Fallout is radioactive dust created when a nuclear weapon explodes. A nuclear explosion vaporizes any material within the fireball, including the ground if it is nearby. In water, the minerals (including sodium in ocean water) are made radioactive by neutrons from the bomb core. Fallout from seawater is unusually dangerous because it is difficult to remove by washing.
When this material condenses in the cloud, it forms dust and light sandy material that resembles ground pumice. In the case of seawater, it forms a heavy fog from the base of the mushroom cloud (the "base surge"). This highly irradiated material then falls to earth. The fallout behaves much like thousands of tiny x-ray machines, emitting gamma rays in all directions. A fallout shelter is designed to shield its occupants from this radiation.
The closer to ground an atomic bomb is detonated, the more dust and debris is thrown into the air, resulting in greater amounts of fallout. From a tactical standpoint, this has the disadvantage of hindering any occupation efforts until the fallout clears, but more directly, the impact with the ground severely limits the destructive force of the bomb. For these reasons, ground bursts are not usually considered tactically advantageous, with the exception of hardened underground targets such as missile silos or command centers such as Cheyenne Mountain. "Salting" enemy territory with a fallout-heavy atomic burst could be used to deny enemy access to a contaminated area but such use is generally not considered an ethical military action.
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Initial radiation from fallout can exceed 30,000 rads/hr immediately downwind of a ground burst. A cumulative dose of 450rad-hours is fatal to half of a population of humans. There have been no documented cases of survival beyond 600 rad-hrs. Most people become ill after an exposure to 100 rad-hrs or more. The fetuses of pregnant women are vulnerable and may miscarry, especially in the first trimester. Human biology resists mutation from large radiation exposure: grossly mutated fetuses usually miscarry. Civilian dose rates in peace-time range from .01 to .003 rad-hrs/year.
Fallout radiation falls off exponentially (quickly) with time. Most areas become safe for travel and decontamination after three to five weeks.
The ground track of fallout from an explosion is a long, thin fuzzy ellipse downwind of the explosion. It may be hundreds of kilometers long, and up to 50km (30mi) wide from a single explosion. Rain can cause fallout to settle more quickly. This means that a rainstorm can significantly increase the hazards to those just downwind of a nuclear war.
The most dangerous emissions from fallout are gamma rays, which travel in straight lines, like ordinary light. The fallout particles emit the invisible, deadly gamma rays in the same way that a light bulb emits light. Gamma rays are invisible, and cannot be seen, smelt, or felt, even at very dangerous intensities. Special equipment is required to detect and measure gamma rays.
The gamma rays do not contaminate people or objects. Fallout particles contaminate people or objects, and since they resemble sand, they can be brushed off, or washed off. The radioactive fog from seawater is a notable exception, being very difficult to wash off. The particles should be removed from the shelter, or shielded. Emergency drinking water can be adequately cleaned by filtering contaminated water through more than 25cm (10 in) of dirt. Food in sealed packages is not poisoned by fallout. Stored grain and exposed fruit can be cleaned and peeled. Vehicles are usually washed down with fire-hoses, into drains with removable filters, or deep trenches. Ground is usually decontaminated by bulldozing the fallout into deep, narrow trenches, and then back-filling the trenches.
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The residual radiation hazard from a nuclear explosion is in the form of radioactive fallout and neutron-induced activity. Residual ionizing radiation arises from:
Fission Products. These are intermediate weight isotopes which are formed when a heavy uranium or plutonium nucleus is split in a fission reaction. There are over 300 different fission products that may result from a fission reaction. Many of these are radioactive with widely differing half-lives. Some are very short, i.e., fractions of a second, while a few are long enough that the materials can be a hazard for months or years. Their principal mode of decay is by the emission of beta and gamma radiation. Approximately 60 grams of fission products are formed per kiloton of yield. The estimated activity of this quantity of fission products 1 minute after detonation is equal to that of 1.1 x 1021 Bq (30 million kilograms of radium) in equilibrium with its decay products.
Unfissioned Nuclear Material. Nuclear weapons are relatively inefficient in their use of fissionable material, and much of the uranium and plutonium is dispersed by the explosion without undergoing fission. Such unfissioned nuclear material decays by the emission of alpha particles and is of relatively minor importance.
Neutron-Induced Activity. If atomic nuclei capture neutrons when exposed to a flux of neutron radiation, they will, as a rule, become radioactive (neutron-induced activity) and then decay by emission of beta and gamma radiation over an extended period of time. Neutrons emitted as part of the initial nuclear radiation will cause activation of the weapon residues. In addition, atoms of environmental material, such as soil, air, and water, may be activated, depending on their composition and distance from the burst. For example, a small area around ground zero may become hazardous as a result of exposure of the minerals in the soil to initial neutron radiation. This is due principally to neutron capture by sodium (Na), manganese, aluminum, and silicon in the soil. This is a negligible hazard because of the limited area involved.
Worldwide Fallout: After an air burst the fission products, unfissioned nuclear material, and weapon residues which have been vaporized by the heat of the fireball will condense into a fine suspension of very small particles 0.01 to 20 micrometers in diameter. These particles may be quickly drawn up into the stratosphere, particularly if the explosive yield exceeds 10 Kt. They will then be dispersed by atmospheric winds and will gradually settle to the earth's surface after weeks, months, and even years as worldwide fallout.
