ML25104A001

Submittal of Approved Terrapower, LLC Topical Report Plume Exposure Pathway Emergency Planning Zone Methodology

ML25104A001
Person / Time
Site:	Kemmerer File:TerraPower icon.png
Issue date:	04/10/2025
From:	George Wilson TerraPower
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
TP-LIC-LET-0416
Download: ML25104A001 (1)

Text

15800 Northrup Way, Bellevue, WA 98008 www.TerraPower.com P. +1 (425) 324-2888 F. +1 (425) 324-2889 April 10, 2025 TP-LIC-LET-0416 Docket Number 50-613 U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 ATTN: Document Control Desk

Subject:

Submittal of Approved TerraPower, LLC Topical Report: Plume Exposure Pathway Emergency Planning Zone Methodology

References:

1.

U.S. Nuclear Regulatory Commission, TerraPower, LLC - Safety Evaluation of Natrium Topical Report NAT-3056, Plume Exposure Pathway Emergency Planning Zone Sizing Methodology, Revision 3 (ML25076A653)

The U.S. Nuclear Regulatory Commission (NRC) provided the final safety evaluation for the TerraPower, LLC Natrium1 Topical Report, Plume Exposure Pathway Emergency Planning Zone Sizing Methodology, in Reference 1.

The enclosure to this letter provides the accepted version of the Topical Report with the additional content incorporated per the NRC staff request, designated NAT-3056-A.

This letter and the associated enclosures make no new or revised regulatory commitments.

If you have any questions regarding this submittal, please contact Ian Gifford at igifford@terrapower.com.

1Natrium is a TerraPower and GE-Hitachi Technology.

Date: April 10, 2025 Page 2 of 2

Sincerely, George Wilson Senior Vice President, Regulatory Affairs TerraPower, LLC

Enclosure:

TerraPower, LLC Topical Report NAT-3056-A, Revision 3, Plume Exposure Pathway Emergency Planning Zone Sizing Methodology cc:

Mallecia Sutton, NRC Josh Borromeo, NRC Nathan Howard, DOE Jeff Ciocco, DOE

ENCLOSURE TerraPower, LLC Topical Report NAT-3056-A, Revision 3, Plume Exposure Pathway Emergency Planning Zone Sizing Methodology

TerraPower,LLC 15800NorthupWay Bellevue,WA98008 ATerraPower&GEHitachiTechnology Plume Exposure Pathway Emergency Planning Zone Sizing Methodology NAT-3056-A Revision3 April 10,2025 SUBJECT TO DOE COOPERATIVE AGREEMENT NO. DE-NE0009054 Copyright © 2025 TerraPower, LLC. All rights reserved.

March 28, 2025 George Wilson Senior Vice President, Regulatory Affairs TerraPower, LLC 15800 Northup Way Bellevue, WA 98008

SUBJECT:

TERRAPOWER, LLC - FINAL SAFETY EVALUATION OF NATRIUM TOPICAL REPORT NAT-3056, PLUME EXPOSURE PATHWAY EMERGENCY PLANNING ZONE SIZING METHODOLOGY REVISION 3 (EPID L-2023-TOP- 0024)

Dear George Wilson:

By letter dated March 20, 2023 (Agencywide Documents Access and Management System (ADAMS)

Accession No. ML23080A045), TerraPower, LLC (TerraPower) submitted a Topical Report (TR) NAT-3056, Plume Exposure Pathway Emergency Planning Zone Sizing Methodology, Revision 0, for the U.S. Nuclear Regulatory Commission (NRC) staffs review. The TR provides a methodology describing how the Plume Exposure Pathway (PEP) Emergency Planning Zone (EPZ) sizing is determined. The TR follows the guidance in NRC Regulatory Guide (RG) 1.242, Performance-Based Emergency Preparedness for Small Modular Reactors, Non-Light-Water Reactors, and Non-Power Production or Utilization Facilities, Revision 0 (ML23226A036).

By email dated June 12, 2023 (ML23158A203), the NRC staff informed TerraPower that the TR provided sufficient information for the NRC staff to conduct a detailed technical review. On August 11, 2023, the NRC staff transmitted an audit plan to TerraPower (ML23199A317) and subsequently conducted an audit of materials related to the TR from August 17, 2023, to September 15, 2023. By letter dated November 16, 2023, Revision 1 of the TR (ML23321A036) was submitted by TerraPower that addressed items discussed in the audit. On September 2, 2024, the NRC staff issued the draft safety evaluation (SE) to TerraPower (ML24242A101). In a letter dated October 29, 2024, TerraPower submitted Revision 2 of the TR (ML24303A147), which clarified information discussed during the Advisory Committee for Reactor Safeguards (ACRS) subcommittee (Design-Centered Review: TerraPower) meeting held on September 19, 2024 (ML24296A058).

The audit summary was issued on January 8, 2024 (ML24008A057). To provide further insight into the description of Plume Exposure Pathway EPZ sizing methodology, TerraPower submitted Revision 3 of the TR (ML24304B034) by letter dated October 30, 2024.

The enclosed final SE is provided to TerraPower because the NRC staff found the TR acceptable for referencing in licensing actions to the extent specified and under the limitations and conditions delineated in the TR. The final SE defines the basis for the NRC staffs acceptance of the TR.

The NRC staff requests that TerraPower submit an approved version of this TR within 3 months of receipt of this letter. The approved version should incorporate this letter and the enclosed SE after the title page. The approved version should include a -A (designating approved) following the TR identification symbol.

G. Wilson If you have any questions, please contact Mallecia Sutton at (301) 415-0673 or via email at Mallecia.Sutton@nrc.gov.

Sincerely,

/RA/

Joshua Borromeo, Chief Advanced Reactor Licensing Branch 1 Division of Advanced Reactors and Non-Power Production and Utilization Facilities Office of Nuclear Reactor Regulation Project No.: 99902100

Enclosure:

As stated cc: TerraPower Natrium via GovDelivery

Package: ML25076A653 Letter: ML25076A657

Enclosure:

ML25065A250 OFFICE NRR/DANU/UAL1:PM NRR/DANU/UTB2:TR NRR/DANU/UAL1:LA NAME MSutton MHart DGreene DATE 3 / 17/2025 3/18/2025 3/18/2025 OFFICE OGC NRR/DANU/UTB2:BC NRR/DANU/UAL1:BC NAME NMertz CdeMessieres JBorromeo DATE 3/10/2025 3/12/2025 3/28/2025

1 TERRAPOWER, LLC - SAFETY EVALUATION OF NATRIUM TOPICAL REPORT NAT-3056, PLUME EXPOSURE PATHWAY EMERGENCY PLANNING ZONE SIZING METHODOLOGY, REVISION 3 (EPID L-2023-TOP-0024/CAC 000431)

SPONSOR AND SUBMITTAL INFORMATION Sponsor:

TerraPower, LLC Sponsor Address:

15800 Northup Way, Bellevue, WA 98008 Project No.:

99902100 Submittal Date:

March 20, 2023 Submittal Agencywide Documents Access and Management System (ADAMS) Accession Nos.: ML23080A045, ML23321A036, ML24304B034 Brief Description of the Topical Report: By letter dated March 20, 2023, TerraPower, LLC (TerraPower) submitted a Topical Report (TR) entitled, Plume Exposure Pathway Emergency Planning Zone Methodology, Revision 0 (ML23080A045), for the U.S. Nuclear Regulatory Commission (NRC) staffs review. By email dated June 12, 2023, the NRC staff informed TerraPower that the TR provided sufficient information for the NRC staff to begin its detailed technical review (ML23158A203). From August 11, 2023, through October 17, 2023, the NRC staff conducted an audit to gain a detailed understanding of the TR methodology and identify any additional information that required docketing to support the NRC staffs safety evaluation (SE) for the TR (ML23199A317). By letter dated November 16, 2023, TerraPower submitted Revision 1 of the TR (ML23321A036) that addressed items discussed during the NRC staff audit and other minor editorial revisions. The audit report summarizing the NRC staffs observations was issued on January 8, 2024 (ML24008A057). By letter dated October 30, 2024, TerraPower submitted Revision 3 of the TR (ML24304B034) to provide further insight into the description of the plume exposure pathway emergency planning zone (EPZ) sizing methodology.

This SE and the staffs determinations are based on Revision 3 of the TR. The TR describes TerraPowers methodology to be used to determine the plume exposure pathway EPZ for the proposed Natrium reactor design. TerraPower requested the NRC staffs review and approval on the plume exposure pathway EPZ sizing methodology TR.

For background, TerraPowers overall licensing approach for the Natrium reactor design follows the Licensing Modernization Project (LMP) methodology described in Nuclear Energy Institute (NEI) 18-04, Revision 1, Risk-Informed Performance-Based Technology Inclusive Guidance for Non-Light Water Reactor Licensing Basis Development (ML19241A472). Regulatory Guide

2 (RG) 1.233, 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, Revision 0 (ML20091L698) endorses the LMP methodology described in NEI 18-04. TerraPower based the methodology described in this TR on the proposed requirements in SECY-22-0001, Final Rule: Emergency Preparedness for Small Modular Reactors and Other New Technologies and a draft version of RG 1.242, Performance-Based Emergency Preparedness for Small Modular Reactors, Non-Light-Water Reactors, and Non-Power Production or Utilization Facilities published only to support an NRC staff meeting with the Advisory Committee on Reactor Safeguards (ACRS)

(ML21285A035). The final version of RG 1.242, Revision 0, (ML23226A036) was issued in November 2023 to support the final rule and did not differ in content from the draft version with respect to guidance relevant to review of the TR EPZ sizing methodology.

REGULATORY EVALUATION NRC regulatory requirements for nuclear power facility emergency planning, including the requirement for a plume exposure pathway EPZ, are given in Title 10 of the Code of Federal Regulation (10 CFR) 50.47, Emergency plans and 10 CFR 50, appendix E, Emergency Planning and Preparedness for Production and Utilization Facilities.

Sections 50.33(g)(1) and 50.47(c)(2), describe the requirements for the size of EPZs for a nuclear power reactor as follows:

Generally, the plume exposure pathway EPZ for nuclear power reactors shall consist of an area with about 10 miles (16 km) in radius and the ingestion pathway EPZ shall consist of an area with about 50 miles (80 km) in radius. The exact size and configuration of the EPZs surrounding a particular nuclear power reactor shall be determined in relation to the local emergency response needs and capabilities as they are affected by such conditions as demography, topography, land characteristics, access routes, and jurisdictional boundaries. The size of the EPZs also may be determined on a case-by-case basis for gas-cooled reactors and for reactors with an authorized power level less than 250 MW thermal. The plans for the ingestion pathway shall focus on such actions as are appropriate to protect the food ingestion pathway.

Alternative emergency preparedness (EP) requirements for small modular reactors (SMRs) and other new technologies (ONTs) are given in 10 CFR 50.160, Emergency preparedness for small modular reactors, non-light-reactors, and non-power production or utilization facilities, also referred to as EP for SMRs and ONTs rule (88 FR 80050). This rule includes a scalable approach to determining the size of the plume exposure pathway EPZ for SMRs and ONTs that is performance-based, consequence-oriented, risk-informed, and technology-inclusive. For facility applications complying with 10 CFR 50.160, 10 CFR 50.33(g)(2) requires that the application must include the analysis used to determine whether plume exposure pathway EPZ criteria in 10 CFR 50.33(g)(2)(i)(A) and (B) are met, and if so, the size of the plume exposure pathway EPZ.

Under 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 [Definitions] of this chapter [Chapter I to Title 10] is projected to exceed 10 mSv (1 rem) total effective dose equivalent [TEDE] over 96 hours0.00111 days <br />0.0267 hours <br />1.587302e-4 weeks <br />3.6528e-5 months <br /> from the release of radioactive materials from the facility considering accident likelihood and source term, timing of the accident sequence, and meteorology; and

3 (B) Pre-determined, prompt protective measures are necessary.

RG 1.242, Revision 0, Performance-Based Emergency Preparedness for Small Modular Reactors, Non-Light-Water Reactors, and Non-Power Production or Utilization Facilities (ML23226A036) provides guidance on consequence analyses to aid in facility-specific plume exposure pathway EPZ size determination for facility applications complying with 10 CFR 50.160.

NUREG-0396, Planning Basis for the Development of State and Local Government Radiological Emergency Response Plans in Support of Light Water Nuclear Power Plants (ML051390356), provides the technical basis for the requirement for a 10-mile plume exposure pathway EPZ in 10 CFR 50.33(g)(1) and 50.47(c)(2). The regulatory basis for the EP for SMRs and ONTs rule considered the dose assessment methodologies that informed NUREG-0396 to offer an EPZ size determination process that is consistent with the philosophy in NUREG-0396, as discussed in the Federal Register (FR) notice for the final 10 CFR 50.160 rule (88 FR 80058). RG 1.242, appendix A, General Methodology for Establishing Plume Exposure Pathway Emergency Planning Zone Size, describes an acceptable approach to meet the EPZ sizing requirements for 10 CFR 50.160 that was generalized from the consequence assessment that informed NUREG-0396.

The emergency plan supports planning and preparedness to enable emergency response organizations and State and local governments to take necessary actions to provide dose savings and protect the public health and safety in the event of an accidental release of radioactive material from a nuclear power plant. The plume exposure pathway EPZ, which is the area where predetermined, prompt protective measures are necessary, is one tool in the emergency plan.

Guidance Related to the Risk-informed Approach TerraPower states its probabilistic risk assessment (PRA) will address all modes of operation and external hazards using the guidance in RG 1.233, Revision0 and NEI 18-04, Revision 1.

NEI 18-04 references the American Society of Mechanical Engineers (ASME)/American Nuclear Society (ANS) PRA Standard, RA--S-1.4, Probabilistic Risk Assessment Standard for Advanced Non-Light Water Reactor Nuclear Power Plants," which the NRC staff endorsed with exceptions in RG 1.247 (for trial use), Acceptability of Probabilistic Risk Assessment Results for Non-Light-Water Reactor Risk-Informed Activities (ML21235A008).1 Industry-developed guidance for content of applications using NEI 18-04 is provided in NEI 21-07, Technology Inclusive Guidance for Non-Light Water Reactors - Safety Analysis Report: For Applications Utilizing the NEI 18-04 Methodology, Revision 1 (ML22060A190). NEI 21-07 is endorsed with clarifications and additions by RG 1.253, Guidance for a Technology-Inclusive Content of Application Methodology to Inform the Licensing Basis and Content of Applications for Licenses, Certifications, and Approvals for Non-Light-Water Reactors, Revision 0 (ML23269A222).

1 This RG has been issued for trial use. The NRC staff may use a trial RG as a reference in its regulatory processes. However, the staff may withdraw or add positions from the trial use guide after the trial use period ends. Moreover, the trial use RG does not establish a staff position for the purposes of backfitting as that term is defined in 10 CFR 50.109, Backfitting, and as described in NRC Management Directive 8.4, Management of Backfitting, Forward Fitting, Issue Finality, and Information Requests. The trial RG also does not constitute forward fitting as that term is described in Management Directive 8.4.

4 TECHNICAL EVALUATION TR section 1, Introduction, provides the purpose and scope of the TR and includes a list of abbreviations. TR section 2, Regulatory Basis, provides the regulatory basis for the plume exposure pathway EPZ methodology. TR section 3, Accident Screening Methodology, through section 7, Summary and Conclusions on Methodology, provide the methodology on which TerraPower requests the NRC staffs review and approval. The NRC staff treated the information in TR sections 1 and 2 as background material supporting the methodology and takes no position on the information therein.

