Nuclear fallout

Fallout is the residual radiation hazard from a nuclear explosion and is named from the fact that it "falls out" of the atmosphere in to which it is spread during the explosion. It commonly refers to the radioactive dust created when a nuclear weapon explodes. This radio-active dust, consisting of hot particles, is a kind of radioactive contamination.

Types of fallout

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 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.

Related Topics:
Air burst - Nm - µm - Stratosphere - Kt

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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.

Related Topics:
Radioisotopes - Strontium - Caesium

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This type of fallout is featured in the novel On the Beach by British author Nevil Shute.

Related Topics:
On the Beach - Nevil Shute

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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.

Related Topics:
Vaporized - Radioactive cloud

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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 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.

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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, such as the rather short-lived iodine-131, which is accumulated in the thyroid.

Related Topics:
Radiologically contaminated - Iodine-131 - Thyroid

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