A systematic approach to identify initiating events and its relationship to Probabilistic Risk Assessment: Demonstrated on the Molten Salt Reactor Experiment

Abstract

One of the first steps in developing a risk assessment model is an exhaustive search for initiating events, which is a systematic and comprehensive starting point to answer the question “what can go wrong?” for a given system design. Identifying Postulated Initiating Events (PIEs) for a reactor design that is at a conceptual or preliminary stage facilitates the incorporation of risk insights into the next iteration of the design process and allows for the early establishment of more quantifiable risk assessment models, such as event sequence diagrams and event tree analysis. Liquid-Fueled Molten Salt Reactors (LF-MSRs) are an example of an advanced reactor technology that does not benefit from having a wealth of operating experience or prior risk-informed safety assessment efforts. Furthermore, design details, such as normal operating conditions and the composition of radioactive material inventories, can deviate substantially from those in other reactors, such that a systematic and comprehensive approach to identifying PIEs for an LF-MSR may highlight accident initiators that have not previously been identified. In the present work, the Master Logic Diagram (MLD) and Hazards and Operability (HAZOP) study approaches were used, together, to identify and consider PIEs for multiple inventories of radioactive material across various Plant Operating States (POSs) in a specific LF-MSR design -- the Molten Salt Reactor Experiment (MSRE). Potentially risk-significant PIEs identified during the analyses of the MSRE design are presented. Furthermore, considerations for exhaustively identifying PIEs for advanced reactor designs are discussed; for example, the combination of inductive and deductive methods was found to provide a robust identification of PIEs in a way that is conducive to the analysis of a nuclear reactor design at an early design stage.