related 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 change in air pressure, called 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, but less overpressure than an air burst at somewhat longer ranges.

When a nuclear weapon is detonated on or near Earth’s surface, the blast digs out a large crater. Some of the material that used in 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 radioactive 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. For the most part, a nuclear blast kills people by indirect means rather than by direct pressure.

Thermal Radiation Effects

Approximately 35 percent of the energy from a nuclear explosion is an intense burst of thermal radiation, i.e., heat. The effects are similar to the effect of a two-second flash from an enormous sunlamp. Since the thermal radiation travels at roughly the speed of light, the flash of light and heat precedes the blast wave by several seconds, just as lightning is seen before thunder is heard.

The visible light will produce “flashblindness” in people who are looking in the direction of the explosion. Flashblindness can last for several minutes, after which recovery is total. If the flash is focused through the lens of the eye, a permanent retinal burn will result. At Hiroshima and Nagasaki, there were many cases of flashblindness, but only one case of retinal burn, among the survivors. On the other hand, anyone flashblinded while driving a car could easiIy cause permanent injury to himself and to others.

Skin burns result from higher intensities of light, and therefore take place closer to the point of explosion. First-degree, second-degree and third-degree burns can occur at distances of five miles away from the blast or more. Third-degree burns over 24 percent of the body, or second-degree burns over 30 percent of the body, will result in serious shock, and will probably prove fatal unless prompt, specialized medical care is available. The entire United States has facilities to treat 1,000 or 2,000 severe burn cases. A single nuclear weapon could produce more than 10,000.

The thermal radiation from a nuclear explosion can directly ignite kindling materials. In general, ignitable materials outside the house, such as leaves or newspapers, are not surrounded by enough combustible material to generate a self-sustaining fire. Fires more likely to spread are those caused by thermal radiation passing through windows to ignite beds and overstuffed furniture inside houses. Another possible source of fires, which might be more damaging in urban areas, is indirect. Blast damage to Stores, water heaters, furnaces, electrical circuits or gas lines would ignite fires where fuel is plentiful.

Direct Nuclear Radiation Effects

Direct radiation occurs at the time of the explosion. It can be very intense, but its range is limited. For large nuclear weapons, the range of intense direct radiation is less than the range of lethal blast and thermal radiation effects. However, in the case of smaller weapons, direct radiation may be the lethal effect with the greatest range. Direct radiation did substantial damage to the residents of Hiroshima and Nagasaki. Human response to ionizing radiation is subject to great scientific uncertainty and intense controversy. It seems likely that even small doses of radiation do some harm.

Types of Nuclear Explosions

The effects of a nuclear explosion depend in part to the height of the detonation. There five general classifications of bursts: air, high-altitude, underwater, underground, and surface bursts.

An air burst is defined as one in which the explosion occurs in the air at an altitude below 100,000 feet (30,480 meters), but at such a height that the fireball does not touch the surface of the earth. A detontation above that altitude is generally refered to as a high-altitude burst.

A nuclear explosion that occurs at or slightly above the actual surface of the land or water is known as a surface burst. If the explosion happens beneath the surface of the land or water, then it is known as underground or underwater respectively. The design of Robust Nuclear Earth Penetrator (RNEP) uses the charaterastics of an underground burst in an attempt to destroy buried targets.

One of the greatest results of the type of burst is the amount of radioactive debris and fallout, and the force of the blast wave.

The Blast Wave

A fraction of a second after a nuclear explosion, the heat from the fireball causes a high-pressure wave to develop and move outward producing the blast effect. The front of the blast wave, i.e., the shock front, travels rapidly away from the fireball, a moving wall of highly compressed air.

The effects of the blast wave on a typical wood framed house.

The air immediately behind the shock front is accelerated to high velocities and creates a powerful wind. These winds in turn create dynamic pressure against the objects facing the blast. Shock waves cause a virtually instantaneous jump in pressure at the shock front. The combination of the pressure jump (called the overpressure) and the dynamic pressure causes blast damage. Both the overpressure and the dynamic pressure reach to their maximum values upon the arrival of the shock wave. They then decay over a period ranging from a few tenths of a second to several seconds, depending on the blast’s strength and the yield.

Overpressure

Blast effects are usually measured by the amount of overpressure, the pressure in excess of the normal atmospheric value, in pounds per square inch (psi).

After 10 seconds, when the fireball of a 1-megaton nuclear weapon has attained its maximum size (5,700 feet across), the shock front is some 3 miles farther ahead. At 50 seconds after the explosion, when the fireball is no longer visible, the blast wave has traveled about 12 miles. It is then traveling at about 784 miles per hour, which is slightly faster than the speed of sound at sea level.

As a general guide, city areas are completely destroyed by overpressures of 5 psi, with heavy damage extending out at least to the 3 psi contour.

These many different effects make it difficult to provide a simple rule of thumb for assessing the magnitude of injury produced by different blast intensities. A general guide is given below:

Blast Effects on Humans

Blast damage is caused by the arrival of the shock wave created by the nuclear explosion. Humans are actually quite resistant to the direct effect of overpressure. Pressures of over 40 psi are required before lethal effects are noted.

The danger from overpressure comes from the collapse of buildings that are generally not as resistant. Urban areas contain many objects that can become airborne, and the destruction of buildings generates many more. The collapse of the structure above can crush or suffocate those caught inside. Serious injury or death can also occur from impact after being thrown through the air.

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