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.
Highlights
Probability calculation from minimal cut sets (MCSs) can be categorized as follows: (1) If the MCS size is small, accurate probability can be calculated by converting MCSs into sum-of-disjoint products (SDPs) [19,20] or applying the inclusion–exclusion principle (IEP) [21] to MCSs. (2) If the MCS size is huge, accurate probability can be calculated by converting MCSs into a binary decision diagram (BDD) [21]
The author of this paper developed the Advanced Cutset Upper Bound Estimator (ACUBE) tool [21] for the US Electric Power Research Institute (EPRI), which converts MCSs into a BDD and calculates the probability from the BDD
For the correct probability calculation of a fault tree that has partitioning events, a proper modeling method of these events was proposed in Section 3, and the strength and simplicity of this modeling method were demonstrated by the applications in Sections 4 and 5
Summary
Probabilistic safety assessments (PSAs) that calculate core damage frequency (CDF) are divided into single-unit PSAs (SUPSAs) and multi-unit PSAs (MUPSAs). The initial MUPSA studies [2,3] were performed due to the gradually increasing concern regarding multi-unit nuclear accidents. In MUPSA [17], multi-unit core damage frequency (MUCDF), site core damage frequency (SCDF), and single-unit core damage frequency (SUCDF) are defined as accident. In MUPSA [17], multi‐unit core damage frequency (MUCDF), site core damage fre‐ quency (SCDF), and single‐unit core damage frequency (SUCDF) are defined as accident ffrreeqquueenncciieessiinnwwhhicichhaat tlelaesatsttwtwo oNNPPPsP, as,t aletalsetaosnt eonNePNP,PaPn,danondlyonolnyeoNnPePNaPrePianrae cinorae core ddaammaaggee ssttaatteeffoolllloowwininggaannininitiiatitaintigngeveevnetn, rte, srpesepcteivcteilvye. Multiple NPPs in a single nuclear site are considered a group of alternating operation NPPs. Since the Kori nuclear site in Korea has nine NPPs and each LPSD PSA has 15 plant operating states (POSs), there might be 169 combinations of plant-level partitioning events in the MCSs of MUPSA
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