Knoxville, Tenn., next July 15-21 will be the place to be for the nuclear community, with two ANS-sponsored meetings being held together. This is a first for ANS-sponsored meetings and will prove to be a can't-miss event. That said, the call for abstracts for the 2023 Nuclear Plant Instrumentation, Control & Human-Machine Interface Technologies (NPIC&HMIT) and Probabilistic Safety Assessment and Analysis (PSA) meetings is now open and is set to close soon—submissions will be accepted until November 18.

The Nuclear Regulatory Commission is seeking input from the public regarding the first set of results from its multiyear Level 3 Probabilistic Risk Assessment Project to analyze risk at a reference two-reactor nuclear power plant site.

The project, which began in 2012, updates risk insights obtained from work done more than 30 years ago, according to the NRC. The project examines a reference site with two large pressurized water reactors and associated spent fuel. It builds on Level 1 (possible reactor core damage scenarios) and Level 2 (possible radioactive material release scenarios) analyses to estimate potential health effects and economic impacts.

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).

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.

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.