Text

Response to Request for Additional Information Regarding Expedited Application for Approval to Use a Growth Rate Temperature Adjustment When Implementing the Generic Letter 95-05 Analysis for the Watts Bar Nuclear Plant
	ML21134A225
Person / Time
Site:	Watts Bar ; (NPF-096)
Issue date:	05/14/2021
From:	Polickoski J; Tennessee Valley Authority
To:	; Document Control Desk, Office of Nuclear Reactor Regulation
References
	CNL-21-043, EPID L-2021-LLA-0026, EPID L-2021-LRO-0003
	Download: ML21134A225 (22)
	v • d • e

1101 Market Street, Chattanooga, Tennessee 37402 CNL-21-043 May 14, 2021 10 CFR 50.90 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D.C. 20555-0001 Watts Bar Nuclear Plant Unit 2 Facility Operating License No. NPF-96 NRC Docket No. 50-391

Subject:

Response to Request for Additional Information Regarding Expedited Application for Approval to Use a Growth Rate Temperature Adjustment When Implementing the Generic Letter 95-05 Analysis for the Watts Bar Nuclear Plant, Unit 2 Steam Generators (WBN TS-391-21-002)

(EPID L-2021-LLA-0026 and EPID L-2021-LRO-0003)

References:

1. TVA letter to NRC, CNL-21-011, Expedited Application for Approval to Use a Growth Rate Temperature Adjustment When Implementing the Generic Letter 95-05 Analysis for the Watts Bar Nuclear Plant (WBN) Unit 2 Steam Generators (WBN TS-391-21-002), dated February 25, 2021 (ML21056A623 and ML21056A624)

2. TVA letter to NRC, CNL-21-040, Supplement to Expedited Application for Approval to Use a Growth Rate Temperature Adjustment When Implementing the Generic Letter 95-05 Analysis for the Watts Bar Nuclear Plant Unit 2 Steam Generators (WBN TS-391-21-002) (EPID L-2021-LLA-0026), dated March 23, 2021 (ML21082A118 and ML21082A119)

3. TVA letter to NRC, WBL-21-006, Watts Bar Nuclear Plant (WBN) Unit 2 -

Refueling Outage 3 Generic Letter 95-05 Voltage-Based Alternate Repair Criteria Final Report, dated February 11, 2021 (ML21042B342)

4. NRC Electronic Mail to TVA, Request for Additional Information re Generic Letter 95-05 90-Day Report and LAR to Adjust Growth Rate for Thot (EPIDs L-2021-LRO-0003 and L-2021-LLA-0026), dated April 2, 2021 (ML21095A040)

5. NRC Electronic Mail to TVA, RE: RE: Request for Additional Information re Generic Letter 95-05 90-Day Report and LAR to Adjust Growth Rate for Thot (EPIDs L-2021-LRO-0003 and L-2021-LLA-0026), dated May 7, 2021

U.S. Nuclear Regulatory Commission CNL-21-043 Page 2 May 14, 2021 In Reference 1, Tennessee Valley Authority (TVA) submitted a request for an amendment to Facility Operating License No. NPF-96 for the Watts Bar Nuclear Plant (WBN), Unit 2 to revise the WBN dual-unit Updated Final Safety Analysis Report (UFSAR) to apply a temperature adjustment to the growth rate calculation used to determine the end-of-cycle (EOC) distribution of indications of axial outside diameter stress corrosion cracking at tube support plates.

In Reference 2, TVA supplemented Reference 1, by providing Westinghouse Electric Company LLC (Westinghouse) Report, SG-CDMP-20-23-P, Revision 2, Watts Bar U2R3 Steam Generator Condition Monitoring and Final Operational Assessment, which is the operational assessment for the steam generator inspection conducted during the WBN Unit 2 Cycle 3 refueling outage (U2R3). In Reference 3, TVA submitted the 90-Day Steam Generator Inspection Report for WBN Unit 2 Cycle 3 in accordance with the requirements of WBN Unit 2 Technical Specification (TS) 5.9.9, Steam Generator Tube Inspection Report.

