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          About NRC  How We Regulate  Risk AssessmentRisk-Informed Activities Technical Guidance Program
On this page:
PRA Acceptability
PRA Consensus Standards
PRA Peer Review Process
Risk Methods
Glossary of Terms
Application-Specific GuidanceThe increased use of Probabilistic Risk Assessments (PRAs) in the NRC’s regulatory decision-making process requires increased confidence that the completeness, correctness, accuracy, and fidelity of the technical contents of a PRA are sufficient for its intended use, also referred to as the acceptability of the PRA. Such confidence in the PRA is necessary for the decision-making process, whether the PRA is developed by a regulated stakeholder to support specific risk-informed regulatory activities, or by the NRC to analyze specific technical issues or to support Commission decisions. To this end, the NRC, professional societies, and industry stakeholders support the development and maintenance of PRA consensus standards and associated guidance for the development and use of PRA. Figure 1 is intended to help illustrate the relationships between documents described in this webpage. Each of the elements shown in the figure will be described in further detail on this page and future iterations of the page.

Figure 1: Risk-Informed Regulatory Activities Relationship Diagram.Figure 1: Risk-Informed Regulatory Activities Relationship Diagram.PRA Acceptability
**Base PRA Acceptability**

The NRC has described a general framework for achieving PRA acceptability in the context of risk-informed regulatory activities, which involves 1) meeting staff regulatory positions in the relevant regulatory guidance document, 2) using an NRC-endorsed PRA consensus standard in the development and maintenance of a PRA, and 3) using an NRC-endorsed peer review process to independently peer review the PRA against the requirements in the NRC-endorsed PRA consensus standard. A key feature of meeting this general framework is that it allows NRC staff to focus more on the PRA’s key assumptions and areas of concern identified by the peer review and less on performing an in-depth review of a given PRA. Figure 2 illustrates the relationship among these three aspects of the general framework.

layout table for Figure 2: NRC general framework for achieving PRA acceptability (source: RG1.200, Revision 3, Figure 1)Figure 2: NRC general framework for achieving PRA acceptability (source: RG 1.200, Revision 3, Figure 1)The NRC published two documents that provide guidance on determining the acceptability of PRA for use in risk-informed regulatory activities. Regulatory Guide 1.200, "Acceptability of Probabilistic Risk Assessment Results for Risk-Informed Activities," Revision 3 (RG 1.200) provides the NRC staff position on one acceptable approach for determining the acceptability of a base PRA used in support of regulatory decision making for light water reactors (LWRs). The trial Regulatory Guide 1.247, "Acceptability of Probabilistic Risk Assessment Results for Non-Light Water Reactor Risk-Informed Activities" (Trial RG 1.247) provides the analogous guidance for the acceptability of non-light water reactor (NLWR) PRA.

Because of the extensive experience gained with the development and use of RG 1.200, the Trial RG 1.247 was modeled after RG 1.200. As such, there are a number of similarities between the Regulatory Guides (RGs). Both RG 1.200 and the Trial RG 1.247 describe what is needed for an LWR base PRA and an NLWR PRA, respectively, to be considered acceptable by establishing the acceptable attributes and characteristics of the PRA; allowing the use of NRC-endorsed PRA consensus standards and an industry guidance on a PRA peer review process; describing what is needed to demonstrate acceptability of a PRA in support of a regulatory application; and establishing the documentation necessary to support a regulatory submittal. Both RGs allow for the use of PRA consensus standards and industry peer review programs, as endorsed by the NRC in the RGs, as one way to meet the staff positions on PRA acceptability in the respective RGs. In endorsing industry standards and related guidance, the NRC staff may have no objection or may take an exception, in which case the issue associated with the exception is described and a proposed resolution to the exception is provided in the RG. However, from their respective affiliations with LWRs and NLWRs, there areseveral key differences between RG 1.200 and the Trial RG 1.247 regarding how they have been and are expected to be used and their scope.

Figure 3 shows relationships between RG 1.200, application-specific regulatory guidance such as RG 1.174, PRA consensus standards, and industry guidance documents. Some application-specific guidance documents related to the application of NLWR PRA have been developed such as Nuclear Energy Institute (NEI) 18-04, "Risk-Informed Performance-Based Technology Guidance for Non-Light Water Reactors," Revision 1, and others are still under development.

