ML22020A226
| ML22020A226 | |
| Person / Time | |
|---|---|
| Site: | Summer  |
| Issue date: | 01/20/2022 |
| From: | Lawrence D Dominion Energy South Carolina |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| 21-411 | |
| Download: ML22020A226 (19) | |
Text
Dominion Energy South Carolina, Inc.
5000 Dominion Boulevard, Glen Allen, VA 23060 Dominion Energy.com Attn: Document Control Desk January 20, 2022 U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 DOMINION ENERGY SOUTH CAROLINA (DESC)
VIRGIL C. SUMMER NUCLEAR STATION (VCSNS) UNIT 1 LICENSE AMENDMENT REQUEST LAR-20-203 ft_ Dominion ii a,, Energy" Serial No.:
21-411 NRA/YG:
RO Docket No.:
50-395 License No.:
NPF-12 "APPLICATION FOR TECHNICAL SPECIFICATION CHANGE REGARDING RISK-INFORMED JUSTIFICATION FOR THE RELOCATION OF SPECIFIC SURVEILLANCE FREQUENCY REQUIREMENTS TO A LICENSEE-CONTROLLED PROGRAM" RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION (RAil By letter dated April 8, 2021 (Agencywide Document Access and Management System Package Accession No. ML21102A127), Dominion Energy South Carolina, Inc. (DESC),
submitted a license amendment request for a Technical Specifications (TS) change regarding risk-informed justification for the relocation of specific surveillance frequency requirements to a licensee-controlled program for Virgil C. Summer Nuclear Station (VCSNS), Unit 1. VCSNS is proposing to modify the TS by relocating specific surveillance frequencies to a licensee-controlled program with the implementation of Nuclear Energy Institute (NEI) 04-10, "Risk-Informed Technical Specifications Initiative Sb, Risk-Informed Method for Control of Surveillance Frequencies."
In an email dated November 29, 2021 (Agencywide Document Access and Management System Package Accession No. ML21333A189), from Mr. Ed Miller, NRG Senior Project Manager, to Mr. Yan Gao of Dominion Energy, the Nuclear Regulatory Commission (NRG) staff requested additional information to facilitate their review of the subject LAR.
The NRC's request for additional information (RAI) and the DESC responses are provided in the Enclosure to this letter. contains the items provided in the RAI, the corresponding responses, and the applicable references.
Serial No.21-411 Docket No. 50-395 Page 2 of 3 Should you have any questions, please contact Mr. Yan Gao at (804) 273-2768.
Douglas wrence Vice President - Nuclear Engineering and Fleet Support COMMONWEAL TH OF VIRGINIA COUNTY OF HENRICO The foregoing document was acknowledged before me, in and for the County and Commonwealth aforesaid, today by Douglas C. Lawrence, who is Vice President - Nuclear Engineering and Fleet Support of Dominion Energy South Carolina, Inc. He has affirmed before me that he is duly authorized to execute and file the foregoing document on behalf of that Company, and that the statements in the document are true to the best of his knowledge and belief.
Acknowledged before me this 2Dth day of Jtu-tuar:;:1, 2022.
My Commission Expires:
t 2.l.3l I 2-'t Commitments made in this letter: None.
Enclosure:
- 1. Response to NRC Request for Additional Information CRAIG D SLY Notary Public Commonwealth of Virginia Reg. # 7518653 My Commission Expires December 31, 20!!
cc:
U.S. Nuclear Regulatory Commission, Region II Marquis One Tower 245 Peachtree Center Avenue, NE Suite 1200 Atlanta, Georgia 30303-125 7 Mr. G. Edward Miller NRG Senior Project Manager U.S. Nuclear Regulatory Commission One White Flint North Mail Stop 09 E-3 11555 Rockville Pike Rockville, Maryland 20852-2738 NRG Senior Resident Inspector V.C. Summer Nuclear Station Ms. Anuradha Nair-Gimmi Bureau of Environmental Health Services Serial No.21-411 Docket No. 50-395 Page 3 of 3 South Carolina Department of Health and Environmental Control 2600 Bull Street Columbia, SC 29201 Mr. G. J. Lindamood Santee Cooper - Nuclear Coordinator 1 Riverwood Drive Moncks Corner, SC 29461 Serial No.21-411 Docket No. 50-395 Response to NRC Request for Additional Information Virgil C. Summer Nuclear Station (VCSNS) Unit 1 Dominion Energy South Carolina, Inc. (DESC)
TABLE OF CONTENTS Serial No.21-411 Docket No. 50-395 : Page 1 of 15
1.0 BACKGROUND
............................................................................................................... 2 2.0 APLB RAls and Responses............................................................................................. 2 2.1 APLB RAl Open PRA Facts and Observations (F&O).......................................... 2 2.2 APLB RAl-02-Open IEPRA Findings......................................................................... 6 3.0 APLC RAls and Responses............................................................................................. 8 3.1 APLC RAl Use of Addendum B of the PRA Standard (2013)............................... 8 3.2 APLC RAl Seismic PRA Open F&Os.................................................................. 10 3.3 APLC RAl Considerations of High Winds............................................................ 11 3.4 APLC RAl Considerations of External Flooding.................................................. 12
