## 6 Conclusions  
In this study, a procedure for the seismic fragility assessment of NPPs by applying ground motion compatible with the probability distribution of CS was presented with a case study for the containment building of an NPP. To this end, three CSs was defined for three control frequencies, and ground motions were selected and scaled to simulate the conditional probability distribution of spectral acceleration. Horizontal component-to-component directional variability was considered as a source of randomness, in addition to spectral shape variability. Nonlinear LMS models were constructed using parameters determined by the LHS to model uncertainty. The seismic fragility curves were calculated from the IDA for seismic inputs based on the CS and compared with those obtained using a conventional seismic input based on the UHRS. The conclusions of this study are summarized as follows.  
1. By selecting a control frequency close to the dominant frequency in the structural response, the seismic input model based on the CS produces a similar median capacity compared to the UHRS-based seismic input model and reduces the logarithmic standard deviation due to randomness.
2. When a CS-based seismic input is applied to structural models simulating uncertainty, the randomness due to seismic input accounts for a large proportion of the combined logarithmic standard deviation, and the proportion of uncertainty decreases compared with the UHRS-based seismic input.
3. When the CS-based seismic input with a control frequency close to the dominant frequency is adopted, the HCLPF capacity is enhanced, and the probability of failure is reduced considerably compared with the result from the UHRS-based seismic input.
4. In addition, the CS-based seismic input has an advantage in that the basis for selecting the seismic hazard curve for seismic risk assessment is clear.  
Figure 15: Probability density of failure.  
5. Spectral shape variability and component-to-component variability are important sources of randomness that characterize the CS and need to be introduced in a rigorous manner as applied to UHRS. For the application of the CS to the practical SPRA of NPPs based on the findings of this study, it is necessary to develop detailed guidelines that addresses the issues of selecting the control frequency and strictly matching ground motions with prescribed component-to-component variability. Additionally, although this study was conducted using a simple inelastic LMS model, the seismic fragility and risk assessment of NPP structures using CSs can be conducted using more rigorous FE models that account for inelastic behavior directly or indirectly using inelastic energy absorption factors.  
Conceptualization, J.-H.P.; methodology, J.-H.P.; formal analysis, D.-H.S., S.-H.J.; investigation, D.-H.S., S.-H.J.; writing --original draft preparation, review, and editing, J.-H.P.; supervision, J.-H.P. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry, and Energy (MOTIE) of the Republic of Korea (No. 20201510100020).  
The authors declare no conflict of interest.