Layout table for Figure 3: Relationships between RGs, PRA consensus standards, and guidance for LWR risk-informed regulatory activities (source: RG1.200, Revision 3, Figure 2)Figure 3: Relationships between RGs, PRA consensus standards, and guidance for LWR risk-informed regulatory activities (source: RG 1.200, Revision 3, Figure 2)PRA acceptability is defined in RG 1.200 and Trial RG 1.247 in terms of the scope, the technical elements, level of detail, and plant representation of a base PRA , but with some differences. RG 1.200, Revision 3, defines the scope of an acceptable base LWR PRA as including a reactor, Level 1 and limited or full Level 2 analysis, all hazards, and at-power and low power & shutdown (LPSD) types of plant operating states. The Trial RG 1.247 defines the scope of an acceptable NLWR PRA as including all radiological sources, all hazards, all plant operating states, and all levels of analysis (i.e., from initiating event to radiological consequence). Both RGs provide minimum sets of technical elements and associated attributes for an acceptable PRA and require that the PRA model represents the plant to a level of detail appropriate for an application, incorporate plant-specific operating experience (i.e., if it exists), and realistically reflects the expected plant response for the hazards considered. And both RGs describe that an acceptable PRA needs to be developed, maintained, and upgraded to ensure the PRA model represents the as-built, as-operated, or as-designed plant. Finally, both RGs endorse industry guidance on how to perform a PRA peer review process.

RG 1.200 was initially developed starting in the early 2000’s to support the broader use of risk and PRA in regulatory decision making. For the last two decades, RG 1.200 has been used primarily in support of voluntary risk-informed regulatory activities associated with operating LWRs such as license amendment requests and implementation of risk-informed programs, some of which include requirements on the use of PRA (e.g., risk-informed categorization of structures, systems, and components under 10 CFR 50.69, "Risk-informed categorization and treatment of structures, systems and components for nuclear power reactors"). For RG 1.200, the approach for demonstrating the acceptability of a base PRA is independent of its application and, as such, the base PRA always needs to be acceptable. However, this inherently means an application PRA need only have the scope, technical elements, level of detail, and plant representation necessary to support the application for which it is being used. For a given regulatory application of a PRA that uses RG 1.200, the scope of the PRA (i.e., risk characterization, plant operating states, initiating events and hazards) needs to be commensurate with the specific risk-informed application under consideration. For example, some regulatory applications may only use a portion of the base PRA (e.g., extension of diesel generator allowed outage time), whereas other applications may require the complete model (e.g., safety significance categorization of structures, systems, and components). When used in support of an application, a major goal of RG 1.200 is to obviate the need for an in-depth review of a base LWR PRA by NRC reviewers, allowing them to instead focus their review on key assumptions and areas identified by peer reviewers as being of concern and relevant to the application. Consequently, the use of RG 1.200 is intended to foster a more consistent review process that is focused on the most important aspects of risk.

The Trial RG 1.247 endorses the following documents:

The 2021 American Society of Mechanical Engineers (ASME)/American Nuclear Society (ANS) NLWR PRA consensus standard, ASME/ANS RA-S-1.4-2021, Probabilistic Risk Assessment Standard for Advanced Non-Light Water Reactor Nuclear Power Plants
NEI NLWR PRA peer review guidance in NEI 20-09, "Performance of PRA Peer Reviews Using the ASME/ANS Advanced Non-Light Water Reactor Standard," Revision 1The Trial RG 1.247 was recently published in March 2022 and is expected to be used in support of future new applications for NLWRs that would include requirements on the use of PRA in different stages of the licensing process as well as other applications of PRA. Because of its anticipated use in new applications, unlike RG 1.200, the Trial RG 1.247 does not refer to the acceptability of a base NLWR PRA as the concept is less compatible with a PRA developed for a stage of the plant lifecycle where the plant design or actual operating configuration has yet to be established or finalized. This distinction between the uses of RG 1.200 and the Trial RG 1.247 is one of the key differences between the two documents.

