Text

NEI - Request for NRC Endorsement of NEI 25-08, Revision a, Graded Approach to Seismic Hazard Analysis and Corresponding Site Investigations for Licensing Nuclear Power Plants
	ML25335A211
Person / Time
Site:	99902028, Nuclear Energy Institute
Issue date:	12/01/2025
From:	Nichol M; Nuclear Energy Institute
To:	Jeremy Bowen; Office of Nuclear Reactor Regulation, Document Control Desk
References
	NEI 25-08, Rev A
	Download: ML25335A211 (0)
	v • d • e

Marc Nichol Executive Director, New Nuclear Phone: 202.739.8131 Email: mrn@nei.org December 1, 2025 Jeremy Bowen Director, Division of Advanced Reactors and Non-Power Production and Utilization Facilities Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington, DC 20555-000

Subject:

Request for NRC Endorsement of NEI 25-08, Revision A, Graded Approach to Seismic Hazard Analysis and Corresponding Site Investigations for Licensing Nuclear Power Plants Project Number: 689

Dear Mr. Bowen:

The Nuclear Energy Institute (NEI)1, on behalf of its members, respectfully submits NEI 25-08, Revision A, titled Graded Approach to Seismic Hazard Analysis and Corresponding Site Investigations for Licensing Nuclear Power Plants, for NRC review and endorsement. This document was developed in collaboration with industry and technical experts to provide a streamlined and risk-informed framework for site seismic characterization, consistent with the NRCs goals for regulatory efficiency and effectiveness.

NEI 25-08 builds directly upon the conceptual framework outlined in SECY-25-0052, which proposes a graded approach to site characterization based on hazard margin and consequence. The methodology supports the use of preliminary seismic screening, indexed gradation tiers, and tailored site investigation requirements to improve regulatory predictability and reduce licensing burden where safety margins are high. NEI 25-08 also builds upon the NEI proposed approach in the proposal entitled, Regulations of Rapid High-Volume Deployable Reactors in Remote Applications (RHDRA) and Other Advanced Reactors, submitted July 31, 2024 (ML24213A337). The approach is applicable to all types of new 1 The Nuclear Energy Institute (NEI) is responsible for establishing unified policy on behalf of its members relating to matters affecting the nuclear energy industry, including the regulatory aspects of generic operational and technical issues. NEIs members include entities licensed to operate commercial nuclear power plants in the United States, nuclear plant designers, major architect and engineering firms, fuel cycle facilities, nuclear materials licensees, and other organizations involved in the nuclear energy industry.

Mr. Jeremy Bowen December 1, 2025 Page 2 Nuclear Energy Institute reactors and is graded based upon the design and site characteristics. The NRC, in a letter dated December 25, 2024 (ML24317A174) concluded: The NRC staff agrees with the high-level concepts outlined in your letter and the staff has not identified any fundamental gaps with the NRCs ongoing and planned activities that would deter implementation of the planned business models associated with microreactor deployment. Furthermore, the staff supports the NEI position that the strategies and guidance being developed to support microreactor deployment may be applied in a graded manner to other advanced reactor designs.

This submittal is aligned with Section 206 of the ADVANCE Act of 2024, which directs the Commission to consider how licensing reviews for production facilities or utilization facilities at covered sites may be expedited by considering matters relating to siting and leverage the availability of historical site-specific environmental data;. In response to the ADVANCE Act, NRC provided a report to Congress titled Regulatory Issues for Nuclear Facilities at Brownfield and/or Retired Fossil Fuel Sites and endorsement of this guidance would support the action ID5: Provide guidance on a graded approach to site characterization for advanced and microreactor designs using bounding site parameters as part of the screening analysis for external hazards. Additionally, Executive Order 14300, issued in May 2025, calls for expedited regulatory pathways for reactors previously demonstrated by the Department of Energy or Department of Defense and to Establish a process for high-volume licensing of microreactors and modular reactors. These mandates highlight the urgent need to include this graded approach into NRC guidance in a manner that enables commercial deployment. A recent NRC Advanced Reactor Stakeholder meeting (ML25237A182) noted that recent Level 3 SSHAC studies have taken 2-5 years and

$6-10 million which is unnecessarily burdensome and not conducive to rapid, high-volume deployment.

NEIs RHDRA Supplement, dated July 14, 2025 (ML25195A307), Attachment C concluded that facilities that have potential consequences and risks that are lower than NRC requirements and many industrial facilities, should be able to use alternative approaches that avoid unnecessary burden. NEI 25-08 proposes the use of USGS and other methods that avoid the timely and costly SSHAC Level 3 for facilities that achieve these lower potential consequences and risks.

Given these imperatives, and the NRCs on-going efforts to develop a wholesale rulemaking within 18 months, we request NRC endorsement of NEI 25-08, Rev A, via a Regulatory Guide or comparable vehicle issued with the Final Rulemaking by November 2026. We are submitting this report as Revision A in anticipation of several proposed rules in early 2026 which may require revision to the report. We look forward to NRC engagement to support a final submittal following NRCs publishing of proposed rule changes. Timely endorsement will ensure the final rule language implementation is clear and predictable, and that an approach is available for applicants preparing early site work and license applications in 2027 and beyond. NEI also requests the NRC provide feedback by the end of Q2 2026 that

Mr. Jeremy Bowen December 1, 2025 Page 3 Nuclear Energy Institute identifies whether there are any fatal flaws or major address to be addressed, to enable companies to use this approach as they develop and begin to execute their strategies for site characterization. NEI believes a fee waiver is warranted pursuant to 10 CFR 170.11(a)(1)(ii), as the NRCs review of NEI 25-08, Rev 0 will "assist the NRC in generic regulatory improvements or efforts."

The attached document reflects extensive industry collaboration and is designed to align with current regulations (10 CFR 100.23) while offering enhanced clarity and flexibility in how applicants demonstrate reasonable assurance. We appreciate the NRCs previous engagement on this topic and look forward to further collaboration throughout the review process.

Please contact Jon Facemire (jwf@nei.org) with any questions or requests for clarification.

Sincerely, Marc Nichol Executive Director, New Nuclear

Attachment:

NEI 25-08, Rev A, Graded Approach to Seismic Hazard Analysis and Corresponding Site Investigations for Licensing Nuclear Power Plants C:

Jeremy Groom, NRR Jon Grieves, NRR Mike Wentzel, NRR Hannah McLatchie, NRR Allen Fetter, NRR NRC Control Desk

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NEI 25-08, Rev A Graded Approach to Seismic Hazard Analysis and Corresponding Site Investigations for Licensing Nuclear Power Plants Prepared by the Nuclear Energy Institute December 2025

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Revision Table Revision Description of Changes Date Modified Responsible Person 0

Initial Issue 12/1/2025 Jon Facemire

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Acknowledgements This document was developed by the Nuclear Energy Institute and Simpson Gumphertz and Heger. NEI acknowledges and appreciates the contributions of NEI members and other organizations in providing input, reviewing, and commenting on the document, including the Siting Task Force, New Nuclear Engineering Working Group and the New Nuclear Licensing Working Group. Special thanks to Oklo, NuScale, and others for table-topping the methodology.

NEI Project Lead: Jon Facemire Notice Neither NEI, nor any of its employees, members, supporting organizations, contractors, or consultants make any warranty, expressed or implied, or assume any legal responsibility for the accuracy or completeness of, or assume any liability for damages resulting from any use of, any information apparatus, methods, or process disclosed in this report or that such may not infringe privately owned rights.

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Table of Contents Introduction..................................................................................................................................... 1 Approach.......................................................................................................................................... 2 Definitions........................................................................................................................................ 3 Screening Framework for Graded Seismic Hazard and Site Characterization................................. 4 Seismic Index.................................................................................................................................... 5 5.1 Hazard................................................................................................................................. 6 5.2 Seismic Screening Margin................................................................................................... 6 5.2.1 Target Seismic Capacity......................................................................................... 6 5.2.2 Scoping Seismic Demand....................................................................................... 7 5.2.3 Seismic Screening Margin...................................................................................... 7 5.3 Confidence.......................................................................................................................... 9 5.4 Seismic Index.................................................................................................................... 10 5.5 Justification and Basis for Seismic Index........................................................................... 11 Consequence Index........................................................................................................................ 14 6.1 DBA Dose Margin as Consequence Index......................................................................... 15 6.2 Emergency Planning Zone Sizing as Consequence Index.................................................. 18 6.3 Unmitigated Accident Release as Consequence Index..................................................... 19 Gradation Tiers.............................................................................................................................. 20 Graded Approach........................................................................................................................... 23 8.1 Specifics for the Tiers........................................................................................................ 23 8.2 Justification and Basis for the Graded Approach.............................................................. 27 Examples........................................................................................................................................ 29 9.1 Seismic Index Examples.................................................................................................... 30 9.2 Consequence Index Examples........................................................................................... 43 9.3 Gradation Tier Examples................................................................................................... 43 9.4 Discussion......................................................................................................................... 43 Recommendations and Conclusions.............................................................................................. 44 References..................................................................................................................................... 45 Table of Figures Figure 1: Framework for Graded Seismic Hazard and Site Characterization................................................ 5

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Figure 2: Seismic Index Margin Parameter................................................................................................... 8 Figure 3: Mapping of Seismic Screening Margin, Hazard, and Confidence Parameters to Seismic Index. 11 Figure 4: Gradation Tier Mapping through Seismic and Consequence Index............................................ 21 Figure 5: Target Seismic Capacity Examples............................................................................................... 31 Figure 6: MCER at Example Locations per Site Classification...................................................................... 34 Figure 7: Site amplification factors for five example locations................................................................... 35 Figure 8: MCER at five example locations at rock....................................................................................... 36 Figure 9: Idealized SRA MCER at five example locations............................................................................. 36 Figure 10: Gulf Coast, TX (STP) Scoping Seismic Demand vs. Target Seismic Capacity.............................. 37 Figure 11: North Anna Scoping Seismic Demand vs. Target Seismic Capacity........................................... 37 Figure 12: Clinch River Scoping Seismic Demand vs. Target Seismic Capacity........................................... 38 Figure 13: Dresden Scoping Seismic Demand vs. Target Seismic Capacity................................................. 38 Figure 14: Columbia Scoping Seismic Demand vs. Target Seismic Capacity............................................... 39 Figure 15: St. Louis, MO, Scoping Seismic Demand vs. Target Seismic Capacity........................................ 39 Table of Tables Table 1: Hazard Parameter Thresholds......................................................................................................... 6 Table 2: Consequence Index....................................................................................................................... 15 Table 3: Gradation Tier Definitions (details in Section 8.1)........................................................................ 22 Table 4: Graded Approach - Tiered Scope of Site Investigation................................................................ 24 Table 5: Target Seismic Capacities, Target Locations, and Site Amplification Method for Seismic Index Examples..................................................................................................................................................... 30 Table 6: Seismic Parameters (Hazard ToolBox).......................................................................................... 34 Table 7: Primary Margin Characterization.................................................................................................. 40 Table 8: Seismic Hazard for six example locations..................................................................................... 42 Table 9: Example Seismic Index Summary.................................................................................................. 42 Table 10: Example Gradation Tier Summary.............................................................................................. 43

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List of Acronyms ADVANCE Accelerating Deployment of Versatile, Advanced Nuclear for Clean Energy AEF Annual Exceedance Frequency AF Amplification Factor ANS/ANSI American Nuclear Society/American National Standards Institute AR Advanced Reactor ASCE American Society of Civil Engineers C10%

10% non-exceedance probability Capacity CBR Center, Body, and Range CEUS Central and Eastern United States CFR Code of Federal Regulation CI Consequence Index CSDRS Certified Seismic Design Response Spectrum DBA Design Basis Accident DBEHL Design Basis External Hazard Level DBGM Design Basis Ground Motion DBHL Design Basis Hazard Level DOD Department of Defense DOE Department of Energy DRS Design Response Spectrum EAB Exclusion Area Boundary EPRI Electric Power Research Institute EPZ Emergency Planning Zone GDRS Generic Design Response Spectrum GMC Ground Motion Characterization GMM Ground Motion Model GMRS Ground Motion Response Spectrum H

High IAEA International Atomic Energy Agency IBC International Building Code INL Idaho National Laboratory L

Low LHT Low Hazard Threshold LLWR Large Light Water Reactor LMP Licensing Modernization Project LWR Light Water Reactor M

Medium or Moderate MHF High Frequency Margin MLF Low Frequency Margin MP Primary Margin MCER Risk-Targeted Maximum Considered Earthquake - The most severe earthquake effects considered by ASCE 7, determined for the orientation that results in the largest maximum response to horizontal ground motions and with adjustment for targeted risk (ASCE 7-22).

MHA Maximum Hypothetical Accident MRD Modulus Reduction and Damping NEHRP National Earthquake Hazards Reduction Program

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NEI Nuclear Energy Institute NEIMA Nuclear Energy Innovation and Modernization Act NOAK Nth-of-a-Kind NPP Nuclear Power Plant NPUF Non-power Production or Utilization Facility NRC Nuclear Regulatory Commission NSHM National Seismic Hazard Model NTTF Near Term Task Force PAG Protective Action Guide PEP Plume Exposure Pathway PGA Peak Ground Acceleration PSHA Probabilistic Seismic Hazard Analysis RG Regulatory Guide RHDRA Rapid High-Volume Deployable Reactors in Remote Applications RI Risk-Informed SD1 Design, 5% damped, spectral response acceleration parameter at a period of 1 s (ASCE 7-22).

SDS Design, 5% damped, spectral response acceleration parameter at short periods (ASCE 7-22).

