ML24117A013

Letter to Paula Gerfen-Diablo Canyon Units 1 and 2-Regulatory Audit Regarding Severe Accident Mitigation Alternatives for the License Renewal Application

ML24117A013
Person / Time
Site:	Diablo Canyon
Issue date:	05/20/2024
From:	Kimberly Conway NRC/NMSS/DREFS/ELRB
To:	Gerfen P Pacific Gas & Electric Co
References
EPID L-2023-LNE-0004
Download: ML24117A013 (1)

Text

Paula Gerfen Senior Vice President, Generation and Chief Nuclear Officer Pacific Gas and Electric Company Diablo Canyon Nuclear Power Plant P.O. Box 56 Mail Code 104/6 Avila Beach, CA 93424

SUBJECT:

DIABLO CANYON NUCLEAR POWER PLANT, UNITS 1 AND 2 - SEVERE ACCIDENT MITIGATION ALTERNATIVES AUDIT REGARDING THE LICENSE RENEWAL APPLICATION (EPID NUMBER: L-2023-LNE-0004)

Dear Paula Gerfen:

By letter dated November 7, 2023 (Agencywide Documents Access and Management System Package ML23311A154), Pacific Gas and Electric Company submitted to the U.S. Nuclear Regulatory Commission (NRC) an application for the renewal of Facility Operating License Nos.

DPR-80 and DPR-82 for Diablo Canyon Nuclear Power Plant, Units 1 and 2 (Diablo Canyon, DCPP) pursuant to Section 103 of the Atomic Energy Act of 1954, as amended, and Part 54 of Title 10 of the Code of Federal Regulations, Requirements for renewal of operating licenses for nuclear power plants.

The NRC staff plans to conduct an audit of information related to the severe accident mitigation alternatives (SAMA) for the Diablo Canyon license renewal application. This audit will be conducted remotely by NRC staff during the week of June 24, 2024. The audit activities will be conducted in accordance with the enclosed Environmental Audit Plan (Enclosure 1). To the extent possible, the NRC staff requests the information identified in the SAMA Audit Needs List (Enclosure 2) be made available on the Diablo Canyon online reference portal prior to the audit.

May 20, 2024 P. Gerfen If you have any questions, please contact me via email at kimberly.conway@nrc.gov.

Sincerely, Kimberly Conway Environmental Project Manager Environmental Project Management Branch 1 Division of Rulemaking, Environmental, and Financial Support Office of Nuclear Material Safety and Safeguards Docket Nos. 50-275 and 50-323

Enclosures:

As stated cc w/encls: Listserv Signed by Conway, Kimberly on 05/20/24

ML24117A013 OFFICE REFS/EPMB1 REFS/EPMB2 NRR/ARCB REFS/EPMB1 REFS/EPMB1 NAME KConway AWalker-Smith KHsueh SKoenick KConway DATE 05/06/2024 05/07/2024 05/13/2024 05/20/2024 05/20/2024

Audit Plan

License Renewal Environmental Review Severe Accident Mitigation Alternatives Diablo Canyon Nuclear Power Plant Units 1 and 2

June 2024

Division of Rulemaking, Environmental, and Financial Support Office of Nuclear Material Safety and Safeguards U.S. Nuclear Regulatory Commission

Enclosure 1 2

License Renewal Environmental Audit Plan Severe Accident Mitigation Alternatives Diablo Canyon Nuclear Power Plant, Units 1 and 2

1. Background

By letter dated November 7, 2023, Agencywide Documents Access and Management System Package No. ML23311A154, Pacific Gas and Electric Company (PG&E or the applicant),

submitted to the U.S. Nuclear Regulatory Commission (NRC) an application for license renewal of Renewed Facility Operating License Nos. DPR-80 and DPR-82, Diablo Canyon Nuclear Power Plant, Units 1 and 2 (Diablo Canyon or DCPP). A Federal Register notice (88 FR 80780) dated November 20, 2023, noted the receipt and availability of the application, including the environmental report (ER).

As part of the staffs environmental review, a severe accident mitigation alternatives (SAMA) audit will be conducted for Diablo Canyons license renewal application. The purpose of this audit is to improve understanding, to verify information, and to identify information related to the review of SAMAs that will require docketing to support the basis of the regulatory decision.

Specifically, the NRC staff will be identifying pertinent information and obtain clarifications regarding information provided in the ER.

2. Audit Bases

License renewal requirements for ERs are specified in Title 10 of the Code of Federal Regulations (10 CFR) Part 51, Postconstruction environmental reports. As specified by 10 CFR 51.53(c): Operating license renewal stage, (1) Each applicant for renewal of a license to operate a nuclear power plant under Part 54 of this chapter shall submit with its application a separate document entitled "Applicant's Environmental ReportOperating License Renewal Stage." Review guidance for the staff is provided in NUREG-1555, Supplement 1, Revision 1, Standard Review Plans for Environmental Reviews for Nuclear Power Plants: Supplement 1 -

Operating License Renewal.

