Validation of seismic probabilistic risk assessments of nuclear power plants

Abstract

A seismic probabilistic risk assessment (PRA) of a nuclear plant requires identification and information regarding the seismic hazard at the plant site, dominant accident sequences leading to core damage, and structure and equipment fragilities. Uncertainties are associated with each of these ingredients of a PRA. Sources of uncertainty due to seismic hazard and assumptions underlying the component fragility modeling may be significant contributors to uncertainty in estimates of core damage probability. Design and construction errors also may be important in some instances. When these uncertainties are propagated through the PRA, the frequency distribution of core damage probability may span three orders of magnitude or more. This large variability brings into question the credibility of PRA methods and the usefulness of insights to be gained from a PRA. List of Tables vii List of Figures ix 6.0 Role of error in safety evaluation 103 6.1 Design/construction errors 103 6.2 Significance of error for nuclear plant safety 104 6.3 Sensitivity of risk estimates to postulated errors 107 6.3.1 Limerick generating station 108 6.3.2 Millstone 3 109 6.4 Summary 110 7.0 Conclusions and recommendations 113 8.0 Acknowledgements 121 9.0 References vi Tables 2.1(a) Component and seismic fragility parameters-Limerick 6 2.1(b) Nonseismic random failure rates-Limerick 7 2.2(a) Component and seismic fragility parameters-Millstone 3 2.2(b) Nonseismic random failure rates-Millstone 3 8 2.3(a) Component and seismic fragility parameters-Maine Yankee 9 2.3(b) Nonseismic random failure rates-Maine Yankee 9 A3.1 Sampling distribution of MLE Y1 A3.2 Sampling distribution of Y2-(XLXu) 1/2 A3.3

Fragility Analysis of Concrete Gravity Dams

“…This point estimate does not necessarily yield the mean of Pf, as the uncertainties are not propagated in the same way. However, this estimate is usually close to the mean of Pf (Ellingwood, 1990;1994a).…”

“…The fragility concept has found widespread usage in the nuclear power industry, where it has been used in seismic probabilistic safety and/or margin assessments of safety-related plant systems (Kennedy and Ravindra, 1984, Ellingwood, 1990, 1994a, 1994b). These fragilities have been determined in a variety of ways: simple heuristics, scaling upward from design calculations, and (rarely) stochastic nonlinear finite element analysis.…”

“…A clear understanding of the mechanics of dam performance and identification of limit states is required to devise the computational platforms that support the uncertainty analysis and propagation (Ellingwood, 1994a). Furthermore, probabilistic models for the description of the various parameters that play significant roles in the safety assessment are needed.…”

“…Nonetheless, it is possible to establish insights on the nature of risk and on strategies for its mitigation (Whitman, 1984;Ellingwood, 1994a).…”

Impact of structural aging on seismic risk assessment of reinforced concrete structures in nuclear power plants

“…Safety evaluations of new and existing structures can be conducted rationally and systematically within a probabilistic framework (Melchers, 1987). During the past four years, a methodology has been developed that provides a reliability-based framework for condition assessment and probability-based life prediction of concrete structures in NPP Mori and Ellingwood, 1993;1994a;1994b). The reliability analysis takes into account time-dependent stochastic changes in resistance as well as randomness in structural loads.…”