An approach to the application of subjective probabilities in level 2 PSAs

Accident Progression Event Trees are commonly used in Probabilistic Safety Assessments (PSAs) for nuclear power plants, chemical process plants and offshore installations. Their quantification necessitates the application of subjective probabilities in dealing with certain issues, such as physical phenomena. Although the subjective probability concept has been widely used in PSAs, its acceptability remains questionable mainly due to the current lack of formalism in the process of subjective probability derivation. A procedure was developed for the Sizewell ‘B’ Level 2 PSA to improve the traceability and scrutability of this process. The key elements include:

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  • development of a degree of belief scheme

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  • issue decomposition

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  • supporting data derivation

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  • documentation route.

A brief discussion of this procedure and two illustrations are provided in this paper.