ML21203A297

Safety Evaluation of Vogtle Electric Generating Plant Units 1 and 2 License Amendment Requests for Risk-Informed Resolution to GSI-191

ML21203A297
Person / Time
Site:	Vogtle
Issue date:	07/26/2021
From:	Matthew Sunseri Advisory Committee on Reactor Safeguards
To:	Margaret Doane NRC/EDO
Wang W
References
Download: ML21203A297 (10)

Text

UNITED STATES NUCLEAR REGULATORY COMMISSION ADVISORY COMMITTEE ON REACTOR SAFEGUARDS WASHINGTON, DC 20555 - 0001 Ms. Margaret M. Doane Executive Director for Operations U.S. Nuclear Regulatory Commission Washington, DC 20555-0001

SUBJECT:

SAFETY EVALUATION OF VOGTLE ELECTRIC GENERATING PLANT UNITS 1 AND 2 LICENSE AMENDMENT REQUESTS FOR RISK-INFORMED RESOLUTION TO GSI-191 Dear Ms. Doane During the 687th meeting of the Advisory Committee on Reactor Safeguards, July 7-9, 2021, we completed our review of the draft safety evaluation (SE) for Southern Nuclear Operating Company (SNC) license amendment requests (LARs) related to risk-informed resolution of Generic Safety Issue (GSI) -191, Assessment of Debris Accumulation on Pressurized Water Reactor [PWR] Sump Performance, at Vogtle Electric Generating Plant Units 1 and 2. In support of these LARs, SNC submitted a related exemption request. A joint subcommittee, consisting of members from our Accident Analyses & Thermal-Hydraulic and the Reliability &

Probabilistic Risk Assessment (PRA) Subcommittees, also reviewed this topic on May 19, 2021.

During these meetings, we met with the U.S. Nuclear Regulatory Commission (NRC) staff and representatives from SNC. We also had the benefit of the referenced documents.

CONCLUSION AND RECOMMENDATIONS 1.

The SNC proposed changes to the Vogtle Units 1 and 2 licensing bases and the related exemption request are acceptable.

2.

The SE report should be issued, and the related exemption should be approved.

BACKGROUND GSI-191 History During a loss-of-coolant accident (LOCA), a two-phase water jet from a pipe break can dislodge and fragment nearby materials such as thermal insulation. This debris can be transported by steam and water to the containment sumps and strainers. Loss of long-term core cooling (LTCC) could occur if debris were to clog the strainers causing recirculation and containment spray pump failures or were to bypass the strainers and block reactor core cooling channels.

We have commented on the containment sump plugging issue (GSI-191) for many years and written several letter reports on the staff and industry research and implementation (see Attachment). The staff established GSI-191 in 1996 and issued Generic Letter (GL) 2004-02 to July 26, 2021

M. Doane request that licensees address the effects of debris accumulation on pressurized water reactor (PWR) sump performance during design basis accidents. The Commission directed the staff to consider a risk-informed approach as an option for resolution of GSI-191.

SNC GSI-191 Approach Vogtle Units 1 and 2 contain Westinghouse four-loop PWRs rated at 3626 MWt. To address safety issues discussed in GSI-191 and GL 2004-02, SNC proposed a risk-informed approach using the five key principles in Regulatory Guide (RG) 1.174. SNC subsequently submitted two LARs to implement their proposed risk-informed approach at these two Vogtle units. In addition to amending several technical specifications, SNC requested an exemption to allow the use of risk-informed information as an alternative to certain requirements associated with the deterministic evaluation required by Title 10 of the Code of Federal Regulations (10 CFR)

Section 50.46(a)(1).

DISCUSSION Our review focused on several topics: modifications and testing, engineering evaluations, and monitoring.

Modifications and Testing Over the period since issuance of GSI-191, the licensee has already implemented various physical and procedural changes. These modifications included installing new strainers with increased surface areas and reduced opening sizes, increasing Refueling Water Storage Tank (RWST) inventory, removing problematic insulation materials, changing procedures to delay isolation of Residual Heat Removal (RHR) pumps from the RWST, and modifying program controls to ensure debris load limits are not exceeded. SNC has proposed reducing the RHR strainer screen height, allowing these strainers to become fully submerged for an increased number of postulated scenarios.

SNC will also add a new technical specification to enhance containment sump reliability. The proposed technical specification is simpler and more flexible than found at plants with shared strainers and sumps. This new technical specification allows an affected train to be declared inoperable rather than shutting down a unit to implement corrective actions. Several other technical specifications (see Monitoring) will be modified to enhance surveillance for detecting debris at the sump suction and structural distress or abnormal corrosion of the sump screen.

