Civil Engineering Reference
In-Depth Information
Decontamination measures in urban areas almost always involve waste prob-
lems or a carry-over of contamination. The actually very effective measure of top
layer removal, be it lawn or road pavement or roof, for instance, always gives rise to
large volumes of waste that must be stored under controlled conditions
(i.e. protected from the weather). These measures are very costly, too. Hosing
down paved surfaces, on the other hand, is less effective but relatively inexpensive
and applicable for large areas. However, it often causes activity to be flushed into
the sewer system, thus shifting removed contamination over to sewage treatment
plants.
Last but not least, all these actions will not eliminate entirely the contamination
from an affected area, as their effectiveness is limited, if not the entire surface
together with all buildings is removed. Hence, there is always the necessity to
weigh measures which can still be carried out without affecting the quality of life
too much towards the extent to which the goals of protecting the population by
reducing the dose can be attained.
20.4 Modeling the Radiological Situation (Terrestrial
Pathways)
20.4.1 Atmospheric Dispersion Models
The mathematically simplest atmospheric dispersion model is the Gaussian plume
model [
9
]. It can be applied in plain topographies within a range of approximately
20 km under steady state conditions, i.e. a uniform release with a constant rate,
geometry, and altitude, and constant atmospheric conditions.
In the Gaussian plume model, the horizontal and vertical concentration profiles
of the dispersing plume are modeled by Gaussian distributions, cf. Fig.
20.3
. The
widths of these distributions are sensitive quantities in the dispersion calculation,
because they describe the dilution of radioactive material during transport in the air,
with direct consequence for the resulting doses. The widths are described by
diffusion parameters that depend on the distance from the origin and on the
turbulent state of the atmosphere. There are numerous field experiments in which
diffusion parameters were determined [
10
,
11
]. The Gaussian plume model is for
example used in the manual methodology for assessing the radiological situation.
Dispersion calculations for variable conditions require models able to take into
account the variability in space and time of both release and atmosphere. Mathe-
matical realizations are the so-called “puff models” which decompose the release
into time steps, i.e. a continuous release is replaced by a sequence of puff emissions,
and describe the nuclide concentrations within the puffs by three-dimensional
Gaussian distributions. Each puff is passed on a trajectory step by step through a