SM1 MCER, 5% damped, spectral response acceleration parameter at a period of 1 s adjusted for site class effects as determined in accordance with Section 11.4.3 of ASCE 7 (ASCE 7-22).

SMS MCER, 5% damped, spectral response acceleration parameter at short periods adjusted for site class effects as determined in accordance with Section 11.4.3 of ASCE 7 (ASCE 7-22).

SAF Site-specific Amplification Factor SDC Seismic Design Category SEI Structural Engineering Institute SHSR Seismic Hazard and Screening Report SMR Small Modular Reactor SPID Screening, Prioritization and Implementation Details SR Safety Related SRA Site Response Analysis SRP Standard Review Plan SSAR Site Safety Analysis Report SSC Structure, System, and Component or Seismic Source Characterization SSE Safe Shutdown Earthquake SSHAC Senior Seismic Hazard Analysis Committee STP South Texas Project SI Seismic Index TDI Technically Defensible Interpretations TEDE Total Effective Does Equivalent UHRS Uniform Hazard Response Spectrum USGS United States Geological Survey V/H Vertical over Horizontal VS30 Shear wave Velocity in the top 30 meters WUS Western United States

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INTRODUCTION The seismic aspects of site characterization for nuclear power plants (NPPs) can be resource-intensive and are often on the critical path for siting. A graded approach for seismic site characterization can reduce deployment time for some new NPPs. Streamlining the deployment schedule aligns with the objectives of the Nuclear Energy Innovation and Modernization Act (NEIMA), the Accelerating Deployment of Versatile, Advanced Nuclear for Clean Energy (ADVANCE) Act, and May 2025 Executive Orders related to nuclear technology. Specifically, the ADVANCE Act encourages maximizing existing data for siting work.

Current regulations (10 CFR 100.23 [1]) and regulatory guidance (NRC RGs 1.132 [2], 1.138 [3], and 1.208

[4]) were developed considering gigawatt-scale (i.e., large) light water reactors (LLWRs). These RGs provide guidance for defining site-specific performance-based seismic design basis ground motions, characterizing sites, and investigating near-surface materials & soils for NPPs. However, new NPPs may differ significantly from LLWRs in reactor technology, power output, quantity of radiological materials on site, and potential consequences in the event of failure. Characteristics of these new NPPs can make them less vulnerable to seismic events. For example, they may utilize passive systems to operate, which do not rely on ac power or operator actions to shut down processes during or following an earthquake, and/or they may use fuel and coolant forms which limit radionuclide dispersion in the event of an extreme accident. Since these design measures reduce the likelihood and/or severity of a radioactive release, a less intensive site characterization program may provide the necessary level of regulatory assurance.

Regulations currently governing geologic and seismic siting criteria (10 CFR 100.23) specify requirements but do not prescribe a methodology for gathering seismic hazard data or developing a site investigation program. Therefore, judgment is required to define the scope of the site investigation given specific site and plant characteristics. It may not be necessary or appropriate for all NPPs to meet the full scope of the guidance in NRC RG 1.132, 1.138, and 1.208 to meet the requirements of 10 CFR 100.23. A graded approach can be established with a combination of RG clarifications and revisions. For example, NRC RG 1.132 acknowledges the attractiveness of tailoring site investigation programs to specific site conditions based on professional judgement. Additionally, the NRC has been considering updating NRC RG 1.208 for several years, with one of the primary drivers being the need for guidance on small modular reactor (SMR) applications. The NRC has identified other necessary updates [5] to RG 1.208, including providing guidance on selecting the appropriate site response analysis approach for a site.

The NRC staff has acknowledged the merits of a graded approach to site characterization with screening based on a combination of margin and failure consequence (e.g., Nth-of-a-Kind (NOAK) Micro-Reactor Licensing and Deployment Considerations, ML24268A317 [6], ML24355A104 [7], and notably SECY 0052 (ML24309A260) [8]). SECY-25-0052 establishes key concepts of a graded approach to seismic site characterization. Within that context, NRC has also expressed an openness to Senior Seismic Hazard Analysis Committee (SSHAC) Level 1 seismic hazard analyses commensurate with the risk profile and seismic design categories of new and advanced reactors [9], but has not specified entry criteria for the use of SSHAC Level 1.

NEI proposed an alternative methodology for site geo-characterization in the July 31, 2024, Proposal Paper 'Regulation of Rapid High-Volume Deployable Reactors in Remote Applications (RHDRA) and Other Advanced Reactors' and the supplement dated July 14, 2025 [29]. The detailed methodology

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established in this paper incorporates the approach proposed therein. Specifically, the proposed framework would permit many facilities with site boundary emergency planning zones to establish the site's Safe Shutdown Earthquake (SSE) based on the United States Geological Survey (USGS) National Seismic Hazard Model (NSHM), with minimal collection of site-specific surface and subsurface geophysical and geotechnical information.

APPROACH This technical report describes a graded approach for site seismic characterization of new NPPs. The approach can be applied to any NPP, including SMRs, microreactors, new Light Water Reactors (LWRs),

Advanced Reactors (ARs), and LLWRs. The approach employs a screening step to determine the level of refinement necessary for seismic site characterization in a specific licensing application. The screening step jointly addresses the importance of seismic hazard to a plant design, as well as the relative dose consequence of postulated accidents. Screening outcomes are described in a series of gradation tiers, with specific seismic site characterization requirements defined for each tier.

The goal of site characterization is to provide information for enabling reasonable assurance of adequate protection from a seismic event. Reasonable assurance is dependent on postulated accident consequences, the magnitude of the seismic hazard, the available seismic margin, and the degree of confidence in the site characterization and margin estimation. The graded approach provides guidance to ensure that the level of rigor required for site characterization provides the necessary reasonable assurance, given that adequate protection is a function of the potential consequences. Existing site characterization guidance provides reasonable assurance based on the consequences associated with a seismically-initiated release from an LLWR, but this may not be necessary for reactors with lower consequences or whose siting or seismic design margin make a seismically initiated release unlikely. By providing guidance on achieving reasonable assurance for new NPPs, this report helps applicants mitigate project risk during the licensing process. If the requirements for site characterization are better defined, it will be easier to ensure these requirements are met and avoid rejection of the license.

Additionally, the guidance will help applicants compare and contrast a portfolio of sites by providing an expedient estimate of resources required for site characterization.

The approach in this report builds upon concepts introduced in other studies, such as SECY-25-0052 and the RHDRA Proposal Paper. SECY-25-0052 discusses how a graded approach can help standardize programs, simplify regulatory reviews, and expedite licensing timeframes. The approach also considers recent NRC investigation and exploration of SSHAC Level 1 studies for licensing of new NPPs (e.g.,

ML25237A182 [9]). In this context, three primary objectives guided the development of the graded approach described here.

First, the approach seeks to provide clarity on site characterization requirements for new NPPs. It utilizes preliminary estimates of seismic hazard and margin at a given site to define the scope of site characterization. The intent is to provide objective guidance and avoid subjective judgments, reducing the risk of a rejected application or protracted review period. Since seismic site characterization occurs early in the overall licensing process, prescriptive guidance will help provide confidence in the acceptability of seismic characterization, allowing for the advancement of plant design.

Second, the approach seeks to specify refinement where such refinement most influences plant seismic safety, but not where reasonable confidence of seismic safety exists even without such refinement. For example, it is understood that soil can amplify or deamplify seismic waves as they travel to the ground

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surface, a phenomenon referred to as site effects. Site effects can have a significant impact on the definition of potential ground motion inputs, and the amount of effort devoted to characterizing the uncertainty associated with this behavior is highly variable. Therefore, particular emphasis is placed on providing a graded approach to characterizing site effects in the screening process, so the applicant can choose to expend resources on reducing uncertainty or accommodating higher input ground motions.

Finally, the approach seeks to leverage existing and easily accessible sources of information in the screening step to the maximum extent practical. This objective stems from the desire to quickly ascertain the viability of various candidate sites, based in part on the level of seismic site characterization that would be required. In other words, applicants cannot always wait for the result of a detailed seismic site characterization program to determine the relative importance of seismic conditions for the application. Leveraging a graded approach is most helpful when there is relatively low effort involved in identifying the appropriate gradation tier. The use of existing data for seismic siting work is consistent with the provisions of the ADVANCE Act and Section 3.0 of RG 1.132.

DEFINITIONS Consequence Index A value used to capture the consequences of a potential radiological release from a facility. Severity of the consequences decrease from high (1) to low (4) and are based on Design Basis Accident (DBA) Dose, Plume Exposure Pathway (PEP) Emergency Planning Zone (EPZ), or unmitigated dose.

Confidence Characterized as High, Medium, or Low to reflect the level of certainty in the Scoping Seismic Demand and Target Seismic Capacity. It depends on the availability of existing information and the maturity of the design.

Gradation Tier A value, A through E, mapped from the Seismic and Consequence Indices. Higher Seismic and Consequence Indices map to Gradation Tiers requiring less refined/complex analysis and investigation; lower Indices map to Gradation Tiers with more rigorous/detailed requirements.

Graded Approach The seismic hazard analysis and site investigation activities that an applicant performs for licensing their facility, which are more or less rigorous depending on the Gradation Tier determined during screening.

Hazard A measure describing the estimated level of ground shaking at a site due to potential future earthquake activity. Seismic hazard is characterized by seismic sources in the region and the attenuation of seismic waves as they travel from the source to the site.

Screening Rapid assessment using a standardized process to evaluate facility designs and sites based on their potential seismic vulnerability and hazard. It is a preliminary step that results in a Gradation Tier, which, in turn, defines the scope of the seismic hazard and site investigation activities for licensing.

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Seismic Screening Margin The minimum frequency-dependent ratio of Target Seismic Capacity to Scoping Seismic Demand. Higher ratios (greater margin) correspond to higher Seismic Indices.

Scoping Seismic Demand The response spectrum used for the initial screening assessment to represent potential ground shaking at the site. It is developed from existing and readily available information for the site, including a preliminary estimate of the sites seismic hazard according to the USGS NSHM.

Seismic Index A value used to represent the influence of seismic hazard on plant design for a given site. It is based on Hazard, Seismic Screening Margin, and Confidence. A higher Seismic Index (4) represents a lower importance of seismic hazard on overall plant design, while a lower Seismic Index (1) represents a significant importance.

Seismic Site Characterization The seismologic, geologic, geotechnical, and geophysical properties of the site that inform the interpretation of the seismic hazard and the seismic design of the proposed facility.

Target Seismic Capacity A horizontal ground motion spectrum defined in the preliminary plant design criteria representing the minimum level for which certain structures, systems, and components will be designed to remain functional.

SCREENING FRAMEWORK FOR GRADED SEISMIC HAZARD AND SITE CHARACTERIZATION The graded approach described herein establishes a framework for seismic site characterization, including a screening/scoping assessment to identify the appropriate Gradation Tier for the facility/site, followed by data collection for licensing, which varies in level of rigor depending on the Gradation Tier.

Figure 1 presents the four main steps of the framework. The framework is not iterative. The steps are summarized below and detailed in the remaining sections:

1. Seismic Index (Box 1 of Figure 1; Section 5): Rapidly determine a Seismic Index used for screening based on readily available information. This effort is intended to reflect normal due diligence when selecting a site for a new facility. The Seismic Index reflects the importance seismic hazard will have for plant design for a given site and the availability of existing information. It is selected with an estimation of seismic Hazard, Seismic Screening Margin, and Confidence. Hazard is a metric intended to capture the relative severity of potential ground shaking across the United States. If the hazard is low, less effort should be required to characterize ground shaking and the impact on overall design. Seismic Screening Margin is the ratio between the plants Target Seismic Capacity and Scoping Seismic Demand. Confidence reflects the applicant's understanding of the Seismic Screening Margin based on existing data and design maturity. The applicant can choose to increase the Seismic Index by reducing the hazard (e.g., selecting a different site), increasing the Seismic Screening Margin (e.g., increasing plant seismic capacity), or increasing Confidence (e.g., gathering additional data).

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2. Consequence Index (Box 2 of Figure 1; Section 6): Determine a Consequence Index reflecting the significance of seismically induced accidents for a given facility design;. The Consequence Index requires that some form of plant safety analysis has been performed, including quantification of potential dose.

3. Gradation Tier (Box 3 of Figure 1; Section 7): Identify the appropriate screening-level Gradation Tier based jointly on the Seismic Index and Consequence Index. Higher Seismic Index and Consequence Index trends to lower Gradation Tier, and vice versa.

4. Graded Approach (Box 4 of Figure 1; Section 8): Characterize the appropriate approach and details for site investigation and seismic hazard analysis methodologies, for the applicable Gradation Tier. Each Gradation Tier defines the specific minimum seismic site characterization requirements that must be performed for a licensing application of that plant at that site.

Figure 1: Framework for Graded Seismic Hazard and Site Characterization SEISMIC INDEX The Seismic Index captures the importance seismic hazard will have for plant design for a given site and the availability of existing information. The Seismic Index is assigned a value of one through four. A value of 1 represents a combination of site, design, and knowledge state where seismic hazard could challenge Seismic Screening Margin, and therefore stricter site characterization is appropriate. A Seismic Index of

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4 represents conditions where seismic hazard is very unlikely to challenge a design for a specified site and therefore, limited additional site characterization is required. It is based on three characteristics:

Hazard, Seismic Screening Margin, and Confidence, which are, in turn, based on the Target Seismic Capacity and the Scoping Seismic Demand. The Target Seismic Capacity depends on the facility design, and the Scoping Seismic Demand depends on the site of interest. The Hazard, Seismic Screening Margin, and Confidence are each designated as High (H), Medium (M), or Low (L), and these designations are used to map to the corresponding Seismic Index. Some Seismic Index examples are illustrated in Section 9.1.

