ML20296A378

NRR E-mail Capture - Browns Ferry Nuclear Plant, Units 1, 2, and 3 - Request for Additional Information Regarding Request to Incorporate TMRE

ML20296A378
Person / Time
Site:	Browns Ferry
Issue date:	10/22/2020
From:	Michael Wentzel NRC/NRR/DORL/LPL2-2
To:	Orf T Tennessee Valley Authority
References
L-2020-LLA-0099
Download: ML20296A378 (8)

Text

From: Wentzel, Michael Sent: Thursday, October 22, 2020 7:23 AM To: Orf, Tracy J

Subject:

Browns Ferry Nuclear Plant, Units 1, 2, and 3 - Request for Additional Information Regarding Request to Incorporate TMRE (EPID L-2020-LLA-0099)

Attachments: Browns Ferry Nuclear Plant, Units 1, 2, and 3 - Request for Additional Information Regarding Request to Incorporate TMRE (L-2020-LLA-0099).pdf

Dear Mr. Orf:

By application May 6, 2020 (Agencywide Documents Access and Management System (ADAMS)

Accession No. ML20127H904), as supplemented by letter dated July 31, 2020 (ADAMS Accession No. ML20213C669), the Tennessee Valley Authority (TVA) submitted a license amendment request for Browns Ferry Nuclear Plant, Units 1, 2, and 3 (Browns Ferry). The proposed amendments would modify the licensing and design bases as described in the Updated Final Safety Analysis Report to include a new methodology based on NEI 17-02, Revision 1 (ADAMS Accession No. ML17268A023) for determining the structures, systems, and components that require protection from tornado-generated missiles.

The NRCs Probabilistic Risk Assessment Licensing Branch B (APLB) staff are reviewing the application and have identified areas where they need additional information to support their review. The NRC staffs request for additional information (RAI) is attached. As previously discussed with you, the NRC staff requests your response to the RAI by January 29, 2021.

If you have any questions, please contact me at (301) 415-6459 or michael.wentzel@nrc.gov.

Sincerely, Michael Wentzel, Project Manager Plant Licensing Branch II-2 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation

Hearing Identifier: NRR_DRMA Email Number: 850 Mail Envelope Properties (MN2PR09MB5867A02AE6F6D9B93EDC64F5E71D0)

Subject:

Browns Ferry Nuclear Plant, Units 1, 2, and 3 - Request for Additional Information Regarding Request to Incorporate TMRE (EPID L-2020-LLA-0099)

Sent Date: 10/22/2020 7:23:03 AM Received Date: 10/22/2020 7:23:03 AM From: Wentzel, Michael Created By: Michael.Wentzel@nrc.gov Recipients:

"Orf, Tracy J" <tjorf@tva.gov>

Tracking Status: None Post Office: MN2PR09MB5867.namprd09.prod.outlook.com Files Size Date & Time MESSAGE 1346 10/22/2020 7:23:03 AM Browns Ferry Nuclear Plant, Units 1, 2, and 3 - Request for Additional Information Regarding Request to Incorporate TMRE (L-2020-LLA-0099).pdf 75292 Options Priority: Normal Return Notification: No Reply Requested: No Sensitivity: Normal Expiration Date:

REQUEST FOR ADDITIONAL INFORMATION REGARDING THE REQUEST TO UTILIZE THE TORNADO MISSILE RISK EVALUATOR TO ANALYZE TORNADO MISSILE PROTECTION NON-CONFORMANCES TENNESSEE VALLEY AUTHORITY BROWNS FERRY NUCLEAR PLANT, UNITS 1, 2, AND 3 DOCKET NOS. 50-259, 50-260 AND 50-296 By letter dated May 6, 2020 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML20127H904), as supplemented by letter dated July 31, 2020 (ADAMS Accession No. ML20213C669), the Tennessee Valley Authority (TVA),

submitted a license amendment request (LAR) to incorporate the Tornado Missile Risk Evaluator (TMRE) into the licensing basis for Browns Ferry Nuclear Plant, Units 1, 2, and 3 (Browns Ferry).

