When designing the safety system for the next generation of pressurized water reactors (PWRs), it is essential to rationalize the safety system by taking factors such as safety, reliability, and economy into account. To do so, a comprehensive methodology for designing an accident mitigation system was developed on the basis of the following studies. Threats to the reactor core, which are inherent to PWRs, were systematically analyzed. Following this, efforts to specify the requirements needed to mitigate the threats were made with the specification of components composing the mitigation systems. On the basis of a loss-of-coolant accident as an example of the severest accident, thermohydro analyses without any mitigation systems were made to determine the requirements needed to keep the core safe. Information related to the system's design parameters were successfully obtained. On the basis of these studies, candidates for mitigation systems that respond in accordance with the scales and phases in progress of accidents were systematically selected and discussed. In the future, the methodology presented herein may be extended to cover the structuring of overall plant safety systems.