Geoscience Reference
In-Depth Information
Exposure from terrestrial radionuclides present at trace levels in all soils is
specific and relates to the types of rock from which the soils originate. Higher
radiation levels are associated with igneous rocks, such as granite, and lower levels
with sedimentary rocks. There are exceptions, however, as some shale and
phosphate rocks have relatively high contents of radionuclides. Radon and its
short-lived decay products in the atmosphere are the most important contributors
to human exposure from natural sources.
The main human-made contribution to the exposure of the world's population
has come from the testing of nuclear weapons in the atmosphere, from 1945 to
1980 (UNSCEAR
2000
). Each nuclear test resulted in unrestrained release into the
environment of substantial quantities of radioactive materials, which were dis-
persed widely in the atmosphere and deposited everywhere on the earth's surface.
Underground testing caused exposures beyond the test sites only if radioactive
gases leaked or were vented. Most underground tests had much lower yields than
atmospheric tests, and it usually was possible to contain the debris. Underground
tests were conducted at the rate of 50 or more per year from 1962 to 1990. During
the time when nuclear weapon arsenals were being expanded, especially in the
earlier years (1945-1960), radionuclide releases exposed local populations
downwind or downstream of nuclear installations. At the time, there was little
recognition of exposure potentials, and monitoring of releases was limited. More
recent controls on the military fuel cycle have now diminished exposures to very
low levels.
Several industries process or utilize large volumes of raw materials containing
natural radionuclides. Discharges from these industrial plants to air and water and
the use of by-products and waste materials may contribute to enhanced exposure of
the general public. A list of radionuclides responsible for most environmental
concerns is given in Table
3.2
. Estimated maximum exposures arise from phos-
phoric acid production, mineral sand processing industries, and coal-fired power
stations. Except in the case of accidents or at sites where wastes have accumulated,
causing localized areas to be contaminated to significant levels, no other practices
result in important exposures of radionuclides released into the environment.
Estimates of releases of isotopes produced and used in industrial and medical
applications are being reviewed, but these seem to be associated with rather
insignificant levels of exposure.
When accidents occur, environmental contamination and exposures may
become significant. The accident at the Chernobyl nuclear power plant was a
notable example. Exposures were highest in local areas surrounding the reactor,
but low-level exposures could be identified for the European region and the entire
northern hemisphere. In the first year following the accident, the highest regionally
averaged annual doses in Europe, outside the former Union of Soviet Socialist
Republics, were less than 50 % of the natural background dose. Subsequent
exposures decreased rapidly.
