Developing a Probabilistic Risk Assessment Framework for Sodium-Cooled Thermal Reactors: Addressing Unique Challenges and Uncertainties

This paper offers a Probabilistic Risk Assessment (PRA) framework designed specifically for sodiumcooled thermal reactors using Uranium zirconium hydride (UZrH) fuel, which currently
lack full quantitative risk assessments. Recognizing the significance for a customized approach to
risk assessment for a new type of advanced reactor, a complete PRA framework has been developed specifically to
navigate the challenges presented by sodium-cooled thermal reactors and to support
the reactor design evolution. The framework is designed to provide a systematic, quantifiable process for
assessing the hazards associated with internal initiating events, which are vital for understanding the reactor's
operational and safety profile throughout its life cycle.
The process starts with a comprehensive hazard assessment to identify internal initiating events,
leveraging existing sodium-cooled reactor data and interviews with design engineers. Subsequent phases include
modeling plant responses using fault tree and event tree analyses, as well as using reliability data and conservative
assumptions to fill data gaps unique to this reactor type. An iterative assessment of safety systems evaluates their
effectiveness in mitigating challenges, considering diversity, redundancy, and common cause failures. The
findings identify potential accident sequences that could have an impact on safety and inform design
enhancements. Findings highlight potential accident sequences that could impact safety, informing design
enhancements. This framework, in accordance with the ASME/ANS RA-S-1.4-2021 and NEI 18-04 guidance,
facilitates continuous improvements in risk-informed decision-making for advanced reactor designs.