ANSI/ASME/ANS RA-S-1.1-2024, Standard for Level 1/Large Early Release Frequency Probabilistic Risk Assessment for Nuclear Power Plant Applications, has just been published by the American Nuclear Society. The document, which is a joint standard developed with the American Society of Mechanical Engineers by the ANS/ASME Joint Committee on Nuclear Risk Management, received approval of the American National Standards Institute on February 29, 2024, and was issued on March 15, 2024.

Last month at the American Nuclear Society’s Risk-informed, Performance-based Principles and Policy Committee’s (RP3C’s) Community of Practice (CoP), Brandon Chisholm presented “Development of a Risk-Informed and Performance-Based Safety Case for TerraPower’s Molten Chloride Reactor Experiment (MCRE).” RP3C holds a CoP on the last Friday of the month from 3:00 p.m. to 4:00 p.m. (ET), and participation is open to all professionals interested in RIPB principles and practices. Chisholm’s January 26 presentation is available to stream on YouTube.

Dennis Henneke

The BWRX-300 is the 10th generation of boiling water reactor designed by GE Hitachi and has a number of evolutionary features. We learned from the design of the Economic Simplified Boiling Water Reactor (ESBWR), the BWRX-300’s predecessor, that implementation of a plant design that utilizes passive safety features can result in a relatively large containment. From the inception of the BWRX-300, our goal was simplification of the design and the reduction in overall size of the “safety footprint,” which includes both containment and the safety-related--component plant areas. The design team was empowered to consider any and all simplification efforts, which were evaluated by the GEH team. One key to implementing this goal—design of a plant that can be licensed anywhere in the world—is the use of a probabilistic risk assessment (PRA) that helps ensure the resulting plant risk is low.

The American Nuclear Society will host the online event “Hispanic Excellence in the Nuclear Field” on September 20 at 10:00 a.m. (EDT), featuring a distinguished panel of nuclear experts. The panelists will share unique insights from their careers and discuss opportunities and challenges facing the future nuclear workforce, including what they see as future opportunities for the Hispanic community in the nuclear field.

In a notice published in the August 21 Federal Register, the Nuclear Regulatory Commission is requesting public input on the latest set of results from a multiyear project to study risk at a two-unit nuclear power plant site.

Askin Guler Yigitoglu (yigitoglua@ornl.gov) is a reactor systems modeling and safety analysis staff member at ORNL, chair of ANS’s Nuclear Installations Safety Division, and technical program chair for the PSA 2023 conference.

Probabilistic risk assessment is a mature technology that has benefited the safety of the current fleet of light water reactors in the United States since the 1970s. Most utilities have used PRA models as part of risk-informed in-service inspection programs to identify degraded plant conditions for more than two decades. The trends indicate an increasing use of risk-informed applications to support safe and cost-effective long-term operations.

Data science and predictive analytics innovations offer the opportunity to assess, monitor, and manage risk effectively. PRA models are coupled with digital twins informed by sensors and system simulators that provide real-time risk insights. Dynamic PRA approaches were initially introduced to beyond-design-basis event models for LWRs and explicitly model time-dependent operator behavior by simulating the actual plant response. Enhancing the quantification speed and memory usage of the PRA computational tools (both dynamic and traditional) is crucial for future risk-informed efforts.

Human factors engineering and risk analysis are part of every instrumentation and control upgrade and new reactor plan—from design and licensing to implementation and operation—and applications continue to evolve.

At the 2023 ANS Annual Meeting, Steven Arndt (as of the close of the meeting, ANS immediate past president) led a president’s session on the mission of the Nuclear Regulatory Commission—a not particularly surprising topic, given that he spent over 30 years at the agency in various roles.

Eddie M. Guerra (eddie.guerra@rizzointl.com) is vice president of civil infrastructure development at Rizzo International.

Eddie Guerra, VP of Civil Infrastructure: Our energy infrastructure is undergoing an unprecedented transformation, which is in turn opening a new wave of challenges for deploying the next generation of nuclear reactors.

The decentralization of power generation will require nuclear plants to be sited closer to demand centers. As the future grid becomes more distributed, energy--intensive customers will demand proximity and flexibility, and new--generation reactors will need to accommodate the intermittency and load--following requirements that a greener and more dynamic grid will pose. Added to that, nuclear projects will need to compete economically within a more liberalized electricity market. Advanced reactor deployments will face unprecedented challenges in today’s world, and in the future.

Despite challenges, advances in engineering and technology point to a very bright future. Smaller reactors with enhanced safety features will allow stakeholders to rethink proximity criteria on siting, opening doors for deployment in new scenarios: university campuses, municipalities in remote areas, or industrial conglomerates, just to name a few.

The American Society of Mechanical Engineers/American Nuclear Society Joint Committee on Nuclear Risk Management (JCNRM) has issued a new edition of its flagship standard, ANSI/ASME/ANS RA-­S-­1.1-­2022, Standard for Level 1/Large Early Release Frequency Probabilistic Risk Assessment for Nuclear Power Plant Applications. This standard was approved by the JCNRM, the ANS Standards Board, and the ASME Board on Nuclear Codes and Standards before being approved on May 11 by the American National Standards Institute (ANSI), earning the title of an American National Standard. With most of the text stable for the past year, the production process was started early, allowing the 400-­page standard to be published on May 31, 2022.

