liquid, solid, or gaseous waste containing radioisotopes in concentrations exceeding a particular country’s standards.
Liquid radioactive wastes are formed during the operation of atomic power plants, the regeneration of nuclear fuel from spent fuel elements, and the use of sources of radioactive emissions in science, industry, and medicine. In the USSR, the disposal of radioactive wastes into open water networks is prohibited by law in all cases where the concentration of radioisotopes in the wastes exceeds the average annual permissible concentration. The permissible concentrations are set at a level such that contact with substances containing radioisotopes will not cause harm to the human body or the environment. Therefore, all radioactive wastes in the USSR are purified, in which process the content of radioisotopes is reduced to the average annual permissible concentration, or are sent to places of safe, permanent containment.
Liquid radioactive wastes are divided into three categories, depending on their activity: (1) low-level, with specific activity not exceeding 10–5 curie per liter; (2) intermediate-level, from 10–5 to 1 curie per liter; and (3) high-level, with activity exceeding 1 curie per liter. More than 99.9 percent of all the activity caused by the operation of atomic power plants during regeneration of nuclear fuel passes into high-level liquid wastes, which, after concentration to small volumes, are stored in sealed stainless steel tanks, usually underground, thus preventing the wastes from entering the environment. In addition, studies on the further improvement of the safety of storing high-level wastes by converting them into solid water-insoluble forms are being conducted in all countries that have atomic industry.
Low-level liquid wastes, called nonindustrial wastes, which result from the cleaning of work areas and the washing of protective clothing, may undergo industrial reprocessing or may be released into the sewer network after careful removal of radioisotopes by coagulation methods and ion exchange or by distillation. The radioisotopes extracted from these wastes, which are concentrated into sludges or still residues (about 0.5 percent of the original volume), are medium-level wastes and are stored in steel tanks. Methods are being developed for converting these concentrates into solid forms by mixing them with bitumen or other materials with good water-insulation properties.
Solid radioactive wastes include contaminated materials and used protective clothing that cannot be washed. All such materials are transferred for permanent storage in concrete-lined trenches and are usually sealed with cement.
In addition to liquid and solid wastes, atomic industrial and power plants may produce emissions that contain volatile compounds of radioisotopes or the isotopes themselves, such as 131I, 129I, and 85Kr; the formation of radioactive aerosols is also possible. Such emissions are passed through a special purification system and are then released into the atmosphere through stacks. The total amount of radioisotopes after purification should not exceed the permissible level of waste established for the installation, taking into account the prevailing winds, local topography, and the nature of the surrounding vegetation. The height of the stack (usually 100–150 m) is selected to ensure that when the radioisotopes from the gaseous wastes settle into the bottom layers of the atmosphere, they will be diluted to limits that prevent even trace effects on the human body (both directly and indirectly, through plants and the soil).
B. S. KOLYCHEV