Regional Basic Professional Training Course in Korea

❙ 601 ❙
9. Probabilistic Safety Analysis
puffs (or the plume which is the global representation of the successive puffs) grow
vertically and horizontally due to atmospheric turbulence, and it is generally assumed that
the puffs have a Gaussian profile, characterized by standard deviations (depending on
meteorological condition and on distance). This assumption has been found to be
sufficiently valid in many situations and also is easy to model; transport and diffusion
processes are summarized by the expression atmospheric dispersion. With an
atmospheric dispersion model, for specified meteorological conditions (possibly varying
with time), it is possible to calculate, at any location, the airborne concentrations and the
deposited activities due to dry (surface effect due to impaction, diffusion, absorption by
vegetation), and wet (due to rain falling through the puffs) depositions.
The content of each puff (for each radionuclide concerned) is depleted by radioactive
decay and by deposition, depending of transfer time and of meteorological conditions.
9.5.4. Meteorological data and sampling of these data
To obtain a probabilistic assessment of the radiological consequences, it is necessary to
take into account all the various meteorological conditions during the release. So, for a
specific site, it is necessary to have representative meteorological data recorded on an
hourly basis for at least one year. These data are obtained on the site itself (by
meteorological mast) or near the site by a station of the meteorological network. For long
distance calculations and for dispersion models more sophisticated than the Gaussian one,
it is required to have recorded meteorological data at several locations around the site, to
take into account topographic effects, for example.
The meteorological data, for each hour, include:
The wind speed and the wind direction;