The emergency core cooling system is a component part of engineered safety feature of a nuclear power plant.During normal operation,it is a stand-by facility.When a loss-of-coolant accident occurs,it injects emergency cooling water into the core to ensure the safety of a nuclear power plant. In this paper,we construct the synthetic fault tree of emergency core cooling system by using phased mission analysis method,calculate the exact so- lution of the unavailability of emergency core cooling system,and pres...