The TR methodology determines a plume exposure pathway EPZ based on the area within, which the dose to a member of the public is projected to exceed 10 millisievert (mSv) (1 rem)

TEDE over an exposure period of 96 hours0.00111 days <br />0.0267 hours <br />1.587302e-4 weeks <br />3.6528e-5 months <br /> from the release of radioactive materials from the facility, considering accident likelihood and source term, timing of the release sequence, and meteorology. TR section 3.1, Process Overview, gives an overview of the methodology, which includes the following steps:

Compile release sequences from the PRA for all internal and external initiators (TR sections 3.4, Development of the Probabilistic Risk Assessment, and 3.5, Hazards and Initiating Events);

Perform screening of non-seismic release sequences based on frequency, including uncertainty (TR section 3.6, Selection of Non-Seismic Release Sequences);

Perform screening of seismic release sequences with a unique set of selection criteria, including uncertainty (TR section 3.7, Selection of Seismic Release Sequences);

Collect meteorological data (outside the TR scope) and incorporate into the radiological consequence analysis (TR section 5.1, Meteorological Input);

Perform source term and radiological consequence analysis (outside the TR scope), with the projected plume exposure pathway EPZ boundary and 96-hour event timing (TR sections 4, Source Term Methodology, and 5.3, Radiological Consequence Analysis);

Evaluate the radiological dose consequences against the plume exposure pathway EPZ dose criteria (TR sections 3.3, Dose-Based Criteria, and 6.1, Criteria for Plume Exposure Pathway Emergency Planning Zone Sizing);

o Determine if design changes, analysis refinements, or expansion of EPZ size are needed; o Address any changes by repeating the accident and consequence analyses; Determine the final plume exposure pathway EPZ distance based on meeting the criteria described in TR section 6.1, Criteria for Plume Exposure Pathway Emergency Planning Zone Sizing.

RG 1.242, appendix A, describes an acceptable approach for determining a plume exposure pathway EPZ size to meet the requirements in 10 CFR 50.33(g)(2) for SMR, non-light-water reactors (non-LWRs), or non-power production or utilization facility applicants complying with 10 CFR 50.160. This approach includes the following general steps for consequence analysis to support the determination of the plume exposure pathway EPZ:

Identify events and radiological release scenarios for the facility; Develop meteorological data; Develop atmospheric transport, dispersion, and deposition model; Model potential exposures to offsite populations;

5 Model potential doses to offsite populations; and Aggregate dose distance information.

The NRC staff reviewed TR sections 1 and 2 and determined that the TerraPowers considerations regarding the process for determining plume exposure pathway EPZ sizing methodology are consistent with the considerations in the basis for the scalable plume exposure pathway EPZ in the EP for SMRs and ONTs rule, 10 CFR 50.160. In addition, the NRC staff found that the steps of the TR methodology described in TR sections 3 through 6, as summarized in TR section 3.1, are consistent with the plume exposure pathway EPZ size analysis methodology guidance in appendix A to RG 1.242. Specifically, the TR methodology is consistent with the considerations discussed in both NUREG-0396 and RG 1.242 for determination of a plume exposure pathway EPZ that supports the objective of emergency response plans to provide dose savings for a spectrum of accidents that could produce offsite doses in excess of the Environmental Protection Agency (EPA) Protective Action Guides (PAGs)2 for those members of the public who would most likely receive exposure as a result of a significant release.

The following sections of this SE describe the NRC staffs technical evaluation of the TR methodology steps.

1.0 Accident Screening Methodology Section 3 of the TR provides the methodology used to determine the spectrum of accidents to include in the consequence analysis to support plume exposure pathway EPZ sizing. The NRC staffs evaluation of the steps of the accident screening methodology is described in each subsection of section 3 of the TR.

1.1 Application of risk-informed methods in event selection for EPZ sizing TR section 3.2, Application of Risk-Informed Methods in Event Selection, describes the use of risk information to select the events used in the plume exposure pathway EPZ sizing consequence analysis. The TR states that the risk-informed approach applies a dose-based framework with a consequence-based approach, event selections with an acceptable spectrum of consequences, and the use of a spectrum of accidents as the basis for the plume exposure pathway EPZ size. This approach is consistent with the guidance in RG 1.242. TR section 3.2 states that consistent with the guidance in RG 1.242, the user of the methodology will have adequate information on licensing basis events (LBEs), radiological source terms, and PRA to be applied in the plume exposure pathway EPZ sizing methodology. The TR methodology also states that event selection will be risk-informed based on release frequency using the site-and design-specific PRA. The NRC staff determined that the TR methodology to apply risk-informed methods in event selection is acceptable because it is consistent with the guidance in RG 1.242.

1.2 Dose-based criteria TR section 3.3 provides the dose-based criteria, as applied to specific event categories. There 2 The EPA PAGs are reference values for radiation doses that warrant preselected protective actions (e.g., evacuation or sheltering-in-place) for public protection, if the projected dose received by an individual in the absence of protective action exceeds the PAGs. The most recent version of the PAGs is given in the January 2017 EPA PAG Manual (EPA-400/R-17/001), available at https://www.epa.gov/radiation/protective-action-guides-pags.

6 are three dose criteria used for the methodology in the TR. Criteria A and B in section 3.3 of the TR necessitate that the projected doses from the design basis accidents (DBAs) and most radiological release sequences would not exceed PAG levels outside the plume exposure pathway EPZ, respectively. The NRC staff notes that the TR use of the EPA PAGs as a basis for Criteria A and B is consistent with the goals for emergency planning as described in NUREG-0396, the plume exposure pathway EPZ sizing methodology guidance in RG 1.242, and the dose criterion for the EPZ size in 10 CFR 50.33(g)(2)(i). Therefore, the NRC staff determined that the dose-based Criteria A and B and their bases are acceptable.

Criterion C necessitates that immediate life-threatening doses from the worst-case radiological release sequences would generally not occur outside the plume exposure pathway EPZ. The EPZ size criteria in 10 CFR 50.33(g)(2)(i) are based on determination of the area where the lower end of the EPA PAG levels (i.e., 10 mSv (1 rem) TEDE over 96 hours0.00111 days <br />0.0267 hours <br />1.587302e-4 weeks <br />3.6528e-5 months <br />) would be exceeded from release of radioactive materials from the facility considering accident likelihood and source term, timing of the accident sequence, and meteorology. Although the regulatory requirement in 10 CFR 50.33(g)(2)(i) does not specifically provide dose metrics that vary with likelihood of the release, the probabilistic dose aggregation guidance in RG 1.242, appendix A, states that the likelihood of exceeding the plume exposure pathway EPZ dose criterion should be consistent with the evaluation in appendix I to NUREG-0396. TR Criterion C gives an immediately life-threatening dose metric for very low probability radiological release sequences.

TR Criterion C is similar to the criterion for the evaluation of the worst core melt sequences in NUREG-0396, where it was determined that there was a low likelihood that immediately life-threatening doses would be projected outside of the 10-mile plume exposure pathway EPZ for the evaluated set of large light water reactor severe accidents with core melt and containment bypass or failure. As stated in TR section 6.1.3, Worst Radiological Release Sequences, Criterion C is evaluated using a 24-hour exposure 200 rem red bone marrow acute effective dose. The NRC staff determined that this dose metric is comparable to the dose metric of 200 rem whole body acute dose used in the NUREG-0396 analyses for the worst-case core melt sequences.

The release frequencies for events selected to be compared to TR Criterion C are important when the intent of the TR methodology criterion is to be consistent with the NUREG-0396 evaluation, which considered the event frequencies as well as scenario characteristics for the worst core melt sequences.

TR sections 3.6 and 3.7 describe the risk-informed selection of release sequences to compare to each of the criteria, as supported by information in TR sections 3.4 and 3.5. The NRC staffs evaluation of the TR event selection methodology is described in the following SE subsections 1.3 through 1.6. SE subsection 4.1 describes the NRC staffs evaluation of the TR section 6.1 description of the TR methodology evaluation of the dose results against the dose-based criteria.

1.3 Development of PRA RG 1.242, appendix A, section A-3.1 provides guidance on the selection of events to use in the consequence analysis to determine the facility-specific plume exposure pathway EPZ size. The guidance states that the applicant should consider the LBEs relevant to the facility described in the facility safety analysis report as candidates for development of potential radiological releases. As discussed in TR section 3.4, the technology-inclusive, risk-informed, and performance-based methodology in NEI 18-04, as endorsed by RG 1.233, is used to determine LBEs. While TerraPower chose not to explicitly use the LBEs in its methodology, the PRA

7 developed for implementation of NEI 18-04 is used. RG 1.242, appendix A, acknowledges that the development of the licensing basis in conformance with RG 1.233 is an option for non-LWRs.

The development and use of PRA is a fundamental part of the NEI 18-04 methodology. TR section 3.4 states that the PRA will be developed using the guidance in the non-LWR PRA standard (ASME/ANS RA-S-1.4) and will address the full spectrum of internal events and external hazards, as well as all operating modes. TerraPower also stated that the PRA will be peer reviewed and meet the non-LWR PRA standard before the submittal of the final plume exposure pathway EPZ sizing analysis for a facility application. RG 1.233 states that the methodology in NEI 18-04 includes an expanded role for PRA and that the NRC staffs review of the PRA prepared by a reactor designer could be facilitated by the designers use of the NRC staff-endorsed consensus codes and standards. RG 1.247 (for trial use) describes the acceptability of PRA for purposes such as supporting the NEI 18-04 methodology.

As discussed in TR section 3.4, the PRA is used to select the spectrum of events used in the plume exposure pathway EPZ sizing methodology. TerraPower states that event sequences for all internal and external events, and for all operating modes will be considered. The TR methodology uses PRA information directly instead of starting with the LBEs determined through the NEI 18-04 process. The TR methodology includes a review of the assumptions and sources of uncertainty in the PRA to identify and address any effect on the plume exposure pathway EPZ sizing methodology. TR sections 3.5 through 3.7 describe the selection of events for the plume exposure pathway EPZ sizing methodology.

1.4 Hazards and initiating events TR section 3.5 describes the methodology for evaluating a broad spectrum of events including hazard groups from the non-LWR PRA standard ASME/ANS RA-S-1.4. Events that are screened out will be identified and documented with justification. The TR methodology will include accident phenomena as analyzed in the PRA which are found to be applicable to the Natrium reactor design. As stated in TR section 3.5.3, Other Risk Events the methodology will also evaluate other risks which are design-or site-specific that may lead to potential offsite radionuclide releases that may impact plume exposure pathway EPZ sizing. The NRC staff determined that the TR identification of hazards and initiating events is acceptable because the scope of potential events is consistent with the guidance in RG 1.242.

Although not addressed by the PRA standard ASME/ANS RA-S-1.4, the TR methodology considers security events for completeness. TR section 3.5.2 discusses how security events and accidents resulting from security events will be addressed. Specifically, the TR methodology states that a qualitative or quantitative assessment of security events will be documented in the plume exposure pathway EPZ size calculation to ensure that security events are addressed, and the associated risks are captured within the calculation. The NRC staff determined that the TR treatment of security events in the calculation is aligned with and provides additional information compared to the guidance in RG 1.242 regarding event selection and is therefore acceptable.

As discussed in TR section 3.5.4, Event Groupings, the PRA will support the categorization and evaluation of events to be used in the plume exposure pathway EPZ sizing analysis. The plume exposure pathway EPZ sizing events will be identified by using the PRA event sequences, event sequence families, and groupings, as included in the PRA documentation.

The NRC staff determined that the use of PRA to support categorization and evaluation of

8 plume exposure pathway EPZ events is acceptable because it is consistent with the guidance in RG 1.242 and RG 1.247.

1.5 Selection of non-seismic release sequences TR section 3.6.1, Criterion, discusses the screening criteria for selection of non-seismic release sequences. The DBAs determined in the LBEs will be included. Non-seismic release sequences with mean frequencies greater than or equal to 1x10-7 per reactor-year and those sequences that contribute 1% or more to the overall release frequency will be included.

Individual events and groups with mean frequency sums greater than 1x10-8 per reactor-year will be considered for cliff-edge effects. The NRC staff determined that this non-seismic release sequence screening approach is acceptable because it would result in a spectrum of events consistent with that evaluated in NUREG-0396 and, with the difference analyzed below, discussed in RG 1.242.

Cliff-edge effects evaluation involves identifying and addressing scenarios where small changes in input parameterssuch as initiating event frequencies; structures, systems, components (SSCs) failure probabilities; or assumptionscan lead to disproportionately large changes in risk outcomes. The staffs review identified a difference between TerraPowers proposed methodology to be used to determine the PEP EPZ sizing for proposed Natrium reactor design and item B3 in Appendix B of RG 1.242 regarding the cliff-edge effects evaluation. The proposed methodology specifies that individual events and groups with combined frequencies between 1x10-7 per reactor-year and 1x10-8 per reactor-year will be considered for cliff-edge effects and that individual events and groups with combined frequencies 1x10-8 per reactor-year or less would be discarded. RG 1.242, however, retains event sequences with frequencies below the cutoff threshold to confirm the absence of cliff-edge effects.

Unlike RG 1.242, which does not reference the non-light water reactor (NLWR) PRA standard or impose conformance to it, the TerraPower proposed methodology specifies that the PRAs will follow the guidance in the NLWR PRA standard, undergo peer review, and meet all NLWR PRA standard requirements. Furthermore, the NRC staff imposed Limitations and Conditions in this SE to ensure that PRAs supporting the proposed methodology address all applicable hazards, all modes, all sources of radioactive material, and maintain technical acceptability.

The NLWR PRA standard includes specific requirements for addressing cliff-edge effects, considering various approaches to minimize these associated risks. By conforming to the NLWR PRA standard:

All applicable initiating events, including equipment failures, human errors, and external hazards, are properly considered; Modeling captures low-frequency events, concurrent initiating events, secondary hazards, and combinations of hazards, ensuring comprehensive representation of possible severe outcomes; Conservative estimates for event frequencies, failure probabilities, success criteria, etc.,

account for worst-case scenarios; Intersystem and intrasystem common-cause failures and intersystem and intrasystem dependencies are modeled to ensure that redundant systems are thoroughly analyzed and not simultaneously significantly affected by a same event; Uncertainty and sensitivity analyses comprehensively identify critical parameters driving risk and focus efforts on reducing risk; and

9 The oversimplification of event sequences that might obscure severe consequences would be avoided.

The proposed Natrium power plant design includes inherent redundancy and diversity, supported by defense-indepth strategies. These design features provide multiple independent protective layers and sufficient safety margins to mitigate cliff-edge effects and address uncertainties effectively. Moreover, importance measures, such as risk achievement worth, provide insights into SSC performance to minimize the risks associated with cliff-edge effects. In addition, peer review of PRA models will further consider expert evaluation to confirm that cliff-edge effects are adequately addressed.

Based on the above, regardless of the 1x10-8 per reactor-year cutoff frequency cited in the TR methodology, an applicant referencing this TR will justify that cliff-edge effects have been thoroughly evaluated and identify and address the risks associated with low-frequency events.

Therefore, considering the factors mentioned above and conformance to NLWR PRA standard requirements and RG 1.247, the NRC staff determined that cliff-edge effects will be appropriately identified and addressed when implementing the proposed PEP EPZ sizing methodology.

TR section 3.6.2, Parameter Uncertainty, describes the treatment of uncertainties in the screening of non-seismic release sequences. The NRC staff determined that the uncertainty treatment is acceptable because it is consistent with guidance in NUREG-1855, Revision 1, Guidance on the Treatment of Uncertainties Associated with PRAs in Risk-Informed Decisionmaking (ML17062A466), which is a reference in RG 1.242.

1.6 Selection of seismic release sequences TR section 3.7 describes the criterion for seismic event selection using insights from a site-specific scoping level seismic PRA (SPRA) to establish a limiting peak ground acceleration (PGA) for the site for use in the plume exposure pathway EPZ sizing methodology. The TR states that the limiting PGA would be aligned to achieve at least two times the ground motion response spectrum (GMRS) for the site with the intent to limit the range of seismic hazard under consideration within the credible range of ground motions. However, the TR also states an upper bound PGA of 1.0 gravitational acceleration (g) will be used to acknowledge the limitations of the SPRA and uncertainties associated with the availability of local and state emergency response infrastructure at large ground motions.

