Studies on the optimal mitigation strategy of extended SBO for the isolated multi-unit NPP site

•The coping time can be maximized by the optimal strategy with proper operator’s actions.•We proposed the resources-based mitigation strategy framework based on PSA and DSA.•14 cases were investigated in terms of available four resources.•The success criteria of PSA need to be updated with the coping time analysis.•The probabilistic model guided by DSA shows the reduction of total CDF and SBO-CDF. The Nuclear Power Plant in the isolated Multi-Unit site eventually reaches the core damage if it undergoes the extended Station Blackout with the depletion of available resources. However, the coping time can be maximized by the optimal strategy with proper operator’s actions so that the operator can prepare further mitigation action to prevent core damage. The resources-based mitigation strategy framework was developed from the insights of the probabilistic model and deterministic model, which consider monitoring of plant conditions, available decay heat removal systems, available power sources, and water sources depending on the time. The maximum coping time is estimated for various cases with MARS-KS1.4 code for APR1400 and combined with a corresponding probabilistic model for each accident sequence based on the accident conditions and the resources with additional human actions.