The NRC staff is required to prepare a site-specific supplement to NUREG-1437, Generic Environmental Impact Statement for License Renewal of Nuclear Plants. During the scoping process required in 10 CFR 51, the NRC staff is required to define the proposed action, identify significant issues which must be studied in depth, and to identify those issues that can be eliminated from further study.

3. Audit Scope

The scope of this environmental audit for the Diablo Canyon license renewal review is to discuss the NRC staffs specific questions regarding the SAMA analysis and results documented in the ER. Audit team members will review the documents and other requested information made available on the Diablo Canyon online reference portal identified on the SAMA audit needs list (Enclosure 2) and discuss any questions and additional information needs with PG&Es subject matter experts.

4. Information and Other Material Necessary for the Audit

As identified on the SAMA audit needs list (Enclosure 2).

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5. Audit Team Members and Assignments

The environmental audit team members and their assignments are shown in the table below.

NRC reviewers will be supported by staff from Pacific Northwest National Laboratory (PNNL).

Discipline Responsibility Environmental Project Manager Kim Conway, NRC Safety Project Manager Brian Harris, NRC SAMA John Parillo, NRC Steve Short, PNNL William Ivans, PNNL

6. Logistics

The SAMA audit will be conducted remotely during the week of June 24, 2024. An entrance meeting will be held at the beginning of the virtual audit via Microsoft Teams. Discussions will be held regarding topics and questions identified in the attached list of audit questions. An exit meeting for this audit will be scheduled at a mutually agreed upon time for NRC staff and PG&E.

7. Special Requests

The staff requests to interview knowledgeable PG&E staff or contractors on Levels 1, 2, and 3 probabilistic risk assessment developments and results, as well as SAMA identification and evaluation. The interviews will discuss the information on the SAMA audit needs list (Enclosure

2) and any other questions that may arise from the review of material during the virtual audit.

8. Deliverables

An audit summary report will be issued by the NRC staff within 90 days from the end of the environmental audit.

Diablo Canyon Nuclear Power Plant, Units 1 and 2 License Renewal Environmental Site Audit Needs List Severe Accident Mitigation Alternatives

Please be prepared to discuss the following questions and make the requested documents available during this audit.

Severe Accident Mitigation Alternatives Audit Questions:

1. Provide the following information regarding the Level 1 Probabilistic Risk Assessment (PRA) used for the Severe Accident Mitigation Alternative (SAMA) analysis. The basis for this request is as follows: Applicants for license renewal are required by Title 10 of the Code of Federal Regulations (10 CFR) Section 51.53(c)(3)(ii)(L) to consider SAMAs if not previously considered in an environmental impact assessment, related supplement, or environmental assessment for the plant. As part of its review of the Diablo Canyon Power Plant (DCPP)

SAMA analysis, the U.S. Nuclear Regulatory Commission (NRC) staff evaluates the applicants treatment of internal events and calculation of core damage frequency in the Level 1 PRA model. The requested information is needed in order for the NRC staff to reach a conclusion on the sufficiency of the applicants Level 1 PRA model for supporting the SAMA evaluation.

a. Section G.2.1.17 of the environmental report (ER) states that a universal truncation limit of 1.00E-13 was set for all initiators rather than the truncation limit at which convergence was defined to occur per ASME/ANS [American Society of Mechanical Engineers/American Nuclear Society] PRA Standard requirements, and Section G.4 confirms that this higher truncation limit was used in support of the SAMA analysis.

These same sections of the ER also demonstrate that use of this higher truncation limit results in a decrease in risk estimates (e.g., from 8.80E-05 per year to 8.68E-05 per year for the case of core damage frequency [CDF]). Provide justification that this truncation level is sufficiently low to show convergence or stable results, and that truncation does not distort the risk results. Additionally, provide justification that the convergence criterion does not impact the SAMA analysis.

b. Provide a breakdown of the contribution to internal events CDF by initiating event.

Confirm that the total CDF is the same as that reported in the ER for the DC05A Application Model. Additionally, provide the contribution to CDF from station blackout (SBO) events and anticipated transients without scram (ATWS) events.

c. Provide a breakdown of the contribution to internal flooding CDF by initiating event.

