Seismic correlation is a kind of dependency among seismically induced failures of structures, systems and components. Therefore, the technically adequate modeling of the seismic correlation is essential for Seismic PRA to have reasonable realistic results. For the efficient analysis of correlated seismic failures in a seismic PRA, a computer program COREX was developed in the previous study. COREX has two main functions. COREX calculates combination probabilities (joint or union probabilities) of correlated seismic failures and solves simultaneous equations for generating probabilities of single seismic failure and seismic CCF basic events for direct fault tree application. COREX calculates combination probabilities (joint or union probabilities) of correlated seismic failures by SSMRP or Reed-McCann integration. COREX casts these combination probabilities into LHS of simultaneous equations, and MCUB or REA probability equations into RHS of simultaneous equations that consist of single seismic failure and seismic CCF basic events. COREX solves these simultaneous equations, and generates the probabilities of single seismic failure and seismic CCF basic events. So far, the standard thumb rule by Bohn has been applied for seismic PRAs in Korea. Though a great deal of conservatism was involved in the seismic PRA results with the application of the standard thumb rule, it has not caused any big issues because the seismic PRA results were not widely used for any risk-informed applications but were mainly used to identify plant vulnerabilities to seismic events. However, a new regulation to require utilities to prepare general accident management program has become effective in which the effectiveness of the accident management program should be verified by PRAs. This leads to the request for refined PRA models development. In this study, we performed seismic PRAs with COREX for a virtual plant which has typical modeling characteristics and SSCs similar to the domestic nuclear power plants in Korea. We identified components which have been assumed to have full seismic correlation but can be modeled as partially correlated based on the reasonable technical bases which are derived from the state-of-the-art literatures review about seismic correlation. Our pilot application for seismic PRA with COREX focused on 1) the guidance development for partial correlation analysis implementation in seismic PRAs 2) comparison of the results difference between the application of standard thumb rule and partial seismic correlation implementation. Based on the results for pilot application for seismic PRAs with COREX, this paper suggests some practical application guidance for using COREX and presents possible results difference between standard thumb rule application and partial seismic correlation implementation.

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