An ANS virtual event titled "Perspectives from Past NRC Chairs" featured four former members of the Nuclear Regulatory Commission who focused on the future of nuclear energy in the United States and the NRC’s role as a regulator of small modular and advanced reactors.

The virtual event gave us an idea for #ThrowbackThursday to search through the Nuclear News archives (available to all ANS members) for an interview with George Apostolakis, published in the March 2000 issue. At the time, Apostolakis was a professor at the Massachusetts Institute of Technology and a member of the NRC’s Advisory Committee on Reactor Safeguards. Ten years later, he was sworn in as an NRC commissioner.

In March 1972, Stephen Hanauer, a technical advisor with the Atomic Energy Commission, met with Norman Rasmussen, a nuclear engineering professor at the Massachusetts Institute of Technology. The AEC had recruited Rasmussen to develop a report, The Reactor Safety Study (WASH-1400), to estimate the probabilities and consequences of a major nuclear power plant accident. With thousands of safety components in a modern reactor, the task was mind-boggling. Rasmussen proposed a novel approach based on more powerful computers, “fault tree” methodology, and an expanding body of operational data. By calculating and aggregating probabilities for innumerable failure chains of components, he believed he could develop a meaningful estimate of overall accident risk. WASH-1400 would be a first-of-its-kind probabilistic risk assessment (PRA).

Probabilistic risk assessment has been around for over 40 years, helping us understand the amazing, complex engineering systems we design, build, and operate. It’s a powerful tool, but the time has come to consider how we can modernize it. There are important gaps in PRA, including in areas such as human reliability, dynamics, natural hazards, and cybersecurity. However, there are three things that are even more important to do:

Matthew Denman

Probabilistic risk assessment is a systematic methodology for evaluating risks associated with a complex engineered technology such as nuclear energy. PRA risk is defined in terms of possible detrimental outcomes of an activity or action, and as such, risk is characterized by three quantities: what can go wrong, the likelihood of the problem, and the resulting consequences of the problem.

Matthew Denman is principal engineer for reliability engineering at Kairos Power and the chair of the American Nuclear Society and American Society of Mechanical Engineers Joint Committee on Nuclear Risk Management’s Subcommittee of Standards Development. As a college student at the University of Florida, Denman took a course on PRA but didn’t enjoy it, because he did not see its connection to the nuclear power industry. Later, during his Ph.D. study at the Massachusetts Institute of Technology, his advisor was Neil Todreas, a well-known thermal hydraulics expert. Todreas was working on a project with George Apostolakis, who would leave MIT to become a commissioner of the Nuclear Regulatory Commission. The project, “Risk Informing the Design of the Sodium-Cooled Fast Reactor,” was a multi-university effort funded through a Department of Energy Nuclear Energy Research Initiative (NERI) grant. Todreas and Apostolakis were joined in this project by a who’s who of nuclear academia, including Andy Kadak (MIT, ANS past president [1999–2000]), Mike Driscoll (MIT), Mike Golay (MIT), Mike Lineberry (Idaho State University, former ANS treasurer), Rich Denning (Ohio State University), and Tunc Aldemir (Ohio State University).

Probabilistic risk assessments (PRAs) have advanced the safe operation of the U.S. reactor fleet over many decades. Risk insights from PRAs have provided information from many different perspectives, from what is most important to maintain at a facility to a better understanding of how to address new information regarding safety issues. The methods and tools that have supported the creation and enhancement of PRA models were established through multiple decades of research, starting with WASH-1400, The Reactor Safety Study,¹ published in 1975, through the comprehensive plant-specific models in use today.

Craig Piercy
cpiercy@ans.org

This month’s issue of Nuclear News focuses on the role of probabilistic methods in assessing and mitigating the risk of adverse events at nuclear plants and facilities. It’s a timely topic as we move to launch a new generation of nuclear technologies, but it is only half of a larger question that is universal to the human condition: Are the rewards of a particular thing worth its attendant risks?

Nuclear engineers use hard technical terms like “probabilistic risk assessment” and “core damage frequency,” but other industries have much more colorful ways of describing the holistic risk-reward construct in their world. In finance, it’s known simply as “alpha.” A zero alpha investment suggests that its returns are commensurate with the associated risks. Negative alphas get pushed to the curb, and “high alpha” deals get Wall Street hedge fund managers their house in the Hamptons.

ANSI/ASME/ANS RA-S-1.4-2021, “Probabilistic Risk Assessment Standard for Advanced Non-Light Water Reactor Nuclear Power Plants,” has just been issued. Approved by the American National Standards Institute (ANSI) on January 28, 2021, this joint American Society of Mechanical Engineers (ASME)/American Nuclear Society (ANS) standard sets forth requirements for probabilistic risk assessments (PRAs) used to support risk-informed decisions for commercial nuclear power plants and prescribes a method for applying these requirements for specific applications.

ANSI/ANS-RA-S-1.4-2021 and its preview are available in the ANS Standards Store.