Tests were performed to determine the head loss induced by debris accumulation on the sump strainers using NRC guidance. A plant-specific containment computer-aided design model was used to quantify the amount of debris generated for a range of pipe break locations, sizes, and orientations. Using these results, the licensee performed a risk analysis (see Engineering Evaluations) that demonstrates a negligible increase in risk by the generated debris. The completeness of this analysis supports issuing the requested exemption.

Engineering Evaluations In their approach, SNC combined PRA with traditional engineering analysis to evaluate change in core damage frequency and change in large early release frequency. This approach used Vogtle Units 1 and 2 PRA models for internal and seismic events.

M. Doane In evaluating LTCC, the licensee compared parameters, including fuel types, fiber limits, sump switchover time, rated thermal power, and minimum time for chemical effects to result in precipitation, with industry findings (WCAP-17788) and staff guidance. The amount of fiber that may reach the core inlet exceeded the WCAP limit in some cases. By crediting the reduced head loss due to non-uniform material buildup associated with coolant flow, SNC showed that additional fiber could accumulate before core inlet channels become blocked. The staff concurred that in-vessel debris will not block the core inlet prior to the availability of cooling through alternate flowpaths and found there is reasonable assurance that in-vessel debris will not limit LTCC.

For these assessments, the licensee modified the PRA model to consider additional structures, systems, and components (e.g., sump strainers) and events (e.g., core blockage scenarios). A screening approach was used to focus analysis on a discreet set of breaks that were significant risk contributors.

In accordance with RG 1.174 and NUREG-1855, the applicant performed an uncertainty evaluation, which focused on CDF and included parametric, model, and completeness considerations. Parametric and model uncertainties were evaluated through sensitivity runs.

Completeness uncertainty was qualitatively evaluated. All parametric and model uncertainty/sensitivity results were acceptable based on RG 1.174 guidance.

Independent staff calculations, using conservative inputs, confirmed that there were adequate margins in the SNC results. The staff concluded that the SNC analysis scope and level of detail were sufficient and that the PRA model used for the evaluation complies with RG 1.200, Revision 2, and can be applied in regulatory decision-making.

Monitoring The licensee implemented procedures and programs for monitoring, controlling, and assessing changes having the potential to impact inputs and assumptions used in the PRA and associated engineering analysis supporting these LARs. These procedures and programs ensure that potential debris accumulation in containment is limited, the impact of observed changes is assessed, and the capability of the sump strainers is maintained. SNC provided a list of relevant programmatic requirements including its Maintenance Rule program, its online configuration risk management procedure, and its quality assurance program. The licensee stated that no changes would be made to ASME Section Xl inspection programs or mitigation strategies.

Consistent with recommendations in the staff evaluation of Technical Specification Task Force Traveler, TSTF-567, SNC proposed a new technical specification (see Modifications and Testing) that provides operability requirements and expands existing surveillance requirements for the containment sump. The new technical specification requires verification, by visual inspection, that the containment sump does not show structural damage, abnormal corrosion, or debris blockage. It also requires inspection of the containment drainage flow paths, the containment sump strainers (or screens), the pump suction trash racks, and the inlet to relevant piping.

M. Doane The licensee committed to performing periodic updates of its risk-informed assessment every 48 months. If such updates reveal non-conforming situations with respect to GSI-191 (i.e.,

acceptance criteria of RG 1.174 are exceeded), SNC will address these items in their corrective action program.

SUMMARY

The SNC Vogtle GSI-191 submittals provide confidence that safety margins and defense-in-depth will be maintained and that the risk associated with LOCA-generated debris is very low.

The SNC proposed changes to the Vogtle Units 1 and 2 licensing bases and the related request for an exemption are acceptable. The SE report should be issued, and the related exemption should be approved.

We are not requesting a formal response from the staff to this letter report.

Sincerely, Matthew W. Sunseri Chairman

Enclosure:

Prior ACRS Letter Reports Addressing GSI-191 REFERENCES 1.

U.S. Nuclear Regulatory Commission, Safety Evaluation by the Office of Nuclear Reactor Regulation Related to a License Amendment Request for a Risk-Informed Resolution to GSI-191 Amendment No. [Xxx] to Facility Operating License No. NPF-68 and Amendment No. [Xxx] to Facility Operating License No. NPF-81 Southern Nuclear Operating Company, Inc. Vogtle Electric Generating Plant, Units 1 and 2 Docket Nos. 50-424 and 50-425, Draft, May 2021 (ML20268A070).

