Insights about Truncation in Probabilistic Safety Assessment

Truncation is a term, which defines cutting off the negligible contribution of the results of the fault tree/event tree analysis, when evaluating complex facilities. This paper shows that consideration of requirements from Probabilistic Risk Assessment Procedures Guide at the largest acceptable truncation may lead to more than 40% differences from the results with no truncation at results for Fussel-Vesely importance and to more than 80% at results for Risk Increase Factor. As Risk Increase Factor is very much an indicator of acceptability of changes according to Regulatory Guide 1.174, such large differences may lead to misleading conclusion about the acceptability of proposed changes. The paper shows that requirements on truncation should be more severe, e.g. considering also accuracy of results of Risk Increase Factor. In this sense, a method for setting up the truncation limit is developed. Application of the method removes the ambiguity of the PSA results due to truncation and it enables to apply PSA results for decision-making.