Background: Probabilistic safety analysis (PSA) methodology is widely used to assess the risk of nuclear power plant (NPP). Based on the results of level 1 PSA

the vulnerability of In-containment Refueling Water Storage Tank (IRWST) sub-system makes the initiating event Safety Injection Line Break (SI-LB) contributing significantly to core damage frequency (CDF). Purpose: This study aims to find the vulnerability of IRWST design by PSA assessment

and provide design improvement suggestion to reduce the risk of NPP. Methods: PSA methodology is used to evaluate the design improvement of IRWST

which is implemented by adding a third strainer (strainer C)

and the three strainers are connected to each other. Fault trees of IRWST are rebuilt

and related event trees and level 1 PSA model are recalculated. Results: Quantitative result shows that the contribution of SI-LB initiating event to CDF is reduced by 53.5% and the total CDF is reduced by 21.5%. Conclusion: Design improvement of IRWST sub-system greatly reduced the core damage risk. Abstract