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General Descriptions of Nuclear Weapons Effects
The energy of a nuclear explosion is released in a number of different ways:
The distribution of the bombs energy among these effects depends on its size and on the details of its design, but a general description is possible.
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] Effects of a nuclear explosion 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 between people and nearby objects.
The magnitude of the blast effect (generally measured in pounds per square inch) diminishes with distance from the centre 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 centre 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. Consequently, 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 over-pressure 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 maximise the area (about 28 mi2 [7,200 hectares]) that would receive 10 psi or more of overpressure.
Table 3 shows the ranges of overpressures and events 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 Earths surface than the radius of the fireball does not dig a crater and produces negligible fallout.
Table 3 - Blast Effects of a 1-Mt Explosion 8,000 ft Above the Earth's Surface
For the most part, blast kills people by an 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 blastthe 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 over-pressure 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 over-pressure. 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.
Nuclear weapons inflict ionising radiation on people, animals, and plants in two different ways. Direct radiation occurs at the time of the explosion; it can be very intense, but its range is limited. 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. Fallout is discussed in a subsequent section.
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 ionising radiation is subject to great scientific uncertainty and intense controversy. It seems likely that even small doses of radiation do some harm, To understand the effects of nuclear weapons, one must distinguish between short- and long-term effects:
Short-Term Effects. A dose of 600 rem within a short period of time (6 to 7 days) has a 90-percent chance of creating a fatal illness, with death occurring within a few weeks. (A rem or " roentgen-equivalent-man" is a measure of biological damage: a "rad" is a measure of radiation energy absorbed; a roentgen is a measure of radiation energy; for our purposes it may be assumed that 100 roentgens produce 100 rads and 100 rem. ) The precise shape of the curve showing the death rate as a function of radiation dose is not known in the region between 300 and 600 rem, but a dose of 450 rem within a short time is estimated to create a fatal illness in half the people exposed to it; the other half would get very sick, but would recover. A dose of 300 rem might kill about 10 percent of those exposed. A dose of 200 to 450 rems will cause a severe illness from which most people would recover; however, this illness would render people highly susceptible to other diseases or infections. A dose of so to 200 rem will cause nausea and lower resistance to other diseases, but medical treatment is not required. A dose below so rem will not cause any short-term effects that the victim will notice, but will do long-term damage.
Long-Term Effects. The effects of smaller doses of radiation are long term, and measured in a statistical way. A dose of 50 rem generally produces no short-term effects; however, if a large population were exposed to so reins, somewhere between 0.4 and 2.5 percent of them would be expected to contract fatal cancer (after some years) as a result. There would also be serious genetic effects for some fraction of those exposed. Lower doses produce lower effects. There is a scientific controversy about whether any dose of radiation, however small, is safe. Chapter V discusses the extent of the long-term effects that a nuclear attack might produce. It should be clearly understood, however, that a large nuclear war would expose the survivors, however well sheltered, to levels of radiation far greater than the U.S. Government considers safe in peacetime.
Approximately 35 percent of the energy from a nuclear explosion is an intense burst of thermal radiation, ie. heat. The effects are roughly analogous to the effect of a 2-second flash from an enormous sunlamp. Since the thermal radiation travels at the speed of light (actually a bit slower, since it is deflected by particles in the atmosphere), the flash of light and heat precedes the blast wave by several seconds, just as lightning is seen before the thunder is heard.
The visible light will produce "flash blindness" in people who are looking in the direction of the explosion. Flash blindness can last for several minutes, after which recovery is total. A 1-Mt explosion could cause flash blindness at distances as great as 13 miles [21 km] on a clear day, or 53 miles [85 km] on a clear night. 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 flash blindness, but only one case of retinal burn, among the survivors. On the other hand, anyone flash blinded while driving a car could easily 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. A 1-Mt explosion can cause first-degree burns (equivalent to a bad sun-burn) at distances of about 7 miles [11 km], second-degree burns (producing blisters that lead to infection if untreated, and permanent scars) at distances of about 6 miles [10 km], and third-degree burns (which destroy skin tissue) at distances of up to 5 miles [8 km]. 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, specialised 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 distance at which burns are dangerous depends heavily on weather conditions. Extensive moisture or a high concentration of particles in the air (smog) absorbs thermal radiation. Thermal radiation behaves like sunlight, so objects create shadows behind which the thermal radiation is indirect (reflected) and less intense. Some conditions, such as ice on the ground or low white clouds over clean air, can increase the range of dangerous thermal radiation.
