The when, where, why, and how of RIPB design

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 risk-informed, performance-based (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. CoPs are used frequently by organizations to help break down barriers that impede the flow of information.

The speaker: Former RP3C chair N. Prasad Kadambi opened the May 30 CoP presentation with brief introductory remarks about the committee and the need for new approaches to nuclear design that go beyond conventional and deterministic methods. He also highlighted that Congress reiterated and reinforced the need for RIPB in achieving effective regulation and oversight of nuclear technology in 2024 with the ADVANCE Act before he welcomed this month’s speaker: Megan Harkema, a research engineer at Vanderbilt University who presented “Risk-Informing: When, Where, and How to Start,” a talk that focused on what risk-informed design means at the very beginning of the process.

The NEERG SiD framework.

Safety-in-Design: Before diving into the details of RIPB, Harkema explained the critical function that the Nuclear Environmental Engineering Research Group (NEERG) at Vanderbilt serves. Over the past 15 years, she said, NEERG has performed research that has integrated risk and safety analysis into the early stages of design for advanced reactors through a process called safety-in-design methodology (SiD), which the group developed in collaboration with the Electric Power Research Institute in 2014. Aside from EPRI, the group has collaborated with commercial advanced reactor customers and the DOE’s Office of Nuclear Energy to implement SiD.

Harkema explained that SiD was developed to help meet advanced reactor safety expectations and avoid delays and costs arising from unexpected issues that could require design retrofitting to meet safety requirements. SiD is done through the early and iterative incorporation of safety analysis using process hazards analysis (PHA) methodologies, which allow for incremental progression of a safety case and eventually can support quantitative risk assessment.

The SiD framework has no demarcated starting point in its flowchart—an intentional absence necessitated by the fact that “the way you move through the SiD framework is dependent on design- and environment-specific factors,” Harkema explained.

However, that necessary absence leaves the big question: When, where, and how do we start using SiD as part of advanced reactor design?

When to start? In short, start as soon as possible. However, there are some considerations to take into account before beginning. Because PHA methods are flexible and are not created equal, it’s important to first evaluate the resources available for a safety analysis in order to decide which PHA method will be used.

Those considerations fall into two groups: team analysis objectives and design maturity. On the objectives side there are basic, practical questions of capacity in staff and expertise alongside time considerations and documentation needs.

On the design maturity side there are questions of the breadth of hazards, preferences for qualitative or quantitative results, and preferences for identification of single- or common-cause failures. Attending to questions in both these groups can serve as an entry point into the SiD process.

Where is the data? The PHA approach depends on the amount and nature of design and hazard information available, and at the early stages of design, design-specific data for first-of-a-kind advanced reactors may not be available. Therefore, an important question is raised: Where do we get the data to build a knowledge base to support the performance of a PHA?

Harkema listed three categories of sources that can help designers “get creative” in finding data despite these challenges: historical and modern operating experience of similar systems, stylized accidents in scientific literature, and phenomenological data. She then detailed the best practices and most effective methods and tools for capturing this data and screening it for relevancy and applicability.

How to start? With data in hand and resources evaluated, Harkema then explored how the process of risk informing actually starts. She offered up two case studies: the molten salt sampling system (MSSS) development and the preliminary initiating event (PIE) identification for General Atomics’ Fast Modular Reactor (FMR).

These two projects contrasted each other significantly. The MSSS was a subsystem project, while the General Atomics project was an entire reactor. The former had significant historical context, while the latter didn’t. The projects also had varying goals and resources. However, both projects were well served by SiD, showing the flexibility and versatility of the process.