The U.S. Nuclear Regulatory Commissions (NRCs) Probabilistic Risk Assessment Licensing Branch B (APLB) staff is reviewing the application and has identified areas where it needs additional information to support its review.

In September 2018, the Nuclear Energy Institute (NEI) issued Technical Report, NEI 17-02, Tornado Missile Risk Evaluator (TMRE) Industry Guidance Document, Revision 1B (ADAMS Accession No. ML18262A328). NEI 17-02 establishes a method to conservatively assess the risks posed by tornado-generated missiles. The TMRE approach is consistent with Regulatory Guide (RG) 1.174, Revision 3, An Approach for Using Probabilistic Risk Assessment [PRA] in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis, (ADAMS Accession No. ML17317A256). All references to NEI 17-02 or RG 1.174 in this request refer to Revision 1B or Revision 3, respectively, unless specified otherwise.

RAI 01 - TMRE Walkdown Training Section 5 of RG 1.174 states that qualified personnel are necessary to meet the quality control standards for the PRA.

NEI 17-02 Section 3.4.1, Tornado Missile Walkdown Personnel, states that walkdown personnel should be trained in the methods provided in the NEI document and Section 4.3 of the Electric Power Research Institute (EPRI) Report, EPRI 3002008092, Process for High Winds Walkdown and Vulnerability Assessments at Nuclear Power Plants.

The LAR does not address the training of the personnel who conducted the tornado missile walkdown. Provide confirmation that the personnel who conducted the tornado missile walkdown were appropriately trained in accordance with the applicable guidance.

RAI 02 - Tornado Missile Walkdown Area Section C.2 of RG 1.174 states that the engineering analyses conducted to justify any proposed licensing basis change should be appropriate for the nature and scope of the proposed change.

Section 3.4.2 of NEI 17-02 states that in the case of targets greater than 1500 feet from the plant reference point, a qualitative evaluation of the missile inventory within 2500 feet from the outlying target(s) should be done. The intent of this evaluation is to determine whether the missile inventory used for the TMRE is applicable to all the targets.

Section 3.3.3 of the enclosure to the LAR states that the missile walkdown area was defined by a 2500-foot radius around the center point of each reactor building but does not state if all targets satisfied the criterion of NEI 17-02.

Based on the information provided in the LAR, it is unclear to the NRC staff if the qualitative evaluation identified in NEI 17-02 was performed and whether such an evaluation resulted in changes to the missile inventory. Therefore, the NRC staff requests the following:

a. Confirm that the Browns Ferry TMRE methodology includes a qualitative evaluation of the missile inventory within 2500 feet of targets that are further than 1500 feet from the plant reference point and changes the missile inventory, if necessary.

b. If such qualitative evaluation is not included in the Browns Ferry TMRE methodology, justify that exclusion for current and future applications of the Browns Ferry TMRE.

RAI 03 - Turbine Building Assessment Section C of RG 1.174 states a staff expectation that the engineering analyses conducted to justify the proposed licensing basis change should be appropriate for the nature and scope of the change.

Section 3.4.4 of NEI 17-02 states that missiles from turbine buildings should not be inventoried using the tables provided in that section but should be counted separately.

a. Confirm that the turbine building missile inventory walkdowns were performed using the guidance in NEI 17-02 Section 3.4.4 (that is, that the tables in that section were not used).

b. If the turbine missile count was performed using the tables provided in Section 3.4.4 of NEI 17-02, provide justification that the use of this methodology does not impact the TMRE analyses results.

c. Alternatively, to Parts a and b, provide an updated TMRE analysis that incorporates the appropriate turbine building walkdown missile inventory.

RAI 04 - Compliant Case Conservatisms Sensitivity Study Section 2.5.1.2 of RG 1.174 states that in interpreting the results of a PRA, it is important to understand the impact of specific assumptions on those results.

Section 7.2.2 of NEI 17-02 states that the licensee should review the TMRE compliant results to identify conservatisms related to equipment failure only. The section provides some examples of possible sensitivity studies that focus on changes to the compliant case.