The limiting PGA will be used to establish the bounding seismic event for the plume exposure pathway EPZ for a construction permit application. The TR states that this bounding seismic event is expected to capture the important phenomena that will challenge the required safety functions and radiological barriers from a seismic event. The limiting PGA will then be used as the event screening threshold for the selection of plume exposure pathway EPZ events for the seismic plume exposure pathway EPZ sizing calculation for an operating license application.

The TR further states that the limiting seismic scenario identified at the construction permit phase is expected to capture the important phenomena that will challenge the required safety functions and radiological barriers after a seismic event. TR section 3.7.2, Parameter Uncertainty, describes how the bounding seismic scenario accounts for uncertainty and evaluates for cliff-edge effects by using insights from the SPRA.

For the early stages of design (i.e., construction permit stage), the NRC staff determined that

10 the use of the scoping level SPRA is acceptable if it is of sufficient technical adequacy to support its role in the plume exposure pathway EPZ sizing analysis. However, the NRC staff notes that the seismic scenario selection methodology described in section 3.7 of the TR could potentially result in a scenario that may not be representative of the potential accident consequences that should be included in determining the Plume Exposure Pathway EPZ for the Natrium reactor design at a specific site. Specifically, in the case of a site for which two times the GMRS would result in a PGA that exceeds 1.0 g, it is not clear that basing the selection of the seismic release scenario on an upper bound PGA of 1.0 g would be encompassing of most seismic release sequences for the facility without further information on the potential scenarios.

Therefore, the NRC staff imposes Limitation and Condition 5, below, on the use of this TR for applications that utilize the upper bound PGA of 1.0 g to determine the seismic scenario.

Subject to this condition that requires additional site-specific justification, the NRC staff concludes that the TR determination of a bounding seismic event is based on a review of the full spectrum of seismic events, as informed by frequency considerations and impact to the facility.

Additionally, with the inclusion of this condition, the NRC staff determined that the selection of seismic release sequences is consistent with guidance in RG 1.242 and is acceptable because the bounding seismic event used is bounding for most release sequences and accounts for the dose consequences of seismic events as well as uncertainty.

1.7 Release timing TR section 3.8, Release Timing, states that the timing of release of radionuclides is an input to the TR methodology. Release timing will be determined by the source term methodology and is therefore out of scope of the TR methodology. Radionuclide release timing information will be used if necessary to identify the events that require prompt protective measures. The staff determined that this is consistent with the guidance in RG 1.242 and is acceptable.

2.0 Source Term Methodology TR section 4, Source Term Methodology, states that methodology to develop mechanistic source terms associated with the release scenarios selected for plume exposure pathway EPZ sizing is consistent with the overall Natrium reactor assessment and projections. The NRC staff understands that this is referring to the development of mechanistic source terms for application safety analysis report analyses and the Natrium PRA. The source terms for the release scenarios are treated as input to the TR methodology. The TR states that the development of source terms is addressed in a separate TR (NAT-9392, Radiological Source Term Methodology Report (ML24261B944)), and therefore is out of scope of this TR. The NRC staff determined that referencing accident radiological source terms from the safety analysis for the facility is acceptable because it is consistent with guidance in RG 1.242, appendix A, item A-3.2.

The NRC staff will review the development of source terms as part of its review of a related license application and the implementation of the related TR on radiological source term methodology.

3.0 Radiological Consequence Considerations 3.1 Radiological consequence analysis TR section 5, Radiological Consequence Considerations, describes the radiological consequence analysis for the methodology. TR sections 5.1 through 5.3 describe the

11 considerations for analyzing the radiological consequences of the selected events. The TerraPower radiological consequence analysis methodology is described in NAT-9391, Radiological Release Consequences Methodology Topical Report (ML24208A181), which at this time is under NRC staff review. The EPZ sizing methodology TR briefly discusses meteorological and population data, as well as the use of the MELCOR Accident Consequence Code System (MACCS). The EPZ sizing methodology TR also identifies that the initial plume exposure pathway EPZ sizing calculation will use 12 consecutive months of representative data, while the final plume exposure pathway EPZ sizing analysis will use a full two-year set collected from the site-specific meteorological data. The NRC staff determined that this approach is acceptable because it is consistent with the guidance in RG 1.242 and the discussion of meteorology and atmospheric dispersion in the interim staff guidance for the Advanced Reactor Content of Application Project DANU-ISG-2022-02, chapter 2, Site Information (ML23277A140).

The NRC staff evaluated the subject TR discussion with respect the use of the output of the radiological consequences methodology as input to the plume exposure pathway EPZ sizing analysis methodology. The TR states that the consequence analysis methodology will be the same as the LBE consequence analysis methodology with the exception that the doses will be calculated for a 96-hour or 24-hour exposure period instead of the 30-day dose needed for the LBE methodology. The NRC staff determined that the calculation of 96-hour dose is acceptable because it is consistent with the requirements in 10 CFR 50.33(g)(2)(i) and the guidance in RG 1.242.

The TR methodology also evaluates acute whole-body dose for a comparison of the worst-case radiological release sequences to a separate criterion related to immediate life-threatening doses discussed in TR section 6.1.3. The NRC staffs evaluation of this criterion is given below in SE section 4.1. The NRC staff determined that the calculation of a 24-hour exposure for the acute dose is acceptable because it is consistent with the time period used for a similar substantial reduction in early deterministic health effect criterion in NUREG-0396. TR section 5.3 states that the consequence analysis will not model protective actions such that there is no credit for evacuations, relocations, or sheltering of the public. The NRC staff determined that that no modeling of protective actions is acceptable because it is consistent with the guidance in RG 1.242 for plume exposure pathway EPZ sizing consequence analysis.

3.2 Dose estimation for pathway contributors TR section 5.4, Dose Estimation for Pathway Contributors, describes the dose pathways modeled in the consequence analysis, and states that the evaluation against the three TR criteria is done in an iterative process to determine the appropriate distance for the boundary of the plume exposure pathway EPZ for a specific facility license application. If any of the dose-based criteria are exceeded at a chosen EPZ boundary distance (nominally the site boundary) during the design phase, then an applicant using this TR would perform subsequent analyses that either change the EPZ boundary distance or make changes to the reactor design to reduce potential offsite consequences or, for the events screened into the worst-case radiological release sequences, change the release frequencies. Exposure pathways include cloud shine, inhalation, resuspension, and ground shine, which is consistent with the guidance in RG 1.242.

The dose results calculated are TEDE to the individual for Criteria A and B, and red bone marrow effective acute dose to the individual for Criterion C. TR section 5.4 provides a justification for use of red bone marrow effective acute dose in lieu of whole-body acute dose.

The NRC staff determined that that TEDE is consistent with the 10 CFR 50.33(g) requirements and RG 1.242, while the red bone marrow effective acute dose is consistent with the evaluation

12 in NUREG-0396 for very severe accidents.

4.0 Probabilistic Dose Aggregation Section 6, Probabilistic Dose Aggregation, of the TR describes the methodology for aggregating doses from different source terms with consideration of the associated frequencies, to provide confidence that the appropriate plume exposure pathway EPZ size has been determined and that risk to the general public is minimized. The NRC staffs evaluation of the probabilistic dose aggregation including comparison to the dose-related criteria, determination of the necessity of predetermined prompt protective measures and treatment of uncertainty is described below.

4.1 Dose-related criteria for plume exposure pathway EPZ sizing TR section 6.1 describes the evaluation of the three dose-related criteria in the TR methodology and states that the plume exposure pathway EPZ will be established at the furthest distance at which all three criteria will be met. In Criterion A for each DBA in the licensing basis, a mechanistic source term will be developed using the TerraPower radiological source term methodology and the consequences evaluated using the TerraPower radiological release consequence analysis methodology (NAT-9392 and NAT-9391, respectively). The mean 96-hour TEDE will be compared to a dose level of 10 mSv (1 rem) TEDE, which is the lower end of the EPA PAG. The 95th percentile 96-hour TEDE will be compared to a dose level of 50 mSv (5 rem), which is the upper end of the EPA PAG. Criterion B for most radiological release sequences is handled in the same way as DBAs, with consideration of events screened into the plume exposure pathway EPZ sizing analysis with mean release frequencies greater than 1x10-

6. The NRC staff determined that these groupings of events and the associated dose-related criteria are acceptable since they are similar to those evaluated in NUREG-0396 and discussed in RG 1.242.

Events screened into the plume exposure pathway EPZ sizing analysis with mean release frequencies below 1x10-6 but greater than 1x10-7 are compared to Criterion C for the worst-case radiological release sequences. The consequences of these events will be compared to a dose metric of 200 rem red marrow acute effective dose for a 24-hr exposure period. Additionally, these events will be analyzed to ensure that the dose drops rapidly beyond the plume exposure pathway EPZ boundary by generating a dose-distance chart mapping the dose reduction as one moves away from the EPZ. The NRC staff notes that the lower end frequency used to determine the worst-case radiological release scenarios is lower than that used to evaluate beyond design basis events in the NEI 18-04 process. This difference, along with the cliff-edge effect evaluations for events with mean frequencies as low as 1x10-8, as described in TR section 6.3, Uncertainty and Sensitivity Analysis Methodology, will ensure that very low probability events with potentially high consequences will not be inappropriately scoped out of the plume exposure pathway EPZ sizing analysis. The NRC staff determined that this evaluation of worst-case radiological release sequences and the associated dose-related criteria is acceptable because the evaluation is similar to those worst core melt accidents evaluated in NUREG-0396 and referred to in RG 1.242, appendix A, on probabilistic dose aggregation.

4.2 Necessity of predetermined prompt protective measures The plume exposure pathway EPZ sizing requirements in 10 CFR 50.33(g)(2)(i)(B) state that the plume exposure pathway EPZ is that area within which predetermined, prompt protective measures are necessary. TR section 6.2, Necessity of Predetermined Prompt Protective

13 Measures, describes the use of radiological release timing to identify the necessity of prompt protective measures for the events included in the plume exposure pathway EPZ sizing analysis. The timing of each event will be assessed individually and if any event is identified to need prompt protective measures and it exceeds the relevant dose-related criterion, the iterative evaluation process will be followed as described in TR section 6.2 until the appropriate plume exposure pathway EPZ is established. Identified protective measures will inform the emergency plan and procedures. The NRC staff determined that the use of radiological release timing to determine the necessity of prompt protective measures is acceptable because it is consistent with the guidance in RG 1.242, appendix A.

LIMITATIONS AND CONDITIONS The NRC staff imposes the following limitations and conditions with regard to the use and approval of the subject TR:

1. The PRAs used to implement the TR methodology will be design-and site-specific and developed for all applicable hazards, all modes, and all sources of radioactive material, using the guidance in RG 1.247 Acceptability of Probabilistic Risk Assessment Results for Non-Light-Water Reactor Risk-Informed Activities and appendix A to RG 1.253 Guidance for a Technology-Inclusive Content-of-Application Methodology to Inform the Licensing Basis and Content of Applications for Licenses, Certifications, and Approvals for Non-Light-Water Reactors (ML23269A222). Prior to the initial fuel loading, any exceptions to meeting capability categories referred to in RG 1.247 should be justified and documented.

2. An applicant that references this TR must justify the technical acceptability of the PRAs performed for the selected hazards and modes (e.g., site-specific scoping level seismic PRA). Prior to the initial fuel loading, PRAs supporting this methodology must include all applicable hazards and modes.

3. An applicant that references this TR must provide discussions of (1) how PRA key assumptions and key sources of uncertainty for each analyzed hazard, mode, and radioactive source were identified; (2) how the key assumptions and key sources of uncertainty identified as having the potential to significantly impact the PRA results have been characterized in a manner consistent with the current state of knowledge; and 3) how the impact of each identified key assumption and source of uncertainty was assessed and dispositioned.

4. An applicant that references this TR must justify that the scoping level seismic PRA is of sufficient technical acceptability. This means that the model will be design-and site-specific and developed based on acceptable methods and data. The engineering analyses, assumptions, and approximations used in developing the scoping level seismic PRA should be appropriate and should demonstrate the robustness of the conclusions with respect to the uncertainties in the assessment. Prior to the initial fuel loading, an applicant that references this TR must reassess the EPZ size using a seismic PRA that meets the requirements of non-LWR PRA standard, as endorsed in RG 1.247, to the extent necessary to support plume exposure pathway EPZ sizing calculation.

5. An applicant that references this TR will provide site-specific justification for the use of the upper bound PGA of 1.0 g when exercising the methodology in TR section 3.7, Selection of Seismic Release Sequences, to choose the seismic scenario for the EPZ sizing analysis.

6. A periodic evaluation of the plume exposure pathway EPZ sizing analysis must be performed following an update or upgrade to the users PRAs based on a review of changes to the plant structures, systems, and components, operational practices, and applicable plant and industry operational experience. Any changes to the emergency preparedness plan as a result of the evaluation should be conducted under 10 CFR 50.54(q).

14 CONCLUSION The NRC staff has completed its review of TR number NAT-3056, TerraPower, LLC (TerraPower) Natrium' Topical Report: Plume Exposure Pathway Emergency Planning Zone Sizing Methodology. Based on its evaluation the NRC staff determined that NAT-3056, Revision 3, subject to the limitations and conditions discussed above, provides an approach acceptable to the NRC staff to develop analyses to aid in the determination of a site-and design-specific plume exposure pathway EPZ for the Natrium reactor. Accordingly, the NRC staff concludes that the subject TerraPower TR can be used in establishment of the plume exposure pathway EPZ size support emergency planning and preparedness in compliance with the regulatory requirements in 10 CFR 50.33(g) and 10 CFR 50.47(c)(2), as applicable, for prospective TerraPower Natrium reactor construction permit or operating license applications under 10 CFR Part 50.

Principal Contributors: Mallecia Sutton, NRR Michelle Hart, NRR Hanh Phan, NRR Edward Robinson, NSIR Kenneth Mott, NSIR

15

SUBJECT:

TERRAPOWER, LLC - REVISED DRAFT SAFETY EVALUATION OF NATRIUM TOPICAL REPORT NAT-3056, PLUME EXPOSURE PATHWAY EMERGENCY PLANNING ZONE SIZING METHODOLOGY, REVISION 3 (EPID L-2023-TOP-0024/CAC 000431) DATED: November 20, 2024 DISTRIBUTION:

Public RidsNrrDanu Resource RidsNrrDanuUal1 Resource RidsOgcMailCenter Resource HPhan, NRR RBrusselmans, NRR SDevlin-Gill, NRR MSutton, NRR DAtkinson, NRR KWagner, NRR JBorromeo, NRR DGreene, NRR CdeMessieres, NRR MHart, NRR JQuintero, NSIR KMott, NSIR ERobinson, NSIR

SUBJECT TO DOE COOPERATIVE AGREEMENT NO. DE-NE0009054 Copyright © 2024 TerraPower, LLC. All Rights Reserved NATD-1911 Rev. 1 Governing Procedure: NAT-1848 Controlled Document - Verify Current Revision 3bd86c56-17c7-4df1-b641-e3e09a903425 Document

Title:

TerraPower, LLC (TerraPower) Natrium Topical Report: Plume Exposure Pathway Emergency Planning Zone Sizing Methodology Natrium Document No.:

NAT-3056 Rev. No.:

3 Page:

1 of 37 Effective Date:

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TerraPower, LLC (TP)

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MDS.7 Approval Approval signatures are captured and maintained electronically; See Electronic Approval Records in EDMS.

NAT-3056 Rev. 3 TerraPower, LLC (TerraPower) Natrium Topical Report: Plume Exposure Pathway Emergency Planning Zone Sizing Methodology Page 2 of 37 Controlled Document - Verify Current Revision SUBJECT TO DOE COOPERATIVE AGREEMENT NO. DE-NE0009054 Copyright © 2024 TerraPower, LLC. All Rights Reserved REVISION HISTORY Revision No.

