A Method to Avoid Underestimated Risks in Seismic SUPSA and MUPSA for Nuclear Power Plants Caused by Partitioning Events

Abstract

Seismic probabilistic safety assessment (PSA) models for nuclear power plants (NPPs) have many non-rare events whose failure probabilities are proportional to the seismic ground acceleration. It has been widely accepted that minimal cut sets (MCSs) that are calculated from the seismic PSA fault tree should be converted into exact solutions, such as binary decision diagrams (BDDs), and that the accurate seismic core damage frequency (CDF) should be calculated from the exact solutions. If the seismic CDF is calculated directly from seismic MCSs, it is drastically overestimated. Seismic single-unit PSA (SUPSA) models have random failures of alternating operation systems that are combined with seismic failures of components and structures. Similarly, seismic multi-unit PSA (MUPSA) models have failures of NPPs that undergo alternating operations between full power and low power and shutdown (LPSD). Their failures for alternating operations are modeled using fraction or partitioning events in seismic SUPSA and MUPSA fault trees. Since partitioning events for one system are mutually exclusive, their combinations should be excluded in exact solutions. However, it is difficult to eliminate the combinations of mutually exclusive events without modifying PSA tools for generating MCSs from a fault tree and converting MCSs into exact solutions. If the combinations of mutually exclusive events are not deleted, seismic CDF is underestimated. To avoid CDF underestimation in seismic SUPSAs and MUPSAs, this paper introduces a process of converting partitioning events into conditional events, and conditional events are then inserted explicitly inside a fault tree. With this conversion, accurate CDF can be calculated without modifying PSA tools. That is, this process does not require any other special operations or tools. It is strongly recommended that the method in this paper be employed for avoiding CDF underestimation in seismic SUPSAs and MUPSAs.

Sensitivity Study on the Correlation Level of Seismic Failures in Seismic Probabilistic Safety Assessments

“…The initial multi-unit risk study [2] was performed due to the gradually increasing concern regarding multi-unit nuclear accidents. After the Fukushima Daiichi NPP accident in 2011 [3], many studies for multi-unit probabilistic safety assessments (MUPSAs) have been conducted [4][5][6][7][8][9][10][11][12][13].…”

Advancements in Probabilistic Safety Assessment of Nuclear Energy for Sustainability

“…In order to transform the dependence expressed in seismic correlation into CCFs that can be utilized in the failure tree, he described a systematic numerical analytical methodology. Jung [5] pointed out potential errors that may occur during the evaluation of combinations for seismic events and suggested a method to increase the accuracy in connection with seismic correlation in multi-unit PSA, especially in which operation modes are considered complex. Kim et al [6] performed a sensitivity analysis for seismic hazard bins used for quantification.…”