Presentation of Common Cause Failures in Fault Tree Structure of Krško PSA: An Historical Overview

Failure of multiple components due to a common cause represents one of the most important issues in evaluation of system reliability or unavailability. The frequency of such events has relatively low expectancy, when compared to random failures, which affect individual components. However, in many cases the consequence is a direct loss of safety system or mitigative safety function. For this reason, the modeling of a common cause failure (CCF) and its presentation in fault tree structure is of the uttermost importance in probabilistic safety analyses (PSA). During the past decade, PSA model of Krško NPP has undergone many small changes and a couple of major ones in fulfilling its basic purpose, which was serving as a tool for providing an appropriate information on the risk associated with actual plant design and operation. All changes to Krško PSA model were undertaken in order to make it a better tool and / or to make it represent the plant in more accurate manner. The paper provides an overview of changes in CCF modeling in the fault tree structure from the initial PSA model development till present.