Effective Date Affected Section(s)

Description of Change(s)

A 03/02/2023 All Initial Issue - DRAFT 2-28-23 0

03/14/2023 All Initial Issue 1

See EDMS All Change addresses questions from the NRC during the PEP EPZ Audit and minor editorial changes.

2 See EDMS All Changed section 3.6 to account for changes suggested by ACRS subcommittee. Additionally, a minor change was made to 6.1.3 for consistency.

3 See EDMS All Changed to Rev. 3 in the header as it was missed for Rev.

2

NAT-3056 Rev. 3 TerraPower, LLC (TerraPower) Natrium Topical Report: Plume Exposure Pathway Emergency Planning Zone Sizing Methodology Page 3 of 37 Controlled Document - Verify Current Revision SUBJECT TO DOE COOPERATIVE AGREEMENT NO. DE-NE0009054 Copyright © 2024 TerraPower, LLC. All Rights Reserved TABLE OF CONTENTS EXECUTIVE

SUMMARY

................................................................................................. 5 1

INTRODUCTION...................................................................................................... 7 1.1 Purpose....................................................................................................................... 7 1.2 Scope.......................................................................................................................... 7 1.3 Abbreviations............................................................................................................... 7 2

REGULATORY BASIS............................................................................................. 9 2.1 Regulatory Requirements and Guidance Considered............................................... 10 2.2 Development of the Regulatory Basis for the Plume Exposure Pathway Emergency Planning Zone........................................................................................................... 11 2.2.1 10-Mile Plume Exposure Pathway Emergency Planning Zone............................. 11 2.2.2 Reduction in the Plume Exposure Pathway Emergency Planning Zone.............. 12 2.2.3 Development of a Risk-Informed Plume Exposure Pathway Emergency Planning Zone...................................................................................................................... 14 2.3 Proposed Emergency Preparedness Rule for Small Modular Reactors and Other New Technologies..................................................................................................... 15 2.4 Previous NRC Considerations of Reduced Emergency Planning Zone Sizes.......... 16 3

ACCIDENT SCREENING METHODOLOGY.......................................................... 18 3.1 Process Overview..................................................................................................... 18 3.2 Application of Risk-Informed Methods in Event Selection......................................... 19 3.3 Dose-Based Criteria.................................................................................................. 20 3.4 Development of the Probabilistic Risk Assessment.................................................. 21 3.5 Hazards and Initiating Events.................................................................................... 22 3.5.1 Treatment of Hazards Groups.............................................................................. 22 3.5.2 Security Events..................................................................................................... 23 3.5.3 Other Risk Events................................................................................................. 23 3.5.4 Event Groupings................................................................................................... 23 3.5.5 Defense-in-Depth.................................................................................................. 24 3.6 Selection of Non-Seismic Release Sequences......................................................... 24 3.6.1 Criterion................................................................................................................ 24 3.6.2 Parameter Uncertainty.......................................................................................... 24 3.7 Selection of Seismic Release Sequences................................................................. 25 3.7.1 Criterion................................................................................................................ 25 3.7.2 Parameter Uncertainty.......................................................................................... 26

NAT-3056 Rev. 3 TerraPower, LLC (TerraPower) Natrium Topical Report: Plume Exposure Pathway Emergency Planning Zone Sizing Methodology Page 4 of 37 Controlled Document - Verify Current Revision SUBJECT TO DOE COOPERATIVE AGREEMENT NO. DE-NE0009054 Copyright © 2024 TerraPower, LLC. All Rights Reserved 3.8 Release Timing......................................................................................................... 26 4

SOURCE TERM METHODOLOGY........................................................................ 30 5

RADIOLOGICAL CONSEQUENCE CONSIDERATIONS....................................... 30 5.1 Meteorological Input.................................................................................................. 30 5.2 Population Data......................................................................................................... 30 5.3 Radiological Consequence Analysis......................................................................... 30 5.4 Dose Estimation for Pathway Contributors................................................................ 31 6

PROBABILISTIC DOSE AGGREGATION.............................................................. 31 6.1 Criteria for Plume Exposure Pathway Emergency Planning Zone Sizing................. 31 6.1.1 Design-Basis Accidents........................................................................................ 31 6.1.2 Most Radiological Release Sequences................................................................ 32 6.1.3 Worst Radiological Release Sequences............................................................... 32 6.1.4 Establishment of the Plume Exposure Emergency Planning Zone....................... 33 6.2 Necessity of Predetermined Prompt Protective Measures........................................ 33 6.3 Uncertainty and Sensitivity Analysis Methodology.................................................... 33 7

SUMMARY

AND CONCLUSIONS ON METHODOLOGY...................................... 34 8

REFERENCES....................................................................................................... 34 LIST OF FIGURES Figure 3-1: Overall Methodology to Determine PEP EPZ Distance............................................ 27 Figure 3-2: Non-seismic Release Sequence Screening............................................................. 28 Figure 3-3: Seismic Event Analysis Flow Chart.......................................................................... 29

NAT-3056 Rev. 3 TerraPower, LLC (TerraPower) Natrium Topical Report: Plume Exposure Pathway Emergency Planning Zone Sizing Methodology Page 5 of 37 Controlled Document - Verify Current Revision SUBJECT TO DOE COOPERATIVE AGREEMENT NO. DE-NE0009054 Copyright © 2024 TerraPower, LLC. All Rights Reserved EXECUTIVE

SUMMARY

This topical report provides the methodology and criteria that will be used to establish the site-specific plume exposure pathway (PEP) emergency planning zone (EPZ) size for the Natrium reactor. The Natrium reactor is a TerraPower, LLC (TerraPower) and GE Hitachi technology.

This methodology provides a risk-informed approach for determining a PEP EPZ size based on the area within which public dose, as defined in Title 10 of the Code of Federal Regulations, Part 20.1003, Definitions, is projected to exceed 10 mSv (1 rem) total effective dose equivalent over 96 hours0.00111 days <br />0.0267 hours <br />1.587302e-4 weeks <br />3.6528e-5 months <br /> from the release of radioactive materials from the facility, considering accident likelihood and source term, timing of the release sequence, and meteorology.

The methodology utilizes the approach laid out in Appendix A, General Methodology for Establishing Plume Exposure Pathway Emergency Planning Zone Size, of proposed Regulatory Guide 1.242, "Performance-Based Emergency Preparedness for Small Modular Reactors, Non-Light-Water Reactors, and Non-Power Production or Utilization Facilities," as well as supporting information from NUREG-0396, "Planning Basis for the Development of State and Local Government Radiological Emergency Response Plans in Support of Light Water Nuclear Power Plants."

Release sequences (events that lead to a radiological release) to be considered in the PEP EPZ methodology will be selected based on risk-information from the design and site-specific Natrium reactor probabilistic risk assessment (PRA). The PRA will address all modes and hazards, including seismic events, using the guidance in Regulatory Guide 1.233, "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," Nuclear Energy Institute 18-04, Risk-Informed Performance-Based Guidance for Non-Light Water Reactor Licensing Basis Development, and American Society of Mechanical Engineers/American Nuclear Society RA-S-1.4-2021, "Probabilistic Risk Assessment Standard for Advanced Non-Light Water Reactor Nuclear Power Plants."

All non-seismic release sequences contributing one percent (1%) or more to the overall release frequency will be included, as well as all Design Basis Accidents. Individual events and groups with sums greater than the frequency 1E-08 per reactor year will be considered for cliff-edge effects. All non-seismic release sequences with a frequency greater than or equal to 1E-07 per reactor year and contributing 1% or more of overall release frequency will be retained for evaluation.

The seismic events evaluated for the PEP EPZ sizing will be based on the beyond design basis earthquake for the site. The seismic events sequences are based on a spectrum of seismic events and is consistent with NUREG-0396, "Planning Basis for the Development of State and Local Government Radiological Emergency Response Plans in Support of Light Water Nuclear Power Plants". Most of the risk due to seismic events will be captured and the release sequences will ensure that the PEP EPZ size is appropriate for the seismic event.

Once the release sequences are selected, release sequence simulations will be conducted to determine projected doses. The methodology for source term development for the PEP EPZ

NAT-3056 Rev. 3 TerraPower, LLC (TerraPower) Natrium Topical Report: Plume Exposure Pathway Emergency Planning Zone Sizing Methodology Page 6 of 37 Controlled Document - Verify Current Revision SUBJECT TO DOE COOPERATIVE AGREEMENT NO. DE-NE0009054 Copyright © 2024 TerraPower, LLC. All Rights Reserved analysis will be consistent with overall Natrium reactor assessment and projections and will be a direct input into the radiological consequences methodology. This input will establish the specific radionuclide inventory and the quantity released for the events that will be assessed in the PEP EPZ analysis. A mechanistic source term methodology will be used for the source term calculations that provide the radioactive materials released to the environment. A meteorological file will be created by obtaining meteorological data available for a representative location for one year for the calculation submitted with the Construction Permit Application and two years that is most representative of the meteorological conditions at the site for the calculation submitted with the Operating License Application.

Projected doses will be evaluated against three dose-based criteria akin to those in NUREG-0396. During the design phase, if the PEP EPZ sizing criteria are not met, then a determination will be made if design changes or analysis refinements can be made to reduce the PEP EPZ size, or if the PEP EPZ size needs to be expanded. Accident and consequence simulations will be reperformed to address any changes made.

NAT-3056 Rev. 3 TerraPower, LLC (TerraPower) Natrium Topical Report: Plume Exposure Pathway Emergency Planning Zone Sizing Methodology Page 7 of 37 Controlled Document - Verify Current Revision SUBJECT TO DOE COOPERATIVE AGREEMENT NO. DE-NE0009054 Copyright © 2024 TerraPower, LLC. All Rights Reserved 1

INTRODUCTION 1.1 Purpose The purpose of this topical report (ToR) is to provide the methodology and criteria that will be used to establish the site-specific plume exposure pathway (PEP) emergency planning zone (EPZ) size for the Natrium reactor.

The ToR contains the PEP EPZ sizing methodology for which U.S. Nuclear Regulatory Commission (NRC) approval is sought. This methodology provides an approach for determining a PEP EPZ size based on the area within which public dose, as defined in Title 10 of the Code of Federal Regulations (10 CFR) 20.1003, Definitions, is projected to exceed 10 mSv (1 rem) total effective dose equivalent (TEDE) over 96 hours0.00111 days <br />0.0267 hours <br />1.587302e-4 weeks <br />3.6528e-5 months <br /> from the release of radioactive materials from the facility, considering accident likelihood and source term, timing of the release sequence, and meteorology. In addition, it addresses the consideration of the area in which predetermined, prompt protective measures are necessary.

1.2 Scope The PEP EPZ methodology utilizes the approach laid out in Appendix A, General Methodology for Establishing Plume Exposure Pathway Emergency Planning Zone Size, of proposed Regulatory Guide (RG) 1.242, "Performance-Based Emergency Preparedness for Small Modular Reactors, Non-Light-Water Reactors, and Non-Power Production or Utilization Facilities," [1] as well as supporting information from NUREG-0396, "Planning Basis for the Development of State and Local Government Radiological Emergency Response Plans in Support of Light Water Nuclear Power Plants" [2].

This report is based on the following technical considerations:

Methodology is designed to be structured and repeatable, Risk-informed methods are used to determine the spectrum of release sequences to be evaluated, including internal, external, and seismic events, and Analysis of uncertainties.

This PEP EPZ methodology is based upon numerical inputs from the Probabilistic Risk Assessment (PRA), Radiological Consequence Assessment Methodology [3] and Source Term Methodology [4], which are outside the scope of this ToR. The associated uncertainty with each input will be quantified within their own respective assessments, however, the overall uncertainty will be addressed in the PEP EPZ analysis submitted for the Operating License Application (OLA). The methodology for the uncertainty analysis within the scope of this ToR is described within this report.

1.3 Abbreviations ACRS Advisory Committee on Reactor Safeguards ANS American Nuclear Society

NAT-3056 Rev. 3 TerraPower, LLC (TerraPower) Natrium Topical Report: Plume Exposure Pathway Emergency Planning Zone Sizing Methodology Page 8 of 37 Controlled Document - Verify Current Revision SUBJECT TO DOE COOPERATIVE AGREEMENT NO. DE-NE0009054 Copyright © 2024 TerraPower, LLC. All Rights Reserved AOO Anticipated Operational Occurrence ASME American Society of Mechanical Engineers BDBE Beyond Design Basis Event CEMP comprehensive emergency management plan CFR Code of Federal Regulations CPA Construction Permit Application CPG Comprehensive Preparedness Guide DBA Design Basis Accident DBE Design Basis Event DID defense-in-depth DG Draft Guide DL defense line EAB Exclusion Area Boundary EAL Emergency Action Level EOP emergency operations plan EP Emergency Preparedness EPA Environmental Protection Agency EPZ Emergency Planning Zone ESP Early Site Permit F-C frequency-consequence FEMA Federal Emergency Management Agency GEH GE-Hitachi Nuclear Americas, LLC GMRS Ground Motion Response Spectra LBE Licensing Basis Event LWR light-water reactor MACCS MELCOR Accident Consequence Code System NEI Nuclear Energy Institute NRC U.S. Nuclear Regulatory Commission NSRST non-safety-related with special treatment NPUF non-power production or utilization facilities NUREG U.S. Nuclear Regulatory Commission technical report designation

NAT-3056 Rev. 3 TerraPower, LLC (TerraPower) Natrium Topical Report: Plume Exposure Pathway Emergency Planning Zone Sizing Methodology Page 9 of 37 Controlled Document - Verify Current Revision SUBJECT TO DOE COOPERATIVE AGREEMENT NO. DE-NE0009054 Copyright © 2024 TerraPower, LLC. All Rights Reserved NUREG/CR contractor prepared NUREG OLA Operating License Application ONT Other New Technology PAG Protective Action Guide PEP Plume Exposure Pathway PGA Peak Ground Acceleration PIE plant initiating event PRA Probabilistic Risk Assessment rem Roentgen equivalent man RG Regulatory Guide SECY Office of the Secretary SMR Small Modular Reactor SOARCA State-of-the-Art Reactor Consequence Analysis (NUREG-1935)

SPRA Seismic Probabilistic Risk Assessment SR safety-related SRM staff requirements memorandum SSC structure, system, and component SSE Safe Shutdown Earthquake TED total effective dose TEDE total effective dose equivalent TerraPower TerraPower, LLC ToR Topical Report TVA Tennessee Valley Authority 2

REGULATORY BASIS The purpose of this section is to discuss the regulatory basis that supports the PEP EPZ sizing methodology. It also discusses the historical background on the development of the regulatory basis of the 10-mile PEP EPZ for the light water reactor (LWR) operating nuclear plants, the regulatory basis for reducing and risk-informing the PEP EPZ size, and recent NRC rulemaking and guidance documents that address reevaluation of PEP EPZ size and planning elements for small modular reactors (SMRs) and other new technologies (ONTs). Also provided are examples where the NRC has considered and approved reduced PEP EPZ sizes.

NAT-3056 Rev. 3 TerraPower, LLC (TerraPower) Natrium Topical Report: Plume Exposure Pathway Emergency Planning Zone Sizing Methodology Page 10 of 37 Controlled Document - Verify Current Revision SUBJECT TO DOE COOPERATIVE AGREEMENT NO. DE-NE0009054 Copyright © 2024 TerraPower, LLC. All Rights Reserved 2.1 Regulatory Requirements and Guidance Considered The methodology described in this ToR is based upon the proposed requirements for Emergency Preparedness (EP) for SMRs and ONTs in SECY-22-0001, Rulemaking Issue (Affirmation), Final Rule: Emergency Preparedness for Small Modular Reactors and Other New Technologies [5]1 (herein referred to as proposed final EP SMR-ONT rule) and proposed RG 1.242, Performance-Based Emergency Preparedness for Small Modular Reactors, Non-Light-Water Reactors, and Non-Power Production or Utilization Facilities [1].

