NL-18-1179, Response to NRC Request for Additional Information Tornado Missile Risk Evaluator SNC Supplemental

Response to NRC Request for Additional Information Tornado Missile Risk Evaluator SNC Supplemental

ML18260A053
Person / Time
Site:	Vogtle
Issue date:	09/14/2018
From:	Gayheart C Southern Nuclear Operating Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
NL-18-1179
Download: ML18260A053 (8)

Text

~ Southern Nuclear September 14, 2018 Docket Nos.: 50-424 50-425 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D. C. 20555-0001 Cheryl A. Gayheart Regulatory Affa1rs Director Vogtle Electric Generating Plant - Unit 1 and 2 Tornado Missile Risk Evaluator 3535 Colonnade Parkway Birmingham. AL 35243 205 992 5316 tel 205 992 7601 lax cagayhea@ southernco.com NL-18-1179 SNC Supplemental Response to NRC Request for Additional Information Ladies and Gentlemen:

By letter dated October 11, 2017 (Agencywide Documents Access and Management System Accession No. ML17284A348); Southern Nuclear Operating Company, Inc. (SNC) submitted a License Amendment Request (LAR) for Vogtle Electric Generating Plant (VEGP), Units 1 and 2.

This LAR requested approval to use the Tornado Missile Risk Evaluator to resolve non-conformance with the Tornado Missile Protection licensing basis. By letter dated June 1, 2018 (Agencywide Documents Access and Management System Accession No. ML18136A497), the NRC staff notified SNC that additional information is needed for the staff to complete their review. By letter dated July 26, 2018 (Agencywide Documents Access and Management System Accession No. ML18207A876) SNC responded to the NRC request for additional information (RAI).

By memorandum dated August 21, 2018 (Agencywide Documents Access and Management System Accession No. ML18094A263) the NRC documented a summary of a public meeting held between the NRC and SNC on August 2, 2018, where the NRC discussed issues and points of clarification on the SNC RAI responses.

By letter dated August 30, 2018 (Agencywide Documents Access and Management System Accession No. ML18236A445) the NRC staff notified SNC that augmentation of its response to RAI questions 11, 13a, and 19c is needed to fully address NRC staff information needs to continue their review. Additionally, the NRC staff notified SNC that additional changes included in the RAI response were beyond the scope of the original LAR as noticed in 83 FA 13150.

These additional changes were described in the preface to Enclosure 2, "Red-Line Mark-up of NEI 17-02, Revision 1," of the SNC letter dated July 26, 2018 (Agencywide Documents Access and Management System Accession No. ML18207A876). SNC has elected to remove those additional changes from the RAI response to expedite the review and approval of the LAR. to this letter augments the SNC response to RAI 11, 13a, and 19c.

This letter contains no NRC commitments. If you have any questions, please contact Jamie Coleman at 205.992.6611.

U.S. Nuclear Regulatory Commission NL-18-1179 Page2 I dec@re under penalty of perjury that the foregoing is true and correct. Executed on the 141D-day of September 2018.

Respectfully submitted, Cheryl.

Directo, egulatory Affairs Southern Nuclear Operating Company CAG/kgl/scm

Enclosure:

SNC Supplemental Response to NRC Request for Additional Information (RAI) cc:

Regional Administrator, Region II NRR Project Manager-VEGP 1 and 2 Senior Resident Inspector-VEGP 1 and 2 State of Georgia Environmental Protection Division RType: CVC700

Vogtle Electric Generating Plant - Unit 1 and 2 Tornado Missile Risk Evaluator SNC Supplemental Response to NRC Request for Additional Information SNC Supplemental Response to NRC Request for Additional Information (RAI}

to NL-18-1179 NRC RAI11 (DRA) Regulatory Position 2.1.2 in RG 1.17 4, Revision 2, discusses safety margin as one of the key principles of risk-informed integrated decision-making. This Regulatory Position states, in part, that with sufficient safety margin, the safety analysis acceptance criteria in the licensing basis (e.g., final safety analysis report (FSAR), supporting analyses) are met or proposed revisions provide sufficient margin to account for analysis and data uncertainty. Section 7.5, "Defense-in-Depth and Safety Margin," of NEI 17-02, Revision 1, explains that engineering evaluation should be performed to assess whether the proposed licensing basis change maintains safety margin and identify conservatisms in the risk assessment to show that safety margin is maintained.

