A New Modelling Approach for Containment Event Tree Construction: Accident Progression Stage Event Tree Method

Abstract

The Accident Progression Stage Event Tree (APSET) method presented here is a new modelling approach for construction of comprehensive and concise containment event trees to describe physical processes inside containment and accident mitigation actions, yet provide enough detail to analyze important factors for containment responses to severe accidents. In this approach, the accident progression is generally divided into four accident stages, i.e., Pre-stage for Core-melt, Core-melt Progression Stage, Debris Exit Stage, and Long-term Progression Stage, to reflect the timing of containment failure. Physical phenomena which challenge the containment integrity and accident mitigation actions are chronologically represented in event trees for each stage. Event trees for two successive stages are cross-linked by "interface parameter". The interface parameter is defined as a set of plant conditions that have a significant influence on physical processes in the subsequent stage. By quantifying the containment event trees constructed with the APSET method, the respective conditional probabilities of the containment failure modes and the accident termination can be calculated stage by stage for each core melt accident sequence. The quantification results provide the characteristics of each core melt sequence on containment responses such as a dominant containment failure mode, its timing, and the effectiveness of mitigation actions. The usefulness of the APSET method was demonstrated through its application to a containment event tree analysis for BWR with MARK-II containment.