The proposed final EP SMR-ONT rule provides a scalable approach for determining the size of the PEP EPZ based on both the projected off-site public doses from a spectrum of events and the need for predetermined, prompt protective measures. Proposed RG 1.242, Appendix A, General Methodology for Establishing Plume Exposure Pathway Emergency Planning Zone Size, provides a sample methodology acceptable to the NRC for the analysis to establish PEP EPZ size, as required under proposed final EP SMR-ONT rule, specifically 10 CFR 50.33(g)(2).

The methodology also takes into consideration the regulatory information and guidance in the following documents:

NUREG-0396/EPA 520/1-78-016, "Planning Basis for the Development of State and Local Government Radiological Emergency Response Plans in Support of Light Water Nuclear Power Plants" (herein referred to as NUREG-0396) [2], provides a planning basis for off-site emergency preparedness efforts considered necessary and prudent for large power reactor facilities. It also provides the technical basis for the current EPZ regulations for operating power reactors referenced in 10 CFR 50.47, Emergency plans, and a PEP EPZ of about 10 miles. Additionally, it provides the technical basis for the methodology in the proposed final EP SMR-ONT rule.

EPA-400/R-17/001, PAG Manual: Protective Action Guides and Planning Guidance for Radiological Incidents [7] (herein referred to as the EPA PAG Manual), provides radiological protection criteria for application to all incidents that would require consideration of protective actions. The Environmental Protection Agency (EPA)

Protective Action Guideline (PAG) Manual, Section 1.4, Radiological Incident Phases and Applicability of Protective Actions, discusses the phases in which emergency planners divide responses to radiological incidents. The Early Phase of a radiological incident is defined as:

The beginning of a radiological incident for which immediate decisions for effective use of protective actions are required and must therefore be based primarily on the status of the radiological incident and the prognosis for worsening conditions. This phase may last from hours to days.

For the Early Phase PAGs, the established projected dose criteria range from 1 to 5 rem total effective dose (TED)2 over four days.

1 On August 14, 2023, the NRC Commissioners approved the final EP SMR-ONT rule [6]

2 The NRC staff notes that the EPA defined TED is different than the NRC defined TEDE, as it utilizes different dosimetry methodologies. As such, the NRC uses its definition of TEDE for regulatory activities under its statutory authority.

NAT-3056 Rev. 3 TerraPower, LLC (TerraPower) Natrium Topical Report: Plume Exposure Pathway Emergency Planning Zone Sizing Methodology Page 11 of 37 Controlled Document - Verify Current Revision SUBJECT TO DOE COOPERATIVE AGREEMENT NO. DE-NE0009054 Copyright © 2024 TerraPower, LLC. All Rights Reserved NUREG-1855, "Guidance on the Treatment of Uncertainties Associated with PRAs in Risk-Informed Decision Making," [8] provides guidance for the treatment of uncertainties in a risk-informed application. The objectives of the guidance include fostering an understanding of the uncertainties associated with PRA, their impact on the results of a PRA, and provides a pragmatic approach to addressing these uncertainties in the context of the decision-making.

RG 1.174, An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis [9] provides an integrated approach for risk-informed decision-making that considers traditional engineering and risk information and that may be based on qualitative factors as well as quantitative analyses and information. It describes the principles of risk-informed decision-making that include addressing defense-in-depth (DID) and maintaining safety margin in parallel with use of risk analysis techniques.

RG 1.200, Acceptability of Probabilistic Risk Assessment Results for Risk-Informed Activities [10] describes an approach for determining the acceptability of a PRA. This RG provides guidance on the four areas that collectively determine the acceptability of a PRA (i.e., scope, technical elements, level of detail, and plant representation) that can be met using national consensus PRA standards and a peer review.

2.2 Development of the Regulatory Basis for the Plume Exposure Pathway Emergency Planning Zone Existing EP regulations and guidance are primarily focused on large LWRs, and which have developed over time based on gained experience. 10 CFR 50, Appendix E, Emergency Planning and Preparedness for Production and Utilization Facilities, identifies the specific items currently required in emergency plans. Additionally, 10 CFR 50.47 provides EP requirements for nuclear power reactors, including planning standards for on-site and off-site emergency response plans. Other relevant regulations include 10 CFR 50.54, Conditions of licenses, paragraphs (q), (s), and (t).

In 10 CFR 50.47(a)(1), prior to issuing an Operating License, the NRC must find that the there is reasonable assurance that adequate protective measures can be taken in the event of a radiological emergency. The NRC's determination of reasonable assurance is based in part on a Federal Emergency Management Agency (FEMA) review of the adequacy of off-site plans and resulting determinations and findings that adequate off-site protective measures can be implemented, where applicable.

2.2.1 10-Mile Plume Exposure Pathway Emergency Planning Zone The purpose of this section is to provide background on the regulatory basis for development of the 10-mile PEP EPZ for the large LWRs.

NUREG-0396 [2], which was based on NUREG-75/014 (WASH-1400), Reactor Safety Study:

An Assessment of Accident Risks in U.S. Commercial Nuclear Power Plants [11], provides a planning basis for off-site emergency preparedness efforts considered necessary and prudent for large power reactor facilities and also provides the technical basis for a PEP EPZ of about 10 miles (16 kilometers). The NUREG-0396 Task Force concluded that the appropriate planning

NAT-3056 Rev. 3 TerraPower, LLC (TerraPower) Natrium Topical Report: Plume Exposure Pathway Emergency Planning Zone Sizing Methodology Page 12 of 37 Controlled Document - Verify Current Revision SUBJECT TO DOE COOPERATIVE AGREEMENT NO. DE-NE0009054 Copyright © 2024 TerraPower, LLC. All Rights Reserved distance should be determined by consideration of a spectrum of accident consequences, tempered by probability considerations, and that no single reactor accident scenario should drive determination of the EPZ size. The NUREG-0396 Task Force, also concluded that the EPZ should be the area beyond which the projected dose from Design Basis Accidents (DBAs) and less severe core damage accidents (i.e., accidents not involving large releases of radioactive material to the environment) would not likely exceed the then applicable early-phase EPA PAGs in EPA-520/1-75-001, Manual of Protective Action Guides and Protective Actions for Nuclear Incidents [12].3 Additionally, the PEP EPZ should be of sufficient size to provide for substantial reduction in early severe health effects in the event of more severe core-melt accidents (i.e., more severe than the design basis accidents with a release of substantial quantities of radioactive materials to the environment). For the 10-mile PEP EPZ, NUREG-0396 evaluates the projected off-site doses against the EPA guidance of 1 and 5 rem whole body for DBAs, and the significant early injury (health effect) threshold of 200 rem whole body acute dose from more severe core-melt accidents.

The NRC issued a policy statement, Planning Basis for Emergency Responses to Nuclear Power Reactor Accidents, [13] endorsing the NUREG-0396 PEP EPZ of about 10 miles for detailed planning and early response (e.g., 30 minutes to one day after the initiation). The intent of the EPZ was to provide dose savings to the population in areas where the projected dose could be expected to exceed the then applicable EPA PAGs. The PEP EPZ was codified in 10 CFR 50.47 and 10 CFR 50, Appendix E. Both these regulations stipulate the 10-mile PEP EPZ for power reactors, but also allow for a different PEP EPZ size for reactors with a thermal power of less than 250 megawatt thermal on a case-by-case basis.

NUREG-0654/FEMA-REP-1, Criteria for Preparation and Evaluation of Radiological Emergency Response Plans and Preparedness in Support of Nuclear Power Plants [14], which was originally published in 1980, provides guidance and evaluation criteria for the development and evaluation of operating power reactor and off-site response organization radiological emergency response plans.

2.2.2 Reduction in the Plume Exposure Pathway Emergency Planning Zone The NRC staff has provided EPZ-related information and conducted several studies that are useful in the reconsideration of the PEP EPZ size and planning elements for SMRs and ONTs, and the associated process for regulatory change in the following documents:

SECY-97-020, Results of Evaluation of Emergency Planning for Evolutionary and Advanced Reactors [15], provides the rationale upon which EP is based for current reactor designs, stating that potential consequences from a spectrum of accidents is appropriate for use as the basis for EP for evolutionary and passive advanced LWR designs, and is consistent with the Commissions DID safety philosophy.

SECY-10-0034, Potential Policy, Licensing, and Key Technical Issues for Small Modular Nuclear Reactor Designs [16], in part, addresses the possibility of considering the appropriate PEP EPZ size and the extent of on-site and off-site emergency planning, taking into account the characteristics of SMRs (e.g., smaller size, lower power 3 In January 2017, the EPA published an updated version of this document, EPA-400/R-17/001, PAG Manual Protective Action Guides and Planning Guidance for Radiological Incidents, [5] which supersedes EPA-520/1-75-001.

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SECY-11-0152, "Development of an Emergency Planning and Preparedness Framework for Small Modular Reactors" [17], discusses that even though the guidance in NUREG-0396 and EPA-400 was written for large LWRs, the principle of using dose savings to determine PEP EPZ size can also be applied to SMRs. The NRC indicated that it may be appropriate for SMRs to develop reduced PEP EPZ sizes, commensurate with their accident source terms, fission product releases, and accident dose characteristics. The projected approach for PEP EPZ sizing is based on off-site dose considerations and the use of a PRA that includes dose assessments to calculate the probability of exceeding a PAG as function of distance from the Exclusion Area Boundary (EAB) for a spectrum of accidents and establishing criteria for determining the point at which the probability of exceeding the PAG is acceptably low.

SECY-15-0077, "Options for Emergency Preparedness for Small Modular Reactors and Other New Technologies" [18], the NRC staff sought Commission approval to initiate proposed revisions to NRC regulations and guidance for a consequence-based approach, including requirements that would allow SMR and ONT license applicants to demonstrate how their proposed facilities achieve appropriate dose limits at specified PEP EPZ distances, which may be as low as the site boundary. SECY-15-0077 indicates that the regulations can be established generically without site-or design-specific information regarding source term, fission products, or projected off-site dose.

Design and licensing information provided by SMR and ONT applicants would be rigorously reviewed by the NRC to ensure that the off-site dose consequences are commensurate with the requested PEP EPZ size and to ensure that applicable requirements for adequate protection of public health and safety, and the environment, are met. The Commission approved the NRC staff recommendation to initiate the rulemaking in staff requirements memorandum (SRM) to SECY-15-0077 [19].

NUREG-1935, State-of-the-Art Reactor Consequence Analysis (SOARCA) Report [20],

evaluates fission product releases, associated off-site consequences, and hypothetical evacuations in response to potential accidents in operating plants. By applying modern analysis tools and techniques, the SOARCA project developed a body of knowledge regarding the realistic outcomes of select severe nuclear reactor accidents. The study focused on providing a realistic evaluation of accident progression, source term, and off-site consequences for select scenarios for Peach Bottom Atomic Power and Surry Power Stations. By using the most current EP practices and plant capabilities, as well as the best available modeling, these analyses are more realistic than past analyses. These analyses also consider mitigative measures (e.g., emergency operating procedures, severe accident management guidelines, and 10 CFR 50.54(hh) measures), contributing to a more realistic evaluation. All SOARCA scenarios, even when unmitigated, progress more slowly and release much less radioactive material than previously indicated. As a result, the calculated risks of public health consequences from severe accidents modeled in SOARCA are very small.

NAT-3056 Rev. 3 TerraPower, LLC (TerraPower) Natrium Topical Report: Plume Exposure Pathway Emergency Planning Zone Sizing Methodology Page 14 of 37 Controlled Document - Verify Current Revision SUBJECT TO DOE COOPERATIVE AGREEMENT NO. DE-NE0009054 Copyright © 2024 TerraPower, LLC. All Rights Reserved 2.2.3 Development of a Risk-Informed Plume Exposure Pathway Emergency Planning Zone Recent NRC documents also reflect progress in risk-informed methods and applications as applied to emergency planning, including PEP EPZ sizing:

SRM to SECY-98-144, White Paper on Risk-Informed and Performance-Based Regulation [21] defines a risk-informed approach to regulatory decision-making as one that represents a philosophy whereby risk insights are considered together with other factors to establish requirements that better focus licensee and regulatory attention on design and operational issues commensurate with their importance to public health and safety. Additionally, it states that DID is an element of the NRCs Safety Philosophy, which ensures that safety will not be wholly dependent on any single element of the design, construction, maintenance, or operation of a nuclear facility.

SECY-10-0034 [16] states that the NRC staff plans to use a risk informed-and performance-based approach that employs deterministic judgment and analysis complemented by PRA information to review design and license applications for SMRs.

As provided in in the enclosure to SECY-10-0034, the NRC staff will consider a different or revised set of accidents for SMRs (e.g., other than those considered for current LWRs) to provide the basis for judging the adequacy of features such as off-site emergency planning. Additionally, the enclosure to SECY-10-0034 discusses that while the Commission stated that licensing-basis event categories (i.e., abnormal occurrences, design-basis accidents, and beyond-design basis-accidents) would be established based on the expected probability of event occurrence, selection of licensing basis events within each category would be performed using deterministic engineering judgment complemented by insights from the PRA.

SECY-11-0152 [17] states that an appropriate method for addressing PEP EPZ size would involve (1) using a PRA that includes dose assessments based on current insights in severe accident progression to calculate the probability of exceeding a PAG level as function of distance from the EAB for a spectrum of accidents, (2) establishing criteria for determining the point at which the probability of exceeding the PAG level is acceptably low, and (3) concluding that the events provide an acceptable spectrum of consequences.

NUREG-1935 [20] the SOARCA scenarios were selected from the results of previous staff and licensee PRAs. Some of these existing PRAs model accident sequences to the point of radiological release (i.e., Level 2 PRAs); however, most of existing PRAs in the SOARCA were limited to the onset of core damage (i.e., Level 1 PRAs). Ideally the SOARCA project would have included those sequences found to be important to risk as demonstrated by a full-scope Level 3 PRA, which is an assessment of risk of off-site consequences in the event of a severe accident that causes the release of radioactive material to the environment, however, they were not available.

SECY-15-0077 [18] states that the concept of a PEP EPZ size commensurate with the off-site radiological risk is not new to the NRC. The NRC staff anticipated that the technical basis for this EP framework would be developed also as part of rulemaking.

This would include quantitative guidelines and criteria for accident selection and evaluation specific to SMRs and ONTs. As described in SECY-15-0077, These guidelines and criteria would then be used to derive a dose-based, consequence-oriented rationale, similar to that described in SECY-11-0152 [17], which would be used

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NUREG/CR-7154, Risk Informing Emergency Preparedness Oversight: Evaluation of Emergency Action LevelsA Pilot Study of Peach Bottom, Surry and Sequoyah [22],

was the first effort to apply PRA to nuclear power plant Emergency Action Level (EAL) schemes. The methodology and the limited pilot applications described in the report demonstrate the feasibility of using risk-informed approaches to enhance emergency planning. The report notes that regulatory decisions for EP are complex and should not be made solely considering PRA generated risk metrics but should be substantiated by deterministic approaches along with the PRA insights.

SECY-22-0001 [5] states that the new alternative EP requirements and implementing guidance in proposed RG 1.242 [1] adopt a performance-based, technology-inclusive, risk-informed, and consequence-oriented approach.

Proposed RG 1.242 [1], Appendix A, Section A-3.1, Event Selection, provides that for non-LWRs, the applicant may use the technology-inclusive, risk-informed, and performance-based methodology endorsed by RG 1.233, 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 [23], to determine their licensing basis events. RG 1.233 endorses Nuclear Energy Institute (NEI) 18-04, Risk-Informed Performance-Based Guidance for Non-Light Water Reactor Licensing Basis Development [24] as one acceptable method for non-LWR designers to use when carrying out these activities and preparing their applications. NEI 18-04 identifies that the guidance in American Society of Mechanical Engineers (ASME)/ American Nuclear Society (ANS) RA-S-1.4, "Probabilistic Risk Assessment Standard for Advanced Non-Light Water Reactor Nuclear Power Plants," [25] provides an acceptable means to establish the scope and technical adequacy of the PRA.

