SEISMIC FRAGILITY ANALYSIS
Unique knowledge and application of seismic fragility analysis. Seismic fragility analysis stands for a part of a broader concept of seismic probabilistic safety assessment (S-PSA, in the US normative and technical documentation also called seismic probabilistic risk assessment, S-PRA). The main objectives of the seismic PSA study are: 1) to estimate the annual frequency of core damage (CDF) and the annual frequency of occurrence of radioactive releases and 2) to identify the structures, systems and equipment components which contribute most significantly to the seismic risk of operated NPP. RLE earthquakes used in these studies usually come with a return period of 100 000 or 1 000 000 years (1.0 x10E-5/y or 1.0 x10E-6/y).
To meet the stated goals of the seismic PSA study, it is necessary to put so-called seismic fragility parameters into the specific PSA model of the power plant: the HCLPF seismic margin value, the median seismic capacity Am and the logarithmic standard deviations due to randomness bR and due to uncertainty bU, for each structure, system or component included in the PSA model. The fragility analysis, which we perform in accordance with the guidelines EPRI 103959 (Methodology for Developing Seismic Fragilities) and EPRI 1002988 (Seismic Fragility Application Guide) develops the seismic fragility parameters. In accordance with the given guidelines, we determine the seismic fragility parameters by the method of separation of variables or by the hybrid method, in which probabilistic and deterministic approaches are combined.
Typically, the methods used to estimate HCLPF capacities, e.g. as part of a SMA are explicitly aimed at estimating the mean 1% probability failure point on the composite (mean) fragility curve. This is also equivalent to the 5% probability of failure on the 95th percentile confidence fragility curve. The HCLPF capacities with generic estimates of bR and bU can be utilized to develop preliminary estimates of the median capacities Am .