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• #### APPENDIX O-- ACRONYMS 347-352

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APPENDIX I PROBABILISTIC RISK ASSESSMENT This appendix describes probabilistic risk assessment (PRA) and its current uses at Japanese and U.S. nuclear plants I.1 RISK ASSESSMENT Numerous definitions exist on the meaning of risk and risk assessment. A working definition of risk is the “set of triplets” definition (Kaplan and Garrick, 1981). It has been used in many applications, but particularly by the nuclear power industry and the U.S. Nuclear Regulatory Commission. According to this definition, the question “What is the risk?” is really three separate questions: 1. What can go wrong? 2. How likely is that to happen? 3. What are the consequences if it does happen? Risk can be defined mathematically using the following expression: R = {(Si, Li, Xi,)}c Where R denotes the risk attendant to the system or activity of interest, Si denotes the ith risk scenario (a description of the ith ‘what can go wrong’ scenario), Li denotes the likelihood that the ith scenario will happen, with uncertainty, and Xi denotes the consequences if the ith scenario does happen. The outer brackets in {(Si, Li, Xi,)} imply “the set of” triplets and the subscript c indicates that the set of triplets is “complete” (i.e., all, or all of the important triplets, are included in the set). In other words, “risk” is a set of scenarios, likelihoods, and consequences. In practice these can be assembled into a variety of forms to represent the risk of the system being evaluated. Prepublication Copy I-1

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Appendix I: Probabilistic Risk Assessment I.2 PROBALISTIC RISK ASSESSMENT PRA is a process of probabilistic evidential and inferential analysis of the response of events, systems, or activities to different challenges based on the fundamental rules of logic and plausible reasoning. The risk measure is most often a frequency whose uncertainty is represented by a probability distribution. This is often referred to as the “probability of frequency” format. Frequency is based on observations, which could include something as abstract as a thought experiment, whereas probability calibrates the credibility of the frequency based on the supporting evidence. PRA is a thought process for answering the three basic risk questions stated previously. PRAs for light water reactors are classified according to their completeness, or scope:  Level 1 assesses the risk of core damage generally in the form of core damage frequency. Level 1 is sometimes referred to as the plant model.  Level 2 assesses the magnitude and timing of releases of radioactive material from reactor containment and is sometimes referred to as the containment model or the plant plus containment model.  Level 3 assesses the consequences containment releases, for example injuries, fatalities, and economic losses, and is sometimes referred to as the site model or the combination of the plant, containment and site model. Level 3 PRAs are frequently referred to as a “full-scope PRAs,” but there are confusions at times as to whether it does or does not include the full treatment of external events, uncertainty analysis, and low-power and shutdown risk. In this report the terms “full scope” or “Level 3” are used interchangeably to mean the full range of internal and external events, low-power and shutdown risk, as well as a comprehensive treatment of the uncertainties involved taken to the endpoint of injuries, fatalities and economic damage. If multiple reactor units are present at a site, then full-scope and Level 3 PRAs would include multiunit risks, not just the risks from individual units. I.3 USE OF PRA IN JAPANESE NUCLEAR PLANTS The information in this section is distilled from discussions with representatives from Japanese government, industry, and academia at the committee’s November 2012 meeting in Tokyo. PRAs for Japanese nuclear plants were not required to be performed by rule prior to the March 2011 earthquake and tsunami; however, the Japanese regulatory agency (Nuclear and Industrial Safety Agency1) did require plant owners to perform PRAs to support license issuance and renewal. Moreover, in 1992 the Nuclear Safety Commission strongly recommended that nuclear plant operators identify effective measures for risk reduction based on PRAs of representative BWRs and PWRs in Japan. 1 This agency was abolished and a new organization, the Nuclear Regulation Authority, took over its regulatory responsibilities in September 2012. See Chapter 2. Prepublication Copy I-2

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