5.1 Hazard Preliminary estimates of the seismic hazard at a site are defined according to the USGS NSHM and are conveniently characterized using terminology used in the International Building Code (IBC) via the ASCE 7 structural design loads and criteria standard referenced therein. The Hazard parameter reflects the maximum-direction amplitude of the ASCE 7 site-adjusted risk-targeted maximum considered earthquake, MCER. Higher amplitudes relate to lower Seismic Indices. The Hazard parameter is computed based on response spectral acceleration parameters SMS and SM1 corresponding to MCER shaking. The Hazard parameter is designated H, M, or L according to the thresholds shown in Table 1Error! Reference source not found.1. The higher designation between SMS and SM1 is to be used for the Hazard parameter. Including some characterization of the amplitude of the site hazard in determining Seismic Index allows consideration for the site independent of the facility to be built there.

Table 1: Hazard Parameter Thresholds Hazard Parameter (use the highest)

SMS Threshold SM1 Threshold H

SMS > 0.50 SM1 > 0.20 M

0.25 < SMS < 0.50 0.10 < SM1 < 0.20 L

SMS < 0.25 SM1 < 0.10 5.2 Seismic Screening Margin The Seismic Screening Margin parameter represents the amount that the Target Seismic Capacity of the plant exceeds the Scoping Seismic Demand of the site. A higher value of Seismic Screening Margin relates to a higher Seismic Index.

5.2.1 Target Seismic Capacity The Target Seismic Capacity is an established or committed horizontal ground motion spectrum in the plant design criteria for which certain SSCs will be designed to remain functional. It is defined as a site Design Response Spectra (DRS) and is compatible with the standard plant design strategy typically 1 For convenience and precedence, the threshold values are conservatively based on those used in IBC to define the seismic design category of critical structures. More specifically, the threshold values for SMS and SM1 in Table 1 are 1.5x the two lowest thresholds of SDS and SD1 Risk Category IV values in ASCE 7-22 Table 11.6-1 (SDS: 0.167 and 0.33) and Table 11.6-2 (SD1: 0.067 and 0.133) since MCER is 1.5x the ASCE 7-22 Design Response Spectrum corresponding to SDS and SD1.

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pursued today, where an applicant endeavors to develop a standard plant design with sufficient margin such that it can be located at a range of potential sites with minimal design changes. In a downstream step, the DRS is compared to the MCER for computing the Seismic Screening Margin, so the two spectra must be compatible (e.g., define the DRS as a maximum direction spectrum). Examples of Target Seismic Capacities from different licensing approaches could include:

1. A standardized spectral shape scaled to a user-selected peak ground acceleration (PGA) or other anchor point, such as NRC RG 1.60 [10].

2. A custom spectral shape developed from enveloping studies or similar, such as a user-defined Generic Design Response Spectrum (GDRS) selected to bound the expected seismic hazard across one or more targeted sites.

3. A design response spectrum for an approved Design Certification, Manufacturing License, or Standard Design Approval under 10 CFR Part 52 or another multi-site licensing approach, such as a Certified Seismic Design Response Spectrum (CSDRS).

A site-specific DRS, e.g., a ground motion response spectrum (GMRS), would not be a typical Target Seismic Capacity to use for screening because having a GMRS indicates that the applicant has already performed the downstream hazard characterization for the site. However, if an applicant has a GMRS for Site X and wants to consider placing a corresponding design at Site Y, Site Xs GMRS could be used as the Target Seismic Capacity for Site Ys screening evaluation.

5.2.2 Scoping Seismic Demand The Scoping Seismic Demand is the same MCER spectrum underpinning the Hazard parameter described in Section 5.1. It is a risk-targeted horizontal earthquake response spectrum, meaning that it has an underlying target probability of unacceptable performance associated with exceeding a specific limit state. The MCER is a maximum-direction horizontal spectrum. The Scoping Seismic Demand is developed from existing and readily available information for the site. The MCER is site-adjusted by amplifying base motion according to the applicable Soil Class per ASCE/SEI 7-22 [11] Section 11.4. A user can conveniently obtain the MCER for a specific location and Soil Class via the ASCE Hazard Tool.

In the Scoping Seismic Demand context (i.e., in determining the Seismic Screening Margin), the needed inputs for the ASCE hazard tool are the location (e.g., Lat/Long) and the Soil Class. The outputs are (a) the multi-period MCER from the embedded USGS NSHM, and (b) parameters used to define the hazard amplitude thresholds, namely SMS and SM1.

5.2.3 Seismic Screening Margin The Seismic Screening Margin is the minimum frequency-dependent ratio of Target Seismic Capacity to a compatible Scoping Seismic Demand. Higher ratios imply larger margin in seismic design and correspond to higher Seismic Indices. Three such ratios are calculated, each representing the governing ratio across a subset of frequencies, as illustrated in Figure 2.

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Primary Margin parameter, MP: frequency range 1 Hz to 10 Hz2 Secondary Margin parameters:

o Low Frequency Margin, MLF: frequency range < 1 Hz o High Frequency Margin, MHF: frequency range > 10 Hz Figure 2: Seismic Index Margin Parameter Unless unique circumstances necessitate consideration of other frequency ranges (described later in this section), the Design Margin parameter is based on the value of MP as compared to the thresholds below.

If MP is greater than 1.87, Seismic Screening Margin is characterized as High (H).

If MP is between 1.5 and 1.87, Seismic Screening Margin is characterized as Moderate (M).

If MP is between 1.25 and 1.5, Seismic Screening Margin is characterized as Low (L).

If MP is less than 1.25, the selected combination of Target Seismic Capacity and Scoping Seismic Demand is deemed not suitable for use in the graded approach to seismic hazard analysis and site investigations.

2 The use of the frequency range of 1 to 10 Hz for screening is consistent with precedent from the seismic hazard screening process of the EPRI SPID [15], wherein plant capacity was compared to site seismic hazard across the frequency range of 1 to 10 Hz to evaluate the extent of seismic re-evaluation necessary for a given plant to resolve the 50.54(f) information request under the post-Fukushima Near Term Task Force (NTTF) Recommendation 2.1: Seismic.

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The MP threshold increments are based on those used in ASCE/SEI 7-22 [11] for seismic importance factors assigned across different risk categories. Additional discussion of the thresholds is provided in Section 5.5.

If an applicant finds the Seismic Screening Margin or the resulting Seismic Index to be unfavorable, they can increase it by raising the Target Seismic Capacity, improving Confidence, such as via additional site investigation refinement (see Section 5.3), or considering alternative sites. Examples for raising the Target Seismic Capacity include seismically hardening SSCs or incorporating earthquake protection technologies such as seismic base isolation.

Under certain unique considerations, MLF or MHF can affect the Seismic Screening Margin parameter as noted below.

If MP characterizes Seismic Screening Margin as L, the user moves on to the screening process with that characterization.

If MP characterizes Seismic Screening Margin as H or M, the user must evaluate potential reductions/penalties in the Seismic Screening Margin parameter to explicitly evaluate high-frequency or low-frequency exceedances (MHF or MLF < 1.0).

If both MHF and MLF are greater than 1.0 (indicating no exceedances), then the Seismic Screening Margin parameter for the Seismic Index is based on MP.

If MHF < 1.0 (high-frequency exceedance), then EPRI guidance (e.g., EPRI (2015) [13], (2017)

[14], and others) can be used to evaluate potential high-frequency sensitivities. If that evaluation suggests that high-frequency sensitivities are significant, then the user assigns L as the Seismic Screening Margin.

If MLF < 1.0 (low-frequency exceedance), then important low-frequency response behaviors should be identified (e.g., very soft soil, seismic isolation). If that evaluation suggests that low-frequency response behaviors are important, then the user assigns L as the Seismic Screening Margin.

5.3 Confidence The selection of a Confidence parameter captures the confidence the applicant has in their screening-level Target Seismic Capacity and Scoping Seismic Demand parameters, given the availability of existing information and design maturity. Low, Medium, and High Confidence bins are defined, with High providing the greatest assurance that the Target Seismic Capacity and Scoping Seismic Demand have been adequately characterized. Judgment is required to select a Confidence bin, but general guidelines are provided below.

High:

A site-specific site response analysis has been performed to characterize site effects.

Sufficient geotechnical data is available to support the selection of an initial foundation type and approximate depth for characterizing effective seismic input to the system.

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Project is utilizing an existing plant design with clearly established seismic capacity.

Medium:

A Site Class is assigned per ASCE 7 with the use of measured shear wave velocity data (geophysics). Site effects are accounted for in accordance with Section 11.4 of the standard.

Subsurface information is available in the vicinity of the site in conditions that are deemed representative of the proposed plant location.

Design is preliminary, and the Target Seismic Capacity is considered to be achievable.

Low:

A Site Class is assigned per ASCE 7 with the use of shear wave velocity estimates derived from correlations with geotechnical parameters (e.g. standard penetration test blow counts). Site effects are accounted for in accordance with Section 11.4 of the standard.

Geologic/geotechnical characteristics of the site are largely derived from regional data or nearby sites.

Design is conceptual and minimal work has been performed to establish that the Target Seismic Capacity is feasible.

5.4 Seismic Index Together, the combination of Hazard, Seismic Screening Margin, and Confidence parameters map to the Seismic Index as shown in Figure 3. Seismic Indices are a value of one through four, where one represents high seismic importance. For all levels of Confidence, High Hazard with Low Seismic Screening Margin maps to Seismic Index of 1, and Low Hazard with High Seismic Screening Margin maps to Seismic Index of 4. For intermediate combinations of Hazard and Seismic Screening Margin, the mapping to Seismic Index for each Confidence level has been defined based on engineering judgement informed by example application of the screening methodology (see Section 9). In general, Seismic Screening Margin is given more weight than Hazard, and Confidence is more important where Seismic Screening Margin is Low and/or Hazard is High.

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Figure 3: Mapping of Seismic Screening Margin, Hazard, and Confidence Parameters to Seismic Index 5.5 Justification and Basis for Seismic Index This section provides technical justification for the approach defined in the preceding Seismic Index subsections.

Hazard ASCE 7-22 uses Seismic Design Categories (SDCs) to implement a graded approach between simplified design and construction procedures and minimum requirements, and more sophisticated and costly requirements. ASCE 7 SDCs account for both the level of seismic hazard and the consequences of failure.

Analogously, the Seismic Index and Consequence Index are used in the screening approach, herein. For the Seismic Index contribution, the Hazard parameter addresses the level of seismic hazard. Therefore, the hazard thresholds are equivalent to the thresholds between the different SDCs in ASCE 7 when translated to MCER instead of DRS.

In addition to linking the hazard thresholds to those of the ASCE 7 SDCs, nuclear industry guidance was also considered in determining the thresholds. Specifically, the EPRI SPID [15], used during post-Fukushima Seismic Reevaluations in the U.S. Nuclear industry, compared site-specific GMRS to the design basis SSE. In the process, EPRI assigned a Low Hazard Threshold (LHT) to sites where the GMRS does not exceed 0.4g in the 1 to 10 Hz range. This 0.4g threshold is higher than the threshold for L Hazard herein, meaning that the current approach is conservatively biased compared to the precedence, since more sites would be classified in EPRIs LHT than would be assigned a Hazard parameter of L for screening.

In many cases, SMS (or SM1) may not capture the peak spectral acceleration of the ASCE 7-22 Multi-Period Spectrum, which is used herein. In these cases, the peak of the spectra could conservatively be considered in addition to SMS with the same thresholds. However, doing so would introduce considerable conservatism because high-frequency motions are not as significant to SSC response as are lower-frequency motions with higher energy content. Therefore, the peak of the spectra is not recommended to be used in determining the Hazard parameter.

Target Seismic Capacity

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To consider a range of factors contributing to site seismicity and facility sensitivity, the Target Seismic Capacity is not defined as an earthquake magnitude or intensity but as a horizontal design spectrum.

Horizontal excitation is typically both (a) used as a reference to define vertical seismic excitation (i.e., via commonly used V/H ratio models) and (b) more indicative of damage potential than is vertical excitation, so the Seismic Index considers only the horizontal ground motion component. Defining the Target Seismic Capacity as a free-field, near-surface spectrum analogous to the GMRS site horizon at the elevation of the highest competent material3 enables direct comparison to the sites Scoping Seismic Demand.

Scoping Seismic Demand Using ASCE/SEI 7-22 [11] to define the Seismic Index enables appropriately streamlining the process by leveraging existing data and methods. The ASCE 7 framework provides an established, consistent, and repeatable process to access and build upon the USGS NSHM. Use of ASCE 7 in defining the Seismic Index does not intend to imply adequacy for its use in the design of nuclear facilities. However, ASCE 7 is the standard invoked by model building codes, including for critical facilities. The seismic provisions therein are based on a rigorous consensus process guided by NEHRP. Both ASCE 7 and the USGS NSHM are regularly updated to account for new information and improved methods.

ASCE 7 Section 11.4 provides the process and definition of a risk-targeted horizontal earthquake response spectra for the site, defined as the risk-targeted maximum considered earthquake (MCER).

Using a risk-targeted response spectrum rather than a uniform hazard spectrum for defining the Seismic Index is appropriate, as both nuclear and non-nuclear industries have adopted uniform risk principles for developing seismic criteria. Using risk-targeted spectra improves structural performance consistency across diverse regions (e.g., Western U.S. (WUS) vs Central and Eastern U.S. (CEUS)). The MCER is defined to target 1% probability of unacceptable performance (e.g., collapse) over a span of 50 years for typical structures (i.e., RC II), and lower for more critical facilities or those containing hazardous materials (i.e., RC III and RC IV).

