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Radioactive contamination means the distribution in an environment of radioactive material. This differs from direct radiation because the radioactive material may be moved around by wind or water, or it may be taken up by organisms.
A nuclear reaction typically produces some sort of ionizing radiation, which can pose a hazard to nearby people and machinery. However, when the material undergoing the reaction is removed, the radiation ceases, and the hazard is ended (although its effects may take time to show up; see radiation poisoning). However, many of the materials involved in a nuclear reaction are radioactive by themselves. If these materials escape into the environment, they will continue to release radiation after the material undergoing the reaction is removed. If these radioactive materials have a long half-life, then the area in which they were released will be dangerous for a long time.
Radioactive material can be taken into the human body. Once there, it continues to produce radiation, which can lead to radiation poisoning. If the radioactive element is used primarily in one part of the body, then a large concentration of radiation will be emitted in that part of the body, often leading to cancer. For example, iodine is primarily used in the thyroid. When radioactive iodine is released into the environment (as in the Chernobyl accident, for instance) it is taken up by the body and stored in the thyroid, leading to thyroid cancer (which was very common among children who were near the Chernobyl accident).
Many radioisotopes are also heavy metals. Most of these are chemically toxic, and many bioaccumulate, just as ordinary lead and mercury do.
A nuclear reaction can irradiate nearby people directly. But if it releases radioactive material, this material can irradiate people in a variety of ways.
The radioactive material may simply land on a person's clothing, in their home, in their workplace, in the water, on the soil, or in any other place near the person. From these places, its radiation may affect the person over the course of many years.
Radioactive material may be taken up from the soil or water by plants. These plants then become radioactive. If they are eaten by animals, the animals become radioactive. Both these effects are more severe if the radioactive elements bioaccumulate, that is, if they are easily taken up by plants and animals, but not easily disposed of.
The most severe problem occurs when humans drink contaminated water, eat contaminated plants or animals, or breathe contaminated air. The radioactive material may then lodge itself inside the human, releasing its radiation in direct contact with human tissues. Many elements are used primarily in a small part of the body's tissues, so the radioactive materials may be concentrated there.
When it is necessary for workers to visit contaminated sites, they normally wear suits that cover as much as possible of their bodies and breathing filters. These suits cannot effectively stop direct radiation, but they can prevent the worker from taking up radioactive material. Upon leaving the contaminated area, the worker is decontaminated by being scrubbed thoroughly and then checked with a geiger counter. The suit and breathing filter are either discarded or cleaned with extreme thoroughness.
There are a confusing array of units for measuring radiation and radioactivity: grays, sieverts, becquerels, plus a whole different set of non-SI derived units including roentgen, rems and rads. Moreover, there are different conventions for how to measure radioactive contamination.
Grays measure an amount of radiation absorbed in absolute terms. Sieverts measure an amount of radiation absorbed, adjusted for its effects on humans: some radiation has more severe effects on humans, and some has less severe effects on humans. Neither of these describe amounts of radioactive material. One could, in principle, measure amounts of radioactive material the same way one measures any other amount of material; in grams, perhaps. However, some materials are much more radioactive than others per unit weight. Becquerels measure the amount of radioactive material, normalized by the amount of radiation it emits. Thus becquerels are the appropriate measure of radioactive contamination. However, a bequerel is a tiny amount of radioactive material, so one more commonly uses giga-bequerels, "GBq".
Radioactive isotopes have different half-lives. For example, iodine-131 has a half-life of about 8 days, while plutonium-239 has a half-life of over 10,000 years. Thus if 1 GBq of each is released into the environment, after 8 days there will be only 0.5 GBq of iodine-131, but there will still be almost 1 GBq of plutonium left. Thus the number of becquerels of radioactive material released into the environment does not tell the whole story. One way of measuring the release of radioactive material is to give the number of becquerels of material released; another is to give the number of becquerels, adjusted for some fixed period after the release. For example, the Soviet government published 10-day adjusted numbers for the release of material from the Chernobyl accident, which de-emphasizes the effect of short-lived radioactive elements such as iodine-131, which caused many cases of thyroid cancer in children. On the other hand, if the release had been made up entirely of iodine-131, then the area would be safe by now; as it stands, the caesium-137 (which has a half-life of 30 years) is the major problem.
Actual detection of radioactive contamination proceeds by detection of the emitted radiation, using a geiger counter or a dosimeter.
Radioactive contamination can come from many sources, but most are man-made.
Uranium and thorium are radioactive in their naturally-occurring forms, and many elements are mixed with small amounts of radioactive isotopes. In large quantities, these can pose hazards, but normally the concentration of radioactive material is quite low; the radiation these isotopes produce is part of the general background radiation experienced all over the Earth. Normally these isotopes are buried in underground mineral deposits, but they are also present in coal; mining and burning coal releases these into the environment, both into the solid coal ash that must be disposed of and into the air in the form of fine ash.
Nuclear weapons release large quantities of radioactive material. This material comes from both the radioactive materials used to make the weapons and from other materials that become radioactive when exposed to the burst of neutrons that accompanies most nuclear reactions. This material may remain near the site of the weapon's detonation, or it may be carried by the wind until it falls to earth as fallout. Contamination of seawater can also spread radiation; in 2004 soil samples from Sagami Bay near Tokyo, Japan, were found to contain contamination from the testing on Bikini Atoll from 1946 to 1958 .
Nuclear accidents can also release radioactive material. Nuclear reactors are full of radioactive material, consisting of the nuclear fuel itself, its fission byproducts, and reactor material activated by the neutrons produced in the reactor. Nuclear reactors are designed with numerous safety features to prevent escape of radioactive material, but in nuclear accidents, some release can occur. Early Soviet reactors, such as the reactor at Chernobyl, have minimal safety features designed to prevent this. A number of more robust reactors have had quite serious accidents without significant release of radioactive contamination to the environment. Nuclear accidents need not be spectacular: in the Goiânia accident a radioactive caesium rod was left behind when a Brazilian hospital was decomissioned. When scavengers found the rod, it was passed around, spreading radioactive contamination to hundreds of people.
Nuclear waste is radioactive material; if it is disposed of improperly, some of this material could escape to the environment. Some early nuclear waste was disposed of by dissolving it in the ocean, based on the idea that sufficient dilution would reduce the danger due to the waste. This is no longer considered acceptable.
Radiological weapons are explicitly designed to release radioactive contamination into the environment. The least disastrous of these release only isotopes with a short half-life, so that the area in which they are released becomes usable in a relatively short time. Media attention has focused on the possibility of terrorists assembling a dirty bomb, a crude radiological weapon which would probably use long-lived radioactive waste or nuclear fuel. The United States and the Soviet Union also researched nuclear weapons designed to distribute large amounts of radioactive contamination into the environment (see nuclear weapon design).
Fusion power plants, once built, might release radioactive material into the environment. Most contemplated fusion reactions would release many neutrons, making the reactor components radioactive. These effects could be minimized by careful design, and the waste could be made up of short-lived isotopes. However, the only current fusion reaction that shows promise uses tritium, a radioactive isotope of hydrogen. This is a serious concern, because hydrogen (and by extension tritium) is very difficult to confine, escaping through rubber, plastic, and many kinds of steel. Moreover, since tritium behaves very much like hydrogen in chemical and biological systems, it is easily incorporated into organisms.
Depleted uranium has been treated to remove most of the radioactive isotopes, but some remains. This can be a problem when depleted uranium is used in munitions, spreading large quantities around the site of a battle. Some controversy surrounds exactly how much radioactive contamination arises from the use of depleted uranium.
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