Effect of air temperature on passive containment cooling system reliability in AP1000

Passive containment cooling system is innovatively used in AP1000 reactor design to enhance the safety. Since the system operation is based on natural circulation, physical process failure induced by uncertainties of physical parameters becomes one of the important failure modes (e.g. natural circulation cannot establish or keep and system design function cannot be accomplished because some parameters such as air temperature deviate from their design values), which should be considered in system reliability evaluation. As the heat sink, air temperature with high uncertainty has important effect on system reliability. In this article, we analyze the pressure variation in the containment along with the air temperature based on system thermal–hydraulic model, and the effect of air temperature on system operation is closely related to the thermal–hydraulic performance of the system. Moreover, the system thermal–hydraulic capacity is influenced by the system component configuration, so we evaluate the system physical process failure probability by Monte Carlo simulation and analyze the effect of air temperature distribution under different system component configurations. Finally, we evaluate the whole system reliability considering the logical relationship between physical process failure and equipment fault by fault tree method. The results illustrate that air temperature distribution has important influence on the system reliability, the system failure probability may be difference by several orders and the main contributors may be different at different plant locations, so climate should be considered in system design and reliability analysis.