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NUCLEAR FALLOUT

'Fallout' is the residual radiation hazard from a nuclear explosion, so named because it "falls out" of the atmosphere into which it is spread during the explosion. It commonly refers to the radioactive dust created when a nuclear weapon explodes. This radioactive dust, consisting of hot particles, is a kind of radioactive contamination. It can lead to contamination of the food chain. Fallout can also refer to the dust or debris that results from the nuclear explosion.

Factors affecting fallout

Tactical military considerations

Fallout protection

Nuclear reactor accident

Volatility

Half-life

Source

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.

Fission byproducts

Fission products 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 (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 g of fission products are formed per kiloton of yield. The estimated activity of this quantity of fission products 1 minute after detonation is 1.1 ZBq, equal to that of 30 Gg of radium, in equilibrium with its decay products. The mixture of fission product radioisotopes is very complex.

Unfissioned material

Nuclear weapons are relatively inefficient in their use of fissionable material; usually only 2%-40% of the fissionable material undergoes fission 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 activation

Main articles: Neutron activation

If atomic nuclei capture neutrons when exposed to a flux of neutron radiation, they may become radioactive 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 principally a result of neutron capture by sodium, manganese, aluminum, and silicon in the soil. This is a relatively negligible hazard because of the limited area involved.

Actinide

Higher actinides are formed during a nuclear detonation. The neutron flux is very high, so too little time exists between each neutron capture for beta decay. Hence a different group of isotopes is formed to those which are formed in a normal low flux power reactor (S-process). This is an example of the R-process which is also seen in exploding stars. These higher actinides are known as minor actinides in the context of used power reactor fuel. Some of the higher actinides were first found in the fallout from bomb tests, for instance einsteinium (element 99) was first found in the fallout from a hydrogen bomb test.

Types

Atmospheric nuclear weapon tests almost doubled the concentration of radioactive ¹⁴C in the Northern Hemisphere, before levels slowly declined following the Partial Test Ban Treaty.

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

Worldwide

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 10 nm to 20 µm 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.
Initially little was known about the dispersion of nuclear fallout on a global scale. The AEC assumed that fallout would be dispersed evenly across the globe, dispersed by atmospheric winds and will gradually settle to the earth's surface after weeks, months, and even years as worldwide fallout. Nuclear products were deposited in the Northern Hemisphere becoming "far more dangerous than they had originally been estimated."
The radio-biological hazard of worldwide fallout is essentially a long-term one because of the potential accumulation of long-lived radioisotopes (such as strontium-90 and caesium-137) in the body as a result of ingestion of foods containing the radioactive materials. This hazard is much less serious than those which are associated with local fallout, which is of much greater immediate operational concern.

Local

The roughly 280 mile long fallout plume from 15 Mt shot Castle Bravo, ca. 1954

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. Many of the isotopes in the table below will decay into the isotopes that many people are more familiar with.

Table (according to T. Imanaka ''et. al'') of the relative abilities of isotopes to form solids
Isotope	Refractory index
⁹¹Sr	0.2
⁹²Sr	1.0
⁹⁵Zr	1.0
⁹⁹Mo	1.0
¹⁰⁶Ru	0.0
¹³¹Sb	0.1
¹³²Te	0.0
¹³⁴Te	0.0
¹³⁷Cs	0.0
¹⁴⁰Ba	0.3
¹⁴¹La	0.7
¹⁴⁴Ce	1.0

Per capita thyroid doses in the continental United States resulting from all exposure routes from all atmospheric nuclear tests conducted at the Nevada Test Site from 1951-1962.

There will be large amounts of particles of less than 100 nm 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 spill out of the stem and cascade down the outside of the fireball in a downdraft even while the cloud rises, so fallout begins to arrive near ground zero within an hour, and more than half the total bomb debris is deposited on the ground within about 24 hours as local fallout.
The chemical properties of the different elements in the fallout will control the rate at which they are deposited on the ground. The less volatile elements will deposit first.
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 depends on the weather situation from the time of detonation onwards. In stronger winds, fallout travels faster but takes the same time to descend, so although it covers a larger path, it is more spread out or diluted. So the width of the fallout pattern for any given dose rate is reduced where the downwind distance is increased by higher winds. The total amount of activity deposited up to any given time is the same irrespective of the wind pattern, so the overall casualty figures from fallout will generally be independent of the winds. But thunderstorms can bring down activity as rain more rapidly than dry fallout, particularly if the mushroom cloud is low enough to be below, or mixed with, the thunderstorm.
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 from inhalation and ingestion of radiocontaminants, such as the rather short-lived iodine-131, which is accumulated in the thyroid.

