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This report, "Modernization of Technical Requirements for Licensing of Advanced Non-Light Water Reactors: Selection and Evaluation of Licensing Basis Events," represents a key element in the development of a methodology for the efficient licensing of advanced non-light water reactors (non-LWRs). It is the result of a Licensing Modernization Project (LMP) led by Southern Company and cost-shared by the U.S. Department of Energy (DOE). The LMP will result in detailed proposals for establishing licensing technical requirements to facilitate riskinformed and performance-based design and licensing of advanced non-LWRs. Such a methodology acknowledges enhancements in safety achievable with advanced designs and reflects more recent states of knowledge regarding safety and design innovation, creating an opportunity for reduced regulatory complexity with increased levels of safety. The project builds on best practices as well as previous activities through DOE and industry-sponsored advanced reactor licensing initiatives. The LMP objective is to assist the Nuclear Regulatory Commission (NRC) in developing regulatory guidance for licensing advanced non-LWR plants. This report presents a modern, technology-inclusive, risk-informed, and performance-based (TI-RIPB) approach to identifying Licensing Basis Events (LBEs), which cover a spectrum of events considered in the design and licensing of a nuclear power plant. A key licensing outcome of this approach is the structured selection of Design Basis Accidents (DBAs) that are traditionally analyzed in Chapter 15 of the license application.

A companion report described the LMP approach to selecting and evaluating licensing basis events and for identifying structures, systems, and components (SSCs) that are classified as Safety-Related. The current document outlines the approach to performing the following additional tasks that impact the development SSC design performance targets: • Complete the process of SSC safety classification by subdividing non-safety-related SSCs into subcategories to identify additional safety-significant SSCs that require special treatment • Describe the LMP approach to the definition of risk significant SSCs • Describe the LMP approach for defining safety-significant SSCs in terms of their risk significance and role in supporting defense-in-depth (DID) adequacy * As used in this report, the term "PRA safety function" is defined as any function modeled in the PRA that protects one or more radionuclide barriers, or otherwise prevents or mitigates a release of radioactive material to the environment. Those safety functions that are relied on in the design to prevent or mitigate a design basis accident within 10 CFR 50.34 dose limits are referred to as required safety functions.

This report captures lessons learned, best practices, and frequently asked questions with responses gleaned from the experiences of early adopters of the LMP RIPB process. Interviews were conducted in late 2019 with five non-LWR design organizations to gather their feedback on the application of the LMP RIPB process under real production conditions. The overall feedback from the reactor developers is that the LMP RIPB process can be successfully implemented with material benefits to the designer. As with any new processes, numerous challenges and questions were relayed by the designers. These challenges have been translated into lessons learned and best practices. Those two sections address both technical and organizational aspects of implementing the LMP RIPB process. Questions that were asked by multiple designers were turned into "frequently asked questions" and responses were provided by the LMP team.

This document supports the work contained in Nuclear Energy Institute (NEI) 18-04 "Risk-Informed Performance-Based Technology Inclusive Guidance for Advanced Reactor Licensing Basis Development" Revision 0. [19] NEI 18-04 presents a modern, technology-inclusive, riskinformed, and performance-based (TI-RIPB) process for selection of Licensing Basis Events (LBEs); safety classification of structures, systems, and components (SSCs) and associated riskinformed special treatments; and determination of defense-in-depth (DID) adequacy for non-LWRs. The NEI guidance document provides one acceptable means for addressing the aforementioned topics as part of demonstrating a specific design provides reasonable assurance of adequate radiological protection.This report provides the framework and associated methodology guidelines and discussion for establishing, then evaluating, confirming, and documenting the adequacy of defense-in-depth (DID) for advanced non-light-water reactor technologies. It was developed as part of the Licensing Modernization Project led by Southern Company and cost-shared by the United States Department of Energy and has benefited from considerable NRC formal reviews [20][21] and public workshops.The methodology converts the DID philosophy into a structured process that is implementable, embraces existing United States and international definitions and philosophies of DID that set the foundation for the process. It builds on the DID framework developed in the Department of Energy Next Generation Nuclear Plant Project and earlier works on this subject.