Radioactive contamination is the unwanted presence of radioactive material on, inside or around people, objects, surfaces, soil, water, air or other materials. The essential feature is that radioactive atoms or particles have been physically deposited, dispersed or incorporated into a location or organism. Contamination is therefore different from radiation exposure. A person standing near a radioactive source can receive radiation without becoming contaminated if radioactive material does not enter or remain on the person.
Conversely, a person can be contaminated by radioactive dust, liquid or particles and continue to receive radiation from that material until it is removed or sufficiently decays. Contamination can be external, when radioactive material is present on the skin, clothing or other external surfaces, or internal, when radioactive material enters the body through inhalation, ingestion, wounds or other routes. The type and severity of contamination depend on the radionuclide, its activity, radiation emissions, chemical form, physical form, route of entry and duration of retention.
Radioactive contamination can originate from many sources. Nuclear facilities can generate radioactive materials during reactor operation, fuel handling, maintenance and waste processing. Medical procedures can involve radioactive substances that are deliberately administered to patients, after which small amounts may be present in bodily fluids and require appropriate precautions. Industrial radiography, research laboratories and isotope production facilities can also involve radioactive materials.
Natural sources contribute to background radioactivity and can sometimes produce significant environmental exposure, although naturally occurring radioactivity is not normally described as contamination merely because it exists. Human activities can redistribute naturally occurring radionuclides, for example through mining and processing of uranium or other minerals. Nuclear accidents and releases can disperse radioactive materials over wide areas, with deposition patterns influenced by weather, particle size, chemical form and geography. Contamination can therefore be highly localised or distributed across large regions depending on the release mechanism.
External contamination occurs when radioactive material is present on the outside of a person or object. Contaminated clothing, dust on skin or radioactive liquid on a surface are examples. External contamination can often be reduced by physical removal methods such as controlled removal of clothing, washing, cleaning or specialised decontamination procedures. Internal contamination occurs when radionuclides enter the body and may be more difficult to remove because the material can be distributed through biological systems.
The chemical properties of the radionuclide strongly influence its behaviour. Radioactive iodine can be concentrated by the thyroid because the body uses iodine in thyroid hormone production. Caesium can behave in ways related to potassium metabolism, while strontium can behave partly like calcium and accumulate in bone. Plutonium and some transuranic elements have different chemical and biological behaviours. Internal contamination is therefore not simply a matter of radioactive material being present inside the body. Its location, chemical form, biological retention and radioactive emissions determine the resulting dose.
Contamination does not automatically imply a high radiation dose or severe health consequences. Radiation risk depends on the amount of radioactive material, the radionuclide, the type and energy of radiation, the exposure geometry, the route of exposure and the time for which exposure occurs. A small quantity of a short lived radionuclide may produce a different risk profile from a larger quantity of a long lived alpha emitting radionuclide.
External gamma radiation can penetrate tissue from outside the body, while alpha particles generally have very short ranges and pose their greatest radiation risk when alpha emitting material is inhaled, swallowed or otherwise incorporated into the body. Beta particles have intermediate penetration characteristics. Radiation protection therefore uses measurements of activity, dose rate, absorbed dose and equivalent or effective dose as appropriate. Contamination surveys may measure surface activity or airborne activity rather than directly measuring biological dose. Correct interpretation requires understanding which physical quantity has been measured and what it represents.
Decontamination is the process of removing or reducing radioactive material from people, equipment, buildings, soil, water or other surfaces. Techniques vary according to the material and environment. Personal decontamination may involve carefully removing contaminated clothing and washing affected skin while avoiding actions that spread material. Equipment and surfaces can be cleaned mechanically, chemically or through specialised processes. Soil contamination may be managed through removal, containment, fixation, treatment or controlled land use depending on the radionuclide and site.
Water can sometimes be treated using filtration, ion exchange, chemical separation or other methods. Decontamination decisions must balance radiation reduction against the risk of spreading material or creating additional radioactive waste. In many situations, attempting to remove every atom of radioactive material would be impractical and unnecessary. Regulatory standards and risk based criteria determine when contamination levels are sufficiently reduced for a particular use or activity.
Radioactive contamination is monitored using instruments selected for the expected radiation type and energy. Geiger Muller counters can detect many forms of ionising radiation and are useful for surveying, although they do not provide a complete characterisation of contamination. Scintillation detectors can provide greater sensitivity and, depending on design, energy information. Semiconductor detectors can identify gamma ray energies with high resolution, allowing specific radionuclides to be identified.
Air samplers can collect radioactive particles for analysis, while smear or wipe tests can determine whether removable contamination is present on surfaces. Whole body counters and specialised bioassay techniques can assess internal contamination in appropriate circumstances. Environmental monitoring may include measurements of soil, water, vegetation, food and air. The interpretation of measurements requires calibration, background correction, detection limits and knowledge of instrument response. A numerical reading without information about the radionuclide, units and measurement conditions can therefore be misleading.
Radioactive contamination has been an important issue in nuclear accidents and environmental management. Releases from major nuclear incidents have demonstrated that radioactive materials can be transported through atmospheric processes, deposited on land and water and subsequently redistributed through erosion, weathering and biological processes. The behaviour of individual radionuclides varies greatly. Some attach strongly to soil particles, some dissolve readily in water, and others can enter food chains.
Environmental remediation therefore requires site specific measurements rather than assumptions based only on the existence of contamination. Authorities may establish controlled areas, food restrictions, monitoring programmes and decontamination strategies depending on measured activity and projected doses. Long term management can continue for years or decades when long lived radionuclides are present. These cases also demonstrate the importance of distinguishing contamination from radiation itself. Removing or isolating the radioactive material can reduce exposure, whereas shielding alone may reduce exposure from the material without necessarily removing the contamination.
Radioactive contamination must also be distinguished from radioactive activation. Contamination involves radioactive material being deposited on or introduced into another material, whereas activation occurs when a previously nonradioactive material becomes radioactive because nuclear reactions, commonly neutron interactions, transform its nuclei. A reactor component can therefore become radioactive through neutron activation even when no radioactive substance has been physically deposited on its surface. Both contamination and activation may require radiation protection controls, but the appropriate management methods differ.
Contamination can often be reduced through removal or cleaning, while activation is generally an intrinsic property of the material and may require decay storage, shielding, controlled handling or disposal. Understanding this distinction is important in nuclear facilities, laboratories and decommissioning projects. Radioactive contamination is ultimately a material management problem as well as a radiation protection problem because the radioactive atoms must be located, controlled, monitored and, where appropriate, removed, contained or disposed of safely.