4.0 REFERENCES
............................................................................................................... 13
Serial No. 21 -411 Docket Nos. 50-395 : Page 2 of 15 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION -APPLICATION FOR TECHNICAL SPECIFICATION CHANGE REGARDING RISK-INFORMED JUSTIFICATION FOR THE RELOCATION OF SPECIFIC SURVEILLANCE FREQUENCY REQUIREMENTS TO A LICENSEE-CONTROLLED PROGRAM DOMINION ENERGY SOUTH CAROLINA, INC. (DESC)
VIRGIL C. SUMMER NUCLEAR STATION (VCSNS) UNIT 1
1.0 BACKGROUND
By letter dated April 8, 2021 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML21102A127) [4.1 ], Dominion Energy South Carolina, (DESC), submitted a license amendment request (LAR) for the Virgil C. Summer Nuclear Station, Unit 1, (VCSNS), to relocate specific surveillance frequency requirements to a licensee-controlled program in accordance with Technical Specifical Task Force Traveler 425, "Relocate Surveillance Frequencies to Licensee Control - RITSTF Initiative Sb,"
(TSTF-425). The U.S. Nuclear Regulatory Commission (NRC) staff has reviewed the LAR and generated a request for additional information (RAI) [4.2] in order to complete the review. Section 2 and Section 3 below provide the RAI items and the corresponding responses.
2.0 APLB RAls and Responses 2.1 APLB RAl Open PRA Facts and Observations (F&O)
Regulatory Guide (RG) 1.200 An Approach for Determining the Technical Adequacy of Probabilistic Risk Assessment for Risk-Informed Activities," Revision 2. March 2009 (ADAMS Accession No. ML090410014), provides guidance for addressing probabilistic risk assessment (PRA) acceptability and describes the peer review process using the American Society of Mechanical Engineers/American Nuclear Society (ASMEIANS) PRA standard ASMEIANS-RA-Sa-2009. as one acceptable approach for determining the technical acceptability of the PRA. The primary results of peer reviews are the Facts and Observations (f&Os) recorded by the peer review team and the subsequent resolution of these F&Os.
a) of the LAR. "Documentation of PRA Acceptability," provides finding-
/eve/ F&Os for internal events. internal flooding, fire. and seismic PRAs that remain open.
The licensee stated that if a non-trivial impact is expected. then performance of additional sensitivity studies or PRA model changes to confirm the impact on the risk analysis will be included. Please clarify the process and criteria that will be used to determine what constitutes a non-trivial impact on surveillance test interval (ST!) evaluations for open items. Please iustify how the process and criteria is sufficient to determine the impact the open items can have on the ST/ evaluations.
- RAI texts are italic and underlined
Response to 2.1.aJ Serial No. 21 -411 Docket No. 50-395 : Page 3 of 15 Open items impact will be evaluated as follows. The first step in the process is to analyze the proposed STI change using the existing PRA model (i.e., NEI 04-10 [4.3], Risk-Informed Method for Control of Surveillance Frequencies, Step 12) in order to generate a cutset solution for sequences specifically contributing to change in risk.
This is important because the change-in-risk cutset solution will allow the analyst to identify the relevant risk contributors and accident sequences on an STI evaluation-specific basis.
This enables detailed, STl-specific evaluations of PRA acceptability. With this STI cutset solution, a progressive screening approach is used to screen findings, (and similarly, PRA model assumptions and uncertainties) for impact on the analysis. If it can be qualitatively reasoned that the impact of resolving the finding has less than 1 E-8/yr adverse impact on delta (~) Core Damage Frequency (CDF) and 1 E-9/yr on ~Large Early Release Frequency (LERF) for the proposed STI change, then the open item can be considered "trivial" and screened from further consideration in the surveillance frequency evaluation under consideration. Any finding not able to be reasoned in this manner would undergo detailed quantitative evaluation (i.e., sensitivity analysis) in order to affirmatively demonstrate that the acceptance criteria for the surveillance test interval is met.
Sensitivity analyses will be performed by either incorporating finding resolutions or a conservatively bounded approximation of the finding resolution in the internal events PRA (IEPRA) model and re-quantifying the change in CDF and LERF associated with the STI change.
This process is sufficient because it systematically considers PRA acceptability in a comprehensive, STI evaluation-specific manner, that considers cumulative impact. The threshold of 1 E-8/yr on ~CDF and 1 E-9/yr on ~LERF is sufficiently low to ensure that the STI analyst is not prematurely screening out issues which are potentially significant to an evaluation of cumulative significance of PRA acceptability.
b)
Seismic F&O 19-10 regarding assessment of internal events PRA (IEPRA) open findings overall impact on other PRA hazard models.