Trial RG 1.247, Acceptability of Probabilistic Risk Assessment Results for Advanced Non-Light Water Reactor Risk-Informed Activities [26] endorses ASME/ANS RA-S-1.4-2021 [26] with exception. This trial RG also endorses NEI 20-09, Performance of PRA Peer Reviews Using the ASME/ANS Advanced Non-LWR PRA Standard [28] without exception.

2.3 Proposed Emergency Preparedness Rule for Small Modular Reactors and Other New Technologies In May 2020, the NRC published a proposed rule, "Emergency Preparedness for Small Modular Reactors and Other New Technologies" [29] and an accompanying draft RG, DG-13504, 4 DG-1350 was the precursor to draft RG 1.242.

NAT-3056 Rev. 3 TerraPower, LLC (TerraPower) Natrium Topical Report: Plume Exposure Pathway Emergency Planning Zone Sizing Methodology Page 16 of 37 Controlled Document - Verify Current Revision SUBJECT TO DOE COOPERATIVE AGREEMENT NO. DE-NE0009054 Copyright © 2024 TerraPower, LLC. All Rights Reserved "Performance-Based Emergency Preparedness for Small Modular Reactors, Non-Light-Water Reactors, and Non-Power Production or Utilization Facilities," [30] for public comment. The proposed rule provides an alternative option for SMRs and ONTs to determine the PEP EPZ as the area within which the dose to an individual is projected to exceed 1 rem TEDE over an exposure time of 96 hours0.00111 days <br />0.0267 hours <br />1.587302e-4 weeks <br />3.6528e-5 months <br /> from the release of radioactive materials resulting from a spectrum of accidents for the facility.

In January 2022, the NRC provided the proposed final EP SMR-ONT rule to the Commission for approval in SECY-22-0001 [5]. This proposed final rule was approved by the NRC Commissioners on August 14, 2023 [6]. This proposed final rule provides for new alternative EP requirements that adopt a performance-based, technology-inclusive, risk-informed, and consequence-oriented approach as an alternative to using the existing, deterministic EP requirements in 10 CFR 50. The proposed final EP SMR-ONT recognizes advances in design and technologically, safety enhancements in evolutionary and passive systems, and the potential benefits of smaller sized non-LWRs reactors, including slower transient response times and relatively small and slow release of fission products. While it continues to provide reasonable assurance that adequate protective measures will be implemented by an SMR or ONT licensee. In developing the regulation, the NRC considered the existing regulatory framework for EP at non-power production or utilization facilities (NPUFs) since it reflects the lower potential radiological hazards associated with the operation of SMRs and ONTs compared to large LWRs. The proposed final EP SMR-ONT rule provides for a scalable approach for determining the size of the PEP EPZ. NUREG-0396 [2] remains the technical basis for the methodology for the proposed final EP SMR-ONT rule.

As mentioned above, concurrent with the proposed rulemaking described in SECY-22-001 [5],

the NRC issued proposed RG 1.242 [1]. The guidance in proposed RG 1.242 addresses a performance-based, technology-inclusive, risk-informed, and consequence-oriented approach.

Appendix A to proposed RG 1.242 provides a sample methodology acceptable to the NRC for the analysis to establish the PEP EPZ size, as required under 10 CFR 50.33(g)(2). The approach has been generalized from the dose assessment methodologies that informed PEP EPZ size determinations in NUREG-0396 [2].

As discussed in Section 2.1, the proposed final EP SMR-ONT rule and proposed RG 1.242 serve as the primary regulatory basis for the PEP EPZ sizing methodology described in this ToR.

2.4 Previous NRC Considerations of Reduced Emergency Planning Zone Sizes The concept of a PEP EPZ size commensurate with off-site radiological risk is not new to the NRC. The NRC has considered and approved reduced PEP EPZ sizes in the following examples:

The NRC reviewed and approved various PEP EPZ size-related exemption requests from reactor licensees that have permanently ceased operations and defueled based on the reduced risks from the sites. Examples include:

Three Mile Island Nuclear Station, Units 1 and 2 - Exemptions from Certain Emergency Planning Requirements and Related Safety Evaluation [31], and Pilgrim Nuclear Power Station - Exemptions from Certain Emergency Planning Requirements and Related Safety Evaluation [32].

NAT-3056 Rev. 3 TerraPower, LLC (TerraPower) Natrium Topical Report: Plume Exposure Pathway Emergency Planning Zone Sizing Methodology Page 17 of 37 Controlled Document - Verify Current Revision SUBJECT TO DOE COOPERATIVE AGREEMENT NO. DE-NE0009054 Copyright © 2024 TerraPower, LLC. All Rights Reserved In their reviews, the NRC staff concluded that the postulated dose from any applicable DBA would not exceed the EPA early phase PAG levels for the public at the EAB.

Additionally, the NRC concluded that for any highly unlikely Beyond Design Basis Events (BDBEs), the length of time available for the site personnel to implement pre-planned mitigation measures consistent with plant conditions and, if warranted, for off-site agencies to implement protective actions using a comprehensive emergency management plan (CEMP)5, provided confidence that off-site measures for the public could be taken without preplanning. In NSIR/DRP-ISG-02, Emergency Planning Exemption Requests for Decommissioning Nuclear Power Plants [33], the staff concluded that if a minimum of 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> was available to initiate mitigative actions or, if needed, for off-site authorities to implement protective actions using a CEMP approach, and formal off-site radiological emergency plans, required under 10 CFR 50, were not necessary. CLI-19-10, Memorandum and Order, Tennessee Valley Authority (Clinch River Nuclear Site Early Site Permit Application) [34], granted an Early Site Permit (ESP) to the Tennessee Valley Authority (TVA) for the Clinch River Site. In CLI-19-10, the NRC discusses their conclusion that TVAs methodology for establishing a 2-mile and site boundary EPZ is consistent with the methodology used to establish the 10-mile EPZs reflected in our current regulations. The Advisory Committee on Reactor Safeguards (ACRS) concluded that the NRC staff was correct in determining that TVAs EPZ-sizing methodology is consistent with analyses that form the technical basis of the current [10-mile] PEP EPZ and maintains the same level of protection. Section C.3 of CLI-19-10 notes that the Staff does "not view TVA's proposal [of site boundary PEP EPZ] as eliminating an element of defense in depth; rather, emergency planning activities would be appropriately scaled to reflect the potential hazards posed by the facility."

In the Safety Evaluation for NuScale Topical Report, TR-0915-17772, Revision 3, Methodology for Establishing the Technical Basis for Plume Exposure Emergency Planning Zones at NuScale Small Modular Reactor Plant Sites [35] the NRC staff concluded that there is reasonable assurance that the proposed methodology in the NuScale Topical Report is adequate for assessing PEP EPZ size. The NuScale PEP EPZ methodology uses a risk-informed approach to screen appropriate release sequences to be evaluated for the determination of the PEP EPZ size. The screening includes quantitative insights from PRA, including consideration of uncertainty, as well as application of engineering insights emphasizing safety margin and DID. Based on the accident sequence screening, the risk results, including source terms and off-site dose versus distance, serve as the basis for a PEP EPZ size methodology. It includes consideration of internal events, external hazards, and all modes of operation, as well as 5 A CEMP, in this context, also referred to as an emergency operations plan (EOP), is addressed in FEMAs Comprehensive Preparedness Guide (CPG) 101, Developing and Maintaining Emergency Operations Plans [36]. CPG 101 is the foundation for State, territorial, Tribal, and local EP in the United States. It promotes a common understanding of the fundamentals of risk-informed planning and decision making and helps planners at all levels of government in their efforts to develop and maintain viable, all-hazards, all-threats emergency plans. An EOP is flexible enough for use in all emergencies. It describes how people and property will be protected; details who is responsible for carrying out specific actions; identifies the personnel, equipment, facilities, supplies and other resources available; and outlines how all actions will be coordinated. A CEMP is often referred to as a synonym for all hazards planning.

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The NuScale methodology was informed by the 2013 NEI White Paper, White Paper on Proposed Methodology and Criteria for Establishing the Technical Basis for Small Modular Reactor Emergency Planning Zone [37], and incorporates concepts from NUREG-0396 [2] in that the objective goal is based on consideration of off-site dose-based consequences to distance.

The NRC imposed Conditions of Use related to the use of PRA in the NuScale PEP EPZ methodology, including that the PRA will be developed for all modes and hazards (at Capability Category II) and will be peer reviewed using the guidance in RG 1.200

[10]. Additionally, the identification, assessment, and dispositioning of key assumptions and sources of uncertainty should be consistent with the guidance in RG 1.200 and NUREG-1855 [8].

3 ACCIDENT SCREENING METHODOLOGY 3.1 Process Overview Figure 3-1 provides an overview of the PEP EPZ methodology. Each step that will be implemented to determine the final PEP EPZ distance is also presented below with the corresponding ToR section number:

Compile release sequences from the PRA for all internal and external initiators (Sections 3.4 and 3.5).

Perform screening of non-seismic release sequences based on frequency, including uncertainty (Section 3.6).

Perform screening of seismic release sequences with a unique set of selection criteria, including uncertainty (Section 3.7).

Meteorological data will be collected and incorporated into the radiological consequence analysis (Section 5.1). (Outside the scope of this ToR.)

Source term and radiological consequence analysis will be performed with projected PEP EPZ boundary and 96-hour event timing (Sections 4 and 5.3). (Outside the scope of this ToR.)

Evaluate radiological dose consequences against the PEP EPZ dose criteria established from proposed RG 1.242 [1] (Sections 3.3 and 6.1).

o If PEP EPZ sizing criteria are not met, then determine if design changes, or analysis refinements, can be made to improve PEP EPZ sizing considerations or if the PEP EPZ size must be expanded.

o Repeat accident and consequence analysis to address any changes made.

Determine the final PEP EPZ distance based on meeting the criteria described in Section 6.1.

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Risk-informed processes for any regulatory application should combine and balance insights from deterministic and probabilistic assessments. A qualitative evaluation of DID, consistent with regulatory guidance and practice, will be included in the risk-informed approach being used to confirm the existence, functionality, and capability of design features and strategies that balance accident prevention and mitigation to provide confidence in the acceptably low plant risk and demonstrate protection of the health and safety of the public. In the generic process used in NUREG-0396 [2] in the 1970s, the margins of safety provided by the EPZ were based on a combination of risk insights from NUREG-75/014 [WASH-1400], "Reactor Safety Study: An Assessment of Accident Risks in U.S. Commercial Nuclear Power Plants," [11] and "were qualitatively found adequate as a matter of judgment" as indicated in SECY-97-020, "Results of Evaluation of Emergency Planning for Evolutionary and Advanced Reactors" [15]. This qualitative, generic concept for determining the adequacy of the margins of safety can now be updated to include a more accurate risk-informed, design-specific approach with appropriate consideration given to quantitative and qualitative methods, as well as taking into consideration the inherent safety features associated with sodium-cooled fast reactors (SFRs). Since NUREG-0396 was published, the severe accident experimental knowledge base and analytical methods have advanced to the point where more accurate and realistic tools and models are available to support a risk-informed methodology for PEP EPZ sizing.

The risk-informed approach provided in this methodology follows the guidance in proposed RG 1.242, which includes: applying a dose-based framework with a consequence-based approach, event selections with an acceptable spectrum of consequences, and the use of a "spectrum of accidents" as a basis for developing emergency response plans and as the basis for PEP EPZ size.

The proposed risk-informed approach includes steps to achieve a more realistic consequence-based approach without having to resort to unrealistic assumptions or being over conservative.

These elements include:

Design and operational features that provide multiple, independent DID and very low release sequence frequencies with consideration of uncertainty.

Use of mechanistic models to calculate source terms and doses, which greatly reduces the uncertainty compared to older quantitative methods.

Integrated uncertainty analysis to increase confidence in the best estimate source term and consequence results as discussed in Section 6.3.

Application of qualitative means to address uncertainties in the context of very low frequency events including:

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o plant-level, qualitative evaluation of DID to demonstrate adequate balance between accident prevention and mitigation of potential consequences as an extension to emergency planning and NRC's existing DID philosophy and guidance, and o development of an emergency plan that provides a base for expanding mitigation and protective action strategies, if necessary, in accordance with regulatory guidance to provide additional DID.

3.3 Dose-Based Criteria The methodology provided in proposed RG 1.242 [1] has been generalized from the dose assessment methodologies that informed the PEP EPZ size determinations in NUREG-0396 [2].

The probabilistic dose aggregation in NUREG-0396 demonstrated that the PEP EPZ was of sufficient size such that the following conditions were met:

a. Projected doses from the traditional DBAs would not exceed PAG6 levels outside the PEP EPZ,

b. Projected doses from most core melt sequences7 would not exceed PAG levels outside the PEP EPZ, and

c. For the worst core melt sequences, immediate life-threatening doses would generally not occur outside the PEP EPZ.

As stated in proposed RG 1.242, "The methodologies used for event selection, identification of source terms, modeling of releases, and aggregation of potential off-site doses should provide similar confidence that appropriate off-site planning will be identified for small modular reactors, non-light-water reactors, and non-power production or utilization facilities."

The dose criteria employed in the methodology described in this ToR are:

Criterion A: Projected doses from the DBAs would not exceed PAG levels outside the PEP EPZ. (Refer to Section 6.1.1 for a discussion of this criteria.)

Criterion B: Projected doses from most radiological release sequences would not exceed PAG levels outside the PEP EPZ. (Refer to Section 6.1.2 for a discussion of this criteria.)

Criterion C: Immediate life-threatening doses from the worst-case radiological release sequences would generally not occur outside the PEP EPZ. (Refer to Section 6.1.3 for a discussion of this criteria.)

The use of the PAGs as criteria ensure that the PEP EPZ is properly sized by meeting an approved standard public exposure to dose. The metric used in NUREG-0396 for condition C is 6 Whenever the term "PAGs" is used, it refers only to the early phase EPA PAGs.

7 As provided in NEI 18-04, plant damage states in non-LWR may not involve an equivalent metric to the core damage state, therefore as provided elsewhere in this ToR, radiological release frequencies are used.

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The methodology for determining appropriate release sequences to be evaluated against the criteria are addressed in Sections 3.6 and 3.7, and method details for applying the dose criteria are provided in Section 6.1.

The application of these criteria is consistent with the guidance outlined within proposed RG 1.242 and provides a similar level of confidence provided by the criterion originally found in NUREG-0396.

3.4 Development of the Probabilistic Risk Assessment The PRA being developed is design and site-specific. It will address all modes of operation and external hazards, including seismic events, using the guidance in RG 1.233 [23] and NEI 18-04

[24]. RG 1.233 endorses NEI 18-04 as one acceptable method for non-LWR designers to use when selecting licensing basis events (LBEs)9, 10. The PRA will be developed using the guidance in ASME/ANS RA-S-1.4-2021, "Probabilistic Risk Assessment Standard for Advanced Non-Light Water Reactor Nuclear Power Plants" [27] and will address the full spectrum of internal events and external hazards that pose challenges to the capabilities of the plant. Before submittal of the final PEP EPZ sizing analysis, the PRA will be peer reviewed and meet the requirements within the PRA standard. The PRA will identify the facility radiological sources and events. Specific hazards may be screened or addressed in another manner and will be identified and assessed accordingly within the final PEP EPZ sizing calculation as described in Section 3.5.1.

NEI 18-04 describes a systematic process for identifying and categorizing event sequences11 as anticipated operational occurrences (AOOs), Design Basis Events (DBEs), BDBEs for non-LWRs.12 DBAs are derived from DBEs by assuming that only safety-related (SR) structures, systems, and components (SSCs) are available to mitigate the events. The primary determinate for categorizing events is the estimated release frequency of the event sequence.

