## 3 Case Study
### Example MUPSA Cases with Different Numbers of Units  
#### 3.2.1 Scope and Method  
For this study, both site CDF, referring to core damage in one or more units, and multi-unit CDF, referring to core damage in two or more units, due to a MU-LOOP IE were estimated for four cases with different numbers of units at the site: 2-unit, 4-unit, 6-unit, and 10-unit cases. This case study was not intended to perform a MUPSA for an actual NPP site and identify significant contributors to risk but to examine the applicability and effectiveness of the proposed approach. Therefore, the following assumptions were made to reduce the complexity of the case study. * In each of the four cases, the reference site consists of one to five pairs of twin units. * All units at the site are identical OPR1000 units. All structures, systems, and components except diesel generators (DGs) are identical across the units. * Emergency DGs (EDGs) are identical for each pair of twin units (e.g., Units 1 and 2) but not between units in different pairs (e.g., Units 1 and 3). * Unit 2). * The MU-LOOP IE is a weather-related LOOP caused by severe weather conditions such as a typhoon, so it affects all units at the site simultaneously. * Offsite power recovery actions are completely dependent across all units, while the other HFEs are independent between units because each unit has its own operating crew working in a physically separate control room or building. * The adverse effects of core damage or radioactive release from one unit on operator actions in other adjacent units at the site [19] are negligible. * Portable equipment that has already been or is planned to be installed as part of the Multi-barrier Accident Coping Strategy (MACST) for Korean NPPs is not credited. To develop a base integrated single-unit model, the FP, and LPSD Level 1 PSA models for an OPR1000 plant [14] were used, and logics not related to the LOOP IE were removed. The LPSD model consists of 15 sub-models developed for each POS except POSs 1 and 15; hence the base integrated single-unit LOOP (SU-LOOP) model has 16 POSs, including at power (Table 3). The total number of events included in the final base SU-LOOP model is 58,084, with 37,112 gates and 20,972 basic events. To develop a MUPSA model for each case, the base SU-LOOP model was duplicated to reflect the number of units and then differentiated using unit identifiers. Although all units were assumed to be identical, the fraction of time spent in each POS was applied differently for each unit using the values in Table 3. Specifically, for the 2-unit case, the fractions of Units 1 and 2 were applied; for the 4-unit case, those of Units 1 through 4; and for the 6-unit case, those of Units 1 through 6. For the 10-unit case, the fractions of Units 1 to 6 were applied to each corresponding unit, and those of Units 1 to 4 were applied once more to Units 7 to 10, respectively. Inter-unit CCFs were modeled only for the DGs because the MCSs involving failures of one or more DGs contributed to over 90% of the total CDF in the integrated single-unit model. For each pair of twin units, five DGs, specifically four EDGs and one shared AAC DG, were grouped into a common-cause component group, and all possible CCF basic events (2-of-5, 3-of-5, 4-of-5, and 5-of-5) were modeled for two failure modes: failure to start and failure to run. Regarding each AAC DG shared between twin units, its failure logic was developed using the assumption of unit priority mentioned above, namely that the AAC DG is available only to the first unit by number in cases where it is required at both units simultaneously. More details on this approach to modeling a shared component can be found in [5]. In addition, according to the assumptions related to the IE and HFEs, for the MU-LOOP IE and offsite power recovery actions, the same basic event was applied to all individual unit models without using unit identifiers. In this study, MUPSA models were developed using AIMS-PSA software (Release 3, Rev.