Confirm that the total CDF is the same as that reported in the ER for the DC05A Application Model.

d. Provide the freeze date" or the date which corresponds to the DCPP design and operation incorporated into the DCPP PRA used for the SAMA analysis. Identify any design or operational (including fuel cycle) changes that have been made since, or are planned to be made after, this freeze date that might impact the SAMA analysis.

e. Section G.2.1.17 of the ER indicates that Unit 1 risk estimates form the basis of the SAMA analysis. This same section states that Units 1 and 2 are nearly identical in design and operation such that the insights from one model are either the same or are considered representative for the other model such that quantification of one model is

Enclosure 2 2

judged to be adequate to support the cost benefit analysis for both units. The ER further clarifies that the Unit 1 model was chosen as the representative model for the SAMA analysis because the CDF and Level 2 release category frequencies are larger than those for Unit 2. However, risk estimates reported as part of an earlier license amendment request regarding transition to a risk-informed, performance-based fire protection program in accordance with 10 CFR 50.48(c) (i.e., Agencywide Documents Access and Management System ML16035A441) presented a higher fire CDF for Unit 2 (i.e., 5.24E-05 per year) than for Unit 1 (i.e., 4.83E-05 per year). Discuss unit differences (e.g., design and operation, PRA modeling practices and decisions),

characterizing the extent to which these differences could impact the SAMA analysis and providing further justification that use of the Unit 1 model is reasonable for the SAMA analysis (e.g., does not underestimate the estimated benefits of SAMAs).

f. Sections G.2.3.2.1 and G.2.3.2.2 of the ER indicate that the Internal Events PRA and Internal Flooding PRA under full-scope peer reviews in December 2012; however, the standard and process guidelines used to conduct these peer reviews are not identified. Additionally, for the Fire PRA, Section G.2.3.2.3 indicates that a follow-on peer review was completed in December 2010 using ASME/ANS PRA Standard RA-Sa-2009; however, the process guidelines used to conduct this peer review are not clear. Lastly, for the seismic PRA, Section G.2.3.2.4 indicates that a full-scope peer review was completed in June 2017 using a 2013 version of the ASME/ANS PRA Standard; however, the exact version of the standard and the process guidelines used to conduct this peer review are not discussed. Clarify the standards and process guidelines used to conduct the above peer reviews and explain why they are reasonable for the SAMA analysis.

g. Section G.2.1 of the ER provides a description of major changes to the DCPP PRA since the Individual Plant Examination (IPE) submittal; however, the impact that such changes, specifically those made after Model DC02 and before DC05, have had on CDF is not provided. Consistent with Section 3.1.1.2 of Nuclear Energy Institute (NEI) 05-01, Revision A, describe the impact that the major changes presented in Section G.2.1 have had on CDF since the IPE submittal.

h. Section G.2.3.2 summarizes self-assessments and peer reviews underwent by the DCPP PRA. It is indicated that for the Internal Events and Flooding PRAs, a formal closure review was conducted in July 2023 on the resolutions to all Facts and Observations (F&Os). Similar F&O closure reviews were performed for the Fire PRA in September 2018 and the Seismic PRA in February 2019. Address the following with respect to the F&O closure process:

i. Clarify that the F&O closure reviews were conducted in accordance with NRC letter dated May 1, 2017, U.S. Nuclear Regulatory Commission Staff Expectations for an Industry Facts and Observations Independent Assessment Process (ML17121A271), and NRC letter dated May 3, 2017, U.S. Nuclear Regulatory Commission Acceptance on Nuclear Energy Institute Appendix X to Guidance 05-04, 07-12, and 12-13, Close-Out of Facts and Observations (ML17079A427). If the F&O closure reviews were not conducted in accordance with this guidance, assess the impact of any deviations from this guidance on the SAMA analysis.

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ii. Discuss whether the scope of the F&O closure reviews included all finding-level F&Os (for the internal events, internal flooding, fire, and seismic PRAs), including those finding-level F&Os that are associated with Met Supporting Requirements. If not, identify and provide the complete text of the original peer review findings and recommendations for any F&Os that were excluded from the F&O closure review scope, and their associated disposition for the SAMA analysis.

iii. Confirm that the closure review team for each review was provided with a written assessment and justification of whether the resolution of each F&O, within the scope of the independent assessment, constitutes a PRA upgrade or maintenance update, as defined in ASME/ANS RA-Sa-2009, Addenda to ASME/ANS RA-S-2008, Standard for Level 1/Large Early Release Frequency Probabilistic Risk Assessment for Nuclear Power Plant Applications, as qualified by Regulatory Guide 1.200, Revision 2, An Approach for Determining the Technical Adequacy of Probabilistic Risk Assessment Results for Risk-Informed Activities, (ML090410014). If the written assessment and justification for the determination of each F&O was not performed and reviewed by the F&O closure review team, discuss how this aspect of the F&O closure process was met consistent with the staffs acceptance as discussed in the May 3, 2017, letter.

iv. Confirm that the DCPP PRA model DC05A used in the SAMA analysis incorporates the dispositions to all the formally closed F&Os.

i. Section G.2 of the ER provides a historical overview of changes made to the DCPP PRA models leading to the DCPP PRA model used in the SAMA analysis. Address the following with respect to these changes:

i. Clarify whether the PRA modeling of the Generation III Westinghouse Shutdown Seals was performed consistent with guidance in the final safety evaluation for PWROG 14001 (ML17200A116). If not, discuss the significance of not doing so to the SAMA analysis.

