Radioactive Contamination of the Biosphere
Radioactive Contamination of the Biosphere
the introduction of radioactive substances into living organisms and their environment (the atmosphere, hydrosphere, and soil) that occurs as a result of nuclear explosions, disposal of radioactive wastes, treatment of radioactive ores, and accidents at atomic installations. Radioactive contamination is caused by nuclear fission products (for example, 90Sr, 137Cs, and 144Ce), induced radioactive nuclides (3H, 24Na, 59Fe, 60Co, and 65Zn), naturally radioactive heavy metals (uranium, thorium, and radium), and artificial transuranium elements (plutonium, americium, and curium).
The degree of radioactive contamination is determined by methods of radiochemistry, radiometry, spectrometry, and autoradiography and is expressed in units of radioactivity —disintegrations per second per gram of tissue, picocuries per cubic meter of air or water, or microcuries per square kilometer of land or water surface. By 1973, global radioactive contamination was more than 1.5 gigacuries as a result of nuclear explosions and more than 5 megacuries as a consequence of the introduction of radioactive wastes into the world’s oceans. The most contaminated regions are the moderate latitudes, especially in the northern hemisphere.
Radioactive substances that enter rivers, lakes, seas, and oceans are absorbed by aqueous plants and animals both directly from the water and from the preceding link in the food chain. Thus, radioactive substances pass from algae to zoo-plankton, for which the algae serve as food, and then to mol-lusks, crustaceans, and fish. Radioactive substances enter plants through the roots from the soil surface and through the leaves from atmospheric precipitation; they enter the bodies of animals, including farm animals, both through the food chain and with drinking water. The substances are then ingested by humans with meat and milk (in particular, 90Sr enters the human body as a component of vegetables or milk and may accumulate in bones, especially in children).
When radioactive substances are absorbed by plants or animals, their concentration in the organisms increases significantly over their content in the environment. Organisms that accumulate certain radioactive substances in especially high concentrations are called bioindicators of radioactive contamination. Thus, the alga Cladophora accumulates 91Y particularly intensively, and the large pond snail accumulates 90Sr.
The content of radioactive substances differs in different organisms. For example, the concentration of 137Cs increases in the chain from lichens to the muscles of the deer and wolf (30, 85, and 181 picocuries perg, dry weight, respectively), whereas the concentration of 90Sr in the same chain decreases (7.2, 0.1, and 0.04 picocuries per g, dry weight). The chemical form and physical state of the radioactive substances, as well as the temperature and chemical composition of the environment, affect the radioactive contamination of the biosphere.
The conclusion of the Nuclear Test-Ban Treaty of 1963 in Moscow fostered a decrease in radioactive contamination. On the other hand, the increasing role of nuclear power engineering is presenting new problems in the prevention of radioactive contamination related to the possible increase of artificial radioactive substances in the environment. Storage of containers with radioactive substances on the ocean floor has been found to be unreliable, since such containers are destroyed relatively rapidly. As early as 1957, a test at the Oak Ridge National Laboratory in the USA showed that radioactive substances placed in old mine shafts frequently migrate for great distances.
The clarification of the ecological significance of various levels of ionizing radiation and the creation of a scientific basis for recommending measures for preventing the harmful consequences of radioactive contamination, including the formulation of predictions of the possible disruption of the structure, productivity, and self-decontamination of ecological systems, are the subject of radioecology. The medical aspects of radiation contamination are treated in radiation hygiene. The activities of various countries in the prevention of radioactive contamination are coordinated by the International Atomic Energy Agency (IAEA).
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G. G. POLIKARPOV