LEVEL 3 - gnssn - International Atomic Energy Agency

This publication is no longer valid
Please see http://www-ns.iaea.org/standards/
meteorological data from the meteorological station nearest to the release point (socalled
source meteorology). Data compiled at other meteorological stations may,
however, be acceptable if they are representative of the general conditions experienced
by the plume. Consequence analysis codes using atmospheric dispersion
models other than the Gaussian plume model may require additional meteorological
data. Typically, such codes require meteorological data at regular spatial intervals
over the region of interest (often referred to as non-source meteorology); this is
obtained by interpolation of the available meteorological data. This interpolation
process is complex and cannot be pursued here; some of the approaches used are outlined
in Ref. [13]. In conclusion, the choice of meteorological data often represents a
compromise between the ideal, the available and what is adequate for a particular
assessment.
In the case of an hourly meteorological database, each hour’s conditions can be
regarded as unique and therefore the starting point for a sequence of hourly weather
data. Thus, in one year’s worth of meteorological data there are 8760 hours and, therefore,
8760 possible sequences. It is neither practicable nor necessary to consider every
such sequence. Instead, the one or more year’s worth of data is sampled in such a way
that a truly representative set of weather sequences is selected. Each sequence has, of
course, a probability of occurrence. Initially, random and cyclic sampling techniques
(in which sequences are selected with a set time interval between them) were used.
The preferred method today is to use stratified sampling so as to minimize the chance
of omitting significant but infrequent weather sequences.
In stratified sampling the meteorological sequences are grouped into a number
of categories. The objectives of the categorization are: first, to group those meteorological
conditions which would lead to comparable radiological consequences in the
near range for a given release; and second, to select categories such that adequate
resolution is provided over the whole spectrum of consequences that result from the
distribution of meteorological conditions. For many sites, weather sequences, including
rain, will result in the greatest consequences and, accordingly, rain intensity and
the distance from the release at which it starts to rain are the important meteorological
conditions to take into consideration. The technique of stratified sampling is discussed
in more detail in Ref. [2]. Very little work has been done on the meteorological
sampling of non-source meteorological data for intermediate and far range consequences
and for long duration releases. Although this subject is mentioned in
Ref. [14], detailed guidance on the sampling scheme to be used with atmospheric
dispersion models, other than the Gaussian plume model, is not currently available.
2.5. EXPOSURE PATHWAYS AND DOSE ASSESSMENT
There are six principal pathways by which people can accumulate a radiation
dose after an accidental release of radioactive material to the atmosphere: