Performance evaluating of the AP1000 passive safety systems for mitigation of small break loss of coolant accident using risk assessment tool-II software

Highlights: ► The SBLOCA CDF has been estimated to be 1.934E−8/year, using RAT II. ► The simplicity and passive safety systems result in decrease in the probability of CCF. ► However, the importance analysis ranks the CCF as the greatest contributor in CDF. ► Automatic and manual actuation failure probabilities have been founded to have low contribution in the SBLOCA CDF. ► The analysis assesses the performance of passive safety systems and reveals the deficiencies. - Abstract: The successful performance of defense barriers in the operating nuclear power plants is vital to avoid any release of highly radioactive fission products. Passive safety systems, independent of the electrical power, are implemented in modern NPPs to improve their reliability on demand. In this study, the SB-LOCA CDF is evaluated for the AP1000 to assess the performance of passive safety systems. The core damage states are examined to identify the most considerable risk contributors. Besides, importance measures rank the failures. Risk assessment tool-II has been designed and developed, in the safety research center of Shiraz University, to develop the PSA level 1 models. Using redundant passive and diverse reliable safety systems result in the low CDF of SB-LOCA (i.e. 1.934E−08). Considering the dominant sequences indicates that relying on passive automatic actuation mitigating processes, independent of operator actuations and electrical motive power leads to the considerable decrease in the probability of common cause failures and the CDF. However, importance analysis reveals the high contribution of CCFBEs in the SB-LOCA CDF. The results identify the weak points of operation and the most important risk contributors, in order to improve the inadequacies in design, test and maintenance and required human actions.