atw - International Journal for Nuclear Power | 04.2023

atw Vol. 68 (2023) | Ausgabe 4 ı Juni
designation of appropriate safety measures, and
it is also important to inform decision making on
land use around such sites. The risks considered are
those associated to the major accident hazards which
if they were to occur would impact on society; and
may also have significant adverse repercussions for
the organisations that are responsible for putting in
place the required provisions and arrangements for
protecting individuals and the public. These types of
concerns are primarily associated with hazards that
could give rise to risks which provoke a socio political
response if they did occur. This includes scenarios
which can cause widespread detriment or multiple
fatalities from a single event.
Health and safety legislation specific to major accident
hazard sites requires sites/facilities to be
designed, constructed and operated safely. The legislation
also requires that the site operators provide
the Health and Safety Executive (HSE) with information
on how this is done. The COMAH Regulations
(2015) [6] require site operators “to provide HSE with
information about the potential effects of major accidents
at their sites, including the likelihood, how far
the effects might be experienced off-site and how much
harm maybe caused to people in an event”. Although
such events should be extremely unlikely due to the
numerous precautions that operators must put in
place to prevent accident scenarios arising. However,
a significant number of people could be harmed
if such an accident were to take place and therefore
the scenarios need appropriate considerations.
It should also be considered that the societal risk related
to a major accident hazard site can change over
time. If the population density in the surrounding
areas next to a major accident hazard site builds up
over time, then there would naturally be an increase
in the societal risk. It may also be appropriate and
necessary to take account of societal risk when considering
any further development proposals around
a site with a major accident hazard.
There is a well-established process in the UK for ensuring
that risks from major accident hazard sites
are controlled and kept low. These are discussed
explicitly in [7] and takes a three step approach as
summarised below:
p Identification: Understand which sites have
the potential for major accident hazards based
on inventory, toxicity, fire and explosion
hazards;
p Assessment and Controls: through the
COMAH Regulations assessment is undertaken
and measures are applied by the operator to:
A Prevent, so far as is reasonably practicable,
major accidents;
B Reduce the chances of any incident that does
occur escalating to more serious consequences
(e.g. one explosion leading to
another).
p Mitigation: consequences of major accidents
that occur after the loss of prevention and
control. This includes functions carried out by
local authorities such as land use planning
controls and emergency planning. This step is
undertaken to lessen the effects from a major
accident.
This approach recognises the potential for an accident
can never be completely eliminated, so
mitigation, as a final measure, is also required to
ensure responses to an accident have been thought
out and are in place before the occurrence of such
a scenario. This naturally leads to the development
of emergency response planning and arrangements.
In this way UDM would provide a strong evidence
base to support decision making for crucial choices
such as; initial siting, considerations for expansion
or changes to activities based on consequences to
the public, confirming the impact of hazards from
the different facilities located in a co-generation site.
Being able to assess these types of scenarios quickly
and accurately would allow for the societal risk to be
explicitly considered and provide part of the safety
demonstration that shows that the risk to workers
and members of the public remains tolerable for the
planned activities.
Future Development
UDM has been developed over a period of 20 years
and continues to be an integral part of the US military
and governments CBRN modelling capabilities.
During that time UDM has undergone a substantial
verification and validation programme. DSTL have
validated UDM [8] by taking measured and modelled
results from a range of trials. This included a
full-scale experiment to examine meteorology and
atmospheric dispersion within an urban area, containing
over 60,000 buildings. All trials provided a
range of scales to test the model. The results showed
that the UDM performed well against these examples
for short, medium, and long-range field studies, accurately
recreating experimental observations and
demonstrating that the model provides accuracy
over the range.
With the growing interest in new nuclear power
generation, both nationally and internationally,
Riskaware aims to enhance the radiological
ENVIRONMENT AND SAFETY 69
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