Radioactive contamination |
The radiation warning symbol
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.
Dangers
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.
Routes for radioactive exposure
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 which 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.
Measurement
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. 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 cesium-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.
Origins
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.
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 cesium 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.
See also
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