Nuclear fallout

Nuclear fallout is residual radioisotope material that is created by the reactions producing a nuclear explosion or nuclear accident. In explosions, it is initially present in the radioactive cloud created by the explosion, and "falls out" of the cloud as it is moved by the atmosphere in the minutes, hours, and days after the explosion. The amount of fallout and its distribution is dependent on several factors, including the overall yield of the weapon, the fission yield of the weapon, the height of burst of the weapon, and meteorological conditions.

Fission weapons and many thermonuclear weapons use a large mass of fissionable fuel (such as uranium or plutonium), so their fallout is primarily fission products, and some unfissioned fuel. Cleaner thermonuclear weapons primarily produce fallout via neutron activation. Salted bombs, not widely developed, are tailored to produce and disperse specific radioisotopes selected for their half-life and radiation type.

Fallout also arises from nuclear accidents, such as those involving nuclear reactors or nuclear waste, typically dispersing fission products in the atmosphere or water systems.

Fallout can have serious human health consequences on both short- and long-term time scales, and can cause radioactive contamination far away from the areas impacted by the more immediate effects of nuclear weapons. Atmospheric and underwater nuclear weapons testing, which widely disperses fallout, was ceased by the United States, Soviet Union, and United Kingdom following the 1963 Partial Nuclear Test Ban Treaty. Underground testing, which can sometimes causes fallout via venting, was largely ceased following the 1996 Comprehensive Nuclear-Test-Ban Treaty. The bomb pulse, the increase in global carbon-14 formed from neutron activation of nitrogen in air, is predicted to dominate long-term effects on humans from nuclear testing, causing ill effects and death in a small fraction of the population for up to 8,000 years.

Fallout is usually divided into two major types, largely determined by the height of burst of the detonation. If detonated at a sufficient altitude that allows the fireball to avoid mixing significantly with ground debris, the radioactive byproducts in the fallout will generally stay aloft longer than weapons detonated at or near the surface. This additional time aloft allows the most acutely dangerous radioactive elements, with the shortest half-lives, to decay prior to their descending to the surface, reducing the overall radioactive intensity of the fallout which is deposited. This additional time also allows the radioactive cloud to diffuse over a larger area, resulting in less radioactive debris per area of land below. This more diffuse form of fallout is known as "global fallout," because its main effect is to raise background radiation exposure slightly over vast areas, and is contrasted with "local fallout," which produces a plume of concentrated radioactive byproducts downwind of the detonation within minutes or hours.

Meteorological realities can complicate these distinctions; "rainout," for example, can occur when atmospheric conditions cause precipitation from a nuclear cloud. A nuclear detonation underwater also produces a different local fallout than one on land. There have also been weapons detonated below the threshold for avoiding local fallout but which have not done so; the 50 megaton Tsar Bomba test in 1961, for example, had its fireball buoyantly boosted upwards by its reflected shockwave, preventing its mixture.

After the detonation of a weapon at or above the fallout-free altitude (an air burst), fission products, un-fissioned nuclear material, and weapon residues vaporized by the heat of the fireball condense into a suspension of particles 10 nm to 20 μm in diameter. This size of particulate matter, lifted to the stratosphere, may take months or years to settle, and may do so anywhere in the world. Its radioactive characteristics increase the statistical cancer risk, with up to 2.4 million people having died by 2020 from the measurable elevated atmospheric radioactivity after the widespread nuclear weapons testing of the 1950s, peaking in 1963 (the Bomb pulse).^{[unreliable source?]} Levels reached about 0.15 mSv per year worldwide, or about 7% of average background radiation dose from all sources, and has slowly decreased since, with natural background radiation levels being around 1 mSv.

Radioactive fallout has occurred around the world; for example, people have been exposed to iodine-131 from atmospheric nuclear testing. Fallout accumulates on vegetation, including fruits and vegetables. Starting from 1951 people may have gotten exposure, depending on whether they were outside, the weather, and whether they consumed contaminated milk, vegetables or fruit. Exposure can be on an intermediate time scale or long term. The intermediate time scale results from fallout that has been put into the troposphere and ejected by precipitation during the first month. Long-term fallout can sometimes occur from deposition of tiny particles carried in the stratosphere. By the time that stratospheric fallout has begun to reach the earth, the radioactivity is very much decreased. Also, after a year it is estimated that a sizable quantity of fission products move from the northern to the southern stratosphere. The intermediate time scale is between 1 and 30 days, with long term fallout occurring after that.