In Section 3. 4. 1 of Attachment 2 of the LAR, the licensee stated that sixty-five findings remain open and active against the IEPRA model. The seismic PRA (SPRA) peer review team noted that because of the broad nature of the findings (e.g., most high-level requirements (HLRs) have multiple open findings) that they were unable to assess the collective impact on the SPRA model. The NRG staff notes, since the IEPRA model provides the basis used for other hazard models, that this observation would apply to the internal flooding and fire PRA (FPRA) models. Please clarify dependencies of other models on the SPRA.
Response to 2.1.bJ DESC agrees with the NRC staff observation that the Seismic, Fire and Internal Flood PRAs were built using the IEPRA as input and therefore findings against the internal events model are relevant to these other hazard groups. Note that external hazard PRAs do not depend on each other (i.e., Findings against FPRA supporting requirements are not relevant to seismic PRA (SPRA)).
Serial No.21-411 Docket No. 50-395 : Page 4 of 15 c)
The licensee's disposition description states that. for forty-two open findings. each of the related issues will be evaluated in accordance with Steps 5 and 14 of the process detailed in NE/ 04-10. "Risk-Informed Technical Specifications Initiative 5b Risk-Informed Method for Control of Surveillance Frequencies." Revision 1. (ADAMS Accession No. ML071360456). Step 5 consists of identifying sources of PRA modeling uncertainty related to technical adequacy, and Step 14 is the performance of required supplemental sensitivity studies used to determine the impact of the uncertainty issue on ST/
evaluations. Please clarify how the accumulation of these items will be assessed during ST/ evaluations.
- i.
Describe how the additional sensitivity studies related to the open internal events findings will be performed for the internal flooding. fire. and seismic PRA models for each open F&O issue.
Response to 2.1.c).i Refer to the answer to 2.1.c).iii.
ii. Explain the process that will be used to assess the cumulative impact of the multiple finding for each ST/ surveillance evaluation. Include clarification of whether the process will consist of multiple individual sensitivity studies or one cumulative study.
Response to 2.1.c).ii Refer to the answer to 2.1.c).iii.
iii. If the process allows for the performance of multiple individual sensitivity studies rather than one cumulative study, then justify that the process adequately assesses the cumulative impact of the multiple issues in the ST/ evaluation.
Response to 2.1.c).iii Sensitivity analyses will be performed either by incorporating finding resolutions or a conservatively bounded approximation of the finding resolution into the IEPRA model and re-quantifying the change in GDF and LERF associated with the STI change. Because external hazards PRAs are dependent on the IEPRA, internal events resolutions will also be propagated to external hazard PRAs for sensitivity analysis using the external hazards models. In order to account for the cumulative impact of PRA acceptability issues, a single cumulative sensitivity analysis combining all of the applicable resolutions will be performed for each hazard group. The sensitivity analysis will sum the delta GDF and delta LERF from each hazard group for comparison to success criteria. This approach ensures that the sensitivity analysis addresses the cumulative impact of the findings on all hazard groups.
d)
The licensee's disposition states that. for twelve open findings that appear to involve conservative modeling treatments. they will be evaluated in accordance with Steps 9 and 11 of the process detailed in NE/ 04-10 when the issue is determined to fail to meet acceptance guidelines or does not provide meaningful results. The NRG staff notes Step 9 is intended to determine if an ST/ requires incorporation into the PRA model and Step 11 is to update the PRA model.
Serial No.21-411 Docket No. 50-395 : Page 5 of 15
- i.
Explain how this process will assess the cumulative impact of the twelve conservative treatments for each ST/ evaluation.
Response to 2.1.d).i Refer to the answer to 2.1.d).iii.
ii. Confirm that all PRA model updates will meet Capability Category (CC) -
II requirements provided in the ASMEIANS 2009 PRA Standard.
Response to 2.1.d).ii Refer to the answer to 2.1.d).iii.
iii. Confirm that all PRA model updates will be reviewed for PRA upgrades, as defined in the ASMEIANS 2009 PRA Standard, and will be subiect to a focused-scope peer with the associated findings closed prior to performing ST/ evaluations.
Response to 2.1.d).iii Findings that involve conservative modeling treatments are able to be qualitatively screened using the process described in 2.1.a), because resolving a finding that involves a conservative modeling treatment has a beneficial, not adverse, impact on delta CDF/LERF metrics, and thus would not challenge the outcome of an evaluation.
Cumulative impact need not be considered further for screened findings in the 2.1.a) process.
However, this can cause the analysis to not meet acceptance criteria in NEI 04-10, step 12 [4.3], or sensitivity analysis to not meet acceptance criteria in NEI 04-10, step 14 [4.3].
In practice, PRA conservative modeling treatments have been found by DESC to be important considerations which can dictate whether a proposed STI change meets NEI 04-10 acceptance criteria or not. When a proposed STI change does not meet NEI 04-10 [4.3] acceptance criteria and the cause is attributed to conservative PRA modeling treatments, that prompts DESC to resolve conservative PRA modeling treatments to ensure appropriate PRA insights are provided to the Independent Decision-Making Panel (IDP). This method resembles the process described in NEI 04-10, Steps 9 and 11 [4.3],
and the PRA model/STI analysis is iterated as needed to demonstrate acceptable results.
