Probabilistic Seismic Safety Study of an Existing Nuclear Power Plant

Publication Date Aug 01, 1980 by Kennedy R.P., Cornell C.A., Campbell R.D., Kaplan S., Perla H.F.

This study was conducted as part of an overall safety study of the Oyster Creek nuclear power plant.

The earthquake hazard was considered as an initiating event that could result in radioactive release from the site as a result of core melt. The probability of earthquake initiated releases was compared with the probability of releases due to other initiating events.

Three steps are necessary to evaluate the probability of earthquake initiated core melt.1. estimate the ground motion (peak ground acceleration) and uncertainty in this estimate as functions of annual probability of occurrence;2. estimate the conditional probability of failure and its uncertainty for structures, equipment, piping, controls, etc., as functions of ground acceleration; and3. combine these estimates to obtain probabilities of earthquake induced failure and uncertainties in such estimates to be used in event trees, system models, and fault trees for evaluating the probability of earthquake induced core melt.

This paper concentrates on the first two steps with emphasis on step 2. The major difference between the work presented and previous papers is the development and use of uncertainty estimates for both the ground motion probability estimates and the conditional probability of failure estimates.

The ground motion capacity of a structure, component, etc., is treated for simplicity and clarity as a product random variable A given by , where is the best estimate of the median ground acceleration capacity, R and U are lognormal random variables with unit median and logarithmic standard deviation βR and βU, respectively. βR expresses the dispersion in the ground acceleration capacity due to underlying randomness from such sources as (1) the variability of an earthquake time-history and thus of structural response when the earthquake is only defined in terms of the peak ground acceleration; and (2) the variability of structural material properties associated with strength, inelastic energy absorption and damping. Essentially, βR represents those sources of dispersion which cannot be reduced by more detailed evaluation or more data. Uncertainty concerning the ground motion capacity is expressed by βU which results from such things as (1) lack of complete knowledge of structural material properties; and (2) errors in calculating response due to approximate modeling. This paper presents a methodology (with examples) for estimating , βR, and βU for structures and components. These estimates are then used to estimate conditional probabilities of failure with confidence bounds on these estimates.

The conclusion is that a rational approach exists for estimating earthquake induced probabilities of failure. Confidence bounds on such estimates can be developed to express uncertainty in the parameters used. Such an approach is preferable over one in which dispersion due to underlying randomness, and due to uncertainty in the data are combined into a single probability of failure estimate with no estimate of the uncertainty in this probability.

Nuclear Engineering and Design, Vol. 59, Issue 2, pp. 315-338, August 1980.

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