Factors affecting fallout

Nevada nuclear test total fallout outdoor gamma doses, 1950-57. The fallout is to the north and east because testing was only allowed when the wind was blowing in those directions.

Location

A formerly secret RAND Corporation simulation of the Castle Bravo fallout indicating that high levels on Rongelap may have been caused by a hotspot. Hotspots downwind are typical of bursts on coral in humid atmospheres and also occurred in the 1954 ''Yankee'' and ''Nectar'' water surface bursts, and the 1956 coral surface bursts ''Zuni'' and ''Tewa''.

There are two main considerations for the location of an explosion: height and surface composition. A nuclear weapon detonated in the air, called an air burst, will produce less fallout than a comparable explosion near the ground. Less particulate matter will be contaminated by an air burst. 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, producing less local fallout but extending over a greater area. The particles contain mostly sea salts with some water; these can have a cloud seeding effect causing local rainout and areas of high local fallout. Fallout from a seawater burst is difficult to remove once it has soaked into porous surfaces because the fission products are present as metallic ions which become chemically bonded to many surfaces. Water and detergent washing is effective on removing less than about 50% of this chemically bonded activity from concrete or steel (complete decontamination requires aggressive treatment like sandblasting, or acidic treatment). After the ''Crossroads'' underwater test, it was found that wet fallout needs to be immediately removed from ships by continuous water washdown (such as from the fire sprinkler system on the decks).
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 subsiding column, which is cause by an excessive density of dust or water droplets in the air. 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. Although the base surge typically contains only about 10% of the total bomb debris in a subsurface burst, it can create larger radiation doses than fallout near the detonation, because it arrives sooner than fallout, before much radioactive decay has occurred.

Meteorological

Comparison of fallout gamma dose and dose rate contours for a 1 Mt fission land surface burst, based on DELFIC calculations. Because of radioactive decay, the dose rate contours contract after fallout has arrived, but dose contours continue to grow

Meteorological conditions will greatly influence fallout, particularly local fallout. Atmospheric winds are able to bring fallout over large areas. For example, as a result of a ''Castle Bravo'' 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. There are three very different versions of the fallout pattern from this test, because the fallout was only measured on a small number of widely spaced Pacific Atolls. The two alternative versions both ascribe the high radiation levels at north Rongelap to a downwind hotspot caused by the large amount of radioactivity carried on fallout particles of about 50-100 micrometres size [1].
After ''Bravo'', it was discovered that fallout landing on the ocean disperses in the top water layer (above the thermocline at 100 m depth), and the land equivalent dose rate can be calculated by multiplying the ocean dose rate at two days after burst by a factor of about 530. In other 1954 tests, including ''Yankee'' and ''Nectar,'' hotspots were mapped out by ships with submersible probes, and similar hotspots occurred in 1956 tests such as ''Zuni'' and ''Tewa'' [2] However, the major U.S. 'DELFIC' (Defence Land Fallout Interpretive Code) computer calculations use the natural size distributions of particles in soil instead of the afterwind sweep-up spectrum, and this results in more straightforward fallout patterns lacking the downwind hotspot.
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

A wide range of biological changes 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.
The unit of actual ''exposure'' is the Roentgen which is defined in ionisations per unit volume of air, and all ionisation based instruments (including geiger counters and ionisation chambers) measure exposure. However, effects depend on the energy per unit mass, not the exposure measured in air. A deposit of 1 joule per kilogram has the unit of 1 gray. For 1 MeV energy gamma rays, an exposure of 1 roentgen in air will produce a dose of about 0.01 gray (1 centigray, cGy) in water or surface tissue. Because of shielding by the tissue surrounding the bones, the bone marrow will only receive about 0.67 cGy when the air exposure is 1 roentgen and the surface skin dose is 1 cGy. Some of the lower values reported for the amount of radiation which would kill 50% of personnel (the 'LD₅₀') refer to bone marrow dose, which is only 67% of the air dose.

