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atw Vol. 63 (2018) | Issue 3 ı March ENVIRONMENT AND SAFETY 156 | | Fig. 3. Requirements for IRIDM. considered several factors such as human operators, organization, etc. The IRIDM approach to managing safety adopted by many operators worldwide addresses all aspects and the complex interaction between them, Figure 2. | | Fig. 2. Important parameter for IRIDM. Experience has shown that an integrated decision making process, including deterministic and probabilistic analyses with good engineering practices, consideration of operating experience and sound managerial arrangements, is effective in refining and improving safe design and safe operation of nuclear installations. A risk-based on integrated decision-making process provides a defensible basis for making decisions. In addition, it is possible to recognize the greatest risks and prioritize attempts to minimize or omit them. Decision making case DSA method This approach has many benefits including a greatly improved understanding of the safety and identifying safety vulnerabilities that have not identified using standards-based evaluation techniques. IRIDM is a consultative process that applies a set of performance measures, with other considerations, to “inform” decisionmaking. IRIDM is invoked for key decisions, which typically requires setting of requirements. It is applied in many different fields, risk assessment, engineering design decisions and configuration management processes, etc. Using the IRIDM, assures project success and best decision making for risk assessment, etc [4, 17, 19]. The comparison between several methods for safety analysis and decision making is shown in Table 1. Integrated Risk Informed Decision Making is a best practice approach to safety management and decision making. IRIDM is a modular model that considers all relevant and important factors in an appropriate way to reach a balanced decision for taking account of all the risks and hazards posed by the facility. The main goal of the model is to develop an integrated data bank for informed decision making in necessary cases. The integrated data bank includes: operational feedback, organizational factor analysis, human factor, inspection, Deterministic Safety Analysis, Living Probabilistic Safety Analysis, security and safeguard and etc. The scheme of this model is shown in Figure 3. A series of requirements and criteria including different steps are needed for IRIDM process. The first step is defining the any types of issues that can be considered in safety analysis. The second step is identifying the requirements and criteria related to the specific issue. In this step, the mandatory requirements, deterministic and probabilistic insights and other requirements should be determine for PSA method | | Tab. 1. Comparison between several methods for safety analysis. RIDM method IRDM method Comprehensiveness of events considered Ç – – Ç Quality assurance – Ç – Ç Review of SAR report – Ç Ç Ç Emergency preparedness and resparde – – Ç Ç Licensing Ç Ç – Ç implementation of IRIDM process. In third step, weighting of inputs is determined. A specific weight of each parameter attributes based on its importance for different issues. Then, the evaluating methods of safety issues can be recognized by these weights. The forth step is decision making. The aim of this step is to make a decision whether the change should be made in design or operation of the plant, the regulation under consideration, etc. A good decision making process requires conducting the preceding steps. Because improper decision making will result in necessity of redoing all steps. After making a good decision, it should be implemented. The operators should receive proper training and, required changes in the associated instruments should be applied. The final step is monitoring of the process. In order to have proper implementation of the aforementioned issue, a complete regulation should be performed. in the case, the adequate efficiency has not been achieved in the implemented steps, the procedure should be revised or re-planed [4, 5, 7, 19, 44]. Advantages of IRIDM approach is include: • Transparency, as the weighting of the elements and the way resolution is achieved is clear; • Balanced, if all elements are weighted properly; • Logical, if carried out in a structured way; • Consistency, if weighting developed appropriately; • Accountable, if documented properly so the process can be reconstructed; It is necessary to be considered that complexity of integration of quantitative and qualitative information is very high. According to give explanation in this paper, the importance of using the combination of both deterministic and probabilistic approaches in the framework of integrated risk informed decision making is quite evident. For more realistic analysis of nuclear accidents should be done using both deterministic and probabilistic approaches. The required parameters (for example, deterministic success criteria, new design basis event, re-classified SSCs…) for an approach should be used in the other one or vice versa. In addition, the final decision will be made on basis of IRIDM by using deterministic and probabilistic insights. This will lead to greater safety of operators and environment, 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 Determination of greater defense barriers, lower costs, Selection of limiting cases for detailed quantitative analysis, less energy dissipation, Performance of detailed quantitative analysis for each limiting case, Evaluation of compliance to the acceptance criteria and safety margins, etc. Conclusion In general, when the analysis of nuclear accidents are separately done by deterministic and probabilistic approaches are faced with shortcomings and drawbacks. It is not possible to provide a comprehensive prediction for nuclear accidents. In deterministic approach, many effective factors in the event are not considered but in probabilistic approach, most effective factors in the event are used for determining the frequency of occurrence and total error. Therefore, for better understanding and comprehensive analysis of events, deterministic and probabilistic assessment is necessary at the same time. This review indicates that the integrated risk-informed approach has great potential to improve safety level by using probabilistic and deterministic approaches. By using IRIDM approach, determining of initiating events, multiple failures and event sequences are possible. The final decision should be based on integrated risk-informed rather than the risk, itself. Risk assessment should only be part of the decision process. For the final decision, integrated risk informed should be based on combination of deterministic and probabilistic approach. Adopting an IRIDM model is way of helping prevent these incidents and accidents as well as other benefits such as: • Safer and more secure operations have reduced risks through more comprehensive understanding of operational risks; • Greater resilience, including the ability to cope with unforeseen threats and adverse events; • Better integration of operations and technical systems, with financial and human resource management; • Greater efficiency, including more productive operations, higher staff morale, lower staff turnover, more efficient and effective control measures; • Greater ability to identify weaknesses so that they can be actively corrected to prevent opportunities for accidents to happen. Reference 1. IAEA, Applications of Probabilistic Safety Assessment (PSA) for Nuclear Power Plants, IAEA-TECDOC-1200, Vienna 2001. 2. IAEA, Living Probabilistic Safety Assessment (LPSA), IAEA-TECDOC-1106, Vienna 1999. 3. 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