atw 2018-03v6


atw Vol. 63 (2018) | Issue 3 ı March



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

Introduction For many years, decision making on safety issues has been based on either deterministic safety

assessment (DSA) or probabilistic safety assessment (PSA). In recent years, integrated risk informed decision-making

(IRIDM) approach has been suggested to integrate in a systematic manner quantitative and qualitative (deterministic

and probabilistic) safety considerations to attain a balanced decision [1, 2, 3, 4, 5, 6, 7]. The IRIDM and investigation of

the combination of deterministic and probabilistic approaches are important issues, which have attracted much

attention in recent years. United States Nuclear Regulatory Commission (USNRC) has developed reports on integrated

risk-informed decisions and applications of deterministic and probabilistic approaches since 1998 [8, 9, 10, 11, 12, 13].

They considered the high-level criteria for defence-in-depth and all of safety margin by using the IRIDM concept. Collins

[14] investigated risk informed safety and regulatory decision making based on USNRC perspective. He investigated the

methods to enhance the safety criteria, regulatory effectiveness and efficiency, and public confidence. Impediments for

the application of risk-informed decisions making (RIDM) in nuclear safety were considered by Hahn et al [15]. They

suggested that the PSA method could not be replaced or substituted by DSA method. IAEA has overviewed risk- informed

regulation of nuclear facilities [4]. In the overview, the application of RIDM to provide safety level in all types of nuclear

facility is considered. Risk-informed decision making in the context of the National Aeronautics and Space Administration

(NASA) risk management is studied by Dezfuli et al [16, 17, 18]. In this investigation, evolution of risk-related policy and

guidance documents and NASA’s risk management approach are discussed. The International Nuclear Safety Group

(INSAG) has also published a framework for an integrated RIDM process (INSAG 25, 77, etc) [19, 20, 21, 22]. In this

report, the framework, principles and key elements for RIDM are identified and their interrelationship are described. In

another study, Fontes et al [23, 24] considered ITO model of pit corrosion in pipelines by applying RIDM. Talarico [25,

26] indicated RIDM of safety investments by using the disproportion factor, Process safety and environmental protection.

For this purpose a systematic approach, Cost-benefit analysis, determination model and simulations on realistic data

were presented. Veeramany et al [27, 28] investigated a framework for modeling of high-impact and low-frequency

power grid events to support RIDM. In this report, an integrated high-impact and low-frequency risk framework was

applied for improvement of the risk models. Borgonovo and Apostolakis [29, 30] introduced an importance measure,

the differential importance measure (DIM), for RIDM. Using this method, the problems exiting in Fussell-Vesely (FV)

and risk achievement worth (RAW) methods were solved.

A risk-informed defence-in-depth

frame work for existing and advanced

reactors are considered by Fleming

and Silady [31, 32, 33, 34]. A new

definition of defence-in-depth including

the inherent characteristics,

design features of a nuclear reactor,

and the quantification of the design

features importance is suggested.

Mohammad Modarres [35] proposed

and discussed implications of a largely

probabilistic regulatory framework

using best estimate, goal-driven,

risk-informed, and performancebased


The traditional defense-in-depth

design and operation regulatory

philosophy are used to propose a

framework when uncertainty in

conforming to specific goals and

objectives is high. The steps need to

develop a corresponding technologyneutral

regulatory approach from the

proposed framework explained.

Kang and Sung [36, 37] studied

analysis of safety-critical digital

systems for RIDM. The fault tree

analysis framework of the safety of

digital systems are presented and the

relationship between the important

characteristics of digital systems and

the PSA results using mathematical

expressions are described quantitatively.

Kim et al [38, 39, 40] discussed

the risk-informed approach that have

proposed to make a safety case for

advanced nuclear reactors. They also

considered a risk-informed safety

analysis approach suggested by

Westinghouse. In this paper, the

risk-informed approach and its

potential to improve the conventional

and deterministic approaches because

of various desirable characteristics are

discussed. Future nuclear reactor

designs meet an uncertain regulatory

environment. Delaney et al [41, 42,

43] considered the risk-informed

design guidance for this reactor

systems. Some level of probabilistic

insights in the regulations and

supporting regulatory documents for

generation-IV nuclear reactors are

anticipated. This paper presented an

iterative four-step risk-informed

methodology to guide the design of

future-reactor systems.

Deterministic approach

Deterministic safety approach (DSA)

applies a set of conservative rules

and requirements for the design and

operation of a nuclear facility. Thereby

providing a way of taking into account

uncertainties in the performance of

equipment and humans. DSA provides

the defence-in-depth that assures the

successive performance of barrier to

prevent accidents. A safe for operator

of nuclear power plant, and environment

during the normal and abnormal

operation can be achievable by

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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