Section 3.3.9 of the LAR states that the CDF and LERF values for all three Browns Ferry units are below 1x10-7/year and 1x10-8/year, respectively, and that these are below the thresholds where sensitivity studies are required. The NRC staff notes that the NEI 17-02, Section 7.2.2 sensitivity study does not have any screening criteria. In light of these observations:

a. Provide the results of the Section 7.2.2 sensitivity study.

b. Provide justification that the sensitivity study meets the requirements of NEI 17-02.

c. If the results of the study exceed the RG 1.174 threshold values, as stated in Section 7.3 of NEI 17-02, provide justification that the results do not impact the conclusions of the TMRE analysis.

RAI 05 - TMRE Human Reliability and Dependency Analysis Section 6.3 of RG 1.174 states that the licensees submittal should ensure the PRA used is acceptable to support the risk assessment of the licensing basis change being considered.

Section 6.4 of NEI 17-02 states that operator actions performed outside of Category I structures or actions requiring transit outside of the Category I structures more than one hour after the tornado event should be assessed. Operator actions within one hour of the tornado strike and that take place in a non-Category I structure that has failed the tornado event are assumed to be failed.

With regards to impacting the dependency analysis, Section 6.4 of NEI 17-02 states that new operator action dependencies will be created as a result of the TMRE model change requirements or new cutsets associated with tornado initiators. Therefore, the analysis should appropriately account for these changes in the TMRE PRA models. Enclosure 3 of the LAR regarding supporting requirement HR-G7 states that no new combinations were created or credited. However, it does address modifications to existing combinations.

a. Provide justification that none of the original Browns Ferry human reliability analysis (HRA) combinations contain any of the human failure events (HFEs) assumed to be failed in the TMRE PRA cases. If the combinations did contain any of these HFEs, then provide justification that the dependency analysis used for Browns Ferry TMRE analyses meets the requirements of NEI 17-02 or demonstrate that the exclusion of the updated dependency analysis does not impact the results of the TMRE analyses.

b. If the above request cannot be justified, then provide an updated TMRE analysis, including sensitivities (if required), that incorporate the appropriate HFEs and dependency analysis.

RAI 06 - TMRE Compliant and Degraded PRA Models Section 6.3 of RG 1.174 states that the licensees submittal should ensure the PRA used is acceptable to support the risk assessment of the licensing basis change being considered.

Section 6.3 of NEI 17-02 provides elements that make up the compliant and degraded cases.

Some items (not an exhaustive list) are: (1) turbine buildings and exposed non-safety-related systems, structures, or components (SSCs) (e.g., tanks, pipes) should be evaluated to determine their capability to withstand wind pressures and associated failure probabilities, (2) non-Category I structures incapable of withstanding the forces for tornado winds > 103 mph, along with exposed non-safety-related SSCs (e.g., pumps, compressors) are assumed to fail (probability = 1.0), and (3) vulnerable but conforming components (those SSCs exposed to tornado-missiles and compliant with Browns Ferrys licensing basis) include tornado-missile induced failures based on exposed equipment failure probabilities (EEFP) calculations.

Table 6-1 in Section 6.3 provides a succinct summary of elements for both cases.

Section 3.3.6 of the LAR does not appear to address these elements. It is not clear to the NRC staff that all the criteria of NEI 17-02 are met. In light of these observations:

a. Provide clarification whether EEFPs were included in both the compliant and degraded PRA cases for exposed passive non-safety-related SSCs and other vulnerabilities as identified in Section 6.3 of NEI 17-02. Include in this discussion how this analysis was incorporated into both the compliant and degraded cases.

b. Provide details on the analysis related to non-Category I structures and turbine building related to tornado wind survivability. Include in this discussion how this analysis was incorporated into both the compliant and degraded cases.

c. If the compliant and degraded cases did not include the other failure modes required by NEI 17-02, provide justification that the exclusion of these items does not impact the TMRE analyses results.

d. Alternatively to Part c, provide an updated TMRE analysis that incorporates all of the SSC failure modes required by NEI 17-02.