All PRA model updates are generated in accordance with DESC procedures to meet Capability Category (CC) II requirements in the ASME/ANS 2009 PRA Standard [4.6].
This includes PRA configuration control requirements of the ASME Standard that ensure all PRA model updates be assessed to determine if they are PRA maintenance or PRA upgrade. Any changes assessed as PRA upgrades will be subject to a focused-scope peer review with the associated findings closed prior to performing STI evaluations.
e)
Section 4.2 of Attachment 2 of the LAR provides details for open FPRA findings.
The disposition to FO/O CF-A1-01, regarding Circuit Failure Mode Likelihood Analysis, states that FPRA model is not in complete alignment in resolving this issue since Volume 2 of NUREG-7150. Volume 2. "Joint Assessment of Cable Damage and Quantification of Effects from Fire (JACQUE-Fl RE), Expert Elicitation Exercise for Nuclear Power Plant Fire-Induced Electrical Circuit Failure,"
May 2014 (ADAMS Accession No.
Serial No.21-411 Docket No. 50-395 : Page 6 of 15 ML14141A129). has only been partially implemented. The dispositions to FO/Ds ES-B1-01, ES-B1-03. and PRM-B9-02. regarding data and mapping fidelity. state that the closure review team noted that anomalies still exist for these issues. The dispositions for all four findings state that these issues will be evaluated using Steps 5 and 14 of the NE/ 04-10 process (e.g.* sensitivity studies). Given these issues appear to be related to model completeness. please clarify how the sensitivity study process can address these model completeness issues. Identify PRA updates using NUREG-7150 guidance that remain to be implemented and the data fidelity issues that need to be addressed.
Response to 2.1.e)
As part of STI evaluations, DESC will generate a change-in-risk cutset solution as described in the response to 2.1.a). DESC will use the change-in-risk cutset solution to qualitatively review the Circuit Failure Mode Likelihood Analysis (CFMLA) for anomalies and NUREG/CR-7150, Volume 2 [4.9] implementation issues relevant to the proposed STI change. DESC anticipates being able to qualitatively screen each of these four findings on a STI specific basis as described in the response to 2.1.a) because in many cases the relevant issues are not affecting quantification results (i.e., documentation related), not affecting risk significant or STI relevant sequences, or are a conservative treatment. If DESC is not able to qualitatively screen all four of these findings in this manner, then a sensitivity analysis would be performed either by resolving the relevant issue in the CFMLA or applying a conservatively bounded approximation of the resolution into the CFMLA and re-quantifying the analysis. Pending changes to the VCSNS CFMLA are very minor in nature, such as revising circuit failure probabilities from a value of 0.30 to 0.29, or0.62 to 0.56 for the appropriate NUREG-7150 [4.9] "aggregate" value. A single cumulative sensitivity analysis combining the applicable resolutions will be performed for each hazard group as described in the response to APLB RAl-01 c).
2.2 APLB RAl Open IEPRA Findings RG 1.200 provides guidance for addressing PRA acceptability and describes a peer review process using the PRA standard ASMEIANS-RA-Sa-2009. as one acceptable approach for determining the technical acceptability of the PRA. The primary results of peer review are the F&Os recorded by the peer review team and the subsequent resolution of these F&Os. The following findings were dispositioned in the LAR having no impact on the ST/ process. however, for the following F&Os, please clarify how these issues would not impact the program.
a)
F&O 06-19 regarding divergent path analysis.
The disposition for this finding appears to refer to system component screening, which is the subject of the preceding F&O 06-18. Accordingly, the disposition provided for F&O 06-19 does not appear to apply to this finding. Clarify if the disposition to F&O 06-19 is correct for this issue or provide an updated disposition for this finding.
Response to 2.2.aJ Serial No.21-411 Docket No. 50-395 : Page 7 of 15 The disposition of F&O 06-19 is as intended by the author. The process of identifying and incorporating flow divergence paths into the PRA model is very similar to the process of screening components in the systems analysis. When a flow divergence path is identified, this is incorporated into the PRA model by adding components such as check valves in the systems analysis which have a PRA modeled consequence of flow divergence when failed. Flow divergence screening was performed simultaneously with other systems modeling screening described in 06-18, which is why the disposition is identical. The disposition of F&O 06-19 can be clarified as follows (revised text in bold)
PARTIALLY RESOLVED: System model screening was re-reviewed to identify additional components that have the function of precluding flow divergence which need to be added to the PRA systems analysis. The failure of these components has been added to the PRA model.