ii. Some historical changes to the DCPP PRA model appear to only be discussed within the ER in the context of one hazard group or another (e.g., fire, seismic), making it unclear whether such changes were made consistent across each of the individual hazard groups associated with the DCPP PRA model used in the SAMA analysis. Discuss any differences in which the as-built, as-operated plant is represented across the individual hazard groups that could adversely impact the SAMA analysis and assess the impact of these differences on the SAMA analysis. Additionally, discuss the process used for ensuring that models for the different hazards addressed remain consistent.

j. Section G.2.1.17 of the ER mentions that a focused-scope peer review was performed for select internal flooding and large early release frequency (LERF) analysis supporting requirements; however, Section G.2.3.2, which summarizes self-assessments and peer reviews underwent by the DCPP PRA, does not appear to discuss this peer review. Clarify the scope of the peer review, discuss the standard 4

and process guidelines used, and justify that the peer review is reasonable for the SAMA analysis.

k. While the ER suggests that Diverse and Flexible Mitigation Capability (FLEX) strategies are credited, at least in part, within the DCPP PRA (e.g., Sections G.2.1.11, G.5.1.2, etc.), there is no discussion on the PRA modeling of these FLEX strategies, including the guidance utilized. In an NRC memorandum dated May 30, 2017 (ML17031A269), the NRC staffs positions and conclusions are provided regarding use of guidance in NEI 16-06, Crediting Mitigating Strategies in Risk-Informed Decision Making. In an NRC memorandum dated May 6, 2022, Updated Assessment of Industry Guidance for Crediting Mitigating Strategies in Probabilistic Risk Assessments (ML22014A084), the NRC staff identified challenges and strategies for incorporating FLEX equipment into a PRA model in support of risk-informed decision-making when using additional industry guidance issued after NEI 16-06. Specifically, over-crediting FLEX in the PRA model could underestimate the potential benefit of SAMAs. Address the following:

i. Describe the FLEX strategies that were credited in the DCPP PRA model (inclusive of internal events, including internal flooding, fire, and seismic) used to support the SAMA analysis. Include identification of discussion of the FLEX equipment that was credited and whether that equipment is portable or permanently installed and identification of the operator actions that are credited.

ii. Clarify the guidance used to model FLEX strategies within the DCPP PRA model, identify the NRC memorandum that is applicable to the modeling of FLEX in the DCPP PRA model, and explain how each of the 13 NRC staff conclusions are addressed in the PRA.

iii. If the FLEX modeling is not in accordance with the applicable NRC memorandum, describe and provide the results of a sensitivity study that assesses the impact on the SAMA analysis of not crediting FLEX strategies.

2. Provide the following information relative to the Level 2 PRA analysis. The basis for this request is as follows: Applicants for license renewal are required by 10 CFR 51.53(c)(3)(ii)(L) to consider SAMAs if not previously considered in an environmental impact assessment, related supplement, or environmental assessment for the plant. As part of its review of the DCPP SAMA analysis, the NRC staff evaluates the applicants treatment of accident propagation and radionuclide release in the Level 2 PRA model. The requested information is needed in order for the NRC staff to reach a conclusion on the adequacy of the applicants Level 2 PRA model for supporting the SAMA evaluation.

a. The sum of the frequencies for all source term categories tabulated within Section G.2.1.17 (i.e., 8.45E-05 per year) does not equal the total CDF reported (8.68E-05 per year). Provide an explanation for this difference and justification that it does not impact the SAMA analysis.

b. In Section G.2.1.17 of the ER, it is noted that the release category ST1, leading to a large early release, is 4.63E-06 per year while the Unit 1 LERF value reported in this same section for the DC05A Application Model is 1.20E-05 per year. Explain the 5

reasons for the difference. If this is due to Level 2 model changes compared to a LERF-only model, describe the changes made from the peer-reviewed LERF model to the current full Level 2 model.

c. Section G.2.2.1 of the ER indicates that the Level 2 Containment Event Tree (CET) is quantified separately for each plant damage state (PDS) as the CET top event split fractions can be dependent on the PDS being analyzed. However, the development and basis of the CET split fractions does not appear to be discussed. Provide a discussion of the development of the CET, the basis for the CET split fractions, and updates since the individual plant examination to represent the current state of the art.

d. The discussion of release category binning in Section G.2.2.4.1 of the ER includes the statement:

For early-small containment failures, the medium pressure sequences are binned with the high pressure sequences because containment pressures may be greater later in the event and high RCS [reactor coolant system] retention then becomes a liability."

