ML24187A136

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Severe Accident Mitigation Alternatives Audit Report and Information Requests
ML24187A136
Person / Time
Site: Diablo Canyon  Pacific Gas & Electric icon.png
Issue date: 07/16/2024
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NRC/NMSS/DREFS/EPMB1
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ML24187A131 List:
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Download: ML24187A136 (1)
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DIABLO CANYON POWER PLANT SAMA REVIEW AUDIT REPORT AND INFORMATION REQUESTS

Subject Audit of the Severe Accident Mitigation Alternatives (SAMA) analysis portion of the Diablo Canyon Power Plant Units 1 and 2 (DCPP) License Renewal Application Environmental Report (ER)

When

  • June 25, 2024, 10:00 AM to about 3:30 PM Eastern Time (SAMA Virtual Audit)
  • June 26, 2024, 10:00 AM to about 10:30 AM Eastern Time (SAMA Audit Exit Meeting)

SAMA Discussion Participants Kim Conway U.S. Nuclear Regulatory Commission (NRC)

John Parillo NRC Brian Harris NRC William Ivans Pacific Northwest National Laboratory (PNNL)

Steve Short PNNL Philippe Soenen Pacific Gas and Electric Company (PG&E)

Michelle Olsofsky PG&E David Imbaratto PG&E Nathan Barber PG&E Farzin Nouri PG&E Annika MacLeod Jensen-Hughes David Bidwell Jensen-Hughes Grant Teagarden Jensen-Hughes Robert Kirchner Jensen-Hughes

Documents Reviewed on the Applicants Portal

1. L2.2, Revision 1B, DCPP PRA Level 2 Containment Event Tree Model and Release Category Development, June 2023.
2. MAAP22-02, Revision 2, DCPP MAAP Calculation - SAMA Level 3 Inputs, May 2023.
3. LTR-RAM-II-13-002, RG 1.200 PRA Peer Review Against the ASME/ANS PRA Standard Requirements for the Diablo Canyon Nuclear Power Plant Probabilistic Risk Assessment, March 2013.
4. PWROG-23015-P, Revision 0, Diablo Canyon F&O Closure and Focused Scope Peer Review Report, July 2023.
5. P3118-003-001, Revision 0, Focused Scope Peer Review of the Diablo Canyon Nuclear Power Plant Fire PRA Against the ASME/ANS PRA Standard Requirements, August 2018.
6. P3118-004-001, Revision 1, F&O Closeout by Independent Assessment Report for the Diablo Canyon Nuclear Power Plant (DCPP) Fire PRA Model, May 2023.
7. PWROG-17022-P, Revision 0, Peer Review of the Diablo Canyon Units 1 & 2 Seismic Probabilistic Risk Assessment, September 2017.

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8. PWROG-17078-P, Revision 0-A, Diablo Canyon Units 1 & 2 Independent Assessment of Seismic Probabilistic Risk Assessment Facts & Observations, February 2018.
9. G.2, Revision 11, DCPP Human Reliability Analysis - All Hazards, March 2023.

10.H.1.6, Revision 8, DCPRA RISKMAN Initiating Events, June 2014.

11.E.17, Revision 1, DCPP PRA System Analysis Notebook, May 2023.

12.C.9, Revision 16C, Quantification of CDF and LERF for the DCPP PRA Model, September 2023.

13.X.1, Revision 2, DCPP Other External Events, February 2024.

14.PRA91-009, Revision 0, DCPP PRA Containment Event Tree, April 1992.

15.30028-RPT-02, Revision 1, DCPP Level 3 PRA Consequence Analysis (WinMACCS Model), October 2023.

16.DCPP 1C15, ORIGEN Inventory Evaluation, December 2008.

Discussion On November 7, 2023, the NRC received the PG&E application for renewal of operating licenses for DCPP (Agencywide Documents Access and Management System (ADAMS)

ML23311A154). In support of the application and in accordance with Title 10 of the Code of Federal Regulations (10 CFR) Part 51 and Part 54, PG&E also submitted an ER for DCPP.

