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.%根据一级概率安全分析(Probabilistic Safety Analysis, PSA)的结果，安全壳内置换料水箱(In-containment Refueling Water Storage Tank, IRWST)子系统的初始设计导致安注管线破裂(Safety Injection Line Break, SI-LB)始发事件对堆芯损坏频率(Core Damage Frequency, CDF)有较大的贡献。本文提出了 IRWST 子系统的设计改进方案，将 IRWST 水箱内的滤网由两个(A/B)增加为三个(A/B/C)，并通过管线实现滤网之间的相互连接。通过重新构建故障树对改进后的 IRWST 子系统进行建模分析，并对相应的事件树以及一级 PSA 模型进行详细的定量化计算。结果表明，IRWST 子系统这一改进能够显著降低堆芯损坏风险。IRWST 子系统的改进将 SI-LB始发事件的 CDF 降低了53.5%，将整个一级 PSA 的 CDF 降低了21.5%。
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