The radiobiological hazard of worldwide fallout is essentially a long-term one due to the potential accumulation of long-lived radioisotopes, such as strontium-90 and cesium-137, in the body as a result of ingestion of foods incorporating these radioactive materials. This hazard is much less serious than those which are associated with local fallout and, therefore, is not discussed at length in this publication. Local fallout is of much greater immediate operational concern.
Local Fallout: In a land or water surface burst, large amounts of earth or water will be vaporized by the heat of the fireball and drawn up into the radioactive cloud. This material will become radioactive when it condenses, with fission products and other radiocontaminants that have become neutron-activated.
There will be large amounts of particles of less than 0.1 micrometer to several millimeters in diameter generated in a surface burst in addition to the very fine particles which contribute to worldwide fallout.
The larger particles will not rise into the stratosphere and consequently will settle to earth within about 24 hours as local fallout.
Severe local fallout contamination can extend far beyond the blast and thermal effects, particularly in the case of high yield surface detonations.
Whenever individuals remain in a radiologically contaminated area, such contamination will lead to an immediate external radiation exposure as well as a possible later internal hazard due to inhalation and ingestion of radiocontaminants.
In severe cases of fallout contamination, lethal doses of external radiation may be incurred if protective or evasive measures are not undertaken.
In cases of water surface (and shallow underwater) bursts, the particles tend to be rather lighter and smaller and so produce less local fallout but will extend over a greater area. The particles contain mostly sea salts with some water; these can have a cloud seeding affect causing local rainout and areas of high local fallout.
For subsurface bursts, there is an additional phenomenon present called "base surge." The base surge is a cloud that rolls outward from the bottom of the column produced by a subsurface explosion. For underwater bursts the visible surge is, in effect, a cloud of liquid (water) droplets with the property of flowing almost as if it were a homogeneous fluid. After the water evaporates, an invisible base surge of small radioactive particles may persist.
For subsurface land bursts, the surge is made up of small solid particles, but it still behaves like a fluid. A soil earth medium favors base surge formation in an underground burst.
Meteorological Effects: Meteorological conditions will greatly influence fallout, particularly local fallout. Atmospheric winds are able to distribute fallout over large areas. For example, as a result of a surface burst of a 15 Mt thermonuclear device at Bikini Atoll on March 1, 1954, a roughly cigar-shaped area of the Pacific extending over 500 km downwind and varying in width to a maximum of 100 km was severely contaminated.
Snow and rain, especially if they come from considerable heights, will accelerate local fallout. Under special meteorological conditions, such as a local rain shower that originates above the radioactive cloud, limited areas of heavy contamination may be formed.
Blast and thermal injuries in many cases will far outnumber radiation injuries. However, radiation effects are considerably more complex and varied than are blast or thermal effects and are subject to considerable misunderstanding. A wide range of biological changes may follow the irradiation of animals, ranging from rapid death following high doses of penetrating whole-body radiation to essentially normal lives for a variable period of time until the development of delayed radiation effects, in a portion of the exposed population, following low dose exposures.
Median Lethal Dose (LD50): When comparing the effects of various types or circumstances, that dose which is lethal to 50% of a given population is a very useful parameter. The term is usually defined for a specific time, being limited, generally, to studies of acute lethality. The common time periods used are 30 days or less for most small laboratory animals and to 60 days for large animals and humans. It should be understood that the LD50 assumes that the individuals did not receive other injuries or medical treatment.
For yields of 5-10 Kt (or less), initial nuclear radiation is the dominant casualty producer on the battlefield. Military personnel receiving an acute incapacitation dose (30 Gy) will become performance degraded almost immediately and combat ineffective within several hours. However, they will not die until 5-6 days after exposure if they do not receive any other injuries which make them more susceptible to the radiation dose. Soldiers receiving less than a total of 150 cGy will remain combat effective. Between those two extremes, military personnel receiving doses greater than 150 cGy will become degraded; some will eventually die. A dose of 530-830 cGy is considered lethal but not immediately incapacitating. Personnel exposed to this amount of radiation will become performance degraded within 2-3 hours, depending on how physically demanding the tasks they must perform are, and will remain in this degraded state at least 2 days. However, at that point they will experience a recovery period and be effective at performing nondemanding tasks for about 6 days, after which they will relapse into a degraded state of performance and remain so for about 4 weeks. At this time they will begin exhibiting radiation symptoms of sufficient severity to render them totally ineffective. Death follows at approximately 6 weeks after exposure.
Late or delayed effects of radiation occur following a wide range of doses and dose rates. Delayed effects may appear months to years after irradiation and include a wide variety of effects involving almost all tissues or organs. Some of the possible delayed consequences of radiation injury are life shortening, carcinogenesis, cataract formation, chronic radiodermatitis, decreased fertility, and genetic mutations.
For videos and more on the effects of a thermonuclear device check [1]
See also: nuclear terrorism, radioactive waste.
Effects of Fallout
Nuclear fallout
References