Risk Metrics

Purpose

Identify quantitative and qualitative metrics that can be used in risk modeling for Advanced Reactors to make risk-informed decisions on plant design in a similar fashion to how core damage frequency (CDF) and large early release frequency (LERF) are currently used for LWRS.

Scope

Risk metrics should be expected to evolve over time as those used to investigate qualitative risk at an early stage of design for ARs may be different from the ones used to investigate quantitative risk of a more mature design and licensing phase.

Advanced Reactor Risk Metrics

Decades of operation of LWR technologies have developed a consensus on the risk metrics to be used for these technologies; in particular, the use of CDF and LERF as figures of merit. However, for non-LWR advanced reactor technologies, the LWR metrics may not be meaningful or useful for either regulatory or operational / management purposes because of the vastly different designs of ARs. A question that comes out of this conclusion is what changes to PRA methods and tools would be needed to support risk metrics that are more appropriate for advanced reactors (i.e., metrics other than CDF and LERF as specified in the ASME/ANS Non-LWR PRA Standard)? And are there metrics that can be implemented that would remain the same across designs similar to CDF and LERF? A trial Reg Guide 1.247 [7] has been developed to help answer these questions.

Initial research has found several different proposed options for defining risk metrics:

• Reactor specific metrics related to technology safety margin / physics in design.
• Generalize CDF to encompass various AR designs, adapt the definition to be specific per design.
• Apply technology neutral risk metrics based on frequency and consequence related to public health, site boundary dose.
• Apply an economic and safety specific metric related to plant performance that could lead to safety failure or decline in economic performance related to components.
• Reactor specific metrics related to plant purpose (electrical generation, process heat, etc) more related to intermediate metrics rather than end state.

Current Advanced Reactor Roadmap Actions Supported include:

• Developing a Technical Input to Siting Criteria: In the envisioned advanced reactor licensing framework (as described in NEI 18-04), assessment of plant risk (via characterization by F-C curves) provides a primary set of criteria in the licensing process. This R&D task will develop applicable risk metrics that can provide technical basis for site-specific licensing decisions.
• Demonstrating a Risk-Informed and Performance Based Approach: Because the capability to effectively assess and monitor risk is a fundamental element for both plant owner / operator responsibilities and regulatory review / oversight in a risk-informed performance-based framework, this R&D activity can generally support demonstrating this risk-informed and performance-based framework.