ML20357A009

Summary of December 17, 2020, Teleconference with Tennessee Valley Authority Regarding Planned License Amendment Request Related to Use of Alternate Probability of Detection at Watts Bar Nuclear Plant, Unit 2

ML20357A009
Person / Time
Site:	Browns Ferry, Watts Bar, Sequoyah
Issue date:	01/19/2021
From:	Kimberly Green Plant Licensing Branch II
To:	Jim Barstow Tennessee Valley Authority
Green K, NRR/DORL/LPL2-2, 415-1627
References
EPID L-2020-LRM-0114
Download: ML20357A009 (5)

Text

January 19, 2021 LICENSEE: Tennessee Valley Authority FACILITY: Watts Bar Nuclear Plant, Unit 2

SUMMARY

OF DECEMBER 17, 2020, TELECONFERENCE WITH TENNESSEE VALLEY AUTHORITY REGARDING PLANNED LICENSE AMENDMENT REQUEST RELATED TO USE OF ALTERNATE PROBABILITY OF DETECTION AT WATTS BAR NUCLEAR PLANT, UNIT 2 (EPID L-2020-LRM-0114)

On December 17, 2020, the U.S. Nuclear Regulatory Commission (NRC) staff conducted a Category 1 public teleconference with representatives of Tennessee Valley Authority (TVA, the licensee). The purpose of the call was to discuss a planned license amendment request (LAR) for Watts Bar Nuclear Plant (Watts Bar), Unit 2, to use an alternate probability of detection (POD).

The meeting notice and agenda, dated December 7, 2020, are available at Agencywide Documents Access and Management System (ADAMS) Accession No. ML20342A359. TVAs presentation slides for the meeting are available at ADAMS Accession No. ML20352A036. A list of participants is enclosed.

TVA stated that it plans to submit a LAR to revise Watts Bar, Updated Final Safety Analysis Report, Section 5.5.2.4 to allow the use of an alternate POD for Generic Letter (GL) 95-05, Voltage-Based Repair Criteria for Westinghouse Steam Generator Tubes Affected by Outside Diameter Stress Corrosion Cracking, operational assessments. GL 95-05 states that the POD should be assumed to have a value of 0.6, or as an alternative, an NRC approved POD function can be used, if such a function becomes available. TVA will propose a POD of 0.90 for bobbin voltage amplitude of 3.2 volts but < 6.0 volts, and a POD of 0.95 for bobbin voltage amplitude of 6.0 volts. The proposed POD values will only be used until the Unit 2, steam generators (SGs) are replaced. A POD of 0.6 will be used for bobbin voltage amplitude less than 3.2 volts.

TVA recently completed the Watts Bar, Unit 2, cycle 3 refueling outage (U2R3), during which it implemented the GL 95-05 voltage-based alternate repair criteria. TVA stated that the condition monitoring requirements were met for all degradation mechanisms except for the GL 95-05 conditional burst probability in SG number 3 (SG 3). During the inspections, noise measurements were performed primarily in the middle (center) third of the tube support plate (TSP), which is the area in which the majority of the indications were identified. Outside Diameter Stress Corrosion Cracking within the tube support plate is the region of interest in GL 95-05. TVA found that SG 3 is limiting for the operational assessment.

TVA briefly described the signal-to-noise (S/N) ratios that were developed based on Examination Technique Specification Sheet I28411. TVA developed a regression to represent the technique specific S/N ratio distribution. TVA also described the 3-step process it used to develop the Watts Bar, Unit 2 specific voltage POD function. In Step 1, TVA developed a

function relating the S/N to the POD. In Step 2, it developed a linear regression function relating the Watts Bar, Unit 2 distorted support indication (DSI) volts peak-to-peak (Vpp) to volts voltage max (Vvm). In Step 3, it performed a simulation to obtain a 95-percent lower bound POD as a function of voltage accounting for the U2R3 noise distribution. The NRC staff asked for clarification about the Step 2 regression relating DSI Vpp to Vvm. The licensee responded that it included all DSIs from the last inspection to perform the regression.