Short term

When comparing the effects of various types or circumstances, the dose which is lethal to 50% of a given population is a common parameter. The term is usually defined for a specific time, which is generally limited 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. The LD₅₀ figure assumes that the individuals did not receive other injuries or medical treatment.
In the 1950s, the LD₅₀ for gamma rays was set at 3.5 Gy, while under more dire conditions of war (a bad diet, little medical care, poor nursing) the LD₅₀ was 2.5 Gy (250 rad). There have been few documented cases of survival beyond 6 Gy. One person at Chernobyl survived 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 hand dose of 10 Gy or more will likely result in loss of the hand; a British industrial radiographer who 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. Though the human biology resists mutation from large radiation exposure; grossly mutated fetuses usually miscarry, and this often causes gene-faults.
One hour after a surface burst, the radiation from fallout in the crater region is 30 grays per hour (Gy/h). Civilian dose rates in peacetime range from 30 to 100 µGy per year.
Fallout radiation decays exponentially relatively quickly with time. Most areas become fairly safe for travel and decontamination after three to five weeks.
For yields of up to 10 kt, prompt radiation is the dominant producer of casualties 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 not be incapacitated. 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 poisoning of 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 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. ^[1]

Tactical military considerations

Comparison of predicted fallout "hotline" with test results in the 3.53 Mt 15% fission ''Zuni'' test at Bikini in 1956. The predictions were made under simulated tactical nuclear war conditions aboard ship by Edward A. Schuert.

Blast injuries and thermal burns from 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 silos or command centers, however "salting" enemy territory with a fallout-heavy atomic burst can be used to deny ill-equipped civilians/military personnels access to a contaminated area.

Fallout protection

During the Cold War, the governments of the U.S. and USSR attempted to educate their citizens about surviving a nuclear attack. In the U.S., this effort became known as Civil Defense. The government provided procedures on minimizing short-term exposure to fallout, but currently, the popular attitude towards fallout protection is that short-term survival in a global thermonuclear war would be futile, and fallout shelters are no longer maintained even though fallout shelters could almost entirely eliminate the fallout-related casualties of a Chernobyl-type accident.

Nuclear reactor accident

Fallout can also refer to nuclear accidents, although a nuclear reactor does not explode like a nuclear weapon. The isotopic signature of bomb fallout is very different from the fallout from a serious power reactor accident (such as Chernobyl). The key differences are in volatility and half-life.

Volatility

The boiling point of an element (or its compounds) is able to control the percentage of that element which is released by a power reactor accident. In addition the ability of an element to form a solid controls the rate at which it is deposited on the ground after it has been injected into the atmosphere by a nuclear detonation.

Half-life

In bomb fallout, a large amount of short-lived isotopes such as ⁹⁷Zr are present. This isotope and the other short-lived isotopes are being constantly generated in a power reactor, but because the criticality occurs over a long length of time the majority of these short lived isotopes decay before they can be released.
Below is shown a comparison of the calculated gamma dose rates in open air from the fallout of a fission bomb and of the Chernobyl release. It is clear that average half-life of the Chernobyl release is longer than that for the bomb fallout.

References

1.

★ 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) (Available Online)

★ T. Imanaka, S. Fukutani, M. Yamamoto, A. Sakaguchi and M. Hoshi, ''J. Radiation Research'', 2006, '47', Suppl A121-A127.

★ Sheldon Novick, The Careless Atom, (Boston MA: Houghton Mifflin Co., 1969), p. 98

External links

★ Downwinders.org We're all Downwinders Now

★ Java-based simulator of nuclear weapons effects over various regions, from Federation of American Scientists.

★ RadShelters4u Nuclear FAQs

★ KI4U Potassium Iodide Information

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NUCLEAR FALLOUT

Source

Fission byproducts

Unfissioned material

Neutron activation

Actinide

Types

Worldwide

Local

Factors affecting fallout

Location

Meteorological

Effects

Short term

Long term

Tactical military considerations

Fallout protection

Nuclear reactor accident

Volatility

Half-life

See also

References

External links

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