The outstanding resolution of the issue includes systematically incorporating system model screening into PRA model development process and completing model documentation. System model screening is judged to have a small impact on the surveillance frequency control program (SFCP) because risk insights are governed by major components in modeled systems. This issue will be reviewed and assessed on an evaluation-specific basis in accordance with the NEI 04-10 process (steps 5 and 14) until this issue is considered resolved.
b)
F&O 02-06 regarding the documentation of assumptions. appears to be an open finding with no subsequent work performed. Step 5 of NE/ 04-10 states that "identified sources of key uncertainty serve as inputs to identifying appropriate sensitivity cases in Step 14." Please clarify if this issue has been addressed adequately.
- i.
Provide clarification whether identification of key assumptions and sources related to this supporting requirement (SR) have been identified. sufficiently resolved. and documented for the ST/ process.
Response to 2.2.b).i Assumptions and sources of uncertainty have been identified and sufficiently documented to support the SFCP. These assumptions are discussed "in-line" with various analyses they support.
Resolution of finding 02-06 involves compiling all assumptions into a designated assumptions section of relevant documents. The intent of this documentation finding is to facilitate future peer reviews.
ii. If documentation of these assumptions has not been performed. then provide justification that the excluded sources of uncertainty will not impact ST/ evaluations.
Response to 2.2.b).ii The response to 2.2.b).i discusses that the assumptions associated with F&O 02-06 have been documented adequately to support the SFCP therefore no additional justification is needed.
c)
F&O 04-32 regarding the documentation of limitations of the large early release frequency (LERF) analysis in determining the impact on applications. Please clarify if
Serial No.21-411 Docket No. 50-395 : Page 8 of 15 LERF analysis limitations have been identified and analyzed properly for the ST/ process.
If documentation of these limitations has not been performed. then provide justification that the excluded limitations does not impact ST/ evaluations.
Response to 2.2.cJ While some progress has been made to address this finding, the consensus from the F&O closeout team was that some topics on limitations require additional discussion and documentation to fully resolve this issue. DESC fully understands the extent of these limitations and is tracking this issue in DESC's PRA configuration control process. These LERF modeling limitations must be formally documented in order to close this finding.
Areas where LERF assessment methodology is limited are handled with simplifying conservative treatments in the VCSNS PRA. Therefore, the scope of the sensitivity analysis described in the response to 2.1.aJ does not need to be expanded to include undocumented LERF modeling limitations.
Regardless of whether LERF modeling limitations are formally documented or not: when a proposed STI change does not meet NEI 04-10 [4.3] acceptance criteria and the cause is attributed to conservative PRA modeling treatments, that prompts DESC to resolve the impacting conservative PRA modeling treatments as described in 2.1.d) which in this context the resolution would involve improving the fidelity of the LERF model (i.e., eliminating limitations altogether).
3.0 APLC RAls and Responses 3.1 APLC RAl Use of Addendum B of the PRA Standard (2013)
Section 4 of RG 1. 200. Revision 2. states that a risk informed submittal should contain discussions concerning peer reviews. If the peer review is not performed against the established standards. then information needs to be included in the submittal demonstrating that the different criteria used are consistent with the established standards. as endorsed by NRG.
Section 3. 4. 3 of Attachment 2 of the LAR states that the SPRA was peer reviewed against the requirements in the ASMEIANS PRA Standard (ASMEIANS RA-Sb-2013) for the seismic fragility analysis (SFR) element and seismic plant response (SPR) element. RG 1.200, Revision 2. endorses ASMEIANS PRA Standard Addendum A (ASMEIANS RA-Sa-2009). As noted in the NRG letter dated July 6. 2011. "U.S. Nuclear Regulatory Commission (NRG) Comments on "Addenda to a Current ANS: ASME RA-SB - 20XX Standard for Level 1/Large Early Release Frequency Probabilistic Risk Assessment for Nuclear Power Plant Applications" (ADAMS Accession No. ML111720067), the NRG did not endorse Addendum B of the PRA Standard. DESC's SPRA peer review for SFR and SPR elements was performed using a PRA Standard different from that endorsed by the NRG staff in RG 1.200, Revision 2.
The NRG staff requests that the licensee discuss how the SRs in Addendum B, which is not endorsed by the NRG for licensing applications, and the NRG staff's comments in the
Serial No.21-411 Docket No. 50-395 : Page 9 of 15 above cited letter dated July 6. 2011. are consistent with the SRs in Part 5 of Addendum A. for this LAR. If the different criteria are not consistent with the endorsed Standard.
describe how the analogous Addendum A SRs have been met.
Response to 3.1 DESC notes that the NRC has endorsed (Accession No. ML12319A074 [4.51) Electric Power Research Institute - Nuclear Energy Institute (EPRI-NEI) report, "Screening, Prioritization, and Implementation Details (SPID) document (EPRI 1025287), for the purpose of responding to Near-Term Task Force (NTTF) Recommendation 2.1: Seismic"
[4.10].
Because the guidance of the SPID and the criteria of the ASME/ANS Standard differ in some areas, or the SPID does not explicitly address a Supporting Requirement (SR), the NRC staff developed and applied a checklist to help staff members address and evaluate the differences between Addendum A (2009) [4.6] and the Addendum B (2013) [4. 7]
standards during their detailed review of the VCSNS NTTF 2.1 submittal.
