## 3 Case Study
### Example MUPSA Cases with Different Numbers of Units
#### 3.2.2 Results
These results indicate that the application of the screening criteria proposed in this study can effectively reduce the computational burden usually involved in MUPSA without losing much accuracy or realism. Figure 8 shows the top 25 accident sequences in the order of CDF contribution among the 117 sequences included in the base integrated single-unit model and how the number changes as each selection criterion is applied. When the first-level criterion is applied, there are 24 accident sequences with a CDF contribution greater than 0.1%, and the sum of their contributions is 99.0%. By applying the second-level criterion, 11 sequences with a CDF contribution greater than 0.5% remain, and the contribution sum is 96.2%. Finally, when the third-level criterion is applied, only seven sequences with a CDF contribution greater than 1% survive, and the summed contribution is 93.5%. It can be seen that the ratio of the site CDF when applying each screening criterion to the site CDF of the \"no screening\" case--98.9%, 95.6%, and 92.5%--is very similar to the sum of the CDF contributions from the accident sequences selected in the integrated single-unit model: 99.0%, 96.2%, and 93.5%. Here, the sum of the CDF contributions resulting from applying the third-level criterion (93.5%) is less than the 95% specified in that criterion. For this reason, the third-level criterion cannot be applied in this case; its results are included in Table 5 only for comparison purposes. In addition to the above results, it was found that the MCSs obtained in each case covered a large number of POS combinations and also that the MCSs involving very unlikely combinations, such as four or more units being in shutdown POSs except for at-power, were mostly truncated by the cutoff value of 1E-14/yr. For example, in the 6-unit case without screening, a total of 48,489 POS combinations were included in the MCSs. The sum of the contributions to the site CDF of the MCSs involving the most likely combination where all six units are at-power was 46.8%, while the summed contribution from the MCSs where four or more units are in shutdown POSs was less than 0.1%. Since the proposed \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Screening Criteria** & **No Screening** & **>0.1\%** & **>0.5\%** & **>1.0\%** \\ \hline Model size (sum) & 342,299 & 196,583 & 145,967 & 121,067 \\ Number of gates & 215,477 & 125,975 & 93,395 & 77,285 \\ Number of basic & 126,822 & 70,608 & 52,572 & 43,782 \\ events & - & - & - & - \\ \hline Run time (sec.) & 2001 & 551 & 257 & 181 \\ \hline Number of MCS & 7,116,674 & 6,618,173 & 5,604,299 & 5,010,444 \\ \hline Site CDF & \(6.41\times 10^{-6}\) & \(6.35\times 10^{-6}\) & \(6.13\times 10^{-6}\) & \(5.93\times 10^{-6}\) \\  & (-) & (98.9\%) & (95.6\%) & (92.5\%) \\ \hline Multi-unit CDF & \(2.71\times 10^{-7}\) & \(2.70\times 10^{-7}\) & \(2.69\times 10^{-7}\) & \(2.69\times 10^{-7}\) \\  & (-) & (99.7\%) & (99.4\%) & (99.3\%) \\ \hline \hline \end{tabular} Note: Percentages in parentheses are the ratio of CDF to that of the “no screening” case.  
\end{table} 
Table 5: Comparison of quantification results by screening criteria for the 6U-LOOP model. approach models POS basic events as independent random events without considering the interdependence of the POSs for each unit, the resulting MCSs included POS combinations that were not in the actual O/H experience in Table 2. However, their CDF contribution was sufficiently low to be negligible (i.e., less than 0.1%).