8 Red bone marrow (the A-RED MARR MACCS output parameter) is an acceptable effective dose for acute whole-body dose (Reference [NUREG-0396/EPA 520/1-78-016] Section III, Subsection D).

9 LBEs are defined in terms of event sequences comprised of an Initiating Event, the plant response to the Initiating Event (which includes a sequence of successes and failures of mitigating systems) and a well-defined end state.

10 NEI 18-04 uses AMSE/ANS RA-S-1.4-2013 as an acceptable means for a PRA.

11 In the PRA, an "event sequence" refers to the progression from initiating event to an end state within an event tree, with each sequence representing a unique event progression. The term event sequence is used in lieu of the term release sequence used in LWR PRA standards because the scope of the LBEs includes AOOs and initiating events with no adverse impacts on public safety.

12 The definitions of some phrases used in NEI 18-04 are different from the same phrases used in NRC regulations and guidance developed for LWRs. The terms AOO and DBE are examples of similar terms having different definitions. The terminology used in this ToR with respect to event selection reflects the terminology in NEI 18-04.

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All sequences are treated individually within the PEP EPZ event selection process. Use of individual sequences also removes ambiguity in performing source term and dose analyses.

However, screened in sequences may be grouped into release categories to reduce the number of required source term and dose consequence simulations as discussed in Section 3.5.4.

Grouping of sequences into release categories will be identified and justified in PRA documentation.

The methodology in NEI 18-04 includes plotting event sequence families on the frequency-consequence (F-C) target and assessing margins based on event frequency and estimated 30-day dose at the EAB.13 The mean values of the frequencies are used to classify the LBEs into AOO, DBE, and BDBE categories. However, as described in NEI 18-04, Section 3.2.2, LBE Selection Process, when the uncertainty bands defined by the 5th percentile and 95th percentile of the frequency estimates straddles a frequency boundary, the LBE is evaluated in both LBE categories.

The PRA will be used to identify applicable event sequences to be considered in the PEP EPZ methodology. To support the identification of applicable event sequences, event sequences for all internal events and external events, as well as all operating modes, will be compiled.

A review of the assumptions and sources of uncertainty in the underlying PRA will be completed to identify and address any potential impact on the application of the PEP EPZ sizing method.

The uncertainty issues that can be directly related to sizing the PEP EPZ include:

key assumptions in the PRA, model uncertainty, and completeness uncertainty.

3.5 Hazards and Initiating Events 3.5.1 Treatment of Hazards Groups As stated above, the evaluation of events for the determination of PEP EPZ sizing requires that initiators from screened in event sequences include a broad spectrum of events, including internal and external events. Evaluated hazards will include the hazard groups from ASME/ANS RA-S-1.4-2021 [27]:

internal events, internal floods, internal fires, seismic events, 13 For the purposes of assessing the PEP EPZ size, the doses are assessed using a 96-hour (4-day) period.

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Additional hazards will be assessed including site specific hazards and their inclusion or screening will be identified within the associated PRA documentation and within the PEP EPZ sizing calculation. Events will also be evaluated against all modes of operation including full power, low power, refueling, and shutdown.

Accident phenomena will be analyzed as part of the PRA and will be used as input to the PEP EPZ sizing methodology. The PRA will address all hazards, all modes, all sources, and events will be screened according to specific screening criteria outlined in Sections 3.6 and 3.7. Events that are screened out will be identified and justified within the documentation. The PEP EPZ sizing analysis will include the relevant accident phenomena that is found to be applicable to the Natrium reactor design.

3.5.2 Security Events Security events are explicitly not addressed by ASME/ANS RA-S-1.4-2021. Security events will be considered for completeness for the PEP EPZ; however, accidents resulting from security events may be eliminated from detailed consideration in the PEP EPZ by meeting regulatory requirements to protect against threats. Security events are justified by meeting regulatory requirements and describing security-by-design features of the plant. A qualitative or quantitative assessment of the security events will be performed and documented in the PEP EPZ sizing calculation to ensure that these events are addressed, and their associated risks are captured within the PEP EPZ calculation.

3.5.3 Other Risk Events Other risks that are design-specific or site-specific that may also lead to potential off-site radionuclide releases that may impact PEP EPZ sizing will be included. These other risks will be identified and evaluated to ensure an appropriate PEP EPZ size. A full scope of the radionuclide sources will be incorporated into the PRA and the associated events will be assessed against the PEP EPZ dose criteria methodology to ensure proper sizing.

3.5.4 Event Groupings The PRA process supports the categorization and evaluation of PEP EPZ events in terms of estimated frequencies and consequences of event sequences or event families (i.e., groupings of event sequences having similar initiating events, challenges to plant safety functions, plant response, end state, and mechanistic source term). The event sequences and related estimations of frequencies and consequences include equipment malfunctions caused by internal and external hazards. The groupings will be consistent with ASME/ANS RA-S-1.4-2021 and will be identified and justified in PRA documentation. PEP EPZ events will be identified utilizing the PRA event sequences, event sequence families, and groupings. These events will be used for selection of sequences for the radiological consequence analysis.

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3.6 Selection of Non-Seismic Release Sequences 3.6.1 Criterion All non-seismic release sequences contributing one percent (1%) or more to overall release frequency (total release frequency of all release sequences) will be included as well as all DBAs. All non-seismic release sequences with a frequency greater than or equal to 1E-07 per reactor year and contributing 1% or more of overall release frequency will be retained for evaluation in the PEP EPZ analysis. (Seismic events will be evaluated separately and are discussed in Section 3.7.1.) Individual events and groups with sums greater than the frequency 1E-08 per reactor year will be considered for cliff-edge effects. Events and groups considered for cliff-edge effects will be included if the 95th percentile frequencies are above 1E-07 per reactor year, or minor changes to the events lead to large variations in plant response, or the risk profile of the events. PEP EPZ events are created using the event sequence families generated within the PRA.

This screening criteria is consistent with the spectrum of accidents that form the basis for the original PEP EPZ sizing in NUREG-0396 [2], capturing the range of WASH-1400 [11] release category frequencies. Additionally, identifying the spectrum of release sequences based on release frequency conservatively ignores the conditional probability of radionuclide release when compared to the quantification of WASH-1400 release category frequencies.

The sequence release frequency screening threshold of 1E-07 per reactor year is appropriate for the PEP EPZ sizing methodology and properly captures the associated risk to the facility as it is below the frequency-consequence analysis threshold within NEI 18-04 [24]. To ensure that cliff-edge effects and uncertainties are accounted for, events will be considered in the analysis down to a release frequency of 1E-08 per year.

3.6.2 Parameter Uncertainty Parametric uncertainty in initially screened out sequences will be evaluated to ensure a complete set of source terms is retained in the spectrum of release sequences used in the PEP EPZ sizing analysis.

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1. Release sequence frequencies will be calculated as mean frequencies.

2. Proximity of the mean sequence frequency to the 1E-07 per reactor year screening criteria will be identified.

3. If the mean sequence frequency is below a screening threshold, then the upper bound (95th percentile) of the frequency uncertainty range is compared to the screening criteria.

4. Consider for inclusion to the PEP EPZ sizing method those sequences that challenge the screening criteria in the PEP EPZ basis.

In the specific context of the PEP EPZ sizing method, these steps take the following form:

(Refer to Figure 3-2)

1. If the 95th percentile release frequency of the event is greater than 1E-7 per reactor year, then the event is screened into the PEP EPZ evaluation.

2. If the mean release frequency of the event is greater than 1E-8 per reactor year, then the event is considered for cliff-edge effects within the PEP EPZ evaluation.

3. If the mean release frequency of the event is below 1E-8 per reactor year, then the event is screened out of the PEP EPZ evaluation.

3.7 Selection of Seismic Release Sequences 3.7.1 Criterion The seismic event selection criteria will utilize the insights from a site-specific scoping level seismic PRA (SPRA) to establish a limiting peak ground acceleration (PGA) for the site needed for the PEP EPZ sizing analysis. This limiting PGA value determined for the PEP EPZ analysis would be aligned to achieve at least two times the Ground Motion Response Spectrum (GMRS) to limit the range of seismic hazard under consideration within the credible range of ground motions. However, an upper bound PGA of 1.0g will be utilized to acknowledge the limitations of the PRA and uncertainties associated with availability of local and state emergency response infrastructure at large ground motions. The use of limiting PGA value assures the range of ground motion that needs consideration for emergency planning and PEP EPZ sizing is bounding for the site and will be supported by the SPRA to ensure that the value selected adequately encompasses the associated risk from credible seismic events. The limiting PGA selected will identify the specific set of events that need consideration for the bounding seismic scenario used in PEP EPZ sizing based on the site-specific characteristics and design.

This limiting PGA will be utilized in establishing the bounding seismic event that will be utilized to establish the PEP EPZ sizing calculation for the CPA. The limiting PGA will be used as the event screening threshold for the selection of PEP EPZ events for the seismic PEP EPZ sizing calculation for the OLA. The evaluation of the events against the dose criteria will be consistent between the CPA and the OLA. This limiting scenario for CPA is expected to capture the important phenomena that will challenge the required safety functions and radiological barriers

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The bounding seismic scenario chosen, utilizing the insights from the SPRA, and representing the limiting PGA, will encompass most of the sequences and represent the dose consequences and seismic impacts to the facility from high consequence credible events found at the site. The proposed seismic event selection criterion is consistent with the philosophy discussed in NUREG-0396 [2]. Specifically, the seismic event to be analyzed will be determined based on a review of the full spectrum of seismic events, informed by frequency considerations, and used to establish the PEP EPZ. Additionally, this approach is consistent with the guidance in proposed RG 1.242 [1], Appendix A, in that the seismic event used for the PEP EPZ sizing determination is bounding for most radiological release sequences as well as accounts for the dose consequences and seismic impacts to the facility from high consequence credible events.

3.7.2 Parameter Uncertainty The bounding seismic scenario accounts for the phenomena and the consequences based on uncertainty and cliff-edge effects. The proposed approach ensures all credible sequence of events are evaluated utilizing insights from the SPRA including the assessment of the 95th percentiles of frequencies, within the range of the limiting PGA. and within the limiting PGA including results from the assessment of the 95th percentile frequencies. Uncertainty associated with the confidence of the site-specific seismic characteristics are addressed within the SPRA and specific seismic analyses. The SPRA, including the scoping level model, evaluates the distribution in the uncertainty quantification, see Section 6.3. The proposed method selects initiating events and event sequences by accounting for the limitations of PRA technology in the estimation of the frequency of rare events.

3.8 Release Timing The timing of the release of radionuclides for each event is determined as part of the source term methodology and is outside of the scope of this PEP EPZ ToR. The radionuclide release timing information will be utilized in the event groupings to inform the emergency procedures and, if necessary, identify the events that require prompt protective measures (refer to Section 6.2). The timing of events is established when developing the source term analysis for each event and will be identified and documented in the PEP EPZ sizing calculation.

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SOURCE TERM METHODOLOGY The source term methodology will be used to develop mechanistic source terms associated with the release scenarios. The mechanistic source terms are a direct input into the radiological consequences methodology. This input will establish the specific radionuclide inventory and the quantity released for the events that will be assessed in the PEP EPZ analysis. The methodology for source term development for the PEP EPZ analysis will be consistent with overall Natrium reactor assessment and projections. This methodology is addressed separately [4] and is outside the scope of this PEP EPZ ToR.

5 RADIOLOGICAL CONSEQUENCE CONSIDERATIONS 5.1 Meteorological Input Data collection to support the data file needed to assess the radiological consequences will occur in accordance with RG 1.23, Meteorological Monitoring for Nuclear Power Plants [38].

The initial PEP EPZ sizing calculation will use 12 consecutive months of representative meteorological data. The final PEP EPZ sizing will use a full two-year data set collected from the site-specific meteorological data program. The methodologies associated with the collection of the meteorological data and determination of radiological consequences are out of scope for this ToR. Meteorological data will be utilized within the radiological consequence analysis to properly assess the doses at the PEP EPZ boundary.

5.2 Population Data For the PEP EPZ analysis, a uniform population density is assumed rather than an external population data file to make the evaluation independent of site location. A population study was conducted to develop a proper population density assumption for MELCOR Accident Consequence Code System (MACCS) evaluations; however, the specifics of the population data will be addressed separately and is outside the scope of this PEP EPZ ToR.

5.3 Radiological Consequence Analysis The specific radiological consequence analysis methodology [4], including the use of MACCS

[39] and its associated inputs and uncertainty analysis, is outside the scope of the PEP EPZ methodology ToR. The results of that analysis are a direct input into the PEP EPZ methodology ToR and will be used specifically for the dose aggregation evaluation described in Section 6.1.

The radiological consequence specific to PEP EPZ methodology ToR will quantify the dose consequences to the public from the events identified for the PEP EPZ. The methodology for the radiological consequence analysis for the PEP EPZ analysis will be consistent with the LBE consequence analyses, except instead of a 30-day dose consequence calculation a 96-hour (4-day) or 24-hour dose calculation will be assessed. In addition, no protective actions are modeled including, no evacuation, relocations, or sheltering. The public is assumed to continue normal activities during the event. These requirements will be quantified in the radiological consequences ToR.

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In lieu of determining a quantity representing the total effective acute dose (i.e., whole body value), the red bone marrow effective acute dose is chosen for evaluation against the threshold for significant early injuries (Criterion C). The rationale for this assumption is based on the considerations that the red bone marrow has the lowest dose threshold value amongst the three early fatality health effects, spatially represents the entire body, is assumed to be typically the highest effective acute organ dose, and the red bone marrow dose threshold is very close to the NUREG-0396 [2] criterion of 200 rem acute whole-body dose. The use of red bone marrow dose for acute effective dose is supported by NUREG-0396.

The red-bone marrow effective dose and 96-hour public dose criteria are specific to the PEP EPZ sizing. The PEP EPZ exposure pathways include cloud shine, inhalation, resuspension, and ground shine for the red bone effective dose and the TEDE dose.

These consequences will be calculated using the MACCS peak dose on the spatial grid. The methodology for quantifying the dose and exposure pathways and the specifics of using MACCS will be addressed separately in the radiological consequences ToR [4] and is outside the scope of this PEP EPZ ToR.

6 PROBABILISTIC DOSE AGGREGATION The following is the methodology for aggregating the doses from different source terms with consideration of their frequencies. The methodologies outlined in proposed RG 1.242 [1] and NUREG-0396 [2] illustrate the need to provide a level of confidence that the appropriate PEP EPZ size has been established. To do this the projected doses of various events derived from the PRA will be evaluated against a set of specific criteria to ensure that risk to the general public is minimized. In Sections 3.6 and 3.7 it was illustrated how the selection of the events will take place. Once the events have been identified their respective dose consequences will be evaluated to ensure that the risk to the general public is acceptable.

6.1 Criteria for Plume Exposure Pathway Emergency Planning Zone Sizing 6.1.1 Design-Basis Accidents Criterion A: Projected doses from the DBAs would not exceed PAG levels outside the PEP EPZ. (Refer to Section 3.6 and 3.7 for accident selection.)

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6.1.2 Most Radiological Release Sequences Criterion B: Projected doses from most radiological release sequences would not exceed PAG levels outside the PEP EPZ. (Refer to Section 3.6 and 3.7 for accident selection.)

To evaluate Criterion B, PEP EPZ events with a mean release frequency greater than 1E-06 will be evaluated to determine if the PAG levels of 1 rem mean 4-day TEDE and 5 rems 95th percentile 4-day TEDE will be met. As provided in Section 5.4, this will be initially assessed using a nominal distance of the proposed site boundary. If the calculated dose exceeds the PAG levels, then an iterative process will be used during the design phase until design modifications are made to reduce the release frequency (or radiological consequence) or the PEP EPZ distance will be moved further out until the dose consequences meet the PAG levels.