The NRC staff concluded that the differences between the supporting requirements in the Addenda A and B of Part 5 for the hazard technical element were not significant with respect to the review and decision for VCSNS SPRA NTTF Recommendation 2.1:
Seismic (refer to reference [4.11]).
DESC notes that the Risk-Informed Technical Specifications Initiative Sb is an older risk informed application which originated prior to NTTF Recommendation 2.1: Seismic as well as RG 1.200, Revision 2 [4.8] (RG 1.200 Revision 1 was effective at the time). DESC also notes that the NEI 04-10 process allows for qualitative input to assess change in seismic risk when the licensee has not developed a SPRA. Additionally, DESC's experience with implementing SFCP at other sites has demonstrated that surveillance frequency evaluations generally have very low sensitivity to seismic hazards (and, by extension, the Seismic Standards, and requirements) because SPRAs are dominated by seismic fragilities and have low dependence on marginal changes in non-seismic equipment failure rates postulated by the SFCP.
Based on the following:
NRC's detailed review of the VCSNS NTTF 2.1 submittal which explicitly considered application of the Addendum B (2013) standard, NRC endorsement of EPRI-SPID process which, in part, applied Addendum B (2013) standard requirements to address NTTF Recommendation 2.1: Seismic, NEI 04-10 process which does not require SPRAs to be used in Surveillance Frequency Evaluations where SPRAs do not exist, and Low sensitivity of Surveillance Frequency Evaluations to seismic hazards (and, by extension, SPRA standards and requirements),
The VCSNS seismic fragility analysis and seismic plant response (SPR) element meets the intent of RG 1.200, Revision 2 peer review requirements to ensure that the VCSNS SPRA is adequate to support this application.
3.2 APLC RAl Seismic PRA Open F&Os Serial No.21-411 Docket No. 50-395 : Page 10 of 15 Section 4.2 of RG 1.200 states that the LAR should include a discussion of the resolution of the peer review F&Os that are applicable to the parts of the PRA required for the LAR.
This discussion should take the following forms:
a discussion of how the PRA model has been changed. and
- a justification in the form of a sensitivity study that demonstrates the accident sequences or contributors significant to the application decision were not adversely impacted (remained the same) by the particular issue.
Section 4. 3 of Attachment 2 of the LAR provides SPRA F&Os and SFCP Dispositions.
There are four open F&Os discussed in this section.
FO/O 19-10 is an open F&O that covers all unresolved IEPRA F&Os. which may or may not impact the SPRA.
The licensee's SFCP disposition states that the issue will be reviewed and assessed on an evaluation-specific basis in accordance with NE/ 04-10 process (Steps 5. 14) until this issue is considered resolved.
The licensee neither provided a detailed evaluation how each IEPRA F&O has a potential impact on its SPRA model. nor evaluated the impacts on the SFCP.
a)
Provide a detailed evaluation of how each IEPRA F&O has a potential impact. or not. on the SPRA model. For those IEPRA F&Os with impacts on the SPRA. evaluate their impacts on the SFCP or justify why this F&O has no impact on the SFCP.
Response to 3.2.aJ DESC's intent is to apply the process described in the response to 2.1.a) and c) to evaluate SPRA acceptability, given the IEPRA F&Os, on an STI evaluation-specific basis.
DESC notes that until proposed surveillance frequency changes are defined, detailed evaluation of F&O impact on risk insights being input into the SFCP is not possible.
Additionally, DESC notes that the NRC staff has previously audited the impact of internal events findings on the VCSNS SPRA in the context of the 10 CFR 50.54(f) response associated with NTTF Recommendation 2.1: Seismic. The audit is discussed in the NRC letter documenting the staff's SPRA evaluation (Accession No. ML19199A696 [4.11]) and included an aggregate sensitivity analysis of finding impact on Seismic CDF (SCDF),
Seismic LERF (SLERF) and importance metrics. Refer to ML19199A696 [4.11] technical review checklist topic 14.
FO/O 24-07 states that. "The liquefaction potential was not considered in the identification of failure modes that can affect the Service Water system."
b)
Provide justification that this issue would have small impact on the SFCP.
Response to 3.2.b)
Expanded text from the FOID 24-07 basis includes the following:
Serial No.21-411 Docket No. 50-395 : Page 11 of 15 "Although the GEi Project 1411090 report screens out the liquefaction potential for the West Embankment and much of the areas in the Pumphouse and Intake Structure, it does not completely rule out the liquefaction potential for one area where the saprolite was left in place below the Pumphouse and Intake Structure."
Seismic induced failures of the Service Water (SW) Pumphouse/lntake Structure are assumed to directly result in core damage at VCSNS due to an unrecoverable loss of the ultimate heat sink. No systems, structures, and components (SSCs) are modeled in the SPRA for mitigating core damage accident sequences involving seismic induced failure of the SW Pumphouse/lntake Structure. Therefore, this finding has very small impact on the SFCP because SSC reliability is inconsequential to the relevant core damage sequences.
