CoP webinar lays out timeline of RIPB policy

The American Nuclear Society's Risk-informed, Performance-based Principles and Policy Committee (RP3C) recently held another presentation in its monthly Community of Practice (CoP) series. Former RP3C chair N. Prasad Kadambi opened the meeting with brief introductory remarks about the RP3C, the value of risk-informed, performance-based (RIPB) techniques, and the Nuclear Regulatory Commission's current interest in and work on the field.

After these remarks, Kadambi introduced this month's speaker: Steven Krahn, a professor of the practice of nuclear environmental engineering at Vanderbilt University and chair of the RP3C committee. Krahn's presentation was titled "The Evolution of the Understanding of Nuclear Reactor Risk and Risk-Informed, Performance-Based Policy in the U.S."

Watch the full webinar here.

Some background: RP3C is a special committee created by the ANS Standards Board and chaired by Steven Krahn that provides guidance to ANS standards committees on the use of RIPB methods. The CoP is part of RP3C's charter, which includes training and knowledge-sharing of RIPB principles to exchange ideas outside of the normal management and project processes.

Starting off: Krahn began by reminding the audience that he was presenting his work as a professor and not as a spokesperson for ANS. As such, the presentation should not be interpreted as ANS policy. He also emphasized that his project remains a work in progress.

With those disclaimers out of the way, Krahn transitioned into the substance of his presentation, which aimed to frame the issuance of 10 CFR Part 53 not as an isolated incident but instead as a "summation of work that has been going on for quite some time in the industry."

Krahn explained that Part 53 is the newest chapter in a seven-decade-long story told through technical milestones and the problems they helped solve. While a number of points could be chosen as the beginning of the timeline of that story, Krahn elected to begin at the issuance of WASH-740, which he described as "the first major evaluation of the potential consequences of an accident at a commercial-scale nuclear power plant."

Before diving deeper into the next points on the timeline, Krahn stepped back to explain the central message he has gleaned from his work generally and this project in particular: "Risk-informed regulation is really an accumulation of lessons. There was no direct, single point where the regulatory framework flipped from 'deterministic' to 'probabilistic.'" Instead, each generation of regulatory evolution added a new capability while refining safety principles established in previous generations.

Zooming in: Krahn's timeline, stretching from 1957 to 2026, contains 22 major milestones, each of which played a significant role in developing the field of risk-informed regulation. He spent most of the presentation exploring the details of these milestones, their broader contexts, and how they worked to develop the field further.

For a broader summary of this continuous evolution, Krahn broke the timeline into eight overarching phases. These phases should not be interpreted as a linear series of new ideas replacing the old. Instead, over the decades, new methods were layered into the already existing safety philosophy:

1957-1978-this earliest phase focused on building the analytical foundation of risk, consequences, scenarios, frequences, and uncertainties.

1979-1993-operational experience and plant-specific analysis broadened what "risk" meant.

1994-1998-PRA moved from a study product toward a repeatable decision framework.

1999-2007-the NRC formalized its implementation strategy, performance concepts, and decision principles.

2008-2012-the focus expanded toward holistic, beyond-design-basis, risk-informed thinking.

2013-2019-risk-informed methods increasingly shaped how licensing-basis content could be constructed.

2020-2024-RIPB methods became more explicit in non-light water reactor licensing guidance.

2024-2026-RIPB, technology-inclusive concepts converged into a formal licensing framework.

Another angle: At the conclusion of Krahn's presentation, he offered a different lens through which to interpret the chapters of this story. In this reorganization, he boiled each chapter down into one or two words and paired those words with some key milestones, which he then connected to the big-picture question that chapter ultimately sought to answer.

Foundation-WASH-740 and other early work answered the fundamental question: How do we bound severe consequences when data are sparse?

Quantification-WASH-1400, among other milestones, tackled the question: How do we represent many accident paths and their likelihoods?

Realism-The accident at Three Mile Island-2 in 1979, the lessons learned from it, and follow-on NRC policy on severe accidents answered the next question: How do humans, plant specifics, and external hazards alter the risk picture?

Decision Use-The passage of PRA policy was part of the regulatory effort to understand how risk information should influence regulatory decisions without displacing the safety framework.

Program-The development of a PRA plan answered the question: How does an agency systematically implement RIPB regulation?

Generalization-The Licensing Modernization Project, NEIMA, and other activities answered the question: How do we apply the RIPB philosophy to different technologies?

Convergence-Part 53, the latest step in this long story answered the question: How do these principles become an alternative licensing framework?

Go much deeper: In his full presentation, Krahn talked through each step on this long and detailed timeline and takes a closer look into why it matters. It is available to watch on the American Nuclear Society Standards YouTube channel.