After a welcome and introductions on Tuesday, June 25, 2024, the plan and schedule for the SAMA portion of the audit were discussed. NRC, PNNL, and PG&E/Jensen-Hughes staff involved in the SAMA portion of the audit participated in this meeting.

As stated in the audit plan accompanying the May 20, 2024, letter to Paula Gerfen, Senior Vice President, Generation and Chief Nuclear Officer (ML24117A013), the goal of this portion of the audit was to review SAMA and postulated accident supporting information to complete the license renewal supplemental environmental impact statement. During the audit, NRC, PNNL, and PG&E/Jensen-Hughes staff involved in the SAMA review discussed all the SAMA-related audit needs identified in the Audit Plan. Specifically discussed were the audit needs addressing the following topics:

1. The Level 1 internal events Probabilistic Risk Assessment (PRA);
2. The Level 2 PRA accident phenomenology model;
3. The treatment of external events;
4. The Level 3 PRA consequence analysis;
5. The identification and screening of potential SAMAs;
6. The cost-benefit analysis of SAMAs; and
7. Potential lower cost alternative SAMAs.

As the result of the audit, NRC/PNNL staff identified requests for confirmation of information (RCIs) and requests for additional information (RAIs) for which further information will be needed on the docket to complete the license renewal supplemental environmental impact

2 statement. The following table summarizes the results of the audit by delineating the SAMA-related audit needs in the audit plan into: 1) those for which information is not needed on the docket; 2) those for which supplemental information is needed on the docket as RCIs; and 3) those for which supplemental information is needed on the docket as RAIs. The results shown in this table were summarized with PG&E at the conclusion of the SAMA Virtual Audit meeting held on June 25, 2024. The RCIs and RAIs are provided in separate sections below.

A SAMA audit exit meeting was held on June 26, 2024.

Audit Need Part Subpart Information Needed on the Docket SAMA-1 a RAI b RAI c RAI d RAI e RAI f RAI g RAI h i, ii, iii, ivRAI i i, ii Not Needed j RAI k i, ii, iii RAI SAMA-2 a RAI b RAI c Not Needed d Not Needed e RAI f Combined with SAMA-2e g Not Needed h RAI i i, ii, iii, ivRAI j RAI SAMA-3 a RAI b RAI c RAI d i, ii, iii RAI SAMA-4 a RAI b RAI c RAI SAMA-5 a RAI b RAI c RAI d RAI SAMA-6 a RAI b RCI c Not Needed SAMA-7 a RAI SAMA-8 Not Needed SAMA-9 Not Needed

3 Request for Confirmation of Information (RCI)

1) RCI SAMA-6

[Info Need SAMA-6.b] Section G.7.2.1.4 of the ER for SAMA 17, which proposes an alternate emergency diesel generator to power a station battery charger for steam generator level instrumentation and auxiliary feedwater (AFW) control, identifies a modeling assumption, stating that the [p]ower supplies to the turbine driven [TD] AFW pump are in the same location as existing supplies such that new fire and internal flooding modeling are not required. However, during the audit, this and other modeling assumptions associated with SAMA 17 were discussed, and based on these discussions, it was identified that the assumption regarding power supplies for the TDAFW pump is not needed. Confirm that this assumption can be deleted given that the TDAFW instrumentation is not physically affected by SAMA 17 (i.e., the alternate 480V AC generator powers a station battery charger and the existing power distribution system supports the TDAFW pump).

Requests for Additional Information (RAIs)

1) RAI SAMA-1

REQUIREMENT: 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 internal events and calculation of core damage frequency in the Level 1 PRA model.

ISSUE: 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.

