Application of dynamic fault tree technique in safety assessment for the containment spray system of nuclear power plant

Reliability data is necessary for probabilistic safety assessment (PSA) of a nuclear power plant to assess the performance of the safety systems. One well-known modeling technique in PSA is a fault tree analysis (FTA). However, dynamic failure of complex systems cannot be adequately modeled by traditional static fault trees. Therefore, to overcome this deficiency and to effectively assess the reliability of real complex systems, the concept of dynamic fault tree (DFT) is introduced by defining additional gates called dynamic gates to the traditional fault tree. DFTs are now successfully applied in dynamic failure systems reliability evaluation, design and risk management due to their simplicity and powerful dynamic modeling capacity. The containment spray system (CSS) is one of the confinement safety systems in pressurized water reactor. The basic purpose of CSS is to cool the atmosphere of the containment when its internal pressure surpasses a specific limit. The aim of this study is to develop a DFT for the CSS. Monte Carlo simulation approach is applied to implement the dynamic gates of DFT using Python programming language. This study investigates the effectiveness of DFT in presenting accurately the dynamic failure behaviors of CSS and the effectiveness of the simulation to calculate the reliability indices. Also, an importance measure is calculated based on the developed DFT to measure the impact and contribution of the CSS components on the total system risk.