Table G.2-5 indicates that for small early release categories, the medium-pressure sequences are binned with the low-pressure sequences. In addition, table G.3-10 of the ER describes the Modular Accident Analysis Program (MAAP) case RC16 as a low-pressure core melt. Explain this binning and the selection of the representative MAAP case for the small-early release category.

e. Section G.2.2.4.1 of the ER indicates that release category RC22 was defined for unaccounted for Level 2 sequences and conservatively mapped as a large early release; however, table G.2-8 states that the frequency of RC22 is always zero.

Explain the basis of release category RC22, and describe the criteria upon which sequences, if any, are mapped to this category. If sequences are mapped, clarify their frequency contribution, and justify their treatment within the SAMA analysis.

f. Section G.2.2.4.1 of the ER indicates that two additional release categories, RC22 and RC23, defined for unaccounted for Level 2 sequences and fire-induced main control room abandonment scenarios, respectively, were conservatively mapped as a large early release (or source term category ST1); however, Table G.3-12 indicates that the ISLOCA source term category (or ST5) results in a higher dose and offsite economic cost than a large early release. Provide justification that binning release categories RC22 and RC23 into source term category ST1 is reasonable for the SAMA analysis (e.g., an alternate assumption would not result in the identification of any additional potentially cost beneficial SAMAs).

g. Section G.2.2.4.5 of the ER states that only the release categories with a frequency above the cutoff of 1.0E-10/year are reported. This cutoff differs from the universal truncation limit of 1.0E-13 per year applied to the SAMA analysis and discussed in Section G.2.1.17. Clarify the treatment of release categories that fall below this cutoff of 1.0E-10 per year within the SAMA analysis, and if these categories are not addressed, provide justification that this exclusion does not impact the SAMA analysis.

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h. While the SAMA analysis ultimately applies results developed using MAAP, Section G.2.2.4.2 of the ER states that Pacific Gas and Electric Company (PG&E) elected to generate two different sets of source terms, one using ZISOR and another using MAAP, and further clarified that these sets were developed, in part, to decide which method would be more appropriate for use in the SAMA analysis. However, in comparing the release fractions (RFs) obtained from these two computer codes, Section G.2.2.4.5.1 states that ZISOR tends to predict higher release fractions [RFs]

than MAAP and presents numerous conditions for which MAAP underpredicts RFs.

In one example, this section states that MAAP consistently under-predicts the ZISOR predictionsby factors of 50 and greater. Characterize the version of ZISOR used and justify the selection of MAAP and why it is reasonable for the SAMA analysis.

i. Table G.3-10 of the ER summarizes the representative release categories (RCs) and MAAP cases assigned to each source term (or release category group) that forms the basis of the SAMA analysis (e.g., ST1, ST2, etc.) in table G.3-12. However, the basis for these assignments is not clear. Address the following:

i. Define each of the six RC groups in terms of the characteristics (e.g.,

timing and magnitude of release) that are used to perform the binning of the 39 individual RCs, referred to in Section G.2.2.4.5, and justify that the binning is reasonable.

ii. Describe and justify how individual RCs are characterized by key plant damage states (KPDSs), including, if applicable, the basis for assigning representative KPDSs to a given RC.

iii. Describe the logic used in the selection of the representative MAAP case for each RC group and provide justification that the selected cases are reasonable for the SAMA analysis (e.g., does not underestimate the estimated benefits of SAMAs).

iv. While Section G.3.5 of the ER states that representative MAAP cases ultimately selected for each RC group were run until plateaus for RFs were achieved, Section G.2.2.4.5.1 indicates that for all MAAP cases developed for individual RCs and reported in table G.2-8, the RFs were only reported at 50 hours5.787037e-4 days <br />0.0139 hours <br />8.267196e-5 weeks <br />1.9025e-5 months <br /> after event initiation and that at this point, the RFs for some cases may still be increasing. Also, table G.3-10 indicates that some of the representative MAAP cases chosen for RC groups (e.g., ST3 and ST4) were run for much longer (i.e., for 288 and 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, respectively). Identify the RCs in which the release fractions have not plateaued and are still increasing by the end of the MAAP run time. For each of these RCs, identify those in which the MAAP run times are less than 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> after the time of declaration of general emergency. For each of the identified source term categories, assess the impact on the SAMA analysis from extending the run time to 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> after declaration of a general emergency, including on the mapping of individual RCs to RC groups and on the selection of representative MAAP cases for the RC groups upon which the SAMA analysis is based.

j. Summarize the independent and/or peer reviews performed to provide confidence in the Level 2 analysis (apart from LERF portion), summarize the results of these 7

reviews, and summarize any unresolved independent/peer review comments and their impact on the SAMA analysis.

