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Radioactive contamination

From Academic Kids

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The radiation warning symbol (trefoil).

Radioactive contamination is the uncontrolled distribution of radioactive material in a given environment.

Many radioactive isotopes are produced artificially, either for their specific properties (such as medical radioisotopes) or as a byproduct (such as fission products). Some radioisotopes exist in nature, including uranium, thorium, and some isotopes of potassium and carbon. Radioisotopes share the property of spontaneous transformation, where an atom of a given element will transform into a different element, emitting radiation in the process.

Radioactive contamination is typically the result of a loss of control of radioactive materials during the production or use of radioisotopes. For example, if a radioisotope used in medical imaging is accidentally spilled, the material could be spread by people as they walk around. Radioactive contamination may also be an inevitable result of certain processes, such as the release of radioactive xenon in nuclear fuel reprocessing. In cases that radioactive material cannot be contained, it may be diluted to safe concentrations. Nuclear fallout is the distribution of radioactive contamination by a nuclear explosion.

Containment is what differentiates radioactive material from radioactive contamination. Therefore, radioactive material in sealed and designated waste containers is not properly referred to as contamination, although the units of measurement might be the same.

Measurement

Radioactive contamination may exist on surfaces or in volumes of material or air.

Surface contamination is usually expressed in units of radioactivity per unit of area. For SI, this is becquerels per square metre (or Bq/m2). Other units such as dpm/cm2, or disintegrations per minute per square centimetre (1 dpm/cm2 = 166 2/3 Bq/m2) may be used. Surface contamination may either be fixed or removable. (In the case of fixed contamination, the radioactive material cannot by definition be spread, but it is still measurable.)

Volumes of air, water, waste, or earth may contain radioactive contaminants. Volumetric contamination is expressed in units of radioactivity per unit volume (such as Bq/m3, or becquerels per cubic metre).

The level of contamination may be determined by measuring the radiation emitted by the contaminant. In the case of a known radioisotope, it may be possible to accurately determine the activity simply from a dose rate measurement with a radiation meter. Spectral analysis of the radiation can also yield accurate estimates. In cases where the contaminant emits low-energy radiation, however, it may be difficult to determine its activity.

Hazards

The hazards to people and the environment from radioactive contamination depend on the nature of the radioactive contaminant, the level of contamination, and the extent of the spread of contamination.

Low levels of radioactive contamination pose no risk at all, but can still be detected by radiation instrumentation, thereby being more of an annoyance than a threat. In the case of low-level contamination by isotopes with a short half-life, the best course of action may be to simply allow the material to naturally decay. Longer-lived isotopes should be cleaned up and properly disposed of.

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Unintentionally released radiation can reach humans by a variety of means.

High levels of contamination may pose major risks to people and the environment. People can be exposed to potentially lethal radiation levels, both externally and internally, from the spread of contamination following an accident (or a deliberate detonation) involving large quantities of radioactive material. The biological effects of external exposure to radioactive contamination are generally the same as those from an external radiation source not involving radioactive materials, such as x-ray machines, and are dependent on the absorbed dose.

The biological effects of internally deposited radionuclides depend greatly on the activity and the biodistribution and removal rates of the radioisotope, which in turn depends on its chemical form. The biological effects may also depend on the chemical toxicity of the deposited material, independent of its radioactivity. Some radioisotopes may be generally distributed throughout the body and rapidly removed, as is the case with tritiated water. Some radioisotopes may target specific organs and have much lower removal rates. For instance, the thyroid gland takes up a large percentage of any iodine that enters the body. If large quantities of radioactive iodine are inhaled or ingested, the thyroid may be impaired or destroyed, while other tissues are affected to a lesser extent. Radioactive iodine is a common fission product; it was a major component of the radiation released from the Chernobyl disaster, leading to many cases of pediatric thyroid cancer and hypothyroidism. On the other hand, radioactive iodine is used in the diagnosis and treatment of many diseases of the thyroid precisely because of the thyroid's selective uptake of iodine.

Radioactive contamination can enter the body through ingestion, inhalation, absorption, or injection. For this reason, it is important to use personal protective equipment when working with radioactive materials. Radioactive contamination may also be ingested as the result of eating contaminated plants and animals or drinking contaminated water or milk from exposed animals. Following a major contamination incident, all potential pathways of internal exposure should be considered.

Decontamination of external contamination is often as simple as removing contaminated clothing and cleaning contaminated skin. Internal decontamination is much more difficult, depending on the radionuclide in question.

See also

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