A reasonable alternative to using the MCER to define the Seismic Index could have been to use the ASCE 7 Design Response Spectrum (DRS). However, using the DRS would also require different Seismic Screening Margin and Hazard binning thresholds. An advantage of using the MCER is that it provides direct and convenient compatibility with target reliabilities reported in ASCE 7. For example, Risk Category IV structures designed according to ASCE 7 achieve an implied conditional probability of failure at MCER shaking of ~2.5%.

Per ASCE 7 Sections 11.4.5 and 11.4.6, two alternative approaches to developing the MCER are available:

1.5 times the Multi-Period DRS or 1.5 times the Two-Period DRS. The Seismic Index should be defined consistently with the guidance of ASCE 7-22 Section 11.4.5, in which, per Exception 2, a two-period response spectrum is suitable only where a multi-period spectral shape is not already available from the USGS database. The multi-period spectral shape is more representative of the expected frequency content of a sites seismic hazard and is therefore the more appropriate comparison to Target Seismic Capacity.

Since the intent is to leverage existing data, an applicant can use a site-specific Probabilistic Seismic Hazard (PSHA) if one exists. ASCE 7 Section 11.4.7 references Section 21.2 and allows using site-specific 3 Per RG 1.208 [4], Section 5.3: Although the definition of competent material is not mandated by regulation, a number of reactor designs have specified a shear wave velocity of 1,000 fps as the definition of competent material.

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information in developing the MCER. Similarly, more detailed site information than the Soil Class may be used, if available. For example, SRA could be leveraged in the site-specific ground motion procedures to develop MCER per ASCE 7 11.4.7 and 21.1, either in combination with or separate from an existing PSHA.

Defining the MCER using PSHA information and/or SRA would allow an applicant to benefit from using site-specific information while ensuring a consistent framework for developing the Scoping Seismic Demand.

Seismic Screening Margin Using a frequency range of 1-10 Hz as the Primary Margin parameter to evaluate the level of seismic margin and screen plant-level seismic capacity versus estimated seismic hazard has precedence in the EPRI SPID [15], which was used to resolve post-Fukushima NTTF 2.1 seismic hazard evaluations of the operating commercial reactor fleet. High-frequency exceedances need not be considered directly when characterizing the Seismic Screening Margin parameter based on earthquake experience, which indicates that high-frequency ground motions are less damaging to SSCs than seismic events where the lower frequency range has higher energy content (e.g., DC/COL-ISG-01 [16], EPRI 3002004396 [13]).

The Seismic Screening Margin thresholds are assigned with several considerations in mind:

The assigned thresholds retain a nearly equivalent performance expectation as the first (typically governing) performance level of the Design Basis Earthquake definition in ASCE 43-19

[12], which is 1% conditional probability of unacceptable performance at the design ground motion level. The corresponding ASCE 7 reliability target for Risk Category IV structures is a 2.5%

conditional probability of failure at the MCER level. The near equivalency is enforced by defining Seismic Screening Margin thresholds based on applying a factor of 1.25 to the ASCE 7 SDC threshold values. The value of 1.25 is computed as follows.

o A2.5% is the ground motion acceleration causing a 2.5% conditional probability of failure o A1% is the ground motion acceleration causing a 1% conditional probability of failure o Z2.5% = -1.96 & Z1% = -2.33 are the corresponding Z-scores o = 0.6 is the logarithmic standard deviation used in ASCE 7 (e.g., Section C1.3.1.3) o 1% = 2.5%

1%

2.5%

o 1.25 =

2.5%

1%

Therefore, using 1.25 (vs. 1.0) for the L threshold ensures that the minimum value is calibrated so that the capacity is sufficient to achieve no more than about 1% conditional probability of failure at the Scoping Seismic Demand level hazard.

This approach is consistent with the DBE performance target in ASCE 43-19 [12] and NRC RG 1.208 [4]. Thresholds for M and H are based on multiplication increments of 1.25, analogous to those for elevated Risk Categories in ASCE 7 [11].

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The threshold of 1.87 (=1.5x1.25) for High Seismic Screening Margin is analogous to an elevated importance factor increment and targets being sufficiently high to account for potential differences between the USGS NSHM (regional model with broad characterization of site effects) and seismic hazard estimates generated from site PSHA studies (site-specific hazard including site effects). The benefit of having High Seismic Screening Margin accounts for the fact that the USGS Hazard may be higher or lower than the Hazard determined from a site-specific evaluation; if the USGS hazard is lower, having sufficient margin helps avoid the repercussions of underestimating the Hazard during the screening process. This approach to Seismic Screening Margin threshold accommodates the 10 CFR 100.23 requirement for treatment of uncertainty via sensitivity analysis: higher Seismic Screening Margin allows higher Seismic Index because it is less sensitive to inherent uncertainty in the Hazard.

CONSEQUENCE INDEX The Consequence Index intends to characterize, on a relative scale, the potential radiological release consequence caused by a seismic-induced failure. The Consequence Index is assigned a number 1 through 4 as a function of consequence-based accident analysis. Consequence Index 4 represents the highest margin against minimum thresholds (lowest consequence), and Consequence Index 1 represents the least margin against minimum thresholds (highest consequence). The intent is to determine a Consequence Index by leveraging an applicants existing safety analysis strategy that they are utilizing elsewhere in the license application, such as for other risk-informed licensing purposes, to avoid the need for additional safety analyses beyond those the applicant is already pursuing. Therefore, three options are available for assigning the Consequence Index to accommodate different safety analysis strategies, with selection at the applicants discretion: Design Basis Accident (DBA) Dose, Plume Exposure Pathway (PEP) Emergency Planning Zone (EPZ), and Unmitigated Dose.

The DBA dose approach considers the release consequence while crediting the functional performance of certain SSCs. This approach is aligned with the conceptual framework in SECY-25-0052 [8]. This approach may be attractive for inherently safe facilities that credit relatively few SSCs in their safety analysis.

The PEP EPZ approach considers the outcome of risk-informed sizing of an emergency planning zone based on the consequence measures inherent to that process. This approach may be attractive to applicants pursuing the strategy laid out in NEI 24-05, An Approach for Risk-Informed Performance-Based Emergency Planning (ML24184C122) [17].

The unmitigated dose metric approach considers potential release with only minimal credit for engineered safety functions. This approach is analogous to that in ANS 2.26 [18]. This approach may be most applicable to facilities with a small radiological inventory, where the source terms are of minimal concern regarding public health and safety.

The Consequence Index concept draws from the dose margin concept described in SECY-25-0052 [19],

where the NRC staff proposed that Exclusion Area Boundary (EAB) dose margin be calculated based on the margin Between 25 rem and the DBA Dose at the EAB with a footnote that assumes safety-related SSCs [Structures, Systems and Components] are designed to DBEHLs [Design Basis External

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Hazard Levels]. 4 The Consequence Index described herein provides further clarity and flexibility in how dose can be computed using existing established procedures, with each option necessarily having different thresholds and varying credit for functional performance of SSCs consistent with those existing procedures. Additionally, where SECY-25-0052 proposed three levels, we defined the Consequence Index using four categories for increased granularity and better alignment with gradation in the existing procedures being leveraged (e.g., ANS 2.26 [18] and 10 CFR 50.160 [20]).

Table 2 belowError! Reference source not found. provides the Consequence Index levels. More details on the implementation of the three approaches are provided in Sections 6.1 to 6.3.

Table 2: Consequence Index CI DBA Dose*

PEP EPZ**

Unmitigated Dose***

4

<0.1 rem TEDE No PEP EPZ

< 0.25 Sv (25 rem TEDE) 3 0.1 - 1 rem TEDE EPZ at Site Boundary 0.25 - 1 Sv (25 - 100 rem TEDE) 2 1 - 10 rem TEDE EPZ outside of Site Boundary but less than 10 mi 1 - 5 Sv (100 - 500 rem TEDE) 1 10 - 25 rem TEDE EPZ site at 10 miles (traditional)

> 5 Sv (500 rem TEDE)

Total Effective Dose Equivalent (TEDE) at EAB over two 2-hour periods following the onset of the postulated fission product release for bounding DBA with credited SSCs to have reasonable confidence of having seismic capacity at least that considered in Seismic Index

- Outcome of safety analysis used to define graded plume exposure pathway EPZ size (10 CFR 50.160),

or CI-1 for traditional site EPZ sizing

- - TEDE at EAB over 4 days, consistent with ANS 2.26 6.1 DBA Dose Margin as Consequence Index Using the DBA dose approach relies on the definition of DBA Dose at the EAB. We interpret the following statement to mean the criteria in 10 CFR 50.34(a)(1)(ii)(D)(1) is assessed for the bounding DBA dose analyzed in the licensing basis: An individual located at any point on the boundary of the exclusion area for any 2 hour2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months period following the onset of the postulated fission product release, would not receive a radiation dose in excess of 25 rem total effective dose equivalent (TEDE). We interpret the margin criteria to not address beyond design basis events. It should be noted that methodologies for assessing DBAs may evolve over time, and new methodologies may become acceptable to the NRC. The examples below should not be considered exhaustive, and any methodology for developing a 4 DBEHL is used in SECY-25-0052, but NEI 21-07 provides basis for using the term Design Basis Hazard Level (DBHL) instead. An applicant could define their design basis seismic hazard as a DBEHL, DBHL, Ground Motion Response Spectrum (GMRS), Design Basis Ground Motion (DBGM) or something else depending on their licensing approach.

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comprehensive set of DBAs found acceptable by the NRC should be able to support Consequence Index development.

Traditional Standard Review Plan (SRP) DBAs DBA Dose could be defined via a number of methodologies. Traditionally, light-water reactor DBA guidance is provided in the Standard Review Plan (SRP) [21] and is one acceptable method of determining a set of DBAs to be assessed for consideration of a Consequence Index. Similarly, approved deviations from the SRP such as those approved for the NuScale Standard Design Approval [22] or future SRP-informed applications, should be acceptable. For those following an approach more in line with the SRP, the term DBEHL may not be defined since it was introduced in NEI 18-04 [23], but a standard design could have an analogous Certified Seismic Design Response Spectra (CSDRS). Regardless of the term used, the DBEHL or CSDRS for the SR SSCs credited in the DBA analysis would require verification as bounding the site parameters specified in accordance with Section 8 for the design basis ground motion.

Licensing Modernization Project (LMP) DBAs The Licensing Modernization Project (LMP) introduced additional flexibility into the licensing basis by leveraging a probabilistic risk assessment process early in the development of the licensing basis. NEI 18-04 defined a methodology for establishing a sufficient set of design-basis accidents that was endorsed in RG 1.233 [24]. In DANU-ISG-2022-01 [25] NRC provided interim staff guidance that exempts from some of the fission product release requirements of 10 CFR 50.34(a)(1)(ii)(D) and the SR definition in 50.2, 50.49(b) and 10 CFR 50 Appendix S. The exemption from 10 CFR 50.34(a)(1)(ii)(D) removes the requirement to postulate a fission product release from a core damage event and assessment of containment performance, and instead allows the assessment of DBAs derived through the process established by NEI 18-04. That process is sufficient for establishing a set of DBAs to assess in the context of establishing a consequence index.

For the performance of SSCs credited during the design basis accident, the anticipated exemptions from 10 CFR 50 Appendix S should define the specific SSCs credited for performance during the seismic DBHL determined through the NEI 18-04 process. NEI 18-04 guidance for establishing DBEHLs is copied below:

A set of Design Basis External Hazard Levels (DBEHLs) will be selected to form an important part of the design and licensing basis. This will determine the design basis seismic events and other external events that the SR SSCs will be required to withstand. When supported by available methods, data, design, site information, and supporting guides and standards, these DBEHLs will be informed by a probabilistic external hazards analysis and will be included in the PRA after the design features that are incorporated to withstand these hazards are defined. Other external hazards not supported by a probabilistic hazard analysis will be covered by DBEHLs that are determined using traditional deterministic methods.

The guidance in Section 8 provides guidance for how the seismic DBHL would be determined. SSCs credited in the DBA would require verification as having capacity bounding that seismic DBEHL.

Maximum Hypothetical Accident DBAs While not currently endorsed by NRC for commercial reactors, NUREG-1537 provides guidance for research and test reactors to establish a Maximum Hypothetical Accident (MHA). Appendix 12 of the RHDRA proposal proposed that Applicants could analyze a hypothetical maximum accident (or a small

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set of bounding accidents) such that the application does not have to consider all external hazards explicitly. This approach would provide reasonable assurance of adequate protection of public health and safety even under extreme circumstances postulated under extreme hazard conditions. Reactors with potential dose consequences small enough to never threaten public health and safety should be exempt from Part 100 requirements. Companies like Kairos Power, supported by ClearPath have argued that such a methodology should be acceptable to commercial reactors [26]. NRC has indicated some receptiveness to such an approach in their presentations for a proposed low-consequence reactor rulemaking [27]. INL is working with industry partners to develop such an MHA approach for NRC endorsement, leveraging the guidance in NUREG-1537 [28]. NUREG-1537 states that The applicant should also discuss and analyze postulated accident scenario whose potential consequences are shown to exceed and bound all credible accidents. While not a perfect analogue, bounding all credible accidents could be argued to meet the criteria of assessing all design-basis accidents. It is understood that an MHA-like methodology needs to be developed in detail and approved by the NRC for commercial reactors. But once approved, that methodology should be acceptable for determining a bounding DBA for establishing a consequence index.