PRA Consensus Standards

ASME and ANS jointly published a new Level 1/ Large Early Release Frequency (LERF) LWR PRA standard, ASME/ANS RA-S-1.1-2022, Standard for Level 1/Large Early Release Frequency Probabilistic Risk Assessment for Nuclear Power Plant Applications, in May 2022, which addresses the same scope as the 2009 version, ASME/ANS RA-Sa-2009, Addenda to ASMEIANS RA-S-2008 Standard for Level l/Large Early Release Frequency Probabilistic Risk Assessment for Nuclear Power Plant Applications. The scope of these consensus standards addresses a Level 1 and limited Level 2 (i.e., LERF) PRA for at-power conditions, internal events, internal floods, internal fires, seismic, high wind, external flood, and other hazard groups. ASME and ANS continue to develop LWR PRA consensus standards for LPSD modes of operation, advanced LWRs, Level 2, Level 3, and multi-unit PRA. To date, draft standards for trial use have been issued by ASME/ANS for Level 2, Level 3, LPSD, and NLWRs and consensus standards for these types of PRA are currently under development.

ASME and ANS jointly published a new NLWR PRA consensus standard, ASME/ANS RA-S-1.4-2021, in February 2021, which addresses all sources of radiological material, all hazards, all plant operating states, and all levels of analysis, but excludes internal fire PRA for LPSD types of operating modes. Additionally, ASME/ANS RA-S-1.4-2021 addresses all stages of the plant lifecycle from design, to construction, to operation. ASME and ANS previously published a draft NLWR PRA standard for trial use, upon which ASME/ANS RA-S-1.4-2021 was largely based. ASME/ANS RA-S-1.4-2021 was also heavily derived from the content and structure of ASME/ANS RA-S-1.1-2022 and other draft versions of the other LWR PRA standards as they were developed concurrently.

NRC staff are planning the development of the next technical revision of RG 1.200 which is expected to endorse ASME/ANS RA-S-1.1-2022, as well as the Level 2, advanced LWR, and LPSD LWR PRA consensus standards once they become available. Following the conclusion of the trial period for the Trial RG 1.247, NRC staff will analyze stakeholder feedback, lessons learned related to its use and plan for the next revision of RG 1.247, to include consideration of industry plans to revise PRA consensus standards and related guidance.

PRA Peer Review Process

NEI published NEI 17-07, "Performance of PRA Peer Reviews Using the ASME/ANS PRA Standard," Revision 2, which provides guidance on how to perform a PRA peer review to meet the PRA peer review requirements in the ASME/ANS Level 1/LERF LWR PRA standard. NEI 17-07 addresses the same scope of PRA as covered in the ASME/ANS Level 1/LERF LWR PRA standard for initial and follow-on peer reviews. NEI 17-07, Revision 2, also provides guidance on closing out unresolved findings from PRA peer reviews. NEI 17-07, Revision 2, contains a consolidated update of PRA peer review guidance from NEI 00-02, "Industry PRA Peer Review Process Guidelines," NEI 05-04, "Process for Performing Internal Events PRA Peer Reviews Using the ASME/ANS PRA Standard," NEI 07-12, "Fire Probabilistic Risk Assessment (FPRA) Peer Review Process Guidelines," and NEI 12-13, "External Hazards PRA Peer Review Process Guidelines," the first three of which are endorsed in RG 1.200, "Acceptability of Probabilistic Risk Assessment Results for Risk-Informed Activities," Revision 2.

NEI published NEI 20-09, Revision 1, which provides guidance on how to perform a PRA peer review to meet the PRA peer review requirements in the ASME/ANS NLWR PRA standard. NEI 20-09, Revision 1, is largely derived from NEI 17-07, Revision 2, as the PRA peer review process is largely technology-neutral.

Risk Methods

NUREG-1855, "Guidance on the Treatment of Uncertainties Associated with PRAs in Risk-Informed Decision-Making"Glossary of Terms

NUREG-2122, "Glossary of Risk-Related Term in Support of Risk-Informed Decision-Making"Application-Specific Guidance

Regulation Guide 1.175: Risk-Informed Inservice Testing
Regulation Guide 1.177: Risk-Informed Technical Specifications
Regulation Guide 1.178: An Approach for Plant-Specific Risk-Informed Decisionmaking for Inservice Inspection of PipingPage Last Reviewed/Updated Thursday, December 15, 2022
    
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