atw - International Journal for Nuclear Power | 03.2024

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Research and Innovation
Fig. 2.
Overview of the Assessment Process for Chemical Hazards
because the safety management strategies are required
to satisfy the same regulatory expectations, this also
provides the duty holder with the necessary information
for safe management of the facility and to
reduce risks to ALARP.
The fundamental requirement of any radiological
Safety Case is to demonstrate that hazards presenting
radiological exposure or harm to workers or members
of the public can be safely managed and the risks have
been reduced to ALARP.
Assessment of Fault Conditions
Nuclear licenced sites will often use differing nomenclature
for engineered and administrative safety
features based on their historical processes, operating
organisations and management systems. However, all
nuclear licenced sites in the UK are regulated by the
Office for Nuclear Regulation (ONR) inspectors who are
guided by a single set of SAPs. This means that all
license holders regardless of specific activities at each
site will aspire to the same safety principles, numerical
targets and legal limits which define the Basic Safety
Objective (BSO) and the Basic Safety Limit (BSL).
As such, the details discussed below can be readily
applied to any situation where there are hazards and
associated risk which needs to be managed.
1. The individual hazards are identified by an appropriate
and systematic hazard identification
process.
2. Faults are presented in a Fault Schedule with a
number of grouped fault sequences.
3. Each fault sequence which could lead to unwanted
effects has an appropriate safety function, initiating
event and unmitigated consequence calculated.
4. Radiological safety assessments then specify safety
measures which can be engineered or administrative
in nature. The designated safety measures
need to reduce the consequence and/or initiating
frequency to an appropriate level. The concept of
DiD is also important to ensure nuclear safety.
5. The various engineered safety measures are
identified as Structures, Systems and Components
(SSCs) in an Engineering Schedule, which also
includes the safety function and any performance
requirements.
6. SSCs are then substantiated against their safety
function(s) and performance requirements to
ensure they will fulfil their safety role when
required.
Assessment of Chemical Hazards
Assessment of chemical hazards requires demonstration
that hazards presenting harm to health, physical
harm or asphyxiation can be safely managed and the
risk is reduced to ALARP. The NNL approach to the assessment
of chemical (or chemotoxic) hazards, is
discussed in
Reference 5 . Hazards are identified in a
structured and systematic way and Hazard Management
Strategies (HMS) identified to prevent or adequately
manage them. Based on severity (i.e. unmitigated
consequences and initiating event frequency)
safety measures are assigned which are proportional
to risk, both in terms of number and required reli ability.
For high consequence faults, there is a requirement to
compare the mitigated event frequency to relevant
criteria, and in all cases demonstration that the residual
risk has been reduced to ALARP. The process is shown
diagramatically within Figure 2, and aligns closely
with processes applied for radiological safety.
The incorporation of Safety Case development within
the design process helps to ensure that the design
develops with normal and fault condition safety in
mind with the aim of minimising the hazards present,
and reducing the residual risk and ultimately the
overall cost.
Reconciliation of Measures
There is a clear overlap in the approach for radiological
safety and treatment of chemical hazards. Chemical
Regulations, specifically the Control of Major Accident
Hazards (COMAH) Regulation, and The Radiation
Emergency Preparedness and Public Information
Regulation (REPPIR) both assign Emergency Planning
Zones (EPZ) based on nature and consequence of the
hazard. However, there is also a need to recognise the
need for both harmonisation and reconciliation of the
measures claimed. For example, the consequences of a
chemical release can be reduced by dispersion, whereas
for a radiological release confinement of the material
would be preferred. In these instances, reconciliation
should be applied to identify the greatest reduction
in risk and also to reduce the over-designation of
measures. Figure 3 demonstrates the scope of radiological
and chemotoxic assessments and the obvious
overlap between both approaches.
ALARP Discussion
Fundamentally, ALARP is the demonstration to a
decision-making process, making it possible to justify
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