### Limitation of Existing Studies  
One way to deal with the above issue is to select a manageable number of POS combinations and perform MUPSA for each selected combination. The recently published IAEA-TECDOC-1804 provides a comprehensive set of full-scope Level 1 PSA attributes, some of which are related to MUPSA [4]. As far as POS combinations are concerned, the report also describes the selection of representative combinations and further grouping of POSs as follows. * Special attribute (OS-A01-S2): _\"For multi-unit PSAs, a representative set of combinations of POSs for each unit is selected to account for the most likely combinations. The selected combinations include at least one with all reactors in operation at full power and one for each reactor unit in an LPSD POS.\"_
* Special attribute (OS-B01-S1): _\"Further grouping of POSs for each reactor unit in a multi-unit PSA may be required to keep the combination of POSs modeled to a manageable level. Such further grouping is done so as not to mask the potential for initiating events that impact multiple reactor units concurrently.\"_  
Therefore, most existing studies on MUPSA have considered only one combination of all units in FP operation or a very limited number of representative combinations of POSs. Table 1 summarizes recent studies in this regard. \begin{table}
\begin{tabular}{p{85.4 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**References** & **Organization** & **Number of Reactor Units** & **Description of Modeling POS Combinations** \\ \hline IAEA [3] & International Atomic Energy Agency & 4 & \\ [PERSON], et al. [5] & Korea Atomic Energy Research Institute & 6 & Assuming all units were in FP operation, only one combination of all units at FP was modeled. \\ \hline [PERSON], et al. [6] & Hanyang University & 7 & Three site operating states (SOSs) were defined according to the operating mode (at-power or shutdown) for each unit, but only one combination of both units at FP was analyzed in pilot plant studies. \\ \hline [PERSON] and [PERSON] [8] & Westinghouse Electric Company & 2 & A number of logical multi-unit configurations were analyzed, and three SOGs were defined, but only one combination of both units at FP was applied in the case study. \\ \hline [PERSON], et al. [10] & Idaho National Laboratory & 3 & To illustrate a dynamic MUPSA approach, only one combination was selected and modeled, where two units were at FP, and one was in mid-loop. \\ \hline [PERSON], et al. [11] & Korea Hydro \& Nuclear Power & 9 & Five SOSs were defined. All but two selected units were assumed to be at FP, and for each of the two units, only one POS with the highest CCDP was modeled. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of recent studies on MUPSA. However, to select a representative set of POS combinations, one should consider both the frequency or fraction of time of each selected POS combination and its conditional risk, such as conditional core damage probability (CCDP). This is not an easy task, though, particularly when considering a large number of units. To avoid this difficulty, it is common to select some of the most conservative combinations in terms of a given risk metric, such as core damage frequency (CDF), as representatives. The study conducted by Korea Hydro & Nuclear Power Co., Ltd. (KHNP) in Table 1 is an example (see also Annex IV in [3] for details). One issue with such an approach is that it can reduce the information that should be obtained from MUPSA, especially related to multi-unit accident sequences, and another is that it can produce overly conservative results because, commonly, the fraction of time spent in POSs with higher risk is relatively short compared to POSs with lower risk. Figure 1 shows the change in site CCDP given the occurrence of a multi-unit loss of offsite power (MU-LOOP) IE in all units over an O/H period of 56 days for a reference site with six identical units where one unit is in shutdown and five units are at power [14].