6.1.3 Worst Radiological Release Sequences Criterion C: Immediate life-threatening doses from the worst-case radiological release sequences would generally not occur outside the PEP EPZ. (Refer to Section 3.6 and 3.7 for accident selection.)

To evaluate Criterion C, PEP EPZ events with a mean release frequency below 1E-06 but a 95th percentile greater than 1E-07 (see Section 3.6 for specifics) will be selected for evaluation. As provided in Section 5.4, these events will be initially assessed using a nominal distance at the proposed site boundary to identify which events meet the 24-hour, 200 rem red marrow acute effective dose. If the calculated 95th percentile dose exceeds the acute dose, then an iterative process will be used during the design phase until design modifications can be made to reduce the release frequency (or radiological consequence), or the mean event frequency falls below the 1E-07 range, or the PEP EPZ distance will be moved further out until all events in this range have a dose consequence that meets the acute dose levels. Additionally, these events will be analyzed to ensure that the dose drops rapidly from the PEP EPZ boundary. To ensure that the dose drops off rapidly from the PEP EPZ boundary, additional dose quantifications will take place at various distances beyond the PEP EPZ boundary to generate a dose distance chart mapping the reduction in dose as you move away from the PEP EPZ boundary. Probability of exceedance at distances from the site boundary as well as specific dose-distance information will be collected, and the information will be evaluated and documented to ensure the dose drops rapidly from the boundary. Dose distance curves will provide information like that of the dose distance curves found within NUREG-0396.

14 DBAs are derived from DBEs and assume that only SR SSCs are available to mitigate the events.

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6.2 Necessity of Predetermined Prompt Protective Measures As stated in Section 3.8, timing of the release of radionuclides will be captured for analyzed events. This timing information will be utilized to identify the necessity of prompt protective measures for PEP EPZ events and inform the emergency plan and response procedures. For the purposes of the PEP EPZ ToR, the preliminary analysis will begin with the radiological consequence analysis assuming all PEP EPZ events require predetermined prompt protective measures. If the radiological consequence of the event exceeds any of the dose criteria in Section 6.1 at the PEP EPZ boundary, then the timing of the release will be considered against the guidance in proposed RG 1.242 and specific assessment on the need for prompt protective measures will be completed. The timing of each event will be assessed individually to identify whether the need for predetermined prompt protective measures is necessary, incorporating the specific event information. This evaluation will be documented into the PEP EPZ calculation. If any PEP EPZ event is identified to necessitate the need for predetermined prompt protective measures and it exceeds any of the dose criteria, then modifications to the design or an extension of the PEP EPZ boundary are considered and would follow the iterative process described in Section 6.1. The iterative evaluation process will be used to reduce the release frequency (or radiological consequence) or move the PEP EPZ distance further out until the dose consequences meet the criterion dose levels. Identified protective measures will inform the emergency response plan and procedures. This ensures that predetermined prompt protective measure will be assessed for all PEP EPZ events.

6.3 Uncertainty and Sensitivity Analysis Methodology To ensure that the PEP EPZ events themselves are properly being assessed Monte Carlo sampling will be performed on the PRA event frequencies to ensure correct percentiles are properly captured. This ensures confidence in the specific mean and 95th percentile values. The Monte Carlo sampling is part of the PRA process (including the SPRA). The methodology to determine the uncertainties in radiological consequences, including the uncertainty in meteorological conditions is in the radiological consequence ToR [4]. Radiological consequence uncertainties are included in the results of the radiological consequence analyses.

The methodology of the source term uncertainty analysis is in the source term ToR [4]. Source term uncertainties are included in the results of the source term analyses.

Finally, to ensure that event impacts are representing the appropriate level of risk, cliff-edge events will be looked at to ensure that the event risks are properly captured. Events with frequencies down to 1E-08 will be included for cliff-edge effects. The primary focus is single failures that would either dramatically change the effects (e.g., timing, plant response, source terms, or the end states) of the accident sequencies or the risk metrics (e.g., specific doses or consequences). There are no specific criteria for what is considered a dramatic change, this is based on engineering judgement as part of the event analysis. Cliff-edge effect evaluations will be documented in the PEP EPZ sizing calculation.

NAT-3056 Rev. 3 TerraPower, LLC (TerraPower) Natrium Topical Report: Plume Exposure Pathway Emergency Planning Zone Sizing Methodology Page 34 of 37 Controlled Document - Verify Current Revision SUBJECT TO DOE COOPERATIVE AGREEMENT NO. DE-NE0009054 Copyright © 2024 TerraPower, LLC. All Rights Reserved 7

SUMMARY

AND CONCLUSIONS ON METHODOLOGY The proposed methodology for developing the technical basis for the PEP EPZ size utilizes the guidance in proposed RG 1.242 [1]. It is based on the selection of events from a PRA developed in accordance with current regulatory guidance and industry standard. It applies risk-informed methods and applications to determine PEP EPZ sizing. This risk-informed approach includes PRA information (including SPRA), mechanistic source terms, and a qualitative evaluation of defense-in-depth. The methodology applies severe accident knowledge and analytical methods developed since the current PEP EPZ size was determined based on NUREG-0396 [2], while still maintaining the fundamental characteristics of the methodology employed in NUREG-0396 as discussed in proposed RG 1.242.

The methodology is applicable to any PEP EPZ size, including a PEP EPZ within the site boundary. The final PEP EPZ size will be the smallest distance at which the dose consequences of all screened in release sequences are less than their respective dose criteria and requiring prompt protective measures, with a minimum distance at the site boundary. Based on the results of applying the methodology, the final PEP EPZ size may be less than the current 10-mile requirement.

8 REFERENCES

[1]

NRC, [Proposed] RG 1.242, Performance-Based Emergency Preparedness for Small Modular Reactors, Non-Light-Water Reactors, and Non-Power Production or Utilization Facilities, December 2021 [ADAMS Accession No. ML21285A035].

[2]

NRC/EPA, NUREG-0396/EPA 520/1-78-016, "Planning Basis for the Development of State and Local Government Radiological Emergency Response Plans in Support of Light Water Nuclear Power Plants," December 1978 [Agencywide Documents Access and Management System [ADAMS Accession No. ML051390356].

[3]

Letter from Ryan Sprengel (TerraPower) to U.S. Nuclear Regulatory Commission, Transmittal of TerraPower, LLC Topical Report, Radiological Consequences Methodology Report, Revision 0, TP-LIC-LET-0103, November 6, 2023 [ADAMS Accession No. ML23311A140]

[4]

Letter from Ryan Sprengel (TerraPower) to U.S. Nuclear Regulatory Commission, Transmittal of TerraPower, LLC Topical Report, Radiological Source Term Methodology Report, Revision 0, TP-LIC-LET-0093, August 11, 2023 [ADAMS Accession No. ML23223A235]

[5]

NRC, SECY-22-0001, Rulemaking Issue (Affirmation), Final Rule: Emergency Preparedness for Small Modular Reactors and Other New Technologies (RIN 3150-AJ68; NRC-2015-0225), January 3, 2022[ADAMS Accession No. ML21200A055].

[6]

NRC, SRM-M230814 - Affirmation Session - to SECY-22-0001, SECY-22-0001 -

Rulemaking - Final Rule: Emergency Preparedness for Small Modular Reactors and Other New Technologies, August 14, 2023 [ADAMS Accession No. ML23226A184]

NAT-3056 Rev. 3 TerraPower, LLC (TerraPower) Natrium Topical Report: Plume Exposure Pathway Emergency Planning Zone Sizing Methodology Page 35 of 37 Controlled Document - Verify Current Revision SUBJECT TO DOE COOPERATIVE AGREEMENT NO. DE-NE0009054 Copyright © 2024 TerraPower, LLC. All Rights Reserved

[7]

EPA, EPA-400/R-17/001, PAG Manual: Protective Action Guides and Planning Guidance for Radiological Incidents, January 2017 [ADAMS Accession No. ML21044A536].

[8]

NRC, NUREG-1855, Guidance on the Treatment of Uncertainties Associated with PRAs in Risk Informed Decisionmaking, Revision 1, March 2017 [ADAMS Accession No. ML17062A466].

[9]

NRC, RG 1.174, An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis, Revision 3, January 2018

[ADAMS Accession No. ML17317A256].

[10] NRC, RG 1.200, Acceptability of Probabilistic Risk Assessment Results for Risk-Informed Activities, Revision 3, December 2020 [ADAMS Accession No. ML20238B871].

[11] NRC, NUREG-75/014 (WASH 1400), "Reactor Safety Study: An Assessment of Accident Risks in U.S. Commercial Nuclear Power Plants," October 1975 [NUREG-75/014].

[12] EPA, EPA-520/1-75-001, Manual of Protective Action Guides and Protective Actions for Nuclear Incidents, September 1975 [ADAMS Accession No. ML20203M064].

[13] NRC, "Planning Basis for Emergency Responses to Nuclear Power Reactor Accidents,"

NRC Policy Statement, Federal Register, Vol. 44, No.206 [p 61123], October 23, 1979.

[14] NRC, NUREG-0654/FEMA-REP-1, "Criteria for Preparation and Evaluation of Radiological Emergency Response Plans and Preparedness in Support of Nuclear Power Plants," Rev. 2, December 2019 [ADAMS Accession No. ML19347D139].

[15] NRC, SECY-97-020, "Results of Evaluation of Emergency Planning for Evolutionary and Advanced Reactors," January 27, 1997 [ADAMS Accession No. ML992920024].

[16] NRC, SECY-10-0034, Potential Policy, Licensing, and Key Technical Issues for Small Modular Nuclear Reactor Designs," March 28, 2010 [ADAMS Accession Number ML093290245].

[17] NRC, SECY-11-0152, "Development of an Emergency Planning and Preparedness Framework for Small Modular Reactors," October 28, 2011 [ADAMS Accession No. ML112570439].

[18] NRC, SECY-15-0077, "Options for Emergency Preparedness for Small Modular Reactors and Other New Technologies," May 29, 2015 [ADAMS Accession No. ML15037A176].

[19] NRC, SRM SECY-15-0077, "Options for Emergency Preparedness for Small Modular Reactors and Other New Technologies," August 4, 2015 [ADAMS Accession No. ML15216A492].

[20] NRC, NUREG-1935, "State-of-the-Art Reactor Consequence Analyses (SOARCA)

Report," November 2012 [NUREG-1935].

[21] NRC, SRM-SECY-98-144, Staff Requirements - SECY-98-144 - White Paper on Risk-Informed and Performance-Based Regulation, March 1, 1999 [ADAMS Accession No. ML003753601].

NAT-3056 Rev. 3 TerraPower, LLC (TerraPower) Natrium Topical Report: Plume Exposure Pathway Emergency Planning Zone Sizing Methodology Page 36 of 37 Controlled Document - Verify Current Revision SUBJECT TO DOE COOPERATIVE AGREEMENT NO. DE-NE0009054 Copyright © 2024 TerraPower, LLC. All Rights Reserved

[22] NRC, NUREG/CR-7154, "Risk Informing Emergency Preparedness Oversight:

Evaluation of Emergency Action Level-A Pilot Study of Peach Bottom, Surry and Sequoyah," January 2013 [ADAMS Accession No. ML13031A500 (Volume 1) and ADAMS Accession No. ML13031A501 (Volume 2)].

[23] NRC, RG 1.233, "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,"

Revision 0, June 2020 [ADAMS Accession No. ML20091L698].

[24] NEI, NEI 18-04, Risk-Informed Performance-Based Technology-Inclusive Guidance for Non-Light Water Reactor Licensing Basis Development, Revision 1, August 2019

[ADAMS Accession No. ML19241A472].

[25] ASME/ANS, ASME/ANS-Ra-S-1.4-2013, "Probabilistic Risk Assessment Standard for Advanced Non-Light Water Reactor Nuclear Power Plants," December 9, 2013

[Superseded].

[26] NRC, RG 1.247, Acceptability of Probabilistic Risk Assessment Results for Advanced Non-Light Water Reactor Risk-Informed Activities, For Trial Use, NRC, March 2022

[ADAMS Accession No. ML21235A008].

[27] ASME/ANS, ASME/ANS-RA-S-1.4-2021, "Probabilistic Risk Assessment Standard for Advanced Non-Light Water Reactor Nuclear Power Plants," February 8, 2021.

[28] NEI, NEI 20-09, Performance of PRA Peer Reviews Using the ASME/ANS Advanced Non-LWR PRA Standard, Revision 1, May 5, 2021 [ADAMS Accession No. ML21125A284].

[29] NRC, "Emergency Preparedness for Small Modular Reactors and Other New Technologies (10 CFR 50 and 52)," Federal Register, Vol. 85, No. 92, NRC, May 12, 2020, pp. 28436-28466 [FRN-2020-05-12 EP SMR-ONT Proposed Rule].

[30] NRC, DG-1350, "Performance-Based Emergency Preparedness for Small Modular Reactors, Non-Light-Water Reactors, and Non-Power Production or Utilization Facilities," May 2020 [ADAMS Accession No. ML18082A044].

[31] Letter from Theodore B. Smith (NRC) to Bryan C. Hanson (Exelon Generation Company, LLC), "Three Mile Island Nuclear Station, Units 1 and 2 - Exemptions from Certain Emergency Planning Requirements and Related Safety Evaluation (EPID L-2019-LLE-0016)," December 1, 2020 [ADAMS Accession No. ML20244A292].

[32] Letter from Scott P. Wall (NRC) to Pamela B. Cowan, Pilgrim Nuclear Power Station -

Exemptions from Certain Emergency Planning Requirements and Related Safety Evaluation (EPID L-2018-LLE-0011)," December 18,2019 [ADAMS Accession No. ML19142A043].

[33] NCR, NSIR/DPR-ISG-02, Emergency Planning Exemption Requests for Decommissioning Nuclear Power Plants, NRC Interim Staff Guidance (ISG) Office of Nuclear Security and Incident Response (NSIR), Division of Preparedness and Response (DPR), May 11, 2015 [ADAMS Accession No. ML14106A057].

NAT-3056 Rev. 3 TerraPower, LLC (TerraPower) Natrium Topical Report: Plume Exposure Pathway Emergency Planning Zone Sizing Methodology Page 37 of 37 Controlled Document - Verify Current Revision SUBJECT TO DOE COOPERATIVE AGREEMENT NO. DE-NE0009054 Copyright © 2024 TerraPower, LLC. All Rights Reserved

[34] NRC, CLI-19-10, Memorandum and Order, Tennessee Valley Authority (Clinch River Nuclear Site Early Site Permit Application), December 17, 2019 [ADAMS Accession No. ML19351D663].

[35] NRC, Safety Evaluation for NuScale Licensing Topical Report TR-0915-17772, Revision 3, Methodology for Establishing the Technical Basis for Plume Exposure Emergency Planning Zones at NuScale Small Modular Reactor Plant Sites, dated April 28, 2022 [ADAMS Accession No. ML22118A760] and NRC Form 896, -A Topical Report Verification," Report Number: TR-0915-17772, Revision 3, Methodology for Establishing The Technical Basis For Plume Exposure Emergency Planning Zones at NuScale SMR Plant Sites, dated November 14, 2022 [ADAMS Accession No. ML22304A161].

[36] FEMA, "Developing and Maintaining Emergency Operations Plans, Comprehensive Preparedness Guide (CPG) 101," Version 3.0, September 2021

[37] NEI, White Paper on Proposed Methodology and Criteria for Establishing the Technical Basis for Small Modular Reactor Emergency Planning Zone, from Douglas J. Walters (NEI), to Michael E. Mayfield, dated December 23, 2013 [ADAMS Accession No. ML13364A345].

[38] NRC, RG 1.23, "Meteorological Monitoring Programs for Nuclear Power Plants,"

Revision 1, March 2007 [ADAMS Accession No. ML070350028].

[39] Sandia National Laboratories, MACCS User Guide, "4.1.0, Supplement to MACCS Users Guide and Reference Manual," July 19, 2021.

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