Estimation of the Alpha Factor Parameters for the Emergency Diesel Generators of Ulchin Unit 3

Up to the present, the generic values of the Common cause failure (CCF) event parameters have been used in most PSA projects for the Korean NPPs. However, the CCF analysis should be performed with plant specific information to meet Category II of the ASME PRA Standard. Therefore, we estimated the Alpha factor parameters of the emergency diesel generator (EDG) for Ulchin Unit 3 by using the International Common-Cause Failure data Exchange (ICDE) database. The ICDE database provides the member countries with only the information needed for an estimation of the CCF parameters. The Ulchin Unit A3, pressurized water reactor, has two onsite EDGs and one alternate AC (AAC) diesel generator. The onsite EDGs of Unit 3&4 and the AAC are manufactured by the same company, but they are designed differently. The estimation procedure of the Alpha factor used in this study follows the approach of the NUREG/CR-5485. Since we did not find any qualitative difference between the target systems (two EDGs of Ulchin Unit 3) and the original systems (ICDE database), the applicability factor of each CCF event in the ICDE database was assumed to be 1. For the case of three EDGs including the AAC, five CCF events for the EDGs in the ICDE database were identified to be screened out. However, the detailed information for the independent events in the ICDE database is not presented. Thus, we assumed that the applicability factors for the CCF events to be screened out were, to be conservative, 0.5 and those of the other CCF events were 1. The study results show that the estimated Alpha parameters by using the ICDE database are lower than the generic values of the NUREG/CR-5497. The EDG system unavailability of the 1 out of 3 success criterion except for the supporting systems was calculated as 2.76E−3. Compared with the system unavailability estimated by using the data of NUREG/CR-5497, it is decreased by 31.2%.