TVA displayed a graphic for Watts Bar, Unit 2, SG 3, showing the distribution of the voltage in Vpp by POD. The plot shows that the proposed POD for 3.2 and 6 volts is less than the TSP noise. TVA also displayed a table listing the 10 DSI indications with greater than or equal to 3.2 volts for the U2R3 inspection. Based on the DSI indications listed, the NRC staff asked whether it was only SG 3 that failed to meet the GL 95-05 conditional probability of burst condition monitoring requirements, or whether the indications in SG 2 and SG 4 also failed to meet the conditional probability of burst. The licensee confirmed that only DSI indications in SG 3 failed to meet the condition monitoring requirements.

TVA demonstrated the proposed POD by injecting flaw signals into specific tube locations that are noisy and showed the resulting plots. The NRC staff commented that although it is helpful to see the flaw response from signal injection simulations, it would also be helpful if the LAR presented the actual data from the largest indications that demonstrate large amplitude signals are readily detected. In response to a question from the licensee, the NRC staff clarified that it was referring to printouts from the data analysis presented in a similar manner as the slide demonstration, not the raw eddy current data.

Based on the technical analyses it performed, TVA concluded that indications of 3.2 volts and greater can be detected with a high degree of uncertainty.

TVA will propose a revision to Section 5.5.4.2 of the Watts Bar, Updated Final Safety Analysis Report to add that in addition to the POD of 0.6 required by GL 95-05, a POD of 0.9 will be applied to indications of axial outside diameter stress-corrosion cracking (ODSCC) at tube support plates with bobbin voltage amplitudes of greater than or equal to 3.2 volts, but less than 6.0 volts, and a POD of 0.95 will be applied to indications of axial ODSCC at tube support plates with bobbin voltage amplitudes of greater than or equal to 6.0 volts until the Unit 2 SGs are replaced. TVA pointed out that no changes to the technical specifications are needed.

TVAs current schedule for SG replacement activities is during the Watts Bar, Unit 2, fall 2023 refueling outage (U2R5). However, TVA is evaluating an earlier replacement during the spring 2022 refueling outage (U2R4).

TVA stated it plans to submit the LAR by December 23, 2020, and will request approval by February 8, 2021, to support the submittal of the 90-day operational assessment reporting requirements in accordance with Technical Specification 5.9.9. The NRC staff inquired as to why TVA believes it needs approval of the amendment by February 8. TVA replied that the final required operational assessment is driving the need for expedited approval, and that without it, the assessment would indicate that Unit 2 could not operate beyond 90 days.

No comments or public meeting feedback were received. No regulatory decisions were made at this meeting.

Please direct any inquiries to me at 301-415-1627 or by e-mail to Kimberly.Green@nrc.gov.

/RA/

Kimberly J. Green, Senior Project Manager Plant Licensing Branch II-2 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-391

Enclosure:

List of Participants cc: Listserv

ML20357A009 *by email OFFICE DORL/LPL2-2/PM DORL/LPL2-2/LA DNLR/NCSG/BC* DORL/LPL2-2/BC DORL/LPL2-2/PM NAME KGreen BAbeywickrama SBloom UShoop KGreen DATE 12/18/2020 12/23/2020 1/5/2021 1/15/21 1/19/21 LIST OF PARTICIPANTS DECEMBER 17, 2020, TELECONFERENCE WITH TENNESSEE VALLEY AUTHORITY REGARDING WATTS BAR NUCLEAR PLANT, UNIT 2 PLANNED LICENSE AMENDMENT REQUEST RELATED TO USE OF ALTERNATE PROBABILITY OF DETECTION Name Organization Kim Green U.S. Nuclear Regulatory Commission (NRC)

Steve Bloom NRC Andy Johnson NRC Paul Klein NRC Greg Makar NRC Tom Stephen NRC Undine Shoop NRC Clint Ashley NRC Vic Cusumano NRC Russ Wells Tennessee Valley Authority (TVA)

Gordon Williams TVA Kim Hulvey TVA Dean Baker TVA Jeremy Mayo TVA Anne Robinson-Givens TVA Jesse Baron TVA Jonathan Pope TVA Andy Parker TVA Michael Bradley Westinghouse Levi Marcus Westinghouse Jay Smith Westinghouse Gary Whiteman Westinghouse Jonathan Fisher Westinghouse Jim Smith Westinghouse Gary Bingaman State of Tennessee Enclosure