APPENDIX D
PROBABILISTIC RISK ASSESSMENT42

The term Probabilistic Risk Assessment (PRA) refers to the framework and analytical methods widely used for assessing risk in the U.S. nuclear power industries. PRA utilizes event trees and fault trees in a structured analysis process that can be described in five general steps:

  1. Identifying various hazards and the frequency of various levels of each hazard (initiating event).

  2. Identifying the things that could fail or go wrong and their probability, given the various hazards and hazard levels, including the consequential damage associated with each failure. This step involve the development of all of the scenarios of events that could occur, and their sequence and resultant damage. (The event tree is a tool for this.)

  3. Developing the model for each event in the sequences to indicate which of the event's component parts could fail and lead to failure or degradation of the event. (The fault tree is a tool for this.)

  4. Quantifying the fault trees and event trees to determine the frequency of each damage state, and thereby determining the ranked order of scenario and the component contributors to the different damage state's frequency.

42  

This explanation was provided by one of the peer reviewers of the committee's draft report.



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OCR for page 63
Uses of Risk Analysis to Achieve Balanced Safety in Building Design and Operations APPENDIX D PROBABILISTIC RISK ASSESSMENT42 The term Probabilistic Risk Assessment (PRA) refers to the framework and analytical methods widely used for assessing risk in the U.S. nuclear power industries. PRA utilizes event trees and fault trees in a structured analysis process that can be described in five general steps: Identifying various hazards and the frequency of various levels of each hazard (initiating event). Identifying the things that could fail or go wrong and their probability, given the various hazards and hazard levels, including the consequential damage associated with each failure. This step involve the development of all of the scenarios of events that could occur, and their sequence and resultant damage. (The event tree is a tool for this.) Developing the model for each event in the sequences to indicate which of the event's component parts could fail and lead to failure or degradation of the event. (The fault tree is a tool for this.) Quantifying the fault trees and event trees to determine the frequency of each damage state, and thereby determining the ranked order of scenario and the component contributors to the different damage state's frequency. 42   This explanation was provided by one of the peer reviewers of the committee's draft report.

OCR for page 63
Uses of Risk Analysis to Achieve Balanced Safety in Building Design and Operations Determining the uncertainty in the results by calculating a distribution of damage state frequency, using the distribution of possible hazard frequency and failure probability.