Nuclear fallout

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Fallout is the residual radiation hazard from a nuclear explosionand is named from the fact that it "falls out" of the atmosphere into which it is spread during the explosion. It commonly refers to the radioactivedust created when a nuclear weaponexplodes. This radioactive dust, consisting of hot particles, is a kind of radioactive contamination.

Fallout can also refer to nuclear accidents, although a nuclear reactorcannot explode exactly like a nuclear weapon.

Source of fallout

A nuclear explosion vaporizes any material within the fireball, including the ground if it is nearby and this is combined with residual ionizing radiation to produce fallout. The sources of this residual ionizing radiation are:

• Fissionproducts. These are intermediate weight isotopeswhich are formed when a heavy uraniumor plutoniumnucleus 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, that is, 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 radiation, usually accompanied with gamma radiation. Approximately 60 gof fission products are formed per kilotonof yield. The estimated activity of this quantity of fission products 1 minute after detonation is 1.1 ZBq, equal to that of 30 Ggof 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 particlesand is of relatively minor importance.
• Neutron-induced activity. If atomic nuclei captureneutrons 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 zeromay 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 relatively negligible hazard because of the limited area involved.
• Higher actinidesare formed during a nuclear detonation. The neutronflux is very high so too little time exists between each neutron capturefor beta decay. Hence a different group of isotopesis formed to those which are formed in a normal low flux power reactor (S-Process). This is an example of the R-Processwhich is also seen in exploding stars. These higher actinides are known as minor actinidesin the context of used power reactor fuel. Some of the higher actindies were first found in the fall out from bomb tests, for instance einsteinium(element 99) was first found in the fallout from a hydrogen bombtest.

Types of Fallout

There are many types of fallout, ranging from the global type to the more area-restricted types.

Worldwide Fallout

After an air burstthe 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 10 Nmto 20 µmin 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 Caesium-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 here. Local fallout is of much greater immediate operational concern.

This type of fallout is featured in the novels On the Beach by British author Nevil Shuteand Do Androids Dream of Electric Sheep?by Philip K. Dick.

Local fallout

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In a land or water surface burst, large amounts of earth or water will be vaporizedby 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 100 nmto 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.

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Severe local fallout contamination can extend far beyond the blast and thermal effects, particularly in the case of high yield surface detonations. 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 50 km (30 Miles) wide from a single explosion.

Whenever individuals remain in a radiologically contaminatedarea, 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, such as the rather short-lived Iodine-131, which is accumulated in the thyroid.

Factors affecting fallout

There are plenty of factors that may be affecting fallout, first and foremostly geometry and impact of blast.

Location

There are two main considerations for the location of an explosion: height and surface composition. A nuclear weapon detonated in the air (Air burst) will produce less fallout than a comparable explosion near the ground. This is due to the fact that less particulate matter will be contaminated and kicked up by the explosion. Detonations at the surface ("surface bursts") will tend to produce more fallout material.

In case of water surface 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 seedingeffect causing local rainoutand areas of high local fallout. Fallout from seawateris unusually dangerous because it is difficult to remove by washing with clean water.

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. For underwater bursts the visible surge is, in effect, a cloud of liquid (usually 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. Fallout can spread for long distances with the help of wind currents and possible storms or volcanoes.

Meteorological

Meteorologicalconditions will greatly influence fallout, particularly local fallout. Atmospheric winds are able to bring fallout over large areas. For example, as a result of a surface burst of a 15 Mtthermonuclear device at Bikini Atollon 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 radio-active cloud, limited areas of heavy contamination just downwind of a nuclear blast may be formed.

Effects of fallout

A wide range of biologicalchanges may follow the irradiation of animals. These vary 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.

Short term

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.

It was estimated some years ago that that with the best possible medical care that the LD₅₀ for gamma rays is 3.5 Gy, while under the dire conditions of war (Bad diet, little medical care, poor nursing) that the LD₅₀ will be 2.5 Gy (250 Rads).

Initial radiation from fallout can exceed 300 graysper hour (Gy/h) immediately downwind of a ground burst. A cumulative doseof 4.5 Gy is fatal to half of a population of humans. There have been few documented cases of survival beyond 6 Gy. One person at Chernobylsurvived a dose of more than 10 Gy, but many of the persons exposed there were not uniformly exposed over their entire body. If a person is exposed in a non-homogeneous manner then a given dose (averaged over the entire body) is less likely to be of a lethal dose. For instance if a person gets a hand/low arm dose of 100 Gy which gives them an overall dose of 4 Gy then they are more likely to survive than a person who gets a 4 Gy dose uniformly over their entire body. A handdose of 10 Gy or more will likely result in loss of the hand; a Britishindustrial radiographerwho got a lifetime hand dose of 100 Gy lost his hand because of radiation dermatitis. Most people become ill after an exposure to 1 Gy or more. The fetuses of pregnant women are often more vulnerable than the host body and may miscarry, especially in the first trimester. Human biology resists mutationfrom large radiation exposure; Grossly mutated fetuses usually miscarry, and this often causes gene-faults. Civilian dose rates in peacetime range from 30 to 100 µGy/A.