Section 3.2.2, "Safety Margin," of Enclosure 1 to the LAR discusses safety margin and states, in part, that "safety analyses acceptance criteria stated in the UFSAR are not impacted by this change," but provides no basis for that statement.

Section 2.3, "Evaluate Target and Missile Characteristics," of NEI 17-02, Revision 1, states that tornado missile failures do not need to be considered for SSCs protected by 18-inch reinforced concrete walls, 12-inch reinforced concrete roofs, and/or 1-inch steel plate. The guidance requires no analysis for evaluating the risk of non-conforming conditions that are protected as described in Section 2.3 of NEI17-02, Revision 1, and implies that no protection against the tornado-generated missiles is needed for those SSCs. Revision 1 of NEI17-02 provides similar guidance in Sections 5 and 6.5.

a.

Describe the basis for the conclusion that the safety analysis acceptance criteria in the licensee's safety analysis are not impacted by the proposed change.

b.

Discuss any non-conforming conditions that were (or if identified in the future, will be) screened from VEGP TMRE analysis using the criteria in Section 2.3 of NEI 17-02, Revision 1. For those non-conforming conditions, demonstrate that the safety analysis acceptance criteria in the licensing basis are met or that proposed revisions provide sufficient margin to account for analysis and data uncertainty.

SNC Response to RAI 11 The following response supplements the SNC response to RAI 11 provided in the July 26, 2018 letter (Agencywide Documents Access and Management System Accession No. ML18207A876).

The basis for concluding that the safety analysis acceptance criteria in the Vogtle safety analysis are not impacted by the proposed change is supplemented as follows.

The objective for protection from tornado missiles is to bring the reactor to a safe and stable shutdown condition during or following a tornado. As stated in 10 CFR 50, Appendix A, "General Design Criteria," Criterion 2, "Design bases for protection against natural phenomena," "Structures, systems, and components important to safety shall be designed to withstand the effects of natural phenomena such as earthquakes, tornadoes, hurricanes, floods, tsunami, and seiches without loss of capability to perform their safety functions." Unlike other design criteria, such as Criteria 17, assumption of a single failure is not required in the design to protect against natural phenomena. The safety analysis acceptance criteria described in Chapters 6 and 15 of the Vogtle UFSAR are not altered by the proposed change and the acceptance criteria will continue to be met. The safety E1-1 to NL-18-1179 analyses do not assume a tornado occurs coincident with a design basis accident, except to the extent that a tornado may have the potential to initiate some of the design basis accidents but does not alter the response to the accident. The assumption of failure of one train of a safety system is explicit in the accident analysis. Only a very small fraction of the equipment assumed to mitigate an accident is not protected from the effects of tornado missiles. In the event exposed components of one train of safety related equipment are affected by a tornado missile, there is reasonable assurance that a separate train would be available to provide the safety function. In addition to the equipment credited in the safety analysis described in the Vogtle UFSAR, on-site and near-site FLEX equipment is also available, which provides further assurance that the safety analysis acceptance criteria would be met.

NRC RAI13 (ORA) Regulatory Position 3 in RG 1.17 4, Revision 2, states that careful consideration should be given to implementation of the proposed change and the associated performance-monitoring strategies. Section 8.1, "Plant Configuration Changes," of NEI 17-02, Revision 1, states that design control programs meeting 10 CFR Part 50 Appendix 8, "Quality Assurance Criteria for Nuclear Power Plants and Fuel Reprocessing Plants," will ensure subsequent plant configuration changes are evaluated for their impact on non-conforming sse risk using TMRE. Section 8.1 also states, in part, that:

[l]icensees should ensure that they have sufficient mechanisms to assure that any significant changes to site missile sources, such as a new building, warehouse, or laydown area are evaluated for impact to the TMRE basis, even if not in the purview of the site Design Control program.

