Civil Engineering Reference
In-Depth Information
inversion layer
inversion layer
height
height
dependent
dependent
atmospheric turbulence
atmospheric turbulence
atmospheric turbulence
wind
wind
wet
deposition
wet
deposition
wet
deposition
dry
deposition
dry
deposition
release to
atmosphere
release to
atmosphere
contamination - radioactive particles and gases
contamination - radioactive particles and gases
Fig. 20.1
Release, atmospheric dispersion and deposition, and resulting contamination
20.1 From Atmospheric Radioactivity Releases to Human
Radiation Exposure
Atmospheric dispersion begins with the release of gaseous and aerosol-type radio-
active fission products from a reactor. Figure
20.1
illustrates the processes leading
to a radioactive contamination of the air and of surfaces, and Fig.
20.2
the resulting
radiation exposure from terrestrial exposure pathways. The term “radiological
situation” comprises the contamination of air and surfaces as well as the resulting
radiation fields and doses.
Depending on the type of accident, there may be controlled releases through
monitored openings that are designed for such purpose and typically equipped with
filters, or uncontrolled releases from other parts of a building. In any case, the
released volumes come under the influence of external atmospheric processes: they
follow the mean wind flow (advection) and get mixed with the outside air by
turbulent diffusion. The final outcome is an increasing expansion of the cloud
during the transport, and a corresponding dilution of the nuclide concentration in
the cloud. The vertical spreading and mixing of the cloud is limited by the inversion
layer that sets a limit to the mixing layer. If the released volumes have high
temperature, the cloud will rise until equilibrium with the surrounding air has
been achieved.
Unlike the concentration of chemical tracers in air, which is quantified as
“microgram per cubic meter”, the concentration of radionuclides in air is specified
as “decays per second per cubic meter,” Bq/m
3
, because the biological effect of
radiation is caused by the nuclear disintegrations. The total number of
