Successive Accident Frequency Assessment From Initiating Event to Containment Failure

In general, in evaluating large early release frequency (LERF) or containment failure frequency (CFF), level 2 PSA is separated from level 1PSA as described in NUREG-1150 [1]. A brief procedure of this evaluation is that: (1) the core damage frequencies (CDF) to be evaluated for level 1 PSA are grouped into plant damage states (PDSs) based on their accident progression attributes (similar timings to core damage and systems unavailability), (2) frequencies of PDSs are applied to the initial events of event trees (ETs) after the core damage (containment event trees (CETs)), and (3) CETs are developed and quantified. The above method rationally decreases the number of accident sequences to be evaluated in level 2 PSA. In this paper, CETs of internal events for operating state are connected to the end states of the core damage and performed a successive uncertainty analysis from initiating events to containment failures with taking into account for correlations between component failures for a BWR plant (BWR-5 / Mark-II). The rational and some conservative CFF evaluation method are also proposed because the calculation time and computer memory of the above detailed method was very heavy.