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. A rather substantial amount of combustible material must burn vigorously for 10 to 20 minutes before the room, or whole house, becomes inflamed. The blast wave, which arrives after most thermal energy has been ex-pended, will have some extinguishing effect on the fires. However, studies and tests of this effect have been very contradictory, so the extent to which blast can be counted on to extinguish fire starts remains quite uncertain.
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.
The best estimates are that at the 5-psi level about 10 percent of all buildings would sustain a serious fire, while at 2 psi about 2 percent would have serious fires, usually arising from secondary sources such as blast-damaged utilities rather than direct thermal radiation.
It is possible that individual fires, whether caused by thermal radiation or by blast damage to utilities, furnaces, etc., would coalesce into a mass fire that would consume all structures over a large area. This possibility has been intensely studied, but there remains no basis for estimating its probability. Mass fires could be of two kinds: a "firestorm, " in which violent inrushing winds create extremely high temperatures but prevent the fire from spreading radially outwards, and a "conflagration, " in which a fire spreads along a front. Hamburg, Tokyo, and Hiroshima experienced firestorms in World War 11; the Great Chicago Fire and the San Francisco Earthquake Fire were conflagrations. A firestorm is likely to kill a high proportion of the people in the area of the fire, through heat and through asphyxiation of those in shelters. A conflagration spreads slowly enough so that people in its path can escape, though a conflagration caused by a nuclear attack might take a heavy toll of those too injured to walk. Some believe that firestorms in U.S. or Soviet cities are unlikely because the density of flammable materials ("fuel loading") is too lowthe ignition of a firestorm is thought to require a fuel loading of at least 8 lbs./ft2 (Hamburg had 32), compared to fuel loading of 2 lbs./ft2 in a typical U.S. suburb and 5 lbs./ft2 in a neighbourhood of two-story brick rowhouses. The Likelihood of a conflagration depends on the geography of the area, the speed and direction of the wind, and details of building construction. Another variable is whether people and equipment are available to fight fires before they can coalesce and spread.
Electromagnetic pulse (EMP) is an electro-magnetic 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.
The strength of an EMP pulse is measured in volts per meter (v/m), and is an indication of the voltage that would be produced in an exposed antenna. A nuclear weapon burst on the surface will typically produce an EMP of tens of thousands of v/m at short distances (the 10-psi range) and thousands of v/m at longer distances (l-psi range). Airbursts produce less EMP, but high-altitude bursts (above 19 miles [21 km]) produce very strong EMP, with ranges of hundreds or thousands of miles. An attacker might detonate a few weapons at such 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 powerlines or antennas, can undergo either of two kinds of damage. First, there can be actual physical damage to an electrical component such as shorting of a capacitor or burnout of a transistor, which would require replacement or repair before the equipment can again be used. Second, at a lesser level, there can be a temporary operational upset, frequently requiring some effort to restore operation. For example, instabilities induced in power grids can cause the entire system to shut itself down, upsetting computers that must be started again. Base radio stations are vulnerable not only from the loss of commercial power but from direct damage to electronic components connected to the antenna. In general, portable radio transmitter/receivers with relatively short antennas are not susceptible to EMP. The vulnerability of the telephone system to EMP could not be determined.
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 fireball to touch the ground. As chapter V shows in some detail, the fallout from air bursts alone poses long-term health hazards, but they are trivial compared to the other consequences of a nuclear attack. 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 centre 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 in-tensity of the fallout is strongly influenced by local weather conditions. Much of the material is simply blown downwind in a long plume, The map shown in figure 2 illustrates the plume expected from a 1-Mt surface burst in Detroit if winds were blowing toward Canada. The illustrated plume assumed that the winds were blowing at a uniform speed of 15 mph [24 km] over the entire region, The plume would be longer and thinner if the winds were more intense and shorter and somewhat more broad if the winds were slower. If the winds were from a different direction, the plume would cover a different area. For example, a wind from the northwest would deposit enough fallout on Cleveland to inflict acute radiation sickness on those who did not evacuate or use effective fallout shelters (figure 3). Thus, wind direction can make an enormous difference. Rainfall can also 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 their surroundings, When the radiation intensity from fallout is great enough to pose an immediate threat to health, fallout will generally be visible as a thin layer of dust.