RAI 07 - Addition of FLEX to the PRA Model The NEI published NEI 16-06 Crediting Mitigating Strategies in Risk-Informed Decision Making (ADAMS Accession No. ML16286A297), to address risk assessment of mitigating strategies used to support risk-informed applications. The NRC staff assessed this guidance and clarified the staffs position in a memorandum dated May 30, 2017 (ADAMS Accession No. ML17031A269). The NRC staff identified challenges to incorporating Diverse and Flexible Coping Strategies (FLEX) equipment and strategies in support of risk-informed decision using a PRA model that meets the guidance in RG 1.200, Revision 2, An Approach for Determining the Technical Adequacy of Probabilistic Risk Assessment Results for Risk-Informed Activities, (ADAMS Accession No. ML090410014).

Section 3.2 of the LAR refers to FLEX strategies but does not state whether or not TVA has incorporated FLEX mitigating strategies and associated equipment into the PRA models used to support this application. Therefore, it is unclear whether FLEX equipment and operator actions will be used to support this application and, if applicable, whether the incorporation of FLEX equipment and actions into the PRA models was performed in an acceptable manner.

Provide the following information separately for internal events PRA, seismic PRA, and external hazard screening, as appropriate:

a. Clarify whether FLEX equipment and associated actions have been credited in the PRAs used to support this application, identifying the specific PRA(s) that include such credit. If not incorporated, no response to parts (b) and (c) is requested.

b. If the FLEX equipment or operator actions have been credited, provide the following information separately for the internal events PRA (includes internal floods) and seismic PRA, as appropriate:

1. Discuss the extent to which FLEX has been incorporated. That is, summarize the supplemental equipment and compensatory actions that have been quantitatively credited for each of the PRA models used to support this application.

2. If any credited FLEX equipment is dissimilar to other plant equipment credited in the PRA (i.e., SSCs with sufficient plant-specific or generic industry data), discuss the data and failure probabilities used to support the modeling and provide the rationale for using the chosen data. Include discussion on whether the uncertainties associated with the parameter values are in accordance with the PRA Standard as endorsed by RG 1.200.

3. If any operator actions related to FLEX equipment are evaluated using approaches that are not consistent with the endorsed PRA Standard (e.g.,

using surrogates), discuss the methodology used to assess human error probabilities for these operator actions. The discussion should include:

i. A summary of how the impact of the plant-specific human error probabilities and associated scenario-specific performance shaping factors listed in (a)-(j) of supporting requirement HR-G3 of the PRA Standard were evaluated.

ii. Whether maintenance procedures for the portable equipment were reviewed for possible preinitiator human failures that render the equipment unavailable during an event, and if the probabilities of the preinitiator human failure events were assessed as described in high-level requirement HLR-HR-D of the PRA standard.

iii. If the procedures governing the initiation or entry into mitigating strategies are ambiguous, vague, or not explicit, a discussion detailing the technical bases for probability of failure to initiate mitigating strategies.

c. The PRA Standard defines PRA upgrade as the incorporation into a PRA model of a new methodology or significant changes in scope or capability that impact the significant accident sequences or the significant accident progression sequences.

The PRA Standard states that upgrades of a PRA shall receive a peer review in

accordance with the requirements specified in the peer review section of the relevant part of the Standard (Part 1, Sections 1 5). Either:

1. Provide an evaluation of the model changes associated with incorporating non-safety-related SSCs (whether permanently installed or portable) that were included following the FLEX mitigation strategies but are not similar to safety-related SSCs. This evaluation should demonstrate that none of the following criteria are satisfied: (1) use of new methodology, (2) change in scope that impacts the significant accident sequences or the significant accident progression sequences, and (3) change in capability that impacts the significant accident sequences or the significant accident progression sequences; or

2. Confirm that a focused-scope peer review has been performed on the model changes associated with incorporating mitigating strategies, and associated F&Os are resolved to Capability Category II.