On Seismic Risk Analysis of Nuclear Plants Safety Systems

Abstract

The recent United States Atomic Energy Commission study (WASH-1400), using a simplified analysis, has estimated the probability of earthquake initiated accidents of typical U.S. nuclear plants to be in the range 10−5 to 10−8 per year, and has concluded that these are sufficiently small compared to other causes as to be safely ignored in assessing overall plant safety.An event tree model is presented in this paper to assess the failure probabilities from seismic events of single and multiple component safety systems. The ground motion, response spectra, and failure criteria are treated as probabilistic quantities. The model is applied to a Canadian site and failure probabilites are predicted for several levels of design. The sensitivity of uncertainties in the input data on the failure probabilities is also investigated.The results show the importance of considering common mode failures when determining seismically initiated system failure probabilities. Also, in assessing system failure probabilities of the order of 10−6 it is shown that the input with the highest sensitivity is the probability distribution of large magnitude earthquakes and the component failure probabilities.It is recommended that the design basis earthquake be determined from the desired system failure probability level, and that more effort should be placed on estimating the probability of occurrence of large magnitude earthquakes and in determining component failure probabilities.