Benchmark of dynamic approaches and tools for the Probabilistic Safety Assessment of Nuclear Systems

Improving safety has always been a fundamental objective in the nuclear industry and for the development of new reactors. Over the past 30 years, safety evaluations have evolved, with Probabilistic Risk Assessment (PRA) becoming a well-established procedure. Despite its widespread acceptance, PRA has limitations, including the possibility of overconservative results and the exclusion of the explicit interaction among time, repairs, physics and operator interventions. These limitations are particularly evident in specific cases that could benefit from a dynamic study, such as Generation IV Sodium-cooled Fast Reactors (SFR), which France has developed, constructed, and continues to design and study. New and more ambitious safety goals are key pillars in the development of Gen IV reactors. Addressing the traditional PRA limits and developing more adaptive methods to accurately assess risk is essential for their safety demonstration. In SFRs, the coolant’s significant thermal inertia necessitates considering longer mission times, where repairs and operator interventions could play a crucial role. In this context, Dynamic PRA can complement traditional PRA. Dynamic PRA encompasses a wide range of methods that evaluate risk by considering the dynamic evolution of a system, modeling time dependencies, repairs, and, when necessary, physics. These approaches provide a level of detail that traditional PRA cannot accommodate. However, this added complexity poses a significant challenge for Dynamic PRA. Various approaches have been studied and used, but evaluating their suitability for detailed risk assessment without excessive complexity remains an active research field. This study was performed during a 6-months internship. It considers two dynamic approaches: statecharts and Petri nets, with two associated tools, itemis CREATE and GRIF respectively. These graphical methods were used to construct a simplified model of the Decay Heat Removal System (DHRS) of a SFR, based on the provided system’s specifications. The purpose was to compare these methods, to assess their advantages and disadvantages as modeling tools for Dynamic PRA analysis. After constructing the models and assessing the modeling experience, including the contributions of both methods and tools, readability of the models, and accessibility of meaningful results, it was concluded that statecharts offer distinct advantages. Properties such as hierarchy and the ability to produce output in both transitions and states facilitate model conception, resulting in a visually simpler model. The timeline of the internship did not provide a possibility to properly verify constructed models.