3. Provide the following information with regard to the treatment and inclusion of external events in the SAMA analysis. The basis for this request is as follows: Applicants for license renewal are required by 10 CFR 51.53(c)(3)(ii)(L) to consider SAMAs if not previously considered in an environmental impact assessment, related supplement, or environmental assessment for the plant. As part of its review of the DCPP SAMA analysis, NRC staff evaluates the applicants treatment of external events in the PRA models. The requested information is needed in order for the NRC staff to reach a conclusion on the sufficiency of the applicants PRA models for supporting the SAMA evaluation.

a. Provide a breakdown of the contribution to seismic CDF by initiating event and provide a description of the dominant initiating events. Confirm that the total seismic CDF is the same as that reported in the ER for the DC05A Application Model.

b. Provide a breakdown of the contribution to the fire CDF by initiating events (or grouped by fire area) and provide a description of the dominant initiating events.

Confirm that the total fire CDF is the same as that reported in the ER for the DC05A Application Model.

c. A seismic hazard evaluation and analysis update for DCPP, as documented in a report titled Diablo Canyon Updated Seismic Assessment: Response to Senate Bill 846, was performed and released on February 1, 2024. Summarize this update and its outcomes, identifying any guidance, practices, or processes guidelines utilized in its execution (e.g., the Senior Seismic Hazard Analysis Committees, or SSHAC, process). Additionally, discuss the impact of this new information on the DCPP PRA model, its risk results (e.g., CDF, LERF), and the SAMA analysis.

d. Section G.2.3.2.5 of the ER explains that other external events such as high winds, external flooding, aircraft accidents, hazardous materials, and turbine missiles were assessed in the DCPP Individual Plant Examination - External Events (IPEEE) and that an update to this analysis, performed in November 2016, determined that hazards analyzed in the IPEEE were negligible contributors to CDF. Section G.4.6.2 provides quantitative CDF estimates for some of the hazards. Address the following related to other external events:

i. Section G.4.6.2, which indicates that negligible contributors are not included in the external events multiplier, includes many of the hazards deemed negligible to CDF within Section G.2.3.2.5. Additionally, Section G.5.1.7 lists those hazards that were not qualitatively screened but were evaluated using quantitatively bounding estimates; however, the hurricane hazard, which has a quantitative estimate in Section G.4.6.2, is not listed.

Summarize the updated assessment performed that addresses other external hazards and which was used as the basis for the ER conclusion.

In the response, discuss assessment approaches (e.g., conservative, bounding, best-estimate) and screening criteria (qualitatively and/or quantitative) used for all hazards as well as their applicability to and/or impact on the SAMA analysis, including the external events multiplier.

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ii. Section G.2.3.2.5 of the ER deems the external flooding hazard, similar to other listed hazards, negligible to CDF, but its impact on the SAMA analysis, including external events multiplier, is not discussed (e.g., in Section G.4.6.2). It is further observed that the DCPP IPEEE assessment for external flooding identified an upper-bound CDF of 7.2E-07 per year, which is comparable to those hazards incorporated in the external events multiplier. Additionally, following the accident at the Fukushima Dai-ichi nuclear power plant, PG&E responded to an NRC 10 CFR 50.54(f) request for information. This response included a reevaluation of the external flood hazards, the development of mitigating strategies for external floods, and a focused evaluation of the external flooding mechanisms for which the reevaluated flooding hazards is not bounded by the current design basis.

PG&Es evaluations included addressing local intense precipitation (ML17200D161). This focused evaluation was a deterministic (that is, not a probabilistic) evaluation. Provide a discussion of external flood hazards and the associated impact on DCPP to support the conclusion that they would not impact the SAMA analysis, such as not contributing to an external events multiplier nor leading to any cost-beneficial SAMAs.

iii. NRC Regulatory Issue Summary 2015-06, while not requiring regulatory action, identified several instances in which nuclear power plants were determined to not conform with their tornado-generated missile licensing basis. Additionally, staff observed that not all IPEEE hazards (e.g.,

hazardous chemicals with an identified upper-bound CDF of 8.0E-07 per year) appear to be considered by the DCPP SAMA analysis in Sections G.2.3.2.5 and G.4.6.2. Discuss any changes to the DCPP site or surrounding environment that would change the conclusions of the IPEEE and which could impact the SAMA analysis, including the external events multiplier.

4.Please provide the following information regarding the Level 3 PRA used in the SAMA analysis. The basis for this request is as follows: Applicants for license renewal are required by 10 CFR 51.53(c)(3)(ii)(L) to consider SAMAs, if not previously considered, in an environmental impact assessment, related supplement, or environmental assessment for the plant. As part of its review of the DCPP SAMA analyses, the NRC staff evaluates the applicant's analysis of accident consequences in the Level 3 PRA. The requested information is needed in order for the NRC staff to reach a conclusion on the sufficiency of the applicant's Level 3 PRA model for supporting the SAMA evaluations.

a.Section G.3.7 indicates that meteorological data (e.g., wind direction, wind speed, temperature, precipitation) for the year 2022 were used in the consequence analysis.