Section 4.1, "Applicable Regulatory Requirements/Criteria," of Enclosure 1 to the LAR states that the licensee has confirmed that sufficient mechanisms to assure that any significant permanent changes to site missile sources, such as a new building, warehouse, or laydown area, are evaluated for impact to the TMRE basis, even if not in the purview of the site design control program. Permanent changes that increase the site missile burden within the 2500-ft missile radius established for TMRE shall be reviewed for impact on the TMRE analysis.

a.

Describe the mechanism(s) and approach(es) that will be followed by the licensee to determine whether a particular change to the facility is "significant for evaluation of the impact to the TMRE basis.

b.

Describe the licensee's mechanisms that assure temporary and permanent changes to site missile sources will be evaluated.

c.

Describe the process(es) that ensure changes that could affect VEGP TMRE results (e.g., plant design changes, changes made to the licensee's base internal events PRA model and new information about the tornado hazard at the plant) are considered in future implementation of the licensee's TMRE.

d.

Describe, with justification, the treatment of the currently identified non-conforming conditions in future uses of the licensee's TMRE PRA model.

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e.

Describe, with justification, how the cumulative risk associated with unprotected SSCs evaluated under TMRE will be considered in future decision making (e.g.,

10 CFR 50.59 criteria as well as in future risk-informed submittals).

SNC Response to RAI 13 The following response supplements the SNC response to RAI 13a provided in the July 26, 2018 letter (Agencywide Documents Access and Management System Accession No. ML18207A876).

13a.

Changes to previous non-conforming SSCs that would increase the target EEFP (e.g.,

effect the target exposed area by increasing the exposed exhaust pipe height, effect a robust missile percentage by changing pipe material or thickness) are not allowed under TMRE. Only changes that result in increased site missile burden require a TMRE analysis update.

NRC RAI19 (ORA) Regulatory Position 2 in RG 1.17 4, Revision 2, states that the licensee should appropriately consider uncertainty in the analysis and interpretation of findings.

Regulatory Position 3 states that decisions concerning the implementation of licensing basis changes should be made after considering the uncertainty associated with the results of the traditional and probabilistic engineering evaluations.

Regulatory Position 3 in RG 1.17 4, Revision 2, states that careful consideration should be given to implementation of the proposed change and the associated performance-monitoring strategies. This Regulatory Position further states that an implementation and monitoring plan should be developed to ensure that the engineering evaluation conducted to examine the impact of the proposed changes continues to reflect the actual reliability and availability of SSCs that have been evaluated. This will ensure that the conclusions that have been drawn from the evaluation remain valid.

Section 7.2, "Sensitivity Analysis," of N El 17-02, Revision 1, address the steps that should be taken if the change in CDF and LERF from the sensitivity analyses exceed 1 o-s per year and 1 o-7 per year, respectively.

a.

Describe the VEGP process if change-in-risk estimates from sensitivity analyses exceed the RG 1.17 4 acceptance guidelines for very small" change in risk in implementation of TMRE methodology.

The licensee evaluated the impact of considering construction-related missiles by performing a sensitivity analysis in Section 3.3.9.2, Temporary Construction Missile Sensitivity," of Enclosure 1 to the LAR. Construction-related missiles have been at the site for an extended period. The basis for considering construction-related missiles only as a sensitivity analysis and not including them in evaluations to represent the as-built, as-operated condition was not discussed.

b.

For current and future applications of the methodology, address how construction-related missiles are considered in the analyses in Section 3.3.9, "Sensitivities," and E1-3 to NL-18-1179 how future analyses will be represented in the VEGP sensitivity analysis (e.g.,

updating the results in Section 3.3.7).

c.

For any TMRE sensitivity analyses used to address the representation of construction related missiles, describe how the importance measures are determined from the TMRE PRA model in the context of the 'binning' approach for the tornado categories employed in the model. Describe whether and how the same basic events, which were discretized by binning during the development of the TMRE PRA model, are combined to develop representative importance measures. For same basic events that are not combined, provide a justification that includes discussion of any impact on the results.

d.

