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Fig. 10-6 Effect of the correlation of the response of each component on failure probability When the responses of different components are correlated, the failure probability of the parallel system increases and the failure probability of the series system decreases.

Fig. 10-7 Effect of correlation on core damage probability at each seismic motion level A sensitivity study using a new method which considers the effect of correlation on the occurrence probabilities of not only the intersection of components, but also the union of components, has shown that the traditional method of the SSMRP may give a significantly conservative estimate for core damage probability.

In a seismic Probabilistic Safety Assessment (PSA) of a Nuclear Power Plant (NPP), the reliabilities of many safety systems are calculated at various intensity levels of seismic motion to evaluate their core damage probability. Such safety systems consist of many components such as pumps and valves which are often designed in a similar way. When we estimate the failure probability of simultaneous failure of two components installed on the same floor, we have to take account of the fact that if one component fails, another component of the same design is likely to fail; this tendency is called "correlation." Since the safety systems of an NPP are composed of many redundant components of similar design, consideration of correlation is quite important in a seismic PSA. The correlation increases the occurrence probability of simultaneous failure of multiple components, which is expressed as an intersection of component failures, compared with the "no correlation" (independent) case as shown in Fig. 10-6. On the other hand, the correlation decreases the probability of system failure that is defined by the failure of at least one component in a series system, which is expressed as a union of component failures. A calculation method was developed in the Seismic Safety Margins Research Program (SSMRP) to consider the effect of correlation. However, this traditional method considers only the effect of correlation on the occurrence probability of the intersection of component failures, while system failure or core damage is actually expressed as an union of many intersections of component failures. A new calculation method using the Monte Carlo simulation was developed in JAERI; this method is capable of providing more rigorous results since it considers both effects as discussed above. A sensitivity study using the new method, which considers the effect of correlation on the occurrence probabilities of not only the intersection of components but also the union of components, has shown that the traditional method that originates from the SSMRP may give a significantly conservative estimate of core damage probability as shown in Fig. 10-7.

Reference T. Oikawa et al., Development of Systems Reliability Analysis Code SECOM-2 for Seismic PSA, Reliab. Eng. Syst. Saf., 62, 251 (1998).

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