The term “advanced reactor” describes many types of reactor designs, including water-cooled, gas-cooled, molten metal-cooled, and molten salt-cooled technologies (i.e., Generation IV reactors and small modular reactors (SMRs) based on scaled-down versions of Generation III reactors). These advanced reactors are anticipated to utilize more sophisticated systems that operate differently from the traditional large, commercial water-cooled reactors (e.g., pressurised water reactors and boiling water reactors) for which existing risk assessment methods and tools have been developed and effectively utilized.
Probabilistic risk assessments (PRA) are expected to play a key role in advanced reactor development. As such, risk metrics that can be used in the PRA to represent the risk profile of the reactor’s accident behavior need to be defined and trialed.
Advanced Reactors and Risk Metrics
The most globally used and understood risk metrics are Core Damage Frequency (CDF) and Large Early Release Frequency (LERF), which have been underpinned and refined with decades of water-cooled reactor operating experience. For many advanced reactor technologies, the calculation and usage of CDF and LERF may not be as meaningful for all regulatory and operational/risk-management purposes. For advanced water-cooled designs, risk metrics such as these may be calculable but produce numerical results in a lower range of values than those currently used by the industry and regulators to support operational and regulatory decision-making.
For non-water-cooled designs, the definition and concept of core damage as understood for the current operational fleet may be difficult or even impossible to define in terms of describing a “core melt” scenario, such as the case for molten salt-cooled reactors, where the reactor fuel is liquid during normal operation. Regarding LERF, the concept of a “large” release may no longer apply to advanced reactors, especially when considering designs with smaller inventories of radioactive material and advanced safety systems.
In preparation for the development of non-light water reactors and their ultimate deployment, the United States has published alternative approaches for advanced reactor PRA development. The ASME/ANS PRA standard for non-light water reactors (LWRs) provides an approach for risk analysis using frequency and consequences of radioactive material releases as key indicators for an acceptable level of risk [1]. This aligns with the frequency-consequence framework outlined in NEI 18-04 for non-LWR licensing, supported by the U.S. NRC in Regulatory Guide 1.233 [2,3]. Although these are valid approaches, the guidance still indicates that reactor developers can create their own risk metrics to aid in simplifying the analysis, whether the metrics are technology-neutral or specific. Additional risk metrics may also be required to support the plant’s day-to-day operations or globally, in countries that do not adopt a similar approach to that currently being developed in the United States.
Defining Options for Risk Metric Development
Due to the need for risk metric development guidance and understanding, Jensen Hughes is supporting EPRI research into a range of risk metric options for advanced reactor use. The preliminary study defined three options for risk metric development, focusing on technology-neutral metric development going forward.
- Option 1: Develop risk metrics that are directly analogous to CDF/LERF or each unique advanced reactor design.
- Option 2: Develop new, design-specific risk metrics with new bases for prevention and mitigation metrics.
- Option 3: Develop a limited number of technology-neutral risk metrics focused on the protection of the health and safety of the public.
The EPRI report will be published toward the end of 2024 outlining the findings of a technology neutral approach to risk metric development. Further initiatives with the research will benefit from advanced reactor developer collaboration to trial and underpin the approach developed.
References
[1] Probabilistic Risk Assessment Standard for Advanced Non-Light Water Reactor Nuclear Power Plants, ASME/ANS RA-S-1.4-2001. American Society of Mechanical Engineers, New York, NY: 2021.
[2] Risk-Informed Performance-Based Technology Inclusive Guidance for Non-Light Water Reactor Licensing Basis Development, NEI 18-04 (Revision 1). Nuclear Energy Institute, Washington, DC: August 2021.
[3] Guidance for a Technology-Inclusive, Risk-Informed, and Performance-Based Methodology to Inform the Licensing Basis and Content of Applications for Licenses, Certifications and Approvals for Non-Light-Water Reactors, Regulatory Guide 1.233. U.S. Nuclear Regulatory Commission, Washington, DC: June 2020.
