Abstract
During early stage of a nuclear power plant accident, consideration of beyond design basis accidents is an essential component of the defense in depth approach to assure nuclear safety. The probability of occurrence of a beyond design basis accident is very low, but such an accident may lead to significant consequences. Taiwan's fourth nuclear power plants, the Lungmen nuclear power station (LNPS), which is an advanced boiling water reactor (ABWR) with fully digitized instrumentation and control (I&C) system is chosen as the target plant. The LNPS full scope engineering simulator is used to generate the testing data for method development. The following common initiating events are considered in this study: Loss of coolant accidents (LOCA), Loss of all feedwater (TLOFW), Loss of offsite power (LOOP), Station blackout (SBO). This study combines symptom-oriented and event-oriented approach for early warning and decision making. First, the Plant Damage States, which is symptom-oriented, identify the initiating event. It also gives the information of safety system‘s status. Initiating event and safety system‘s status are used in Event Tree (ET), which is event–oriented, to determine accident sequences by using probabilistic risk assessment (PRA) technique. With accident sequences, the core melt frequency is easy to be calculated. Studies have indicated that the combination of the symptom-oriented approach and the event-oriented approach can be helpful for nuclear power plant accident early warning and management.