From Big Medical Encyclopedia

RADIOACTIVE FALLOUT — besieged from the atmosphere on the Earth's surface (firm or liquid) the particles containing radionuclides. A source of ground mass of R. of the lake are hl. obr. nuclear explosions. Natural R.' number of the lake which are formed in the atmosphere under the influence of space radiations and as a result of disintegration of radon and thoron, very restrictedly.

Rubles of the lake represent mix of cleavage products of nuclear fuel, not reacted part of the sharing substance and the activated materials of a design of the explosive device. Depending on the sphere of distribution distinguish local and global losses of R. of the lake. The first drop out around land or underwater explosion, the second extend everywhere.

The huge number of heat energy which is allocated at nuclear explosions creates convection fluxes, with to-rymi products of explosion in a vaporous state are injected in the atmosphere. The radionuclides which are formed at land and underground nuclear explosions come preferential to the troposphere. Large, including and melted off, particles within several hours are besieged on the Earth's surface in a radius of hundreds kilometers from the place of explosion, creating local local losses. The radiochemistry, structure of global losses, the general radioactivity, their quantity and movement in space are defined by conditions of carrying out and parameters of nuclear explosion.

At air explosions to 99% of the formed radionuclides comes to a stratosphere. Under the influence of air currents they can be transferred to long distances, bending around the globe for 2 — 3 weeks and causing formation of global tropospheric and stratospheric losses. Time of stay of radionuclides in the atmosphere is practically defined their time spent in a stratosphere as clarification of the troposphere happens quickly; so, 90 Sr and 137 Cs are in a stratosphere about 1 — 2 years; 14 With — from 1 year to 5 years whereas the troposphere is cleared of them within 20 — 40 days.

The radioactive particles coming from a stratosphere to the troposphere drop out on the Earth's surface as a result of «washing away» by an atmospheric precipitation (so-called wet sedimentation). In droughty districts the dry sedimentation caused by gravitational forces prevails. Global radioactive losses have the expressed width character with reduction of their density to poles and increase to the equator. The maximum of global losses is observed during the spring and summer period, at least — in the fall and in the winter. Gigabyte. the importance of global losses is defined by hl. obr. hit of radionuclides in a human body and internal radiation. To 98% of the radionuclides which got inside gets into an organism in the peroral way and only shares of percent — inhalation.

Due to a variety of the migratory processes caused by distinctions of a soil cover in different geographical districts and also features of food chains (see), the expressed zonality of levels of accumulation of radionuclides in an organism of the people inhabiting various regions is noted. So, e.g., in a tundra zone where migration of radionuclides goes hl. obr. on a chain: a lichen —> a deer —> the person, levels of contents 137 Cs at aboriginals of districts of the North is higher, than at inhabitants of a midland.

Pollution of objects of the environment is radionuclides a consequence of losses of products of nuclear explosion. In the first days after explosion the greatest importance in a gigabyte. the relation represent short-lived nuclides, in particular 131 I, in the next months and years — long-living ( 90 Sr, 137 Cs, etc.).

Idiosyncrasy of the radiation situation caused by global losses of products of nuclear explosions is hron, impact of ionizing radiation in small doses on the big contingents of people.

The local losses of R. of the lake caused by receipt in the atmosphere of radioactive waste of the enterprises of nuclear power, and also natural radionuclides ( 226 Ra), contained in products of combustion of black coal and other types of organic fuel, can pollute the environment only around finding of this source.

Gigabyte. the importance of local rainfall is, as a rule, limited and eventually in process of implementation of preventive actions shall decrease. Improvement tekhnol belongs to such actions. processes, napr, at work with nuclear fuel, in the direction of reduction of amount of the formed radioactive waste (see) and increases in efficiency of cleaning of the deleted emissions of radioactive gases and aerosols.

Bibliography: Gedeonov L. I. Speed of cleaning of the troposphere, in book: Radiocontamination of external environment, under the editorship of V. P. Shvedov and S. I. Shirokov, page 170, M., 1962; Global losses of products of nuclear explosions as a factor of radiation of the person, under the editorship of A. N. Marey, M., 1980; Marey A. N., Barkhudarov R. M. and Novikova N. Ya. Global losses of caesium-137 and people, M., 1974; Moiseyev A. A. and Ramzayev P. V. Caesium-137 in the biosphere, M., 1975; Standards of radiation safety of NRB-76, M., 1978; Radiation protection, Recommendations of MKRZ, the Publication No. 26, the lane with English, M., 1978.

A. N. Marey.