Utilization of Multilevel Flow Modelling to Support Passive Safety System Reliability Assessment

Many advanced Nuclear Reactor designs deploys passive systems for enhancing safety Passive systems rely on natural driving forces, such as natural circulation, gravity, internal stored energy etc., which do not require external power sources. Substantial efforts are underway towards improving reliability assessment methods for passive system, however, consensus is not yet reached. The existing case studies of various passive safety system reliability assessment methodologies usually consider a single passive safety system, but the reliability of one passive safety system should also dependent on other related systems which operate simultaneously. Thus, this work suggests a framework of passive safety system reliability assessment supported by multilevel flow modelling (MFM) methodology, and a preliminary application is conducted on a small modular reactor station blackout sequence to assess the reliability of Core Makeup Tank (CMT) to remove the decay heat. Also, a causal inference of MFM model and a sensitivity analysis is performed to prove the feasibility of the MFM supported passive safety system reliability assessment methodology. The results show the importance of considering all relevant systems when performing reliability assessment. This MFM supported passive safety system reliability assessment methodology can contribute to the integration of reliability assessment results to the probabilistic risk assessment framework of a whole power plant.