## 4 Discussion and Conclusions  
This paper proposed a pragmatic approach to modeling numerous combinations of POSs for each reactor unit, which is one of the unresolved issues in MUPSA. Unlike previous studies, this approach does not select a representative set of POS combinations. Instead, it first models all possible combinations and then screens out the non-risk-significant accident sequences for each unit in a stepwise manner according to the model quantification results. That is, this approach focuses on _accident sequences_ for each unit rather than POS combinations themselves. To demonstrate the effectiveness and applicability of the approach, a case study was conducted in which MUPSA models for estimating the site and multi-unit CDFs due to a MU-LOOP IE were developed and quantified for four cases with different numbers of units: 2, 4, 6, and 10 units. As the base single-unit model, a full-scale SUPSA model integrating the FP and LPSD Level 1 PSA models for an actual NPP was used. The main results of this study are summarized as follows. * Even though the base integrated single-unit model used in this study was very large, i.e., containing about 37,000 gates and 21,000 basic events, compared to a FP or LPSD SUPSA model for NPPs, the quantification of the MUPSA models combining the single-unit models succeeded within a reasonable time without screening any accident sequences for the 2-, 4-, and 6-unit cases. * In the 10-unit case, the quantification failed without screening, but it succeeded after reducing the model size by about 43% by excluding the accident sequences in each integrated single-unit model with a CDF contribution of less than 0.1%. * The MCSs obtained in each case covered a very large number of POS combinations, for example, about 50,000 combinations in the 6-unit case without screening. * It was found that the MCSs involving very unlikely combinations, such as four or more units being in shutdown POSs, were mostly truncated by the cutoff value of  
Figure 8: Change in the number of accident sequences in the single-unit model by screening criteria. 1E-14/yr, while the MCSs involving very likely combinations, such as all units being at-power, accounted for the majority. * A comparison of the quantification results for the 6-unit case according to the three levels of screening criteria showed that as the level increases, the model size and computation time were significantly reduced by eliminating more accident sequences, while the reductions in the number of MCSs and CDF values were relatively small, especially for the multi-unit CDF with little change. From the above results, the practical implications of the proposed approach can be derived as follows. * This approach can be applied to site-level or multi-unit PSAs for actual NPP sites with a large number of reactor units. The case study was conducted for up to 10 units, but the approach applies to MUPSA involving more than 10 units depending on the size and logic complexity of each unit model. * With this approach, there is no need to select a representative set of POS combinations. Therefore, it is possible to reduce the conservatism caused by selecting a small number of POS combinations. * This approach can effectively reduce computational burden without losing much accuracy or realism by screening out non-risk-significant accident sequences in a stepwise manner according to the quantification results. Although the case study demonstrated the effectiveness and applicability of the proposed approach, this study has the following limitations, which need to be addressed in future research. * While the case study assumed all identical units for simplicity, many multi-unit NPP sites have different types of reactor units. For example, the Kori site in the Republic of Korea has four different types of operating units: WH600, WH900, OPR1000, and APR1400 [6; 11].