Addressing Accident Sequence Over-Counting in the Kernkraftwerk Muhleberg Seismic PSA

摘要

Due to the high conditional failure probabilities that can occur given seismic initiating events, the quantification approximations typically employed in Seismic Probabilistic Safety Assessment (SPSA) models result in significant over-counting of accident sequence frequencies. Over-counting of sequence frequency by a factor often or more has been observed during the quantification of seismic models using algorithms which employ the rare event or minimum cutset upper bound (MCUB) approximations. This can occur when the constituent basic events of a system or functional gate in the model sum to greater than one due to high basic event failure probabilities. This paper describes the methods developed to reduce seismic sequence over-counting via use of "AND-NOT" modeling as well as the Advanced Cutset Upper Bound Estimator (ACUBE) computer code.