Development of combined operation strategy of passive safety features with active systems

Abstract

After the Fukushima Daiichi accident, there is an increasing interest in the passive or inherent safety features of nuclear power plants to prevent core meltdown and to mitigate the accidents caused by severe natural disasters such as earthquakes or tsunamis. Passive safety features however have a high level of uncertainty because they are operated by weak driving forces such as natural circulation or gravity. In order to combine both passive and active features of a reactor for the same safety functions, an optimized operation procedure needs to be developed in consideration of system actuation priority and operational method. This study focuses on such optimal combined operation strategies for a pressurized water reactor under a small break loss-of-coolant accident based on probabilistic safety assessment results. The dynamic transient progression including system interactions and operator actions are considered in the event trees (ETs). The performance of the passive and active systems, the reactor coolant system condition and the allowable times for operator actions are analyzed through a thermal-hydraulic computer code. The reliability assessment of the passive systems, the quantification of human errors using the technique for human error rate prediction model and the analysis of the recovery cost and mitigation time according to the operated systems are evaluated from the results. Finally, optimal operation strategies are suggested based on ETs that consider the failure probabilities of the safety features and human reliabilities, the mitigation time and cost analysis. The evaluation results demonstrate that the combination used the passive auxiliary feedwater system and the active safety injection systems is superior to the combination of the passive systems. Additionally, employing the passive safety injection system as a backup to the active safety injection system will reduce plant risk in a considerable manner.