As for the requirement that SR SSCs be designed to withstand the DBEHL, in an MHA framework, it is expected that the guidance would align with NUREG-1537, where credited structures and systems are to be seismically designed for the design-basis earthquake as defined therein in accordance with ANSI/ANS 15.7 and cross-referenced IAEA guidance.

These criteria may be modified depending on whether the NRC accepts the ClearPath arguments for commercial NPUFs or endorses a methodology under the INL project for the low-consequence reactor rulemaking. NEI is not taking a position on the ClearPath or INL proposals in this report; we are acknowledging there are multiple pathways by which an MHA methodology may become acceptable to NRC for commercial reactors. Since there is uncertainty, we do not request NRC endorsement of an MHA DBA pathway at this time, we provide this discussion as a placeholder to be addressed in future revisions if an MHA DBA approach is approved by NRC for commercial reactors.

DBA Thresholds SECY-25-0052 has three categories of DBA Dose Margin, but does not provide any proposed thresholds between the them. We define four categories of the Consequence Index to allow for more granular consideration of consequence potential. The DBA doses are tied to traditional safety limits: 0.1 rem is selected from the annual cumulative exposure limits in 10 CFR 20 and aligns with one of the risk metrics from NEI 18-04; 1 rem is tied to the lower limit requiring protective actions per the EPA Protective Action Guide (PAG); 10 rem is based on the following footnote from the EPA PAG: Studies of human populations exposed at low doses are inadequate to demonstrate the actual magnitude of risk at low doses (about 0.1 Sv or 10 rem and below); 25 rem is the limit established in 10 CFR 50 and referenced in the NOAK White Paper.

CI DBA Dose*

4

<0.1 rem TEDE 3

0.1 - 1 rem TEDE

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2 1 - 10 rem TEDE 1

10 - 25 rem TEDE 6.2 Emergency Planning Zone Sizing as Consequence Index The EPZ approach leverages a July 2024 submittal to the NRC [29], wherein NEI proposed that the risk-informed emergency planning zone sizing allowed under 10 CFR 50.160 allows a framework for considering graded regulatory requirements broadly, including site characterization. Justification for such an approach was provided in more detail in a 2025 supplement [30]. Site characterization and EPZ Sizing are two critical decisions that have driven developers to focus on these areas early in the design process. It is appropriate to leverage the EPZ analysis when considering the Consequence Index for graded site characterization.

When assessing Risk Informed (RI) PEP EPZ sizing under 10 CFR 50.160, there are three outcomes: no EPZ, site boundary EPZ, and an EPZ sizing larger than the site boundary. For those choosing not to pursue 10 CFR 50.160, including large LWRs that do not meet the 10 CFR 50.160 entry criteria, the EPZ is traditionally 10 miles, with deviations justified on a case-by-case basis. The Consequence Index gradations align well with the concept from SECY-25-0052.

CI PEP EPZ**

4 No PEP EPZ 3

EPZ at Site Boundary 2

EPZ outside of Site Boundary but less than 10 mi 1

EPZ site at 10 miles (traditional)

To meet the 10 CFR 50.160 criteria for No PEP EPZ, the dose assessment process must determine that the criteria from 10 CFR 50.33(g)(2)(i), copied below, would never be met:

[10 CFR 50.33(g)(2)](i) The plume exposure pathway EPZ is the area within which:

(A) Public dose, as defined in § 20.1003 of this chapter, is projected to exceed 10 mSv (1 rem) total effective dose equivalent over 96 hours0.00111 days 0.0267 hours 1.587302e-4 weeks 3.6528e-5 months from the release of radioactive

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materials from the facility considering accident likelihood and source term, timing of the accident sequence, and meteorology; and (B) Pre-determined, prompt protective measures are necessary.

While there are different methodologies for meeting this criterion, NEI proposed a dose assessment in NEI 24-05 that searches beyond the design basis for events that could potentially exceed the 1 rem threshold. Imposing a 1 rem threshold between CI-3 & CI-4 for the design basis leads to reasonably prescribing CI-4 for plants with limiting doses < 1 rem assessed considering events beyond the design basis.

The next category of plants assessed under 10 CFR 50.160 are those with a site boundary EPZ. When meeting the criteria for a site boundary EPZ, the requirements of 10 CFR 50.160(b)(1)(iv)(B) for offsite emergency planning do not apply. While we acknowledge that the context of emergency planning and seismic hazard characterization/assessment are very different, it is reasonable that for cases where no offsite emergency planning is required, that could be leveraged as an insight for the graded Consequence Index.

Finally, that leaves the categories of CI-2 and CI-1. We define EPZ sizing as the determining factor, with the traditional 10-mile EPZ corresponding to a CI-1. This would result in most LLWRs resulting in a CI-1, but permit case-by-case deviations, allowing for CI-2 based on already approved EPZ assessments.

One complication is that the EPZ sizing analysis gets informed by the seismic hazard at a given site. NEI has proposed a method for addressing seismic hazard in EPZ sizing in NEI White Paper: Selection of a Seismic Scenario for an EPZ Boundary Determination [31]. That methodology assumes that RG 1.208 [4]

was followed in establishing the Ground Motion Response Spectrum (GMRS). In our graded seismic site characterization methodology, we use a USGS-based proxy (e.g., 3 times the MCER) instead of a detailed GMRS developed per RG 1.208 for seismic event selection in EPZ sizing determinations. This would allow for an initial assessment of Consequence Index utilizing the majority of the process from the proposed NEI seismic site event selection for EPZ sizing approach. The use of an MCER-based proxy for EPZ sizing is an approximate assessment due to the additional uncertainties that might be introduced in the seismic hazard based on the graded site characterization approach.

Similarly, the plant-specific C10% capacities assumed in the NEI seismic site event selection for the EPZ sizing approach may not yet be defined when doing a graded seismic site characterization assessment.

The C10% capacities can be estimated based on the same plant-level seismic capacity target used for assessing margin in the Seismic Index assessment (Section 6.3). The C10% capacities then become design requirements for the associated components.

6.3 Unmitigated Accident Release as Consequence Index The Unmitigated Accident Release approach builds on the safety analysis approach used for seismic classification of DOE facilities prescribed in DOE-STD-1020[43]. Under the DOE standard, commercial C&S, such as ASCE 7 apply to SDC-1 and SDC-2 and Design Criteria for Natural Phenomena Hazards (NPH) are described in Table 2-1 with doses below 25 rem resulting in NPH Design Criteria (NDC) 1 or 2. There has been increasing interest in establishing a pathway from initial DOE or DOD approval to subsequent NRC commercial licensing for new reactors, with Executive Order 14300 having the following in Section 5(d).

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Establish an expedited pathway to approve reactor designs that the DOD or the DOE have tested and that have demonstrated the ability to function safely. NRC review of such designs shall focus solely on risks that may arise from new applications permitted by NRC licensure, rather than revisiting risks that have already been addressed in the DOE or DOD processes.

Consequence Index 4 aligns with the Criteria by which NDC-1 or 2 would be prescribed under DOE-STD-1020 which allows the use of ASCE 7 and site investigation similar to the investigation prescribed for gradation tier A.

ANS 2.26 could provide a framework for seismic design categorization that could be leveraged for graded seismic site characterization.

ANS 2.26 development is ongoing, but the proposed Consequence Indices were informed by working drafts.

CI Unmitigated Dose 4

< 0.25 Sv (25 rem TEDE) 3 0.25 - 1 Sv (25 - 100 rem TEDE) 2 1 - 5 Sv (100 - 500 rem TEDE) 1

> 5 Sv (500 rem TEDE)

SDC-1 & 2 are related to commercial practices and therefore seem reasonable to associate with CI-4.

The other categories follow the ANS 2.26 categorizations. Since there is uncertainty in what will be published in the next revision of ANS 2.26, we do not request NRC endorsement of an unmitigated dose pathway at this time. We provide this discussion as a placeholder to be addressed in future revisions if an unmitigated dose approach for seismic design categorization is approved by NRC for commercial reactors.

GRADATION TIERS The matrix for mapping Seismic Index (Section 5) and Consequence Index (Section 6) to Gradation Tiers is shown in Figure 4. The general philosophy is that higher Seismic and Consequence Indices map to Gradation Tiers with less refined/complex analysis and investigation requirements, whereas lower Seismic and Consequence Indices map to Gradation Tiers with more rigorous/detailed requirements.

Tier A is intended for facilities with either a very low consequence or a sufficiently low consequence at a site with low seismic importance. It is intended for a facility and site where commercial standards are adequate. Tier B is for cases in which existing information provides adequate confidence that seismic risk significance is sufficiently low such that one can demonstrate this position with relatively low effort methodologies. On the other end of the gradation tier spectrum, Tier E is for treating facilities at sites where seismic hazard can significantly influence facility design on a case-by-case basis. Tier D is intended

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for high-consequence facilities (e.g., LLWRs) in moderate seismic sites. Tier C represents a level of gradation in seismic importance or consequence of failure between Tiers B and D. The Seismic and Consequence Indices are used concurrently to map to Gradation Tiers, which in turn fully define the level of detail for the Seismic Hazard and Site Investigation activities, with one exception. The exception is the Return Period of Interest for Tiers B and C, which is graded based on the Consequence Index, even within the given Tier. Otherwise, SI and CI are not revisited following identification of the applicable Gradation Tier.

The screening process is designed to enable applicants to quickly identify a Gradation Tier. The process is once-through (vs. iterative) yet flexible to accommodate sites with varying detail of existing information. This screening process thereby enables users to rapidly evaluate Gradation Tier implications of changes to the facility design, site location, or level of site characterization information available to leverage. For example:

A user may use the screening process to determine a minimum Target Seismic Capacity which achieves sufficiently high Seismic Screening Margin to result in a desired SI.

A user evaluating multiple candidate sites for constructing an existing facility design may opt for the site(s) that enable facility construction without having to adjust its seismic design.

A user may evaluate whether it could be worthwhile for SI to improve their confidence in site characterization by collecting more refined site-specific subsurface properties and/or performing site-specific site response analysis.

A user may seek to leverage existing site-specific seismic hazard information to develop a more refined estimate of site hazard for SI.

The gradation tiers are characterized in more detail in Table 3. Section 8 provides specific requirements associated with site characterization and seismic hazard activities within each Gradation Tier. Section 9 steps through several example sites to determine the SI for those sites, considering varying levels of existing information available to the applicant. It then pairs the resulting SI with two example CI values to arrive at a Gradation Tier for each example site/facility combination.

Figure 4: Gradation Tier Mapping through Seismic and Consequence Index

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Table 3: Gradation Tier Definitions (details in Section 8.1)

Tier Description Examples Design Spectra A

Sufficiently low consequence such that commercial standards apply Substantially eases the regulatory burden, enabling rapid completion of related licensing activities for seismic hazard analysis and site investigations Facility with CI 4 Nuclear battery Small inventory Microreactor Facility with SI 4, CI 3 Passively safe plant with a High Seismic Screening Margin and Low Hazard MCER with design codes B

Low consequence coupled with sufficiently low seismic importance to justify only limited seismic hazard analysis Relatively low-effort methodologies (vs. legacy nuclear industry methods) can be utilized since existing information provides confidence of low seismic risk Facility with SI 2, CI 3 Robust Passively safe plant with a High Seismic Screening Margin but more than Low Hazard Facility with SI 4, CI 1 LLWR with a High Seismic Screening Margin and Low Hazard DBE target performance level:

CI AFE (yr-1) 3 1E-4 2

4E-5 1

1E-5 C

Moderate consequence or moderate seismic importance to warrant moderate effort in seismic hazard analysis A level of gradation between Tiers B & D Facility with SI 1, CI 3 Passively safe plant with a Low Seismic Screening Margin and High Hazard Facility with SI 3, CI 1 LLWR with a Moderate Seismic Screening Margin and Hazard D

Sufficiently high consequence and seismic importance to warrant a detailed seismic hazard analysis Facility with SI 2, CI 1 LLWR with a High Seismic Screening Margin but also a High Hazard DBE target performance level:

1E-5/yr

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Tier Description Examples Design Spectra Analogous to traditional nuclear industry practices for high-consequence facilities E

Unique conditions that warrant case-specific considerations For case-by-case treatment of high-consequence facilities in high seismic regions Facility with SI 1, CI 1 LLWR with a Low Seismic Screening Margin and High Hazard GRADED APPROACH 8.1 Specifics for the Tiers Table 4 identifies the scope of the seismic hazard analysis and corresponding site investigation for each Graded Tier. The specific elements of the seismic hazard and site investigation that are addressed include: compilation and review of existing data; geotechnical foundation recommendations; dynamic site profile characterization; seismic source characterization; ground motion characterization; site response analysis; and return period of interest for the seismic design criteria.

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Table 4: Graded Approach - Tiered Scope of Site Investigation Activity Tier A Tier B Tier C Tier D Tier E Compilation and Review of Existing Data Required - Scope dictated by the need to meet the requirements listed below.

Subsurface Characterization -

Foundations Consistent with best practices and commercial building code (e.g, ASCE 7, IBC).

Sufficient boreholes (with depth per Appendix D of RG 1.132) to justify an interpreted characterization of the materials below the footprint and to confirm the geotechnical foundation requirements. The number of boreholes should scale with the footprint of the facility, spaced no less than 100 ft apart with one near the center of the reactor; it is acceptable for facilities with small footprints to demonstrate area uniformity via surface geophysics.

Sufficient boreholes (with depth per Appendix D of RG 1.132) to construct justifiable site cross sections and confirm geotechnical foundation requirements. The number of boreholes should scale with the footprint of the facility, spaced no less than 100 ft apart but no fewer than five total (e.g., one near the center of the reactor and one near or beyond each corner of the facility).

