The paper studies the fragility of high-pressure emergency cooling water tanks in Loviisa Nuclear Power Plant located on the elevation +25.40 in the reactor building. The seismic fragility is defined as the conditional probability of its failure given a value of the response parameter, such as peak ground acceleration. Using the lognormal-distribution assumption, the fragility (i.e., the probability of failure, f) at any non-exceedance probability level Q can be derived as
where Q = P(f<f|a) is the probability that the conditional probability f is less than f for a peak ground acceleration a. A is the median ground acceleration capacity, βR is the logarithmic standard deviation representing the randomness about A, and βU is the logarithmic standard deviation representing the uncertainty. The quantity Φ(.) is the standard Gaussian cumulative distribution function. In order to assess the fragility of the tanks the strain time histories for tank supports and piping nozzles were calculated using the joint structural-equipment model. The ground motion response spectra shape used in the structural response analysis has been taken from the YVL 2.6 – guide [1]. This shape represents the envelope spectrum for Southern Finland corresponding to the median annual frequency of 10−5. The sampling of the model properties was carried out with the aid of the Latin hypercube sampling method. In order to find the failure modes the strain time histories were calculated for the piping nozzles and for support structures of the tanks. Since strain is the best measure of energy absorption, energy limited events need to be based on the strain acceptance criteria. The adopted failure limit is the cumulated plastic strain of 8% in the tank sheet metal or in the supporting structures. This failure limit has taken from the reference [2]. The end result of the study is the presentation of the median fragility curve for the tanks as well as 95% and 5% fractile curves.
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