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Concept
Actinides in the environment
Résumé
Environmental radioactivity is not limited to actinides; non-actinides such as radon and radium are of note. While all actinides are radioactive, there are a lot of actinides or actinide-relating minerals in the Earth's crust such as uranium and thorium. These minerals are helpful in many ways, such as carbon-dating, most detectors, X-rays, and more.
Generally, ingested insoluble actinide compounds, such as high-fired uranium dioxide and mixed oxide (MOX) fuel, will pass through the digestive system with little effect since they cannot dissolve and be absorbed by the body. Inhaled actinide compounds, however, will be more damaging as they remain in the lungs and irradiate the lung tissue.
Ingested low-fired oxides and soluble salts such as nitrate can be absorbed into the blood stream. If they are inhaled then it is possible for the solid to dissolve and leave the lungs. Hence, the dose to the lungs will be lower for the soluble form.
Actinium can be naturally found in traces in uranium ore as 227Ac, an α and β emitter with half-life 21.773 years. Uranium ore contains about 0.2 mg of actinium per ton of uranium. It is more commonly made in milligram amounts by neutron irradiation of 226Ra in a nuclear reactor. Natural actinium almost exclusively consists of one isotope, 227Ac, with only minute traces of other shorter-lived isotopes (225Ac and 228Ac) occurring in other decay chains.
In India, a large amount of thorium ore can be found in the form of monazite in placer deposits of the Western and Eastern coastal dune sands, particularly in the Tamil Nadu coastal areas. The residents of this area are exposed to a naturally occurring radiation dose ten times higher than the worldwide average.
Thorium is found at low levels in most rocks and soils, where it is about three times more abundant than uranium and about as abundant as lead. On average, soil commonly contains approximately 6 parts per million (ppm) thorium. Thorium occurs in several minerals; the most common is the rare earth-thorium-phosphate mineral monazite, which contains up to 12% thorium oxide.
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