Follow RG 1.132 Appendix D with guidance from Section 4.3 of ANS 2.27.

Dynamic Site Profile Characterization Based on ASCE 7 site class or site-specific analyses using published MRD curves.

Site-specific measured Vs profile from surface geophysics is acceptable. Provide a basis for using published modulus reduction and damping (MRD) curves.

Site-specific measured Vs profile from surface geophysics benchmarked to downhole measurements is acceptable.

Provide a basis for using published MRD curves.

Follow RG 1.132 and 1.208 with guidance from ANS 2.27 and 2.29.

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Activity Tier A Tier B Tier C Tier D Tier E Seismic Source Characterization (SSC)

Based on ASCE 7 with USGS NSHM or ASCE 7 with site-specific PSHA.

SSHAC Level 1 study. It is acceptable to use inputs consistent with USGS NSHM and perform due diligence for including other existing and available data not already in the USGS model.

SSHAC Level 1 or 2 study, depending on the suitability of pre-existing information and data to satisfy the guidance in ANS 2.29 for the SSC Model.

Level 1 study could be only slightly more rigorous than what would be done for Tier B, with due diligence to supplement new investigation of potential additional sources (e.g., aerial photos and/or LiDAR). A Level 2 study would include querying other experts.

SSHAC Level 2 or 3 study, depending on the suitability of pre-existing information and data to satisfy the requirements in RG 1.208 and guidance in ANS 2.29 for the SSC Model.

SSHAC Level 3 study.

Follow RG 1.208 with guidance from ANS 2.29.

Ground Motion Characterization (GMC)

SSHAC Level 1 study using available and applicable GMMs in a logic tree fashion.

SSHAC Level 1 or 2 study, depending on the suitability of pre-existing information and data to satisfy the guidance in ANS 2.29 for the GMC Model.

Level 1 study would be the same as would be done for Tier B. Level 2 study would apply only for unique cases, such as a site on a boundary between existing GMMs (e.g.,

between CEUS and WUS).

SSHAC Level 2 or 3 study, depending on the suitability of pre-existing information and data to satisfy the requirements in RG 1.208, with guidance from ANS 2.29 for the GMC Model.

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Activity Tier A Tier B Tier C Tier D Tier E Site Response Analysis (SRA)

Based on ASCE 7 site class or site-specific analyses (ASCE 7 Chapter 21)

SSHAC Level 1 study. SRA can be decoupled from hazard computation (i.e., post-facto convolution as in Approach 2 of NUREG/CR-6728).

If site-specific Vs measurements show Vs30>1100 m/s (3609 ft/s),

ASCE 7 site class amplification factors may be used. However, the ergodic aleatory variability term must be used in PSHA calculations.

SSHAC Level 1 study with explicit treatment of uncertainty per guidance in ANS 2.29 for the SRA Model.

SRA could be inside or outside of the hazard convolution integral, depending on the complexity of the site with respect to hazard uncertainties, informed by sensitivity analyses.

SSHAC Level 2 or 3 study, depending on the suitability of pre-existing information and data to satisfy the requirements in RG 1.208, with guidance from ANS 2.29 for the SRA Model.

SSHAC Level 3 study.

Follow RG 1.208 with guidance from ANS 2.29.

Return Period of Interest for Seismic Design Criteria5 Default from IBC and ASCE 7 for computing MCER Consequence Index Seismic Performance Target AEF (yr-1)

Implied AEF of UHRS basis for GMRS (or equiv.) (yr-1) 3 1E-4 1E-3 and 1E-4 2

4E-5 4E-4 and 4E-5 1

1E-5 1E-4 and 1E-5 1E-5 / yr.

Implies GMRS based on UHRS at AEF of 1E-4 and 1E-5 / yr.

5 Since the MCER is used to calculate the initial Screening Margin, it is possible that the final DRS may exceed the proposed Target Seismic Capacity. If this is the case, the user is expected to increase the capacity of the final design so that it envelopes the DRS. It is recommended that this be considered as early in the design process as possible. Since a site investigation and/or site-specific PSHA may take time to complete, it is recommended that UHRS at the specified AFEs be retrieved from the USGS Earthquake Hazard Toolbox to get an estimate of the final DRS. However, if the DRS is lower than the proposed Target Seismic Capacity, the user is not expected to lower its design capacity to the DRS.

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8.2 Justification and Basis for the Graded Approach As a whole, the Graded Approach described in this document is intended to be consistent with existing NRC regulations, namely 10 CFR 100.23 [1], and applicable nuclear industry guidance, while giving users clarity regarding how project-specific factors influence the appropriate scope of analysis and investigation. The screening process that results in a specific Gradation Tier assignment is how those project-specific factors provide influence. The detailing of appropriate scope of analysis and investigation within each Gradation Tier provides user clarity on specifically how one can meet the existing regulations and applicable nuclear industry guidance. As noted previously, it may not be necessary for all nuclear power plant projects to meet the full scope of NRC RG 1.132 [2], 1.138 [3], and 1.208 [4] in order to satisfy the requirements of 10 CFR 100.23. Rather, the scope of analysis and investigation in each gradation tier provides higher refinement and rigor where the influence of seismic hazard on public health and safety is highest, while permitting simpler and coarser approaches where the influence of seismic hazard on safety is not as high.

10 CFR 100.23 stipulates that...there is a reasonable assurance that a NPP can be constructed and operated at the proposed site without undue risk to the health and safety of the public. To do so, 10 CFR 100.23 requires, in part, that the site be investigated in sufficient scope and detail to permit an adequate evaluation of the site, [and] to provide sufficient information to support evaluations performed to arrive at estimates of the Safe Shutdown Earthquake Ground Motion. An important and relevant clause from 10 CFR 100.23 is: Uncertainties are inherent in such estimates. These uncertainties must be addressed through an appropriate analysis, such as a PSHA or suitable sensitivity analyses.

Site-specificity and treatment of uncertainty are two key considerations of the 10 CFR 100.23 requirements, and both are addressed here.

First, site-specificity is addressed directly for each Gradation Tier. Assignment of a Gradation Tier explicitly considers the seismic setting of the site (via Hazard Level), the robustness of NPP design (via Seismic Screening Margin), and the potential consequences of hypothetical seismic-induced accidents (via Consequence Index). Each tier includes requirements for a site-specific geotechnical exploration program, including the dynamic characterization of subsurface conditions. Additionally, each tier requires that site effects be considered in defining the design basis ground motion for that site. This focus on local site effects acknowledges the observed influence of local site conditions on site-specific ground motion estimates. Site-specific guidance is also provided for the development of the SSC and GMC models. The intent of the SSC and GMC guidance is that the Hazard Level, which is influenced by the sites location and seismotectonic setting, contributes to the need for more or less rigorous development methods. In summary, site-specificity is addressed in this framework through the selection of an appropriate Gradation Tier and through the definition of a corresponding scope for the Gradation Tier.

Second, treatment of uncertainty is also addressed for each Gradation Tier. The Graded Approach recognizes the importance of uncertainties in site characterization and seismic hazard models. For example, all Gradation Tiers require the use of a PSHA, which characterizes uncertainty associated with the earthquake source (e.g. magnitude, activity, location) and the attenuation of the seismic waves (i.e.

GMC). However, as the Gradation Tier increases from A to E, the rigor of the methodology for performing the PSHA (i.e., SSC and GMC) increases, resulting in enhanced regulatory assurance of the proper characterization of these uncertainties. A similar ideology is reflected in the graded approach for performing SRAs. The resulting site GMRS or DBGM then includes the uncertainty characterization performed for each element in Table 4.

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Importantly, the Graded Approach considers the level of rigor needed for uncertainty characterization in the context of the overall design. For example, the identification of a High Seismic Screening Margin and High Seismic Index implies that the project is relatively insensitive to potential changes in the estimate of Seismic Demand due to a more rigorous characterization of uncertainty in the PSHA or SRA.

Therefore, resources may not be warranted to better characterize uncertainty in the PSHA or SRA. The use of this screening process allows the applicant to determine where they would like to focus their design and investigation resources while still accounting for uncertainty as required by 10 CFR 100.23.

The refinement and rigor within each Gradation Tier is informed by and leverages the different levels of SSHAC hazard studies developed by NRC and described in NUREG-2213 [32] (superseding prior guidance in NUREG/CR-6372 [35] and NUREG-2117 [36]). Consistent with the intent of the SSHAC levels, the fundamental objectives of the seismic hazard evaluation and corresponding seismic site investigation are the same for each Gradation Tier, but with variable level of complexity and cost depending on a series of generic and project-specific factors. The increasing rigor and scope of studies progressing from Gradation Tier A to E results in increased confidence that the Center, Body, and Range of Technically Defensible Interpretations (CBR of the TDI) has been captured in NUREG-2213 [32]. An appropriate assignment of Gradation Tier provides confidence that the specific scope of analysis and investigation is an adequate evaluation of the site to support reasonable assurance that an NPP can be constructed and operated on the proposed site without undue risk to the health and safety of the public.

Gradation Tier A (and the Hazard used for screening to assign Gradation Tier) relies on the latest USGS NSHM, which does not specifically follow SSHAC guidance. The USGS does, however, include a formal evaluation of various types of uncertainty, albeit on a regional scale, and includes these uncertainties in the final NSHM. The NSHM also accounts for the specific location of a site, and its corresponding historical seismicity and probabilistic influence of seismic sources in the region around the site. We recognize that the NSHM may differ from site-specific hazard estimates due to the national scale of the NSHM, but those potential differences are accounted for in the use of the screening process (e.g.,

Gradation Tier A is assigned when significant margin exists between the plants Target Seismic Capacity and the sites Scoping Seismic Demand seismic hazard as estimated via the NSHM). Beyond Tier A, all studies require a site-specific PSHA performed in accordance with the SSHAC process. However, the required SSHAC level is dependent upon the Gradation Tier, with an increase in Level corresponding to a more rigorous Tier. A graded approach to PSHA is consistent with recent NRC practice (NUREG-2213

[32]).

Importantly, several ANS standards also propose a graded approach for seismic design and hazard analysis. Elements of these standards are invoked by the DOE in their national phenomena hazard directives.

In ANS 2.26 [18], a graded approach to seismic design is achieved via a seismic design category (SDC) and limit state that are assigned to structures, systems, and components based on the necessary performance and consequences of failure. A higher SDC would require more stringent design criteria. This is intended to be used in conjunction with ASCE 43-19, where seismic performance in terms of annual frequencies of exceedance are specified. Additionally, lower SDCs can be designed using the commercial code, ASCE 7.

In ANS 2.27 [33], guidance is provided for performing site investigations that support seismic hazard assessments. This standard specifically notes that the site exploration, number and type of samples obtained, and associated testing programs may be relaxed for evaluations of facilities

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of lower SDC classification. Additionally, for SDC-3 or lower facilities subject to relatively low levels of seismic demand, similar less-intensive measures to acquire strain-dependent properties or soil-structure interaction inputs may be justified. The graded approach proposed in this technical report follows the same philosophy but assigns prescriptive guidance.

ANS 2.29 [34], defines guidance for performing a PSHA. Various levels for performing a PSHA is proposed based on the degree of regulatory assurance that is required. A higher PSHA level corresponds to a more rigorous analysis and increased extent of assurance. Although the standard does not specify when different PSHA levels are required, it inherently allows for a graded approach to PSHA. Like ANS 2.27, this technical report follows the same philosophy but assigns prescriptive guidance.

In summary, the specific graded scope of analysis and investigation for seismic siting described here aligns with the requirements of 10 CFR 100.23 [1] while being consistent with the primary objectives described in Section 2 of this report. Namely, this framework encourages refinement where it would most influence seismic safety, but not where reasonable confidence of seismic safety exists without such refinements. Therefore, Gradation Tiers enable use of project-specific factors in establishing user clarity for how one can meet regulatory requirements while supporting the need to provide reasonable assurance that an NPP can be constructed and operated at a proposed site without undue risk to the public.

EXAMPLES The following subsections step through the screening process for several example facilities and sites, resulting in a Gradation Tier for each examined scenario. The selected example sites include five NPP sites (four CEUS and one WUS), as well as one hypothetical greenfield site. For the NPP sites, we varied the level of information assumed to be available to illustrate the various possible user conditions and screening outcomes.

The examples demonstrate the screening process for different scenarios/use cases. For example, a user who is building on an existing NPP site may have access to an existing PSHA. In that case, they could leverage the PSHA when defining the site-specific MCER per ASCE 7-22, or potentially voluntarily adopt a higher Gradation Tier than their screening outcome because that Gradation Tier could be readily supported by their existing PSHA. Another scenario could be building at an industrial site with detailed existing data regarding the subsurface materials (such as a site for mining or drilling operations), but without an existing PSHA. In this case, the applicant may leverage the detailed existing information and could opt to achieve High Confidence in the Seismic Index by performing site response analysis, if they decide the benefit of doing so would be worthwhile. A third example could be a greenfield site with no subsurface information. In this scenario, the applicant would likely conduct a commercial-style due diligence site investigation, such as using surface geophysics, to define the site class, as is typical for facilitating foundation design activities for a non-nuclear structure. This initial site investigation could then inform the screening process. A fourth scenario involves an applicant evaluating a suite of deployment sites. In this case, they could use the screening process to identify and define the Target Seismic Capacity to achieve the desired Margin for their standard plant design.

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9.1 Seismic Index Examples These Seismic Index examples are developed for four seismic capacities, at six site locations, and use two methods for site amplification, as shown in Table 5.

