Basic Effects of Nuclear Weapons
Nuclear explosions produce both immediate and delayed destructive effects. Blast,
thermal radiation, prompt ionizing radiation are produced and 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.
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.
Fallout radiation is received from particles that are made radioactive by the effects
of the explosion, and subsequently distributed at varying distances from the site of the blast. While any nuclear explosion
in the atmosphere produces some fallout, the fallout is far greater if the burst is on the surface, or at least low enough
for the firebalI to touch the ground. The significant hazards come from particles scooped up from the ground and irradiated
by the nuclear explosion. The radioactive particles that rise only a short distance (those in the "stem" of the familiar mushroom
cloud) will fall back to earth within a matter of minutes, landing close to the center of the explosion. Such particles are
unlikely to cause many deaths, because they will fall in areas where most people have already been killed. However, the radioactivity
will complicate efforts at rescue or eventual reconstruction. The radioactive particles that rise higher will be carried some
distance by the wind before returning to Earth, and hence the area and intensity of the fallout is strongly influenced by
local weather conditions. Much of the material is simply blown downwind in a long plume. Rainfall also can have a significant
influence on the ways in which radiation from smaller weapons is deposited, since rain will carry contaminated particles to
the ground. The areas receiving such contaminated rainfall would become "hot spots," with greater radiation intensity than
Electromagnetic pulse (EMP) is an electromagnetic wave similar to radio waves, which
results from secondary reactions occurring when the nuclear gamma radiation is absorbed in the air or ground. It differs from
the usual radio waves in two important ways. First, it creates much higher electric field strengths. Whereas a radio signal
might produce a thousandth of a volt or less in a receiving antenna, an EMP pulse might produce thousands of volts. Secondly,
it is a single pulse of energy that disappears completely in a small fraction of a second. In this sense, it is rather similar
to the electrical signal from lightning, but the rise in voltage is typically a hundred times faster. This means that most
equipment designed to protect electrical facilities from lightning works too slowly to be effective against EMP.
An attacker might detonate a few weapons at high altitudes in an effort to destroy
or damage the communications and electric power systems of the victim. There is no evidence that EMP is a physical threat
to humans. However, electrical or electronic systems, particularly those connected to long wires such as power lines or antennas,
can undergo damage. There could be actual physical damage to an electrical component or a temporary disruption of operation.
Related to this is the government’s
own account of what a nuclear bomb would do at http://www.atomicarchive.com/Docs/pdfs/7906.pdf