2.

Southern Nuclear Operating Company, Inc., Exemption Request and License Amendment Request for a Risk-Informed Resolution to GSI-191, August 17, 2018 (ML20230A346).

3.

Southern Nuclear Operating Company, Inc., Vogtle Electric Generating Plant - Units 1 & 2

- Response to Request for Additional Information Regarding Risk-Informed Resolution to GSI -191, December 17, 2020 (ML20352A228).

4.

Southern Nuclear Operating Company, Inc., Vogtle Electric Generating Plant - Units 1 & 2

- Supplement to Request for Exemption to Support Risk-Informed Resolution to Generic Letter 2004-02, February 15, 2021 (ML21046A094).

5.

Southern Nuclear Operating Company, Inc., Vogtle Electric Generating Plant Proposed Path to Closure of Generic Safety lssue-191, Assessment of Debris Accumulation on Pressurized-Water Reactor Sump Performance, dated May 16, 2013 (ML13137A130).

Signed by Sunseri, Matthew on 07/26/21

M. Doane 6.

Technical Specifications Task Force, TSTF-567, Add Containment Sump TS to Address GSI-191 Issues, Revision 1, August 2, 2017 (ML17214A813).

7.

U.S. Nuclear Regulatory Commission, Final Safety Evaluation by the Office of Nuclear Reactor Regulation Technical Specifications Task Force Traveler TSTF-567, Revision 1 Add Containment Sump TS to Address GSI-191 Issues, using the Consolidated Line Item Improvement Process (EPIC L-2017-PMP-0005), July 3, 2018 (ML18116A606).

8.

U.S. Nuclear Regulatory Commission, Final Staff Evaluation for Vogtle Electric Generating Plant, Units 1 and 2, Systematic Risk-Informed Assessment of Debris Technical Report (EPID L-2017-TOP-0038), September 30, 2019 (ML19120A469).

9.

Southern Nuclear Operating Company, Inc., NL-18-0915, Vogtle Electric Generating Plant

- Units 1 & 2, Supplemental Response to NRC Generic Letter 2004-02, July 10, 2018 (ML18193B163 and ML18193B165).

10. Southern Nuclear Operating Company, Inc., "Vogtle Electric Generating Plant - Units 1 & 2, Supplemental Response to NRC Generic Letter 2004-02," dated April 21, 2017 (ML17116A096).

11. Southern Nuclear Operating Company, Inc., "Vogtle Electric Generating Plant - Units 1 &2, Response to Supplemental Information Needed for Acceptance of Systematic Risk Informed Assessment of Debris Technical Report," dated July 11, 2017 (ML17192A245).

12. Southern Nuclear Operating Company, Inc., "Vogtle Electric Generating Plant, Units 1 & 2, Systematic Risk-Informed Assessment of Debris Technical Report, SNC Response to NRC Request for Additional Information (RAls #1-3)," dated November 9, 2017 (ML17314A014).

13. Southern Nuclear Operating Company, Inc., "Vogtle Electric Generating Plant, Units 1 & 2, Systematic Risk-Informed Assessment of Debris Technical Report SNC Response to NRC Request for Additional Information (RAls #4-10)," dated January 2, 2018 (ML18004A070).

14. Southern Nuclear Operating Company, Inc., "Vogtle Electric Generating Plant, Units 1 & 2, Response to NRC Request for Additional Information (RAls #11-14) to Systematic Risk-Informed Assessment of Debris Technical Report," dated January 9, 2018 (ML18009A841).

15. Southern Nuclear Operating Company, Inc., "Vogtle Electric Generating Plant, Units 1 & 2, Incorporate Seismic Probabilistic Risk Assessment into the 10 CFR 50.69 Categorization Process, Response to Request for Additional Information (RAls 1, 2, 3 & 12)," dated February 6, 2018 (ML18037B121).

16. Southern Nuclear Operating Company, Inc., "Vogtle Electric Generating Plant, Units 1 & 2, Systematic Risk Informed Assessment of Debris Technical Report SNC Response to NRC Request for Additional Information (RAls #16-36)," dated February 12, 2018 (ML18045A094).

M. Doane 17. Southern Nuclear Operating Company, Inc., "Vogtle Electric Generating Plant, Units 1 & 2, License Amendment Request to Incorporate Seismic Probabilistic Risk Assessment into the 10 CFR 50.69 Categorization Process, Response to Request for Additional Information (RAls 4-11)," dated February 21, 2018 (ML18052B342).

