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.
후쿠시마 원전사고 이후 지진이나 쓰나미와 같은 심각한 자연재해로 인한 사고에 대처할 수 있는 피동 또는 고유한 안전 기능에 대한 관심이 향상되었고, 기존 원전 설계에 피동안전계통을 채택하였다. 그러나, 피동안전기능은 자연순환 및 중력과 같이 약한 자연력에 의해 작동하기 때문에 계통 불확실성이 높다. 따라서 동일한 안전 기능을 수행하는 피동 및 능동 계통을 복합 운전하기 위해서는 계통의 특성 및 작동 방법을 고려하여 최적화된 절차가 개발되어야 한다. 본 연구는 확률론적안전성평가의 결과를 바탕으로 냉각재상실사고시 최적의 복합운전전략을 개발하는 것을 목표로 수행되었다. 시스템의 상호작용, 인적오류, 피동 계통의 신뢰성은 고장 수목에 반영되었다. 피동 및 능동시스템의 성능, 원자로냉각재계통의 상태 및 운전원조치시간은 최적 열수력해석코드로 분석하였다. 최적의 운전 전략이 안전계통 설계 및 비용 분석을 고려하여 사건 수목을 기반으로 제안된다.