Identify the non-conforming conditions and vulnerabilities that met all the characteristics of a "highly exposed" SSC per Section 7.2.1, "TMRE Sensitivities,"

of NEI 17-02, Revision 1.

The discussions in Section 7.2 of NEI17-02, Revision 1, do not address whether sensitivity analyses will be aggregated in future implementations of the TMRE methodology. For example, it is unclear whether the licensee will combine the sensitivity analyses related to any future open PRA facts and observations (F&Os), sensitivities that address compliant case conservatism and TMRE sensitivity analyses.

e.

Describe, with justification, whether sensitivity analyses in Section 7.2 of NEI 17-02, Revision 1, will be aggregated in future implementation of the TMRE methodology.

The discussion in Section 7.2.3, "Compliant Case Conservatisms," and Section A.2.1.3, "Non-Category I Structures and Exposed Non-Safety Related SSCs," of NEI17-02, Revision 1, recognizes that the TMRE PRA could produce non-conservative change-in-risk results if conservatively assumed failures in the Compliant Case mask change-in-risk. Accordingly, Section 7.2.3 of NEI 17-02, Revision 1, states, in part, that:

[the] licensee should review cutsets in the top 90% of the TMRE compliant case to identify conservatisms related to equipment failure {opposed to offsite power recovery or operator actions) that could impact results.

Section 7.2.3 of NEI 17-02, Revision 1, also explains that the licensee should perform sensitivity studies associated with these conservatisms as directed in Appendix D of the TMRE guideline for PRA standard supporting requirements (SRs) AS-A 10, LE-C3, and SY -87 to address equipment failures in the compliant case that may be masking change-in-risk but does not provide guidance on how such a sensitivity can be performed.

Section 3.3.9.1, "Conservative Risk Treatments Masking Sensitivity," of Enclosure 1 to the LAR describes a sensitivity assessment performed to ensure conservative modeling treatments in the compliant case do not affect the risk assessment conclusions.

f.

Describe any future sensitivity analysis that will be performed to assess the impact of conservatisms associated with modeling the equipment failures in the compliant case of the TMRE PRA model.

E1-4 to NL-18-1179 Modeling operator actions, could contribute to underestimating the change-in-risk calculation associated with non-conforming SSCs. Appendix D, Technical Basis for TMRE Methodology," of N El 17-02, Revision 1, does not address the concern described above could also apply to conservative human reliability analysis modeling (e.g., SR HR-G3 and HR-G7).

g.

Describe how VEGP will address the potential impact of TMRE assumptions related to certain human error probabilities within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> after the accident on the compliant case.

SNC Response to RAI 19 The following response supplements the SNC response to RAI 19c provided in the July 26, 2018 letter (Agencywide Documents Access and Management System Accession No. ML18207A876).

19c.

The SNC response to RAt 19.c provided in the July 26, 2018 letter (Agencywide Documents Access and Management System Accession No. ML18207A876) describes how basic events are combined to develop representative importance measures. NEI17-02, Revision 1 A, Section 7.2.1 states that this sensitivity "only applies to tornado missile basic events for tornado categories F'4, F'5, and F'6. Basic events for F'2 and F'3 tornado missile failures are not considered in this sensitivity."

The RAW values for the F'4-F'6 basic events are used to determine the sensitivity targets, since the F'4-F'6 basic events are the ones being affected by the sensitivity study. A RAW value of 2 was chosen to reflect the importance of the events with respect to increasing the basic event failure probability. The RAW values of the F'2 and F'3 basic events are indicative of the overall risk increase associated with failure probabilities for just the F'2 and F'3 basic events, which are not relevant to the sensitivity study. They do not provide information about the potential risk increase associated with higher F'4-F'6 basic event values. As an extreme example, if the RAW for an F'2 basic event is greater than 2, but the RAW values for all the other tornado intensities are 1.0, there is no need to perform the sensitivity analysis, since there would be no change to the flCDF or flLERF.

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