Fallout radiation falls off ('decays') exponentiallyrelatively quickly with time. Most areas become fairly safe for travel and decontamination after three to five weeks.

The most dangerously known emissions from fallout are gamma rays, which travel in straight lines, like ordinary light. The fallout particlesemit gamma rays in the same way that a light bulbemits light. Gamma rays are invisible, and cannot be seen, smelled, or felt. Special equipment is required to detect and measure gamma rays (Such as geiger counters, dosimeters).

For yields of up to 10 kt, prompt radiation is the dominant producer of casualty on the battlefield. Humans receiving an acute incapacitating dose (30 Gy) will have their performance degraded almost immediately and become ineffective within several hours. However, they will not die until 5 to 6 days after exposure assuming they do not receive any other injuries.

Individuals receiving less than a total of 1.5 Gy will remain effective. Between those two extremes, people receiving doses greater than 1.5 Gy will become disabled; Some will eventually die.

A dose of 5.3 Gy to 8.3 Gy is considered lethal but not immediately incapacitating. Personnel exposed to this amount of radiation will have their performance degraded within 2 to 3 hours, depending on how physically demanding the tasks they must perform are, and will remain in this disabled state at least 2 days. However, at that point they will experience a recovery period and be effective at performing non-demanding tasks for about 6 days, after which they will relapse for about 4 weeks. At this time they will begin exhibiting symptoms of radiation poisoningof sufficient severity to render them totally ineffective. Death follows at approximately 6 weeks after exposure, although results may vary.

Long Term

Late or delayed effects of radiation occur following a wide range of doses and dose rates. Delayed effects may appear months to years after irradiationand 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, cataractformation, chronic radiodermatitis, decreased fertility, and genetic mutations.

Tactical military considerations

Blast injuriesand thermalburns, due to the use of nuclear weapons for military action, 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.

The closer to ground an atomic bomb is detonated, the more dust and debris is thrown into the air, resulting in greater amounts of local fallout. From a tactical standpoint, this has the disadvantage of hindering any occupation/invading 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 silosor command centerssuch 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 ethicalmilitaryaction by critics.

Cleanup

In severe cases of fallout contamination, lethal doses of external radiation may be incurred if protective or evasive measures are not undertaken. A fallout shelteris designed to shield its occupants from radiation. However, some radioactive contaminationwill probably remain when the inhabitants eventually emerge.

Gamma raysdo not contaminate people or objects. Fallout particlescontaminate 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 watercan be adequately cleaned by filtering contaminated water through more than 25 cm (10 In) of dirt. Food in sealed packages is not poisoned by fallout. Stored grainand exposed fruitcan be cleaned and peeled. Vehicles are usually washed down with fire-hoses (Usually with regular unradiated water, sometimes chemically), 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.

The fallout residue can be used to analyze the source and nature of the weapon used. Materials used in the weapon will have a distinct isotopic signature, which with proper analysis could reveal where or by whom the weapon was made.

See also

• Radioactive contamination
• Hot Particle
• Dirty bomb
• Neutron bomb
• Radiological weapon
• Radiation poisoning
• Radiation biology
• Fallout shelter
• Nuclear terrorism
• Radioactive waste
• Nuclear weapon design
• List of nuclear accidents
• Castle Bravo- largest nuclear fallout accident by United States,
• Fallout (computer game)- a computer game about the results of fallout.
• Human radiation experiments

References

• Glasstone, Samuel and Dolan, Philip J., The Effects of Nuclear Weapons (third edition), U.S. Government Printing Office, 1977. (Available Online)
• NATO Handbook on the Medical Aspects of NBC Defensive Operations (Part I - Nuclear), Departments of the Army, Navy, and Air Force, Washington, D.C., 1996,

(Available Online)

• Smyth, H. DeW., Atomic Energy for Military Purposes, Princeton University Press, 1945. (Smyth Report)
• The Effects of Nuclear War in America, Office of Technology Assessment (May 1979)

External links

• Java-based simulator of nuclear weapons effects over various regions, from Federation of American Scientists.de:Radioaktiver Niederschlag

es:Contaminación nuclear nl:Fallout ja:?????? no:Atom-Nedfall pl:Odpady Promieniotwórcze sv:Radioaktivt Nedfall

This article is licensed under the GNU Free Documentation License.
It uses material from the http://en.wikipedia.org/wiki/Nuclear+fallout Wikipedia article Nuclear fallout.