Specify the sources of the data and, if the plant meteorological tower data was not used, provide justification not using it.

b. Discuss the extent to which there was missing precipitation data and how missing data was accounted for in the SAMA evaluation.

c.Section G.3.7 states that the 2022 meteorological data set was used to support the Level 3 model as it produced the largest dose and economic cost risk. However, atmospheric mixing height values appear to be based on historical values from a 1972 Environmental Protection Agency publication. Explain why the selected data is 9

reasonable for the SAMA analysis (e.g., does not underestimate the estimated benefits of SAMAs), or alternatively provide the results of a sensitivity analysis.

5. Provide the following information with regard to the selection and screening of Phase I SAMA candidates. The basis for this request is as follows: Applicants for license renewal are required by 10 CFR 51.53(c)(3)(ii)(L) to consider SAMAs if not previously considered in an environmental impact assessment, related supplement, or environmental assessment for the plant. As part of its review of the DCPP SAMA analysis, the NRC staff evaluates the applicants basis for the selection and screening Phase I SAMA candidates. The requested information is needed in order for the NRC staff to reach a conclusion on the adequacy of the applicants Phase I SAMA selection and screening process for the SAMA evaluation.

a. Section G.5.1 of the ER indicates that potentially cost-effective Phase 2 SAMAs from select plants were considered, but it is not clear how the list of generic pressurized water reactor (PWR) enhancements identified in NEI 05-01, Revision A, was considered. Explain how this list was considered in the identification of Phase I SAMA candidates for the DCPP.

b. Sections G.5.1.1 and G.5.1.2 of the ER describes what cutoff values were assumed in determining dominant risk contributors within the Level 1 and Level 2 importance analyses, respectively. However, the rationale for the cutoff values applied is not clear. Additionally, this section indicates that correlating cutoff values to the averted cost-risk is not an appropriate basis. Section G.5.1.7, on the other hand, references the importance analyses and provides a dollar review threshold of $358,750 that below which, no SAMAs are said to be warranted. It is noted, though, that the implementation cost of some identified SAMAs (e.g., SAMA 16) is below this threshold. Justify the rationale for the cutoff value used to determine what is a dominant risk contributor in the context of the SAMA analysis. In doing so, explain why SAMAs identified for contributors below the applied cutoff values would be not cost-beneficial, and clarify why averted cost-risk (e.g., the minimum cost of a simple procedure change with associated training) is not an appropriate basis for informing these values.

c. Sections G.5.1.1 and G.5.1.2 of the ER indicates that the plant-specific SAMAs were identified using an importance analysis based on split fractions from the Level 1 and Level 2 DCPP PRA models, respectively. However, Section 5.1 of NEI 05-01, Revision 1, discusses the definition and review of dominant risk contributors in the context of dominant sequences, equipment failures, and operator actions, leading to the development of a list of equipment failures and human actions that have the greatest potential for reducing risk based on importance analysis and for use in the SAMA evaluation process. Justify that a review based on split fractions is reasonable for the SAMA analysis (e.g., the process used ensures that SAMAs are evaluated for each dominant risk contributor, as defined by NEI 05-01). In doing so, discuss how the importance of structures, systems, and components (SSCs) and operator actions that may affect multiple split fractions is addressed.

d. Table G.5-1 of the ER provides the CDF importance of Level 1 PRA split fractions and describes relevant SAMAs that address each. However, there is no such description of relevant SAMAs for split fraction MCR1P. Describe potential plant-specific SAMA candidates that address this dominant risk contributor and discuss their treatment within the SAMA analysis.

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6. Provide the following information with regard to the Phase II cost-benefit evaluations. The basis for this request is as follows: Applicants for license renewal are required by 10 CFR 51.53(c)(3)(ii)(L) to consider SAMAs if not previously considered in an environmental impact assessment, related supplement, or environmental assessment for the plant. As part of its review of the DCPP SAMA analysis, the NRC staff evaluates the applicants cost-benefit analysis of Phase II SAMAs. The requested information is needed in order for the NRC staff to reach a conclusion on the acceptability of the applicants cost estimations for individual SAMAs and cost-benefit evaluation.

a. As discussed in Section G.6 of the ER, the benefits of many SAMAs are modeled by making adjustments to specific split fractions rules or values. For instance, SAMA 13, which aims to improve bleed and feed availability, revises rules for the bleed and feed split fraction (OB) to address SAMA-related changes, including to the air supply to PCV-474 and to 480V AC bus alignment to support battery chargers on each of three electrical divisions. Additionally, SAMA 12, which appears to provide extended and diverse battery capacity, targets specific split fractions associated with loading shedding. However, it is unclear whether all benefits derived from each modeled SAMA (e.g., the battery crosstie capability to vital buses addressed by SAMA 13) are realized, to the extent applicable, for other accident sequences beyond those addressed by the specific split fraction adjustments made. Provide justification that the scope of modeling changes made to assess the benefits of SAMAs is reasonable for the SAMA analysis (e.g., does not underestimate the estimated benefits of SAMAs).