REQUEST: Section 4.15.3 and Attachment G of the ER state that Nuclear Energy Institute (NEI) 05-01-A, Severe Accident Mitigation Alternatives Analysis Guidance Document, was used in developing the SAMA analysis. Section 3.1 of NEI 05-01-A specifies the information that is to be provided in the ER regarding the Level 1 PRA model and analysis. Some of this information is not provided in the ER. Provide the following information relative to the Level 1 PRA analysis.

The requested information is needed for the NRC staff to reach a conclusion on the adequacy of the applicants Level 1 PRA model for supporting the SAMA evaluation.

a. [Info Need SAMA-1.a] Section G.2.1.17 of the 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.

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b. [Info Need SAMA-1.b] Provide a breakdown of the contribution to internal events CDF by initiating event. Confirm that the total internal events 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 events.
c. [Info Need SAMA-1.c] Provide a breakdown of the contribution to internal flooding CDF by flooding area. Confirm that the total internal flooding CDF is the same as that reported in the ER for the DC05A Application Model.
d. [Info Need SAMA-1.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. [Info Need SAMA-1.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 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) (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. [Info Need SAMA-1.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, 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); 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. [Info Need SAMA-1.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,

5 have had on CDF is not provided. Consistent with Section 3.1.1.2 of 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. [Info Need SAMA-1.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.

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. 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. [Info Need SAMA-1.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 and process guidelines used, and justify that the peer review is reasonable for the SAMA analysis.

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j. [Info Need SAMA-1.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) RAI SAMA-2

REQUIREMENT: 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 accident propagation and radionuclide release in the Level 2 PRA model.

ISSUE: 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.

REQUEST: Section 4.15.3 and Attachment G of the ER state that the NEI 05-01-A guidance was used in developing the SAMA analysis. Section 3.2 of NEI 05-01-A specifies the information that is to be provided in the ER regarding the Level 2 PRA model and analysis.

Some of this information is not provided in the ER. Provide the following information relative to the Level 2 PRA analysis. The requested information is needed for the NRC staff to reach a conclusion on the adequacy of the applicants Level 2 PRA model for supporting the SAMA evaluation.

a. [Info Need SAMA-2.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

7 reported (8.68E-05 per year). Provide an explanation for this difference and justification that it does not impact the SAMA analysis.

b. [Info Need SAMA-2.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 6.81E-06 per year.

Explain the 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. [Info Need SAMA-2.e and SAMA-2.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, it is noted that Table G.3-12 indicates that the interfacing system loss of coolant accident source term category (or ST5) results in a higher dose and offsite economic cost than a large early release. Additionally, Table G.2-8 states that the frequency of RC22 is always zero. Explain the basis of release category RC22, describing the criteria upon which sequences, if any, are mapped to this category, and 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).

d. [Info Need SAMA-2.h] While the SAMA analysis ultimately applies results developed using Modular Accident Analysis Program (MAAP), Section G.2.2.4.2 of the ER states that 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.

e. [Info Need SAMA-2.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

8 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.

f. [Info Need SAMA-2.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) RAI SAMA-3

REQUIREMENT: 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.

ISSUE: 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.

REQUEST: Section 4.15.3 and Attachment G of the ER state that the NEI 05-01-A guidance was used in developing the SAMA analysis. Section 3.1.2 of NEI 05-01-A specifies the information that is to be provided in the ER regarding the treatment and inclusion of external events in the SAMA analysis. Some of this information is not provided in the ER. Provide the following information relative to the treatment and inclusion of external events in the SAMA analysis. The requested information is needed for the NRC staff to reach a conclusion on the adequacy of the applicants analyses that support the SAMA evaluation.

a. [Info Need SAMA-3.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.

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b. [Info Need SAMA-3.b] Provide a breakdown of the contribution to the fire CDF 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. [Info Need SAMA-3.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. [Info Need SAMA-3.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.

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.

10 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) RAI SAMA-4

REQUIREMENT: 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, NRC staff evaluates the applicant's analysis of accident consequences in the Level 3 PRA.

