atw - International Journal for Nuclear Power | 04.2023

atw Vol. 68 (2023) | Ausgabe 4 ı Juni
Dynamic Dispersion Modelling to
Enable Informed Decision Making
in a Modern Nuclear Safety Case
Howard Chapman, Joseph Hargreaves, Stephen Lawton, Robert Gordon, Tim Culmer
Introduction
Radionuclide material when discharged into the atmosphere
is carried along by the wind and dispersed
into the environment by the action of turbulent diffusion
[1] . As outlined by Hester and Harrison “the
problem of predicting the distribution of airborne material
released from a source is commonly approached
by solving the diffusion-transport equation. There are
a range of models which have been developed to solve
the equation depending upon simplifying assumptions
made and boundary conditions imposed” [2] .
The National Nuclear Laboratory (NNL) have historically
used computer modelling techniques which
are based upon the National Radiation Protection
Board (NRPB) Model for Short and Medium Range
Dispersion of Radionuclides Released to the Atmosphere,
NRPB-R91 [1] , and Near Wake Modelling,
NRPB-R157 [3] . These approaches are based on a
simplified Gaussian model; but, as with most dispersion
models, there are limitations on the range at
which the model can be appropriately used because
the modelling technique uses weather data from the
release point for the whole release path. Other factors
affecting the model can include release height,
topography and deposition, amongst others.
Riskaware have developed an assessment code called
the Urban Dispersion Model (UDM) which estimates
the downwind dispersion of airborne contaminants.
This higher fidelity model can predict atmospheric
transport and dispersion of chemical, biological and
radiological materials within urban and semi-urban
environments, including modelling the effects
of buildings, wake regions, courtyards, and urban
canyons. UDM also models complex material effects
such as wet and dry particulate deposition, primary
and secondary liquid evaporation, and buoyant and
dense gas modelling.
As the need to respond to climate change leads to
renewed interest in the potential for new nuclear
energy and deployment as co-generation sites (i.e.
where the heat energy from the reactor is used for
multiple purposes such as electricity, district heating,
process heat and hydrogen/chemical production) it
becomes important to start considering both the radiological
and chemical hazards in a more rounded
way. Traditionally, nuclear Safety Cases have focused
predominately on the radiological hazards associated
with the site; and where required assessment of
specific chemical hazards are undertaken when the
facilities in question contained significant quantities
of certain hazardous and/or toxic chemicals.
However, as the likelihood of co generation looks
to become a reality there is a need to consider all
hazards related to the activities on a site in a more
complete manner. It is considered that use of UDM
in the nuclear industry could provide a unique opportunity
to improve the assessment of radiological
and chemical aerial release hazards. The approach
would allow safety assessors to compare the consequences
of radiological and chemical hazards in
the same code making it easier and more effective
to balance the risks associated with a facility and
any neighbouring major accident hazard sites in a
more holistic way.
Proposed Future Application of UDM
The aim of this paper is to provide an overview of
the current position regarding dispersion modelling
and how this might be improved by the use of the
Riskaware UDM to support dispersion calculations
in consequence assessment and Safety Case work.
It is also thought that using the enhanced graphical
outputs, provided by the UDM interface, will assist
the engagement process with Regulators and other
stakeholders.
It is anticipated that the output from UDM may be
used to support and provide evidence in decision
making and the overall safety demonstration for
aerial dispersion in the Safety Case; particularly relating
to the extent of Emergency Planning Zones. An
aspiration for the Advance Modular Reactors (AMRs)
programme in the UK is to reduce these zones to be as
small as possible, so that AMRs can be located closer
to industrial clusters and populated areas. This
would allow for the heat generated by the reactor to
be utilised more effectively and in new ways beyond
that of traditional electricity generation.
ENVIRONMENT AND SAFETY 63
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