atw 2018-03v6

inforum

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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