The amount of radiation produced by fall-out materials will decrease with time as the radioactive materials "decay. " Each material decays at a different rate, Materials that decay rapidly give off intense radiation for a short period of time while long-lived materials radiate less intensely but for longer periods, immediately after the fallout is deposited in regions surrounding the blast site, radiation intensities will be very high as the short-lived materials decay. These intense radiations will decrease relatively quickly. The intensity will have fallen by a factor of 10 after 7 hours, a factor of 100 after 49 hours and a factor of 1,000 after 2 weeks. The areas in the plume illustrated in figures 2 and 3 would become "safe" (by peacetime standards) in 2 to 3 years for the outer ellipse, and in 10 years or so for the inner ellipse.
Some radioactive particles will be thrust into the stratosphere, and may not return to Earth for some years. In this case only the particularly long-lived particles pose a threat, and they are dispersed around the world over a range of latitudes, Some fallout from U.S. and Soviet weapons tests in the 1950s and early 1960s can still be detected. There are also some particles in the immediate fallout (notably Strontium 90 and Cesium 137) that remain radioactive for years. Chapter V discusses the likely hazards from these long-lived particles.
The biological effects of fallout radiation are substantially the same as those from direct radiation, discussed above, People exposed to enough fallout radiation will die, and those exposed to lesser amounts may become ill. Chapter 11 discusses the theory of fallout sheltering, and chapter IV some of the practical difficulties of escaping fallout from a large counterforce attack.
There is some public interest in the question of the consequences if a nuclear weapon destroyed a nuclear powerplant. The core of a power reactor contains large quantities of radioactive material, which tends to decay more slowly (and hence less intensely) than the fallout particles from a nuclear weapon explosion, consequently, fallout from a destroyed nuclear reactor (whose destruction would, incidentally, require a high-accuracy surface burst) would not be much more intense (during the first day) or widespread than "ordinary" fallout, but would stay radioactive for a considerably longer time. Areas receiving such fallout would have to be evacuated or decontaminated; otherwise, survivors would have to stay in shelters for months.
So far, the discussion of each major effect (blast, nuclear radiation, and thermal radiation) has explained how this effect in isolation causes deaths and injuries to humans. It is customary to calculate the casualties accompanying hypothetical nuclear explosion as follows: for any given range, the effect most likely to kill people is selected and its consequences calculated, while the other effects are ignored. it is obvious that combined injuries are possible, but there are no generally accepted ways of calculating their probability. What data do exist seem to suggest that calculations of single effects are not too inaccurate for immediate deaths, but that deaths occurring some time after the explosion may well be due to combined causes, and hence are omitted from most calculations. Some of the obvious possibilities are:
Nuclear Radiation Combined With Thermal Radiation. Severe burns place considerable stress on the blood system, and often cause anaemia. It is clear from experiments with laboratory animals that exposure of a burn victim to more than 100 reins of radiation will impair the bloods ability to support recovery from the thermal burns. Hence, a sublethal radiation dose could make it impossible to recover from a burn that, without the radiation, would not cause death.
Nuclear Radiation Combined With Mechanical Injuries. Mechanical injuries, the in-direct results of blast, take many forms. Flying glass and wood will cause puncture wounds. Winds may blow people into obstructions, causing broken bones, concussions, and internal injuries. Persons caught in a collapsing building can suffer many similar mechanical injuries. There is evidence that all of these types of injuries are more serious if the person has been exposed to 300 reins, particularly if treatment is delayed. Blood damage will clearly make a victim more susceptible to blood loss and infection. This has been confirmed in laboratory animals in which a borderline lethal radiation dose was followed a week later by a blast over-pressure that alone would have produced a low level of prompt lethality. The number of prompt and delayed (from radiation) deaths both increased over what would be expected from the single effect alone.
Thermal Radiation and Mechanical Injuries. There is no information available about the effects of this combination, beyond the common sense observation that since each can place a great stress on a healthy body, the combination of injuries that are individually tolerable may subject the body to a total stress that it cannot tolerate. Mechanical injuries should be prevalent at about the distance from a nuclear explosion that produces sub-lethal burns, so this synergism could be an important one.
In general, synergistic effects are most likely to produce death when each of the injuries alone is quite severe. Because the uncertainties of nuclear effects are compounded when one tries to estimate the likelihood of two or more serious but (individually) non-fatal injuries, there really is no way to estimate the number of victims.
A further dimension of the problem is the possible synergy between injuries and environmental damage. To take one obvious example, poor sanitation (due to the loss of electrical power and water pressure) can clearly compound the effects of any kind of serious injury. Another possibility is that an injury would so immobilise the victim that he would be unable to escape from a fire.
June 23, 2005