A UNIFIED APPROACH FOR CALCULATING CORE MELT FREQUENCY CAUSED BY INTERNAL AND EXTERNAL INITIATING EVENTS

A unified approach for calculating the core melt frequency of a specif ic reactor caused by both internal and external accident initiators is demonstrated. Two classes of internal initiators are examined: transi ents, of which turbine trip is the chosen example, and loss-of-coolant events of various sizes. The concepts of hazard and fragility analysi s first proposed for seismic risk analysis are linked to the frequenci es of internal initiating events, and to the plant response as a funct ion of the event intensity. Uncertainties are propagated using discret e probability distribution (DPD) arithmetic. Advantages of this approa ch include mathematical and conceptual consistency, and an improved un certainty analysis, which are important considerations if risk studies are to be utilized in decision-making based on quantitative safety go als.