FOID 20-01 states that the PSHA forobabilistic seismic hazard analysis[ for the VG Summer site was performed using the existing seismic source model described in NUREG-2115.
NUREG-2115, "Central and Eastern United States Seismic Source Characterization for Nuclear Facilities." (ADAMS Accession No. ML12048A804 ). which is commonly used for the seismic hazard and screening reports in response to NRG request for information to 10 CFR 50.54{f) regarding recommendation 2.1 of the Near-term Task Force Review of insights from the Fukushima Dai-lchi accident. The licensee neither provided information if the updated seismic source model is available for the VG Summer site to close this F&O. nor compared the existing seismic source model with the updated one if it is available.
c)
If the updated seismic source model is available. compare the existing one used in the SPRA with the updated one. and evaluate the impacts on the SFCP. Otherwise.
describe how it would be closed by FOID 20-01 if the updated seismic source model is not available or is otherwise not supported by docketed information.
Response to 3.2.cJ An evaluation report [4.17] was prepared by Lettis Consultants International, Inc which considered recent geologic information regarding earthquake sources, errors in the maximum magnitude values assigned to some seismic sources in the original study, and recent earthquake occurrences in the region around the VCSNS site since the EPRI et al., 2012 study was conducted. The potential effect of this information on the Probabilistic Seismic Hazard Analysis (PSHA) results for VCSNS was evaluated.
The report concluded that including the more recent earthquake data in an analysis would not increase rates or estimates of seismic hazard at VCSNS from the rates and seismic hazard calculated using the CEUS-SSC (EPRI et al., 2012) model. The report concluded that no updates to the VCSNS PSHA are needed. F&O 20-01 is now considered resolved and therefore has no impact on the SFCP.
3.3 APLC RAI Considerations of High Winds NE/ 04-10, states that external events risk impact may be considered quantitatively or qualitatively. The NRG staff's safety evaluation on NE/ 04-10 (ADAMS Accession No.
Serial No.21-411 Docket No. 50-395 : Page 12 of 15 ML072570267) states that a qualitative screening analysis may be used when the surveillance frequency impact on plant risk can be shown to be negligible or zero.
Section 2.5 of Attachment 2 of the LAR states that the VGSNS hurricane. tornado and high winds analyses show that the plant is adequately designed. or procedures exist to cope with the effects of these natural events. However. the licensee did not provide supporting references to show that the high winds can be screened out from this LAR.
Please clarify whether the licensee's high winds analyses continue to be appropriate for use in the proposed program and whether updated information about high winds will be used in the proposed program. Include a description of how recent high winds related information is included in the licensee's proposed program or justify its exclusion.
Response to 3.3 High winds hazard information from the IPEEE continues to be appropriate for VCSNS.
No new or updated information is available with respect to quantification of high winds hazards.
VCSNS high wind design is described in the FSAR Chapter 3, including identification of Safety-Related SSCs that are protected from the effects of missiles, and those SSCs which are outside (i.e., not protected).
Since the IPEEE, VCSNS has augmented its ability to respond to high wind events that are beyond the design basis as a part of industry response to Fukushima event. Refer to VCSNS FLEX Strategy Final Integrated Plan (Accession No. ML16307A390 [4.12]). Collectively, this information will be used to qualitatively assess, and screen proposed STI changes for adverse risk impact with respect to high winds in accordance with NEI 04-10, step 10 [4.3].
3.4 APLC RAl Considerations of External Flooding NE/ 04-10, states that external events risk impact may be considered quantitatively or qualitatively. The NRG staff's safety evaluation on NE/ 04-10, states that a qualitative screening analysis may be used when the surveillance frequency impact on plant risk can be shown to be negligible or zero.
Section 2.5 of Attachment 2 of the LAR states, in part, that the licensee's IPEEE
{Individual Plant Examination of External Events[ process is capable of identifying the most likely severe accidents and severe accident vulnerabilities. The licensee did not provide any current evaluation on other external hazards, including external flooding. In the licensee's Flood Hazard Reevaluation Report {ADAMS Accession No.
ML13073A114), the licensee determined that the local intense precipitation {LIP) is not bounded by the current design basis.
Please clarify whether the licensee's IPEEE information and evaluation for external flooding continues to be appropriate for use in the proposed program. Please clarify how the updated external flooding information for the site will be used in the proposed program.
The NRG staff requests that the licensee discuss how the IPEEE evaluation for external flooding continues to be appropriate for use in the proposed program given recent information on the external flooding hazard at the site. Include a description of how recent external flooding information for the site is included in the licensee's proposed program
Serial No.21-411 Docket No. 50-395 : Page 13 of 15 or justify its exclusion.
Any information supporting the NRG staff's approval of this amendment needs to be submitted on the docket.