Table 5: Target Seismic Capacities, Target Locations, and Site Amplification Method for Seismic Index Examples Capacities Locations Site Amplification RG 1.60, 0.2g PGA Gulf Coast, TX (STP)

Site classification RG 1.60, 0.5g PGA North Anna Idealized SRA CSDRS Design A Clinch River CSDRS Design B Dresden Columbia St. Louis, MO Target Seismic Capacities Four Target Seismic Capacity examples are illustrated using different DRS options, as described in Section 5.1. Two examples use the NRC Regulatory Guide 1.60 spectral shape, one anchored to 0.20g PGA and the other to 0.50g PGA. The other two examples use two different CSDRS options, referred to as CSDRS Design A and CSDRS Design B. In these examples, the Target Seismic Capacities are treated as a maximum direction spectrum. The applicant performing the screening should ensure that they use compatible spectra for the Target Seismic Capacity and Scoping Seismic Demand (i.e., do not compare a maximum direction spectrum to a geomean spectrum). Figure 5 shows the four example Target Seismic Capacities.

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Figure 5: Target Seismic Capacity Examples Scoping Seismic Demands Site Amplification - Soil Classification Site amplification is obtained through site classification. Site classification is determined per ASCE/SEI 7-22 [11] Table 20.2-1 using time-averaged shear wave velocity in the top 30 m (Vs30), which is estimated following Section 20.4.1. Results for the six locations are as follows:

Gulf Coast TX (STP): Site Class D - Estimated Vs30=906 ft/s. Shear wave velocity profiles are obtained from best estimate base cases of STP Tables 2.3.2-1 and 2.3.2-2 of SHSR NTTF 2.1 [35].

North Anna: Site Class B -Estimated Vs30=3092 ft/s. Shear wave velocity profiles are obtained from best estimate base cases of North Anna Table 2.3.2-1 SHSR NTTF 2.1 [38].

Clinch River: Site Class A - Estimated Vs30=7810 ft/s. Shear wave velocity profiles are obtained from Locations A & B from Tables 2.5.4-30 and 2.5.4-31 of Clinch River Early Site Permit Application - Site Safety Analysis Report, Chapter 2 - Site Characteristics [39].

Dresden: Site Class B - Estimated Vs30=3764 ft/s. Shear wave velocity profiles are obtained from Locations 1, 2 &3 from Dresden Table 2.3.2-1 SHSR NTTF 2.1 [40].

Columbia: Site Class CD - Estimated Vs30=1198 ft/s. Shear wave velocity profile is obtained from Base Case from Columbia Table 2.3.2-1 SHSR NTTF 2.1 [41].

St. Louis, MO: Site Class D - Per Williams et. al, Shallow P-and S-wave velocities and site resonances in the St. Louis region, Missouri-Illinois [42]

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MCER and Seismic Parameters - Site Classification As defined in Section 5.2.2, the Scoping Seismic Demand is the MCER obtained from the ASCE Hazard Tool. The necessary inputs for the ASCE Hazard tool include the site classification of the locations defined in Table 5. The hazard tool is used for the six example sites to obtain the seismic parameters from the MCER. A summary of seismic parameters and input data is shown in Table 6, and the MCER for the six sites is plotted in Figure 6.

Gulf Coast, TX (STP)

North Anna

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Clinch River Dresden Columbia

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St. Louis, MO Table 6: Seismic Parameters (Hazard ToolBox)

Locations Latitude Longitude Site Classification Seismic Parameters SMS SM1 Gulf Coast, TX (STP) 28.7954

-96.0490 D

0.089 0.082 North Anna 38.0605

-77.7899 B

0.200 0.055 Clinch River 35.8909

-84.3823 A

0.390 0.120 Dresden 41.3898

-88.2700 B

0.140 0.082 Columbia 46.4708

-119.3342 CD 0.540 0.260 St. Louis 38.6350

-90.2010 D

0.660 0.370 Figure 6: MCER at Example Locations per Site Classification

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Site Amplification - Idealized SRA Site-specific amplification factors (SAFs) are adapted from existing information, as shown in Figure 7:

site amplification functions derived in SSAR (Clinch River) and SHSR NTTF 2.1 (other sites). No site amplification at St. Louis is developed in these examples.

Figure 7: Site amplification factors for five example locations

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MCER - Idealized SRA In the Idealized SRA method for site amplification, the MCER obtained from the ASCE hazard tool for hard rock sites (Site Class A) is used as a proxy for the seismic demand at depth. These rock spectra are shown in Figure 8. The hard rock MCER spectra are then scaled using the SAFs as a proxy for SRA (i.e., as an idealized SRA). The scaled seismic demand, representing the surface demand, is shown in Figure 9.

Figure 8: MCER at five example locations at rock Figure 9: Idealized SRA MCER at five example locations

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Seismic Screening Margin - Site Class and Idealized SRA Amplifications Figure 10 through Figure 15 show the four example Target Seismic Capacities vs. the two methods of estimating the Scoping Seismic Demands, to characterize the Primary Margin, Mp, for each of the six sites.

Figure 10: Gulf Coast, TX (STP) Scoping Seismic Demand vs. Target Seismic Capacity Figure 11: North Anna Scoping Seismic Demand vs. Target Seismic Capacity

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Figure 12: Clinch River Scoping Seismic Demand vs. Target Seismic Capacity Figure 13: Dresden Scoping Seismic Demand vs. Target Seismic Capacity

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Figure 14: Columbia Scoping Seismic Demand vs. Target Seismic Capacity Figure 15: St. Louis, MO, Scoping Seismic Demand vs. Target Seismic Capacity Primary Margin Characterization Primary Margin, Mp, is computed as the minimum capacity-to-demand ratio (Target Seismic Capacity(f)/MCER(f)) at each frequency between 1 and 10 Hz. Mp is then characterized as high, medium, or low, per Table 7 as defined in Section Error! Reference source not found..

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Table 7: Primary Margin Characterization H

Mp > 1.87 M

1.5 < Mp < 1.87 L

1.25 < Mp < 1.5 Site Classification Amplifications The Primary Margin, MP, is computed for the example Scoping Seismic Demands and Target Seismic Capacities using the ASCE 7 site classifications. Rows indicated as N/A indicate MP values below 1.25 within 1 to 10 Hz. The graded approach to seismic hazard is not appropriate for these Capacity-Demand combinations.

Capacity Location MP RG 1.60, 0.5g PGA Gulf Coast TX (STP) 9.70 H

North Anna 3.18 H

Clinch River 1.83 M

Dresden 5.75 H

Columbia 2.36 H

St. Louis 1.51 M

RG 1.60, 0.2g PGA Gulf Coast TX (STP) 3.88 H

North Anna 1.27 L

Clinch River 0.73 N/A Dresden 2.30 H

Columbia 0.95 N/A St. Louis 0.60 N/A CSDRS Design A Gulf Coast TX (STP) 5.82 H

North Anna 1.96 H

Clinch River 1.13 N/A Dresden 3.55 H

Columbia 1.42 L

St. Louis 0.93 N/A CSDRS Design B Gulf Coast TX (STP) 7.79 H

North Anna 3.03 H

Clinch River 1.74 M

Dresden 5.48 H

Columbia 1.89 H

St. Louis 1.44 L

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Idealized SRA Amplifications The Primary Margin, MP, is computed for the example Scoping Seismic Demands and Target Seismic Capacities using the idealized SRA site amplifications. Rows indicated as N/A indicate MP values below 1.25 within 1 to 10 Hz. The graded approach to seismic hazard is not appropriate for these Capacity-Demand combinations. SRA Amplification is not developed for St. Louis site.

Capacity Location MP RG 1.60, 0.5g PGA Gulf Coast TX (STP) 7.44 H

North Anna 2.13 H

Clinch River 2.12 H

Dresden 5.38 H

Columbia 3.16 H

RG 1.60, 0.2g PGA Gulf Coast TX (STP) 2.97 H

North Anna 0.85 N/A Clinch River 0.85 N/A Dresden 2.15 H

Columbia 1.26 L

CSDRS Design A Gulf Coast TX (STP) 4.46 H

North Anna 1.31 L

Clinch River 1.30 L

Dresden 3.32 H

Columbia 1.90 H

CSDRS Design B Gulf Coast TX (STP) 5.97 H

North Anna 2.03 H

Clinch River 2.02 H

Dresden 5.13 H

Columbia 2.36 H

High-Frequency and Low-Frequency Exceedances The Seismic Screening Margin parameter is based on MP. For the examples herein, there are no low-frequency exceedances. We assume there are no high-frequency seismic sensitivities (e.g., chatter-prone relays) inherent in the seismic design of the facilities under consideration, such that the high-frequency seismic demand exceedances do not affect the Screening Margin parameter used for screening. Therefore, the Seismic Index is solely based on Primary Margin Mp, as defined in Section Error! Reference source not found..

Seismic Hazard Seismic Hazard is characterized as high, medium, or low, using the SMS and SM1 seismic parameters obtained from the ASCE Hazard Tool per Error! Reference source not found.. Hazard characterization for the six example locations are shown in Table 8.

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Table 8: Seismic Hazard for six example locations Seismic Parameters Hazard Characterization Locations Latitude Longitude Site Class SMS SM1 SMS SM1 Highest Hazard Gulf Coast, TX (STP) 28.7954

-96.0490 D

0.089 0.082 L

L L

North Anna 38.0605

-77.7899 B

0.200 0.055 L

L L

Clinch River 35.8909

-84.3823 A

0.390 0.120 M

M M

Dresden 41.3898

-88.2700 B

0.140 0.082 L

L L

Columbia 46.4708

-119.3342 CD 0.540 0.260 H

H H

St. Louis 38.6350

-90.2010 D

0.660 0.370 H

H H

Confidence High confidence is assumed for the Idealized SRA method for site amplification. Medium confidence is assumed for the Site Classification method, as described in Section 5.3.

Seismic Index The Seismic Index is characterized in Table 9 based on the Seismic Screening Margin and Hazard parameters above, and on the Confidence level mapping per Figure 3, as defined in Section 5.4.

Table 9: Example Seismic Index Summary Capacity Location Seismic Screening Margin Seismic Hazard Seismic Index Site Class Idealized SRA Site Class Idealized SRA RG 1.60, 0.5g PGA Gulf Coast, TX (STP)

H H

L 4

4 North Anna H

H L

4 4

Clinch River 2 M

H M

3 3

Dresden H

H L

4 4

Columbia H

H H

2 3

St. Louis M

N/A H

1 No Data RG 1.60, 0.2g PGA Gulf Coast, TX (STP)

H H

L 4

4 North Anna 1 L

N/A L

3 N/A Clinch River N/A N/A M

N/A N/A Dresden H

H L

4 4

Columbia N/A L

H N/A 1

St. Louis N/A N/A H

N/A No Data CSDRS Design A Gulf Coast, TX (STP)

H H

L 4

4 North Anna 1 H

L L

4 3

Clinch River 2 N/A L

M N/A 3

Dresden H

H L

4 4

Columbia L

H H

1 3

St. Louis N/A N/A H

N/A No Data CSDRS Design B Gulf Coast, TX (STP)

H H

L 4

4 North Anna H

H L

4 4

Clinch River 2 M

H M

3 3

Dresden H

H L

4 4

Columbia H

H H

2 3

St. Louis L

N/A H

1 No Data 1, 2 See the corresponding discussion in Section 9.4

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9.2 Consequence Index Examples Two Consequence Index examples are illustrated: (1) a typical large light water reactor with a traditional 10-mile EPZ, and (2) a small modular reactor with a small inventory such that the unmitigated dose if the entire inventory is released (i.e., not accounting for engineering safety functions) results in a dose release of 75 rem. The resulting Consequence Indices are 1 and 3, respectively.

9.3 Gradation Tier Examples Table 10: Example Gradation Tier Summary Row No.

Capacity Location Seismic Index Gradation Tier Site Class Idealized SRA LLWR (CI = 1)

SMR w/small inventory (CI = 3) 1 3 RG 1.60, 0.5g PGA Gulf Coast, TX (STP) 4 4

B A

2 3 North Anna 4

4 B

A 3

Clinch River 3

3 C (site class)

B (idealized SRA)

A 4 3 Dresden 4

4 B

A 5 5 Columbia 2

3 D (site class)

C (idealized SRA)

B (site class)

A (idealized SRA) 6 St. Louis 1

No Data E (site class)

C (site class) 7 4 RG 1.60, 0.2g PGA Gulf Coast, TX (STP) 4 4

B A

8 North Anna 3

N/A C (site class)

N/A (idealized SRA)

A (site class)

N/A (idealized SRA) 9 Clinch River N/A N/A N/A N/A 10 4 Dresden 4

4 B

A 11 5 Columbia N/A 1

N/A (site class)

E (idealized SRA)

N/A (site class)

C (idealized SRA) 12 St. Louis N/A No Data N/A (site class)

N/A (site class) 13 3 CSDRS Design A Gulf Coast, TX (STP) 4 4

B A

14 North Anna 4

3 B (site class)

C (idealized SRA)

A 15 Clinch River N/A 3

N/A (site class)

C (idealized SRA)

N/A (site class)

A (idealized SRA) 16 3 Dresden 4

4 B

A 17 5 Columbia 1

3 E (site class)

C (idealized SRA)

B (site class)

A (idealized SRA) 18 St. Louis N/A No Data N/A (site class)

N/A (site class) 19 3 CSDRS Design B Gulf Coast, TX (STP) 4 4

B A

20 3 North Anna 4

4 B

A 21 Clinch River 3

3 C (site class)

B (idealized SRA)

A 22 3 Dresden 4

4 B

A 23 5 Columbia 2

3 D (site class)

C (idealized SRA)

B (site class)

A (idealized SRA) 24 St. Louis 1

No Data E (site class)

C (site class) 3, 4, 5 See the corresponding discussion in Section 9.4 9.4 Discussion Several notable insights can be drawn from the example cases. For example, note that:

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Seismic Index

1. The idealized SRA approach results in a higher Scoping Seismic Demand spectra for North Anna than the Site Class amplification approach. The benefit of using the idealized SRA is the ability to credit increased confidence. Although the selected example sites do not reveal this benefit, a different site that results in a High Seismic Screening Margin using both the SRA and Site Class amplifications could shift the Seismic Index from 3 to 4.

