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, 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.

Types of fallout

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.

Global fallout

After the detonation of a weapon at or above the fallout-free altitude, 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. 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.
Examples of both intermediate and long term fallout occurred after the 1986 Chernobyl accident, which contaminated over of land in Ukraine and Belarus. The main fuel of the reactor was uranium, and surrounding this was graphite, both of which were vaporized by the hydrogen explosion that destroyed the reactor and breached its containment. An estimated 31 people died within a few weeks after this happened, including two plant workers killed at the scene. Although residents were evacuated within 36 hours, people started to complain of vomiting, migraines and other major signs of radiation sickness. The officials of Ukraine had to close off an area with an radius. Long term effects included at least 6,000 cases of thyroid cancer, mainly among children. Fallout spread throughout Europe, with Northern Scandinavia receiving a heavy dose, contaminating reindeer herds in Lapland, and salad greens becoming almost unavailable in France. Some sheep farms in North Wales and the North Of England were required to monitor radioactivity levels in their flocks until the control was lifted in 2012.

Local fallout

During detonations of devices at ground level, below the fallout-free altitude, or in shallow water, heat vaporizes large amounts of earth or water, which is drawn up into the radioactive cloud. This material becomes radioactive when it combines with fission products or other radio-contaminants, or when it is neutron-activated.
The table below summarizes the abilities of common isotopes to form fallout. Some radiation taints large amounts of land and drinking water causing formal mutations throughout animal and human life.

Isotope	⁹¹Sr	⁹²Sr	⁹⁵Zr	⁹⁹Mo	¹⁰⁶Ru	¹³¹Sb	¹³²Te	¹³⁴Te	¹³⁷Cs	¹⁴⁰Ba	¹⁴¹La	¹⁴⁴Ce
Refractive index	0.2	1.0	1.0	1.0	0.0	0.1	0.0	0.0	0.0	0.3	0.7	1.0

Image:US fallout exposure.png|right|thumb|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 to 1962 and from emissions from plutonium production at the Hanford Site in Washington state
A surface burst generates large amounts of particulate matter, composed of particles from less than 100 nm to several millimeters in diameter—in addition to very fine particles that contribute to worldwide fallout. The larger particles spill out of the stem and cascade down the outside of the fireball in a downdraft even as the cloud rises, so fallout begins to arrive near ground zero within an hour. More than half the total bomb debris lands on the ground within about 24 hours as local fallout. Chemical properties of the elements in the fallout control the rate at which they are deposited on the ground. Less volatile elements 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 from the time of detonation onward. 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. Thus, 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 overall casualty figures from fallout are generally independent of winds. But thunderstorms can bring down activity as rain allows fallout to drop more rapidly, 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 leads 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

Location

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, produces less fallout than a comparable explosion near the ground. A nuclear explosion in which the fireball touches the ground pulls soil and other materials into the cloud and neutron activates it before it falls back to the ground. An air burst produces a relatively small amount of the highly radioactive heavy metal components of the device itself.
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 that chemically bond to many surfaces. Water and detergent washing effectively removes less than 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 must be immediately removed from ships by continuous water washdown.
Parts of the sea bottom may become fallout. After the Castle Bravo test, white dust—contaminated calcium oxide particles originating from pulverized and calcined corals—fell for several hours, causing beta burns and radiation exposure to the inhabitants of the nearby atolls and the crew of the Daigo Fukuryū Maru fishing boat. The scientists called the fallout Bikini snow.
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 caused 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 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.