AbstmctProbabilistic risk assessments (PRAs) have been widely used in nuclear, chemical, petrochemical, and several other industries. The probability andlor frequency results of most PRAs are based on average component unavailabilities during the mission of interesL While these average results are useful, they provide no indication of the significance of the facility3 current status when one or more components are known to be out of service. Recentiy, several interactive computational models have been developed for nuclear power plants to allow the user to specifi the plant3 stalus at a particular time (Le., to specifi equipment known to be out of service) and then to receive updated PRA information.As with conventional PRA results, there are uncertainties associated with the numerical updated results. These uncertainties stem from a number of sources, including parameter uncertainty (uncertainties in equipment failure rates and human error probabilities). This paper presents an anaiysis of the impact of parameter uncertainties on the updated PRA results for nuclear power plants.
This paper presents the results of a study that identified how often a probabilistic risk assessment (PRA) should be updated to accommodate the changes that take place at nuclear power plants. Based on a 7-year analysis of design and procedural changes at one plant, we consider 5 years to be the maximum interval for updating PRAs. This conclusion is preliminary because it is based on the review of changes that occurred at a single plant, and it addresses only PRAs that involve a Level 1 analysis (i.e., a PRA including calculation of core damage frequency only). Nevertheless, this conclusion indicates that maintaining a useful PRA requires periodic updating efforts. However, the need for this periodic update stems only partly from the number of changes that can be expected to take place at nuclear power plants-changes that individually have only a moderate to minor impact on the PRA, but whose combined impact is substantial and necessitates a PRA update. Additionally, a comparison of two generations of PRAs performed about 5 years apart indicates that PRAs must be periodically updated to reflect the evolution of PFL4 methods. The most desirable updating interval depends on these two technical considerations as well as the cost of updating the PRA. (Cost considerations, however, were beyond the scope of this study.)
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