18. Southern Nuclear Operating Company, Inc., "Vogtle Electric Generating Plant, Units 1 & 2, Systematic Risk-Informed Assessment of Debris Technical Report SNC Response to NRC Request for Additional Information (RAls #37-39)," dated May 23, 2018 (ML18143B785).

19. Southern Nuclear Operating Company, Inc., "Vogtle Electric Generating Plant, Units 1 & 2, Systematic Risk-Informed Assessment of Debris Technical Report Supplemental Information," dated December 4, 2018 (ML18338A497).

20. U.S. Nuclear Regulatory Commission, "Potential Impact of Debris Blockage on Emergency Recirculation During Design Basis Accidents at Pressurized-Water Reactors," NRC Generic Letter 2004-02, dated September 13, 2004 (ML042360586).

21. U.S. Nuclear Regulatory Commission, Staff Requirements - SECY-10-0113 - Closure Options for Generic Safety Issue - 191, Assessment of Debris Accumulation on Pressurized Water Reactor Sump Performance, December 23, 2010 (ML103570354).

22. U.S. Nuclear Regulatory Commission, SECY-12-0093, Closure Options for Generic Safety Issue - 191, Assessment of Debris Accumulation on Pressurized-Water Reactor Sump Performance, July 9, 2012 (ML121310648).

23. U.S. Nuclear Regulatory Commission, Regulatory Guide 1.174, An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis, Revision 3, January 2018 (ML17317A256).

24. U.S. Nuclear Regulatory Commission, Regulatory Guide 1.200, An Approach for Determining the Technical Adequacy of Probabilistic Risk Assessment Results for Risk-Informed Activities, Revision 2, March 2009 (ML090410014).

25. Westinghouse Electric Company LLC, "Evaluation of Post-Accident Chemical Effects in Containment Sump Fluids to Support GSl-191," WCAP-16530-NP-A, March 2008 (ML081150379).

26. Nuclear Energy Institute, NEI 04-07: Volume 1, Pressurized Water Reactor Sump Performance Evaluation Methodology, dated December 2004; and Volume 2, Safety Evaluation by the Office of Nuclear Reactor Regulation Related to NRC Generic Letter 2004-02, December 2004 (ML050550138 and ML050550156).

27. PWR Owners Group, Evaluation of Downstream Sump Debris Effects in Support of GSl-191," Revision 1, TR-WCAP-16406-P-A, dated March 2008 (ML081000025).

28. Westinghouse Electric Company LLC, Evaluation of Long-Term Cooling Considering Particulate, Fibrous and Chemical Debris in the Recirculating Fluid, TR-WCAP-16793-NP-A, Revision 2, dated July 2013 (ML13239A114).

29. PWR Owners Group, Comprehensive Analysis and Test Program for GSI-191 Closure (PA-SEE-1090), TR-WCAP-17788, July 17, 2015 (ML15210A667).

M. Doane 30. U.S. Nuclear Regulatory Commission, Staff SE on WCAP-17788, Technical Evaluation Report (TER) of In-Vessel Debris Effects, issued June 13, 2019 (ML19178A252).

31. U.S. Nuclear Regulatory Commission, Guidance on the Treatment of Uncertainties Associated with PRAs in Risk-Informed Decisionmaking - Final Report, NUREG-1855, Rev 1, March 2017 (ML17062A466).

ENCLOSURE Prior ACRS Letter Reports Addressing GSI-191 1.

U.S. Nuclear Regulatory Commission, ACRS letter report, ACRS Review of NRC Staff Resolution for Unresolved Safety Issues A-1, Water Hammer, and A-43, Containment Emergency Sump Performance, Sep 7, 1983 (ML19277D926).

2.

U.S. Nuclear Regulatory Commission, ACRS letter report, ACRS Review of Proposed Resolution for USI A-43, Containment Emergency Sump Performance and Regulatory Guide 1.82, Revision 1, Water Sources for Long Term Recirculation Cooling Following a Loss of Coolant Accident, Sep 16, 1985 (ML17006A228).

3.

U.S. Nuclear Regulatory Commission, ACRS letter report, Potential for BWR ECCS Strainer Blockage Due to LOCA Generated Debris, October 14, 1994 (ML16218A295).

4.

