A Risk-Informed Framework for Safety Design of Generation IV Systems - 2023

Novel aspects of numerous advanced reactors would benefit from a systematic and technology-neutral approach for identification and categorization of event sequences to support their design and licensing. The risk-informed approach1 offers an iterative process, complementary to the traditional deterministic approach, for a more comprehensive search of event sequences including their expected frequency and consequences to understand the risk. The approach can also support safety classification of plant equipment and defence-in-depth (DiD) assessment as an integral part of the process to ensure compliance with safety design criteria and establish links between required safety functions and design requirements.
This position paper is intended to provide an example of framework for such a risk-informed approach in application to Generation-IV systems, recognizing that different approaches are also possible to risk-inform a design. The framework borrows from previously proposed risk-informed performance-based guidance for licensing basis development by the Nuclear Energy Institute.[1] While Reference [1] was developed based on the U.S. regulatory codes and standards, our position paper includes broader considerations to generalize the approach for other regulatory frameworks mainly based on International Atomic Energy Agency (IAEA) safety standards. It discusses how a risk-informed design process can combine both deterministic and probabilistic insights into the decision-making in a complementary way to inform the safety design and demonstrate alignment with DiD principles.
In particular, the approach aims to:

• establish generic event sequence categories to be considered in design, and integrate the deterministic input and risk insights to identify and classify the event sequences in each category,
• define the main elements of a generic frequency-consequence target to evaluate the event-sequences against a generic set of regulatory requirements and risk goals,
• establish a process to classify the plant equipment based on their risk-significance and the role in plant safety (prevention or mitigation functions within each event sequence),
• support deterministic phenomenological analysis of the key event sequences considered in design consistent with the safety classification of the responding plant equipment,
• assess the alignment of event sequence categories considered in design with the DiD levels, and
• establish the process for treatment of low-frequency event sequences as residual risk, including the consideration of high-consequence cliff-edge effects to ensure that the possibility of conditions with potential for large or early releases are ‘practically eliminated’.