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atw 2018-03v6


atw Vol. 63 (2018) | Issue 3 ı March 154 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 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 methods. 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

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