b. Section G.6 of the ER describes that modeling assumptions made to simulate the implementation of each proposed SAMA were optimistic in nature as to not inadvertently screen any potentially cost-beneficial SAMA. The discussion in Section G.6.2 for SAMA 4, which contemplates an automated monitoring system to trip the reactor coolant pump (RCP), indicates that the current failure to trip the RCPs is primarily related to the failure of the operators to perform the action, which is said to be complicated by fire-induced damage of instrumentation. However, it is observed that when the instrumentation is failed by fire, associated split fractions are not changed to account for the independent trip system. Additionally, the discussion in Section G.7.2.1.4 for SAMA 17, which proposes an alternate emergency diesel generator to power a station battery charger for steam generator level instruction and alternate feedwater (AFW) control, states that the [p]ower supplies to the turbine driven AFW pump are in the same location as existing supplies such that new fire and internal flooding modeling are not required. However, the basis for this assumption is not clear. Provide additional justification that the modeling assumptions made to assess the benefits of SAMAs are reasonable for the SAMA analysis (e.g., do not underestimate the estimated benefits of SAMAs).

c. Section G.4.5 of the ER indicates that a correction factor was applied to scale up the replacement power cost from the 910 MWe reference plant in NUREG/BR 0184.

Discuss how the cost of replacement power has increased since the issuance of NUREG/BR 0184 as well as how it may be affected by differences in the average capacity factor between DCPP and the reference plant (i.e., 60-65 percent as indicated in Section 5.7.7.1 of NUREG/BR 0184).

7. For certain SAMAs considered in the DCPP Environmental Report, there may be lower cost or more effective alternatives that could achieve much of the risk reduction. In this regard, 11

provide an evaluation of the following SAMA. The basis for this request is as follows:

Applicants for license renewal are required by 10 CFR 51.53(c)(3)(ii)(L) to consider SAMAs if not previously considered in an environmental impact assessment, related supplement, or environmental assessment for the plant. As part of its review of the DCPP SAMA analysis, the NRC staff considers additional SAMAs that may be more effective or have lower implementation costs than the other SAMAs evaluated by the applicant. The requested information is needed in order for the NRC staff to reach a conclusion on the adequacy of the applicants determination of cost-beneficial SAMAs.

a. All but one of the identified SAMAs in table G.5-3 of the ER (i.e., SAMA 11) appear to involve procurement and installation of new SSCs or the permanent modification to existing SSCs. Additionally, many of the 21 Phase 1 SAMAs identified in table G.5-3 (e.g., SAMAs 1, 3, 5, 10, 14, 17, 19, and 21) have costs well in excess of the maximum averted cost-risk. However, NEI 05-01, Revision A, states that SAMAs may also include procedure changes or enhancements to programs, including training and surveillance programs, which are typically a much lower cost alternative. NEI 05-01 further clarifies that hardware changes should not be limited to permanent changes but should also include lower cost alternatives, such as temporary connections using commercial grade equipment (e.g., portable generators and temporary crossties).

Discuss the possibility of these potentially lower-cost SAMAs.

8. Discuss the various systems available to DCPP that were not part of the original design basis or have been added as part of various industry programs to address beyond design basis events and the extent they are credited in the SAMA PRA.

9. Discuss how Level 1 SBO sequences were treated in the Level 2 analysis.

Provide copies of the following documents for the NRC staff to review:

1. The most recent PRA peer review reports, and/or self-assessment reports, and Facts and Observations (F&O) closure review reports for all hazards (i.e., internal events, including internal flooding, fire and seismic), including but not limited to:

o PWROG-23015-P Revision 0, "Diablo Canyon F&O Closure and Focused Scope Peer Review Report", PA-RMSC-1974, July 2023.

2. Documentation associated with the Level 2 and 3 PRA model reviews.

3. Available documentation of the review of the results of the DCPP Level 2 PRA by the team of DCPP experts representing various site organizations (e.g. Operations, System Engineering, Mechanical/Safety Analysis, PRA, and License Renewal) to confirm that the model is representative of the plant and the results are reasonable.

4. PRA notebooks for DC05A model used in the SAMA analysis that document the quantification results and which provide the breakdown of the contributors to CDF.

5. Documentation of updated analysis for other external events, including but not limited to:

o DCPP Other External Events. Pacific Gas and Electric Company, Cale. File X.1, Revision 1, November 2016.

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6. Documentation of the containment event trees (CETs) developed to produce results for each of the CET end states or source term categories reported in the ER, including but not limited to:

o DCPP PRA Containment Event Tree. Pacific Gas and Electric Company, NOSPRA Cale. File PRA91-009, Revision 0, February 1992.

7. Documentation of the Level 3 analysis and the core inventory.

8. Documentation of the identification and disposition of SAMAs identified from the review of SAMAs from other SAMA analyses and from other sources.