Analysis of common cause failure effect on system reliability in seismic PSA

• Causation of CCF under earthquake is analyzed.
• Sensitivity of CCF factor is evaluated under different PGA levels.
• Effect of structure's response can be analyzed by MC simulation.
• Component failure probability is close to 1 under high PGA, CCF is not important.
• CCF model for internal PSA is also suitable for seismic PSA.

In seismic PSA of nuclear power plant, the equipment failure probability is affected by two parts – response and fragility. In this paper causations of CCF in the above two parts is analyzed, the sensitivity of CCF factor is evaluated under different peak ground acceleration levels. When the peak ground acceleration is much lower than the component seismic resistance capacity, difference between the results of CCF factor β = 0.05 and β = 1 is about one magnitude, while the results of β = 0.05 and β = 1 are very similar when the peak ground acceleration is close to or even higher than the component seismic resistance capacity. Then a model for system reliability analysis based on Monte Carlo (MC) simulation is proposed: the effect of structure's response can be analyzed in the system reliability model, and the CCF methods used in internal event PSA are also suitable for CCF in response and fragility of the equipment in seismic PSA. The reliability of accumulator system in AP1000 is analyzed as an example under different seismic hazard curves, the system failure probability difference between uncertainties of structure's response βS = 0.1 g and βS = 0.4 g is about one magnitude, and the results of CCF factor of β = 0.05 and β = 1 are very similar since the component failure probability is close to 1 when peak ground acceleration is high.