atw Vol. 63 (2018) | Issue 3 ı March
applying an appropriate defence- indepth.
It is needed to determine the
design basis accidents to analyze
safety of nuclear facilities in deterministic
approach, that its analysis as
well as presence of DID can increase
the safety margin, which has an
important role in prevention and
mitigation of the accidents. If these
parameter are met, the level of risk to
operators and public from operation
of the nuclear facility will be acceptably
low [4, 5, 7, 19].There are
also uncertainties in deterministic
approach; For example, there are
uncertainties in the analytical models,
computer codes, and the capability of
structures, systems and components,
etc. The involved uncertainties are
determined by applying conservative
assumptions, as well as models and
data. Deterministic approach has
advantages and disadvantages. The
main advantage of deterministic
approach is that it is well developed
for applying to all types of nuclear
facilities [4, 5, 7, 19]. In addition to its
advantages, there are defects like
indicating the rare fault instead of
lesser faults that are more frequent to
the risk, disability to balance a design
and reduction in level of risk.
Probabilistic approach
Probabilistic approach is used for the
analysis of safety of nuclear power
plants. This method has three safety
levels. By application of this approach,
it is possible to analyze all transients
and accidents including fires and
floods, Core Damage Frequency
(CDF) and Large Early Release
Frequency (LERF). In addition, all
sources of radioactive material,
human errors, and levels of risk can be
considered in this method. Probabilistic
approach can be used in all the
modes of operation of the plant. The
scope of the PSA applying may be less
than this and, the limitations of PSA
method must be recognized when it is
used as part of the IRIDM process.
At first, initial events are determined
in probabilistic safety analysis,
then it must calculated whether the
core damage frequency and associated
risk can satisfy the required requirements
or not.
The PSA method uses comprehensive
list of initiating events and determines
all the fault sequences that
could lead to core damage or a large
early release. The levels of risk,
parameters uncertainty, and sensitivity
studies can be also considered
by using PSA approach.
The deficiency in the probabilistic
approach is that the PSA model cannot
determine all the initiating events
and fault sequences that could affect
to the risk. The uncertainties in some
areas of the PSA model are very large.
Nevertheless, The PSA model can explicitly
explain many of uncertainties
by using modern PSA computer codes.
The PSA approach is a part of decision-making
and cannot replace it, individually.
It can only be a contributor
to the decision making.
Integration of PSA and DSA
methods into the integrated
risk informed decisionmaking
The deterministic and probabilistic
approaches must be used to control
the level of nuclear facilities risk to
satisfy the safety of operators. There
are many differences between deterministic
and probabilistic approaches
in evaluation methods and boundary
conditions. The deterministic approach
is conservative but Probabilistic
approach is more realistic and uses
best estimate approach. The deterministic
approach usually uses some
of initiating events and fault
sequences, while the Probabilistic
approach uses a comprehensive set of
initiating events and hazards for
analysis. In deterministic approach,
accident conditions are addressed
separately, so that the PSA approximately
integrates all initiating events
and safety systems in the same model.
DSA approach uses approximate
method for calculating initiating
events frequencies and systems and
components failure probabilities,
while PSA uses explicit methods for
these purposes. Uncertainties are
addressed by conservative assumptions
and can be quantified by using
explicit methods in deterministic and
probabilistic models.
Generally, in view of intiating
events, DSA only considers design
basis accidents, howerver PSA considers
all design basis and beyond
design accidents. By considering the
safety systems, DSA only indicates
singular failure criterion, however PSA
indicates both of singular and combined
failiure criterion . In deterministic
approach, with the respect of the
operator instruction, nothing should
be done in 30 minutes, but afterwards
instructions should be implemented
completely. Whereas in the PSA the
operator's proceeding is more realistic.
In other words, the basis of DSA is
more conservative while the PSA is
realistic as much as possible.
The PSA can complement the
deterministic methods because:
• PSA considers thousands of accident
sequences instead of the
relatively few.
• It analyses more complex failure
modes.
• It quantifies the remaining risk.
• It identifies non-conservative and
overly conservative in the design.
• It quantifies the part of the uncertainties,
contributing to the understanding
of the issues.
Integrated approach can determine
that design is balanced against
initiating events. Also, determines the
importance of structures, systems and
components (SSCs). In all cases, a
combination of deterministic and
probabilistic approaches is made to
achieve acceptable safety level. Each
approach has separate viewpoint, it is
possible to use the result of each
approach for another one instead of
the applying assumptions into them.
In this way, the deterministic success
criteria, which is obtained in the
deterministic approach, can be used
in probabilistic approach. In addition,
the new design basis events and
re-classified structures, systems
and components from probabilistic
approach can be used in the deterministic
approach. Then, deterministic
and probabilistic results are compared
with regulation and the assessed risk
metrics, respectively. Finally, the
acceptable safety level can be achieved
by using the integrated risk-informed
decision. If the safety level is not
satisfied, the measures should be
re-implemented to enhance the safety
level [1, 2, 3, 4, 7, 19, 20, 33, 44],
Figure 1.
Early safety management focused
primarily on the safety of the plant
and equipment (the technology),
while subsequent practices also
| | Fig. 1.
Process of safety analysis by integration of DSA and PSA.
ENVIRONMENT AND SAFETY 155
Environment and Safety
The Importance of Integration of Deterministic and Probabilistic Approaches in the Framework of Integrated Risk Informed Decision Making in Nuclear Reactors
Mohsen Esfandiari, Kamran Sepanloo, Gholamreza Jahanfarnia and Ehsan Zarifi
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