Response to 3.4 VCSNS has most recently reevaluated its external flooding hazard in response to the NRC's March 12, 2012, 10 CFR 50.54(f) request for information. The external flood hazard information in the IPEEE has been superseded by this more recent external flooding hazard evaluation. VCSNS will supplement the IPEEE with information in the following docketed reports (and any subsequent updates performed by VCSNS) in order to perform qualitative risk assessments of the impact of proposed STI changes on external flooding hazards in accordance with NEI 04-10, Step 10 [4.3].
ML13073A114 - Flood Hazard Re-Evaluation Report [4.13]
ML16357A603 - Flood Mitigation Strategies Assessment [4.14]
ML17181A513 - Focused Evaluation for External Flooding [4.15]
These reports describe external flooding hazards and flood mitigation features at VCSNS and will be used as technical input for qualitative external flooding risk assessments in accordance with NEI 04-10, Step 10 [4.3].
With respect to the Flood Hazard Reevaluation Report conclusion that the local intense precipitation (LIP) is not bounded by the current design basis, VCSNS elected to remediate the plant design to account for this insight. In the NRC staff evaluation for the Focused Evaluation for External Flooding (Accession No. ML17272A929 [4.161) NRG staff concluded that the licensee has demonstrated that effective flood protection, if appropriately implemented, exists for the unbounded flooding mechanisms during a beyond-design-basis external flooding event at VCSNS.
This conclusion assumed appropriate implementation of the regulatory commitments identified in the licensee's Focused Evaluation. In response to this RAI, the station corrective action program was reviewed to confirm that all associated regulatory commitments were appropriately implemented at VCSNS.
4.0 REFERENCES
4.1 ML21102A127, License Amendment Request (LAR) for the Virgil C. Summer Nuclear Station (VCSNS), Unit 1, to Relocate Specific Surveillance Frequency Requirements to a Licensee-Controlled Program in Accordance with Technical Specification Task Force Traveler 425, "Relocate Surveillance Frequencies to Licensee Control - RITSTF Initiative 5b," (TSTF-425), April 08, 2021 4.2 ML21333A189, email from Mr. Ed Miller (NRG) to Mr. Yan Gao (Dominion) regarding RAI for VCSNS TSTF-425 LAR, November 29, 2021
Serial No.21-411 Docket No. 50-395 : Page 14 of 15 4.3 ML071360456, Nuclear Energy Institute (NEI) 04-10, Revision 1, "Risk-Informed Technical Specifications Initiative Sb, Risk-Informed Method for Control of Surveillance Frequencies," April 2007 4.4 ML12333A170, Electric Power Research Institute (EPRI) Report 1025285, "Seismic Evaluation Guidance, Screening, Prioritization and Implementation Details (SPID) for the Resolution of Fukushima Near-Term Task Force Recommendation 2.1: Seismic", November 2012 4.5 ML12319A074, NRG letter to NEI, "Endorsement of Electric Power Research Institute Final Draft Report 1025287, "Seismic Evaluation Guidance,"" February 15,2013 4.6 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 4.7 ASME/ANS RA-Sb-2013, Addenda to ASME/ANS RA-S-2008 Standard for Level 1/Large Early Release Frequency Probabilistic Risk Assessment for Nuclear Power Plant Applications 4.8 Regulatory Guide 1.200, Revision 2, March 2009, "An approach for Determining the Technical Adequacy of Probabilistic Risk Assessment results for Risk-Informed Activities" 4.9 ML14141A129, NUREG/CR-7150 Vol 2, "Joint Assessment of Cable Damage and Quantification of Effects", May 22, 2014 4.10 ML12333A170, Electric Power Research Institute - Nuclear Energy Institute (EPRI-NEI) report, "Screening, Prioritization, and Implementation Details (SPID) document (EPRI 1025287), for the purpose of responding to Near-Term Task Force (NTTF) Recommendation 2.1: Seismic," November 2012 4.11 ML19199A696, NRG letter to DESC, "Virgil C. Summer Nuclear Station, Unit 1 -
Staff Review of Seismic Probabilistic Risk Assessment Associated with Reevaluated Seismic Hazard Implementation Of the Near-Term Task Force Recommendation 2.1: Seismic," September 6, 2019 4.12 ML16307A390, SCE&G letter to NRG, BOB Integrated Pan, October 31, 2016 4.13 ML13073A114, SCE&G letter to NRG, Flood Hazard Re-Evaluation Report, March 12,2013 4.14 ML16357A603, SCE&G letter to NRG, VCSNS Flood Mitigation Strategies Report, December 22, 2016 4.15 ML17181A513, SCE&G letter to NRG, Focused Evaluation for External Flooding, June 30, 2017 4.16 ML17272A929, NRG letter to SCE&G, VCSNS Staff assessment of Flooding Focused Evaluation, October 24, 2017
Serial No.21-411 Docket No. 50-395 : Page 15 of 15 4.17 Evaluation report, Lettis Consultants International, Inc., "Review of New Information for V.C. Summer Nuclear Generating Station Probabilistic Seismic Hazard Analysis", December 2018