U.S. Nuclear Regulatory Commission, ACRS letter report, Proposed Final NRC Bulletin 96-XX, Potential Plugging of Emergency Core Cooling Suction Strainer by Debris in Boiling Water Reactors and an Associated Draft Revision 2 of Regulatory Guide 1.82, Water Sources for Long-term Recirculation Cooling Following a Loss-of-Coolant Accident, February 26, 1996 (ML16203A410).

5.

U.S. Nuclear Regulatory Commission, ACRS letter report, Generic Safety Issue-191, Assessment of Debris Accumulation on PWR Sump Performance, September 14, 2001 (ML012610281).

6.

U.S. Nuclear Regulatory Commission, ACRS letter report, Proposed Resolution of Generic Safety Issue-191, Assessment of Debris Accumulation on PWR Sump Performance, February 20, 2003 (ML030520101).

7.

U.S. Nuclear Regulatory Commission, ACRS letter report, Draft Final Revision 3 to Regulatory Guide 1.82, Water Sources for Long-Term Recirculation Cooling Following a Loss-of-Coolant Accident, September 30, 2003 (ML032731487).

8.

U.S. Nuclear Regulatory Commission, ACRS letter report, Proposed Draft Final Generic Letter on Potential Impact of Debris Blockage on Emergency Recirculation During Design Basis Accidents at PWRs, July 19, 2004 (ML042020003).

9.

U.S. Nuclear Regulatory Commission, ACRS letter report, Safety Evaluation of the Industry Guidelines related to Pressurized Water Reactor Sump Performance, October 18, 2004 (ML042920334).

10. U.S. Nuclear Regulatory Commission, ACRS letter report, Safety Evaluation of the Industry Guidelines related to Pressurized Water Reactor Sump Performance, December 10, 2004 (ML043450346).

11. U.S. Nuclear Regulatory Commission, ACRS letter report, Generic Safety Issue 191 -

Assessment of Debris Accumulation on PWR Sump Performance, April 10, 2006 (ML060830362).

12. U.S. Nuclear Regulatory Commission, ACRS letter report, Generic Safety Issue 191 -

Assessment of Debris Accumulation on PWR Sump Performance, August 1, 2006 (ML062140359).

ENCLOSURE

13. U.S. Nuclear Regulatory Commission, ACRS letter report, Status of Resolution of Generic Safety Issue - 191, Assessment of Debris Accumulation on PWR Sump Performance, October 22, 2008 (ML082810710).

14. U.S. Nuclear Regulatory Commission, ACRS letter report, Comments on SECY-10-0113, Closure Options for Generic Safety Issue-191, Assessment of Debris Accumulation in Pressurized Water Reactor Sump Performance, September 17, 2010 (ML102560368).

15. U.S. Nuclear Regulatory Commission, ACRS letter report, Long-Term Cooling for the Westinghouse AP1000 Pressurized Water Reactor, December 20, 2010 (ML103410348).

16. U.S. Nuclear Regulatory Commission, ACRS letter report, Draft Safety Evaluation of WCAP-16793-NP, Revision 2, Evaluation of Long-Term Cooling Considering Particulate, Fibrous and Chemical Debris in the Recirculating Fluid, October 18, 2012 (ML12290A654).

17. U.S. Nuclear Regulatory Commission, ACRS letter report, Long-Term Core Cooling for the South Texas Project Advanced Boiling Water Reactor Combined License Application, November 07, 2012 (ML12312A215).

18. U.S. Nuclear Regulatory Commission, ACRS letter report, Chapters 6 and 7 of the Safety Evaluation Report with Open Items for Certification of the US-APWR Design and Related Long-Term Core Cooling Issues, December 24, 2013 (ML13346A732).

19. U.S. Nuclear Regulatory Commission, ACRS letter report, Safety Evaluation of License Amendment Request by South Texas Project Nuclear Operating Company to Adopt a Risk-Informed Resolution of Generic Safety Issue-191, May 18, 2017 (ML17137A325).

M. Doane

SUBJECT:

SAFETY EVALUATION OF VOGTLE ELECTRIC GENERATING PLANT UNITS 1 AND 2 LICENSE AMENDMENT REQUESTS FOR RISK-INFORMED RESOLUTION TO GSI-191 Accession No: ML21203A297 Publicly Available (Y/N): Y Sensitive (Y/N): N If Sensitive, which category?

Viewing Rights:

NRC Users or ACRS only or See restricted distribution OFFICE ACRS SUNSI Review ACRS ACRS ACRS NAME WWang WWang LBurkhart SMoore MSunseri DATE 7/22/21 7/22/21 7/22/21 7/23/21 7/26/21 OFFICIAL RECORD COPY July 26, 2021