atw - International Journal for Nuclear Power | 04.2023

atw Vol. 68 (2023) | Ausgabe 4 ı Juni
National Radiation Protection Board
(NRPB) Wake Models
If the release originates within a building with leakage
from the building to the outside, or the release
arises from an incident close to a building, the presence
of the building will influence the dispersion
of the released material. Air turbulence from wind
flowing past the building can make the simple expanding
cube model inaccurate, and excessively
conservative. In these circumstances, the wake
models will provide more realistic dispersion and
dose assessments. NRPB-R91 [1] covers people located
at any distance, from immediately adjacent to the
building/incident up to ~10 km distant.
Use of the wake model avoids the need to determine
where the contamination might leak out of a building.
The model assumes that wind creates a ‘wake’
downstream of the building. Within the turbulent air
close to the building, activity released is assumed to
be instantaneously dispersed in the whole volume of
this ‘near wake’. The near wake distance depends on
the wind speed and the building size.
UDM Physics
Within UDM, the dispersion plume is represented
as a set of discrete gaussian ’puffs’ that travel downwind
and disperse. The concentration distribution of
each puff is Gaussian over three axes. The x-axis is
aligned with the wind direction at the centroid of the
puff, the y-axis is perpendicular to the x-axis in the
horizontal plane, and the z-axis lies in the vertical
direction. The size of the puff is represented by its
standard deviations along the three axes, denoted σ x ,
σ y and σ z respectively. These values are referred to
as the longitudinal, lateral and vertical spreads. Figure
3 illustrates the representation of a puff in the x
and y plane; the distribution is similar in the z plane.
ENVIRONMENT AND SAFETY 65
Urban Dispersion Modelling
Approach
The UDM has been in use since 1999 and was originally
developed, by Riskaware under contract to
Defence Science and Technology Laboratory (DSTL)
for the prediction of toxic contaminants in urban environments.
A key requirement of the model was that
the calculations should be fast in order to efficiently
simulate the large range of distances and surface
characteristics likely to be encountered and to enable
simulation of a wide range of complex source
terms. The model was also required to operate in
‘real time’ for some applications. In order to satisfy
these requirements UDM was developed as an
empirical model based on a research programme
providing urban dispersion data from wind tunnel
and field experiments as shown in Figure 2 [5] .
| Fig. 2
Wind Tunnel Experiments used to Parameterise UDM.
| Fig. 3
Representation of a Gaussian puff in the x and y planes.
UDM models the environment at a high level through
the definition of three distinct calculation ’regimes’,
effectively representing varying degrees of urban interaction.
Greater fidelity is provided within each
regime through the consideration of background
land cover and any defined ground areas or urban
ground areas. These regimes are:
p Open Regime: This is the default calculation
regime and assumes a ground area with an
obstacle density of less than 5 %, or that the
puffs are large compared to the average
obstacle size or that the puff is above the urban
canopy.
p Urban Regime: Used when puffs interact
significantly with the urban canopy. This means
an obstacle density of greater than 5 %, or the
puff is small enough to interact with an
obstacle, or the puff is within the urban canopy.
p Recirculating Regime: If a puff interacts with
an obstacle (or building), some of the mass of
the puff may be exchanged into one or more
entrainment regions. These exist downwind of
the obstacle, where they are known as wakes, or
can be contained within the obstacle itself, in
Environment and Safety
Dynamic Dispersion Modelling to Enable Informed Decision Making in a Modern Nuclear Safety Case ı Howard Chapman, Stephen Lawton, Joseph Hargreaves, Robert Gordon, Tim Culmer