ISSUE: 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.

REQUEST: Section 4.15.3 and Attachment G of the ER state that the NEI 05-01-A guidance was used in developing the SAMA analysis. Section 3.4 of NEI 05-01-A specifies the information that is to be provided in the ER regarding the Level 3 PRA model and analysis.

Some of this information is not provided in the ER. Provide the following information relative to the Level 3 PRA analysis. The requested information is needed for the NRC staff to reach a conclusion on the adequacy of the applicants Level 3 PRA model for supporting the SAMA evaluation.

a. [Info Need SAMA-4.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. [Info Need SAMA-4.b] Discuss the extent to which there was missing precipitation data and how missing data was accounted for in the SAMA evaluation.
c. [Info Need SAMA-4.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 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) RAI SAMA-5

REQUIREMENT: 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 basis for the selection and screening Phase I SAMA candidates.

11 ISSUE: 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.

REQUEST: Section 4.15.3 and Attachment G of the ER state that the NEI 05-01-A guidance was used in developing the SAMA analysis. Sections 5 and 6 of NEI 05-01-A specifies the information that is to be provided in the ER regarding the selection and screening of Phase I SAMA candidates. Some of this information is not provided in the ER. Provide the following information with regard to the selection and screening of Phase I SAMA candidates. The requested information is needed for the NRC staff to reach a conclusion on the adequacy of the applicants analyses that support the SAMA evaluation.

a. [Info Need SAMA-5.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 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. [Info Need SAMA-5.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. [Info Need SAMA-5.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. [Info Need SAMA-5.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) RAI SAMA-6

REQUIREMENT: 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 Plant SAMA analysis, NRC staff evaluates the applicants cost-benefit analysis of Phase II SAMAs.

ISSUE: 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.

REQUEST: Section 4.15.3 and Attachment G of the ER state that the NEI 05-01-A guidance was used in developing the SAMA analysis. Sections 7 and 8 of NEI 05-01-A specifies the information that is to be provided in the ER regarding the Phase II cost-benefit and sensitivity evaluations. Some of this information is not provided in the ER. Provide the following information relative to the Phase II SAMA analysis and sensitivity evaluations. The requested information is needed for the NRC staff to reach a conclusion on the adequacy of the applicants analyses that support the SAMA evaluation.

a. [Info Need SAMA-6.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. [New Info Need from Audit] As discussed during the audit, some information requests documented herein may have the potential to impact the results of the baseline SAMA analysis. Examples include information needed to assess the impact of the modeling of FLEX strategies consistent with accepted guidance (RAI 1.j), new seismic information (RAI 3.c), and the reasonableness of modeling changes made to assess the benefits of SAMAs (RAI 6.a). Provide justification that the cumulative impact of such information does not alter the results of the SAMA analysis (i.e., all modeling changes proposed to address the referenced information requests, if incorporated into the baseline SAMA analysis, would not result in the identification of any additional potentially cost beneficial SAMAs).
7) RAI SAMA-7

REQUIREMENT: 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

13 SAMA analysis, NRC staff considers additional SAMAs that may be more effective or have lower implementation costs than the other SAMAs evaluated by the applicant.

ISSUE: The requested information is needed in order for the NRC staff to reach a conclusion on the adequacy of the applicants determination of potentially cost beneficial SAMAs.

REQUEST: For certain SAMAs considered in the DCPP ER, there may be lower cost or more effective alternatives that could achieve much of the risk reduction. In this regard, provide an evaluation of the following SAMA.

Section 4.15.3 and Attachment G of the ER state that the NEI 05-01-A guidance was used in developing the SAMA analysis. Section 2 of NEI 05-01-A notes that potentially cost-beneficial SAMAs are most likely to be procedure changes and minimal hardware changes. 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 for the NRC staff to reach a conclusion on the adequacy of the applicants analyses that support the SAMA evaluation.

[Info Need SAMA-7.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.

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