2. Pursuing the idealized SRA approach for the Clinch River site results in two benefits compared to the Site Class amplification approach: increased Seismic Screening Margin and Confidence.

Gradation Tiers

3. The cases in Table 10, Rows 1, 2, 4, 13, 16, 19, 20 and 22 represent a robust seismic design when comparing the Scoping Seismic Demand to the Target Seismic Capacity. These cases illustrate the benefit available to an applicant for the seismic hazard and site investigation effort, should they opt to design their facility to achieve these relatively high capacities. A standard design to accommodate a high seismic region allows the applicant to install the same robust facility at a typical low to moderate seismic site without performing excessive site investigation studies (e.g., install a standard design facility for the WUS in the CEUS). If a detailed, site-specific SSHAC study were performed, the result would be that the robust design is suitable for these sites, so allowing the applicant to avoid the significant effort associated with that site-specific study is desirable.

4. Similarly, the cases in Table 10, Rows 7 and 10, represent a less robust design, but one with sufficient margin to avoid costly site-specific hazard investigations for low seismic sites.

5. The cases in Table 10, Rows 5, 11, 17, and 23 represent a typical WUS site/seismic region.

Installing an LLWR at that site generally requires performing a detailed site-specific hazard assessment, especially if the Target Seismic Capacity is relatively low or there is low confidence in the Scoping Seismic Demand.

RECOMMENDATIONS AND CONCLUSIONS If adopted, the graded approach to seismic hazard analysis and site investigations for licensing next-generation NPPs would significantly streamline early site investigation work, helping to enable the preparation and review of a high volume of license applications. The approach is technically sound and right-sizes the effort to place emphasis where important and avoid unnecessary effort.

The graded approach presented herein aligns with regulations (i.e., 10 CFR 100.23) and with the conceptual framework laid out by the NRC staff in SECY-25-0052, enabling project-specific considerations to influence the necessary scope of seismic hazard analysis and corresponding site investigations while maintaining the ability to demonstrate reasonable assurance that the sited NPP is without undue risk to the health and safety of the public. Importantly, it also meets the intent of NEIMA, the ADVANCE Act, and the Executive Orders of May 2025 related to nuclear technology by helping streamline the deployment schedule to license and construct nuclear power plants in a high volume. For these reasons, we recommend that the NRC endorse the approach via Regulatory Guide or otherwise.

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REFERENCES

[1] 10 CFR 100.23, https://www.ecfr.gov/current/title-10/part-100/section-100.23

[2] U.S. Nuclear Regulatory Commission, Regulatory Guide 1.132, Revision 3, Geologic and Geotechnical Site Characterization Investigations for Nuclear Power Plants, https://www.nrc.gov/docs/ML2129/ML21298A054.pdf.

[3] U.S. Nuclear Regulatory Commission, Regulatory Guide 1.138, Revision 3, Laboratory Investigations of Soils and Rocks for Engineering Analysis and Design of Nuclear Power Plants, https://www.nrc.gov/docs/ML1428/ML14289A600.pdf.

[4] U.S. Nuclear Regulatory Commission, Regulatory Guide 1.208, A Performance-Based Approach to Define the Site-Specific Earthquake Ground Motion, https://www.nrc.gov/docs/ML0703/ML070310619.pdf.

[5] U.S. Nuclear Regulatory Commission, Regulatory Guide Periodic Review, Regulatory Guide 1.208, https://www.nrc.gov/docs/ML2101/ML21012A197.pdf.

[6] U.S. Nuclear Regulatory Commission, Preliminary White Paper - Nth-of-a-Kind Micro-Reactor Licensing and Deployment Considerations, Enclosure 3, September 2024.

https://www.nrc.gov/docs/ML2426/ML24268A317.pdf.

[7] U.S. Nuclear Regulatory Commission, Applying a Graded Approach and Adapting Guidance on Site Characterization of External Hazards for Advanced Reactor and Microreactor Applications, October 30, 2024. https://www.nrc.gov/docs/ML2435/ML24355A104.pdf.

[8] U.S. Nuclear Regulatory Commission, SECY-25-0052: Enclosure 3 - Technical, Licensing, and Policy Considerations for Nth-of-a-Kind Microreactors, Technical, Licensing, and Policy Considerations for Nth-of-a-Kind Microreactors.

https://www.nrc.gov/docs/ML2430/ML24309A260.pdf.

[9] U.S. Nuclear Regulatory Commission, Advanced Reactor Stakeholder Public Meeting, August 28,

2025, https://adamswebsearch2.nrc.gov/webSearch2/main.jsp?AccessionNumber=ML25237A182.

[10] U.S. Nuclear Regulatory Commission, Regulatory Guide 1.60, Revision 2, Design Response Spectra for Seismic Design of Nuclear Power Plants.

https://www.nrc.gov/docs/ML1321/ML13210A432.pdf.

[11] American Society of Civil Engineers/Structural Engineering Institute 7-22, Minimum Design Loads and Associated Criteria for Buildings and Other Structures.

[12] American Society of Civil Engineers/Structural Engineering Institute 43-19, Seismic Design for Structures, Systems, and Components in Nuclear Facilities.

[13] Electric Power Research Institute (EPRI), High Frequency Program Application Guidance for Functional Confirmation and Fragility Evaluation. EPRI; Palo Alto, CA: 2015. 3002004396.

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[14] Electric Power Research Institute (EPRI), High Frequency Seismic Inelastic Effects on Equipment Anchorage: Quantifying Increase in Anchorage Acceleration Capacities at High Equipment Frequencies. EPRI; Palo Alto, CA: 2017. 3002010665.

[15] Electric Power Research Institute (EPRI), Seismic Evaluation Guidance: Screening, Prioritization and Implementation Details (SPID) for the Resolution of Fukushima Near-Term Task Force Recommendation 2.1: Seismic. EPRI; Palo Alto, CA: 2013. 1025287.

[16] U.S. Nuclear Regulatory Commission, DC/COL-ISG-01, Interim Staff Guidance on Seismic Issues Associated with High Frequency Ground Motion in Design Certification and Combined License Applications. https://www.nrc.gov/docs/ML0814/ML081400293.pdf.

[17] Nuclear Energy Institute, NEI 24-05, An Approach for Risk-Informed Performance-Based Emergency Planning, June 2024. https://www.nrc.gov/docs/ML2418/ML24184C122.pdf.

[18] American Nuclear Society, ANSI/ANS-2.26-2004 (R2010), Categorization of Nuclear Facility Structures, Systems, and Components for Seismic Design.

[19] U.S. Nuclear Regulatory Commission, SECY-25-0052: Nth-of-a-Kind Microreactor Licensing and Deployment Considerations. https://www.nrc.gov/docs/ML2430/ML24309A266.html.

[20] 10 CFR 50.160, https://www.federalregister.gov/documents/2023/11/16/2023-25163/emergency-preparedness-for-small-modular-reactors-and-other-new-technologies.

[21] U.S. Nuclear Regulatory Commission, NUREG-0800, Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR Edition - Transient and Accident Analysis (NUREG-0800, Chapter 15). https://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr0800/ch15/index.html.

[22] NuScale Power LLC, NuScale Standard Plant Design Certification Application, Chapter 15, Transient and Accident Analysis. https://www.nrc.gov/docs/ML2022/ML20224A504.pdf.

[23] Nuclear Energy Institute, NEI 18-04, Modernization of Technical Requirements for Licensing of Advanced Non-Light Water Reactors; Risk-Informed Performance-Based Technology Inclusive Guidance for Non-Light Water Reactor Licensing Basis Development, Revision 1, August 2019.

https://www.nrc.gov/docs/ML1924/ML19241A472.pdf.

[24] U.S. Nuclear Regulatory Commission, Regulatory Guide 1.233 Revision 0, Guidance for a Technology-Inclusive, Risk-Informed, and Performance-Based Methodology to Inform the Licensing Basis and Content of Applications for Licenses, Certifications, and Approvals for Non-Light-Water Reactors. https://www.nrc.gov/docs/ml2009/ml20091l698.pdf.

[25] U.S. Nuclear Regulatory Commission, DANU-ISG-2022-01, Interim Staff Guidance, Review of Risk-Informed, Technology-Inclusive Advanced Reactor ApplicationsRoadmap, March 2024.

https://www.nrc.gov/docs/ML2329/ML23297A158.pdf.

[26] ClearPath, Transformative Regulatory Reform for New Reactors, May 15, 2025.

https://clearpath.org/wp-content/uploads/sites/44/2025/05/catf-clearpath-veriten-nrc-rulemaking.pdf.

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[27] U.S. Nuclear Regulatory Commission, NRC Public Meeting Licensing Requirements for Microreactors and Other Low Consequence Reactors Rulemaking, July 17-18, 2025.

https://adamswebsearch2.nrc.gov/webSearch2/main.jsp?AccessionNumber=ML25196A357.

[28] U.S. Nuclear Regulatory Commission, NUREG-1537, Guidelines for Preparing and Reviewing Applications for the Licensing of Non-Power Reactors Format and Content.

https://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1537/index.html.

[29] Nuclear Energy Institute, NEI Proposal Paper Regulations of Rapid High-Volume Deployable Reactors in Remote Applications (RHDRA) and Other Advanced Reactors. July 31, 2024.

https://www.nrc.gov/docs/ML2421/ML24213A337.pdf.

[30] Nuclear Energy Institute, Supplement to the NEI Proposal Paper Regulations of Rapid High-Volume Deployable Reactors in Remote Applications (RHDRA) and Other Advanced Reactors.

July 14, 2025. https://www.nrc.gov/docs/ML2519/ML25195A307.pdf.

[31] Nuclear Energy Institute, NEI White Paper: Selection of a Seismic Scenario for an EPZ Boundary Determination, May 2024. https://www.nrc.gov/docs/ML2418/ML24187A096.pdf.

[32] U.S. Nuclear Regulatory Commission, NUREG-2213, Updated Implementation Guidelines for SSHAC Hazard Studies. https://www.nrc.gov/docs/ML1828/ML18282A082.pdf.

[33] American Nuclear Society, ANSI/ANS-2.27-2020, Criteria For Investigations of Nuclear Facility Sites for Seismic Hazard Assessments.

[34] American Nuclear Society, ANSI/ANS-2.29-2020, Probabilistic Seismic Hazard Analysis.

[35] U.S. Nuclear Regulatory Commission, NUREG-6372, Recommendations for Probabilistic Seismic Hazard Analysis: Guidance on Uncertainty of Use of Experts. https://www.nrc.gov/reading-rm/doc-collections/nuregs/contract/cr6372/index.

[36] U.S. Nuclear Regulatory Commission, NUREG-2117, Practical Implementation Guidelines for SSHAC Level 3 and 4 Hazard Studies. https://www.nrc.gov/docs/ML1211/ML12118A445.pdf.

[37] STP Nuclear Operating Company, South Texas Project Units 1 and 2 Seismic Hazard and Screening Report (CEUS Sites) Response NRC Request for Information Pursuant to 10 CFR 50.54(f)

Regarding Recommendation 2.1 of the Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident, March 31, 2014.

https://www.nrc.gov/docs/ML1409/ML14099A235.pdf.

[38] Virginia Electric and Power Company, Virgina Electric and Power Company North Anna Power Station Units 1 and 2 Response to March 12, 2012 Information Request Seismic Hazard and Screening Report (CEUS Sites) for Recommendation 2.1, March 31, 2014.

https://www.nrc.gov/docs/ML1409/ML14092A416.pdf.

[39] Clinch River Nuclear Site, Early Site Permit Application Part 2, Site Safety Analysis Report, Revision 2. https://www.nrc.gov/docs/ML1903/ML19030A344.pdf.

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[40] Exelon Generation, Exelon Generation Company, LLC, Seismic Hazard and Screening Report (Central and Eastern United States (CEUS) Sites), Response to NRC Request for Information Pursuant to 10 CFR 50.54(f) Regarding Recommendation 2.1 of the Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident, March 31, 2014.

https://www.nrc.gov/docs/ML1409/ML14091A012.pdf.

[41] Energy Northwest, Columbia Generating Station Docket No. 50-397 Seismic Hazard and Screening Report, Response to NRC Request for Information Pursuant to 10 CFR 50.54(f)

Regarding Recommendation 2.1 of the Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident, March 12, 2015.

https://www.nrc.gov/docs/ml1507/ML15078A243.pdf.

[42] Williams, Robert A., Jack K. Odum, William J. Stephenson, and Robert B. Herrmann. "Shallow P-and S-wave velocities and site resonances in the St. Louis region, Missouri-Illinois." Earthquake Spectra 23, no. 3 (2007): 711-726.

[43] U. S. Department of Energy, DOE-STD-1020-2016, Natural Phenomena Hazards Analysis and Design Criteria for DOE Facilities. https://www.standards.doe.gov/standards-documents/1000/1020-astd-2016/@@images/file.

ML25335A211

Text

Subject:

Dear Mr. Bowen:

Attachment:

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