In Reference 4, the Nuclear Regulatory Commission (NRC) issued a request for additional information (RAI) regarding References 1, 2, and 3, and requested TVA respond by May 2, 2021. In Reference 5, the due date for this RAI response was extended to May 17, 2021. Enclosure 1 to this letter provides the TVA response to the RAI. TVA recognizes that the due date for the RAI response does not support the TVA requested date of May 19, 2021, for the NRC approval of Reference 1.

In response to RAI 2, Enclosure 2 to this letter provides a revision to the WBN UFSAR marked-up pages that were provided in Reference 1. Enclosure 3 to this letter provides a revision to the WBN UFSAR retyped pages that were provided in Reference 1. Enclosures 2 and 3 to this letter supersede the revisions to the WBN UFSAR pages that were provided in Reference 1.

This letter does not change the no significant hazard considerations or the environmental considerations contained in the referenced letter. Additionally, in accordance with 10 CFR 50.91(b)(1), TVA is sending a copy of this letter and the enclosure to the Tennessee Department of Environment and Conservation.

contains the new regulatory commitment associated with this submittal. Please address any questions regarding this request to Kimberly D. Hulvey, Senior Manager, Fleet Licensing, at (423) 751-3275.

I declare under penalty of perjury that the foregoing is true and correct. Executed on this 14th day of May 2021.

Respectfully, James T. Polickoski Director, Nuclear Regulatory Affairs Enclosures cc (See Page 3)

U.S. Nuclear Regulatory Commission CNL-21-043 Page 3 May 14, 2021

Enclosures:

1. Response to NRC Request for Additional Information

2. Revised Proposed UFSAR Changes (Markups) for WBN Unit 2

3. Revised Proposed UFSAR Changes (Final Typed) for WBN Unit 2

4. List of Commitments cc (Enclosures):

NRC Regional Administrator - Region II NRC Project Manager - Watts Bar Nuclear Plant NRC Senior Resident Inspector - Watts Bar Nuclear Plant Director, Division of Radiological Health - Tennessee State Department of Environment and Conservation CNL-21-043 E1-1 of 12 Response to NRC Request for Additional Information NRC introduction By letter dated February 11, 2021 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML21042B342), Tennessee Valley Authority (TVA, the licensee) submitted the fall 2020 Generic Letter (GL) 95-05 Voltage-Based Alternate Repair Criteria (ARC) Steam Generator (SG) Report for Watts Bar Nuclear Plant (Watts Bar), Unit 2. The SG tube inspections were performed during the third refueling outage (U2R3). When the voltage-based ARC methodology is applied during an inspection of the SGs performed in accordance with Technical Specification (TS) 5.7.2.12, Steam Generator (SG) Program, TS 5.9.9, Steam Generator Tube Inspection Report, requires that a report be submitted within 90 days after the initial entry into hot shutdown (MODE 4) following completion of the inspection. TS 5.7.2.12 requires that an SG Program be established and implemented to ensure SG tube integrity is maintained.

By letter dated February 25, 2021 (ADAMS Accession No. ML21056A623), TVA submitted a license amendment request (LAR) to change Section 5.5.2.4, Tests and Inspections, of the Updated Final Safety Analysis Report (UFSAR) for Watts Bar, Unit 2. The proposed changes would allow the use of a temperature adjustment in calculating the voltage growth rate and end-of-cycle voltage distribution for bobbin probe stress corrosion cracking eddy current indications in steam generator tubes evaluated according to GL 95-05, Voltage-Based Repair Criteria for Westinghouse Steam Generator Tubes Affected by Outside Diameter Stress Corrosion Cracking [ODSCC]. These growth rate calculations are used to demonstrate SG tubes meet the technical specification performance criteria for structural and leakage integrity. TVA supplemented its request by letter dated March 23, 2021 (ADAMS Accession No. ML21082A118), and submitted Westinghouse Report, SG-CDMP-20-23-P, Revision 2, Watts Bar U2R3 Steam Generator Condition Monitoring and Final Operational Assessment [CMOA],

which is the operational assessment for the SG inspection conducted during U2R3.

Fundamental regulatory requirements with respect to the integrity of the SG tubing are established in Title 10 of the Code of Federal Regulations (10 CFR) Part 50. Specifically, the general design criteria (GDC) in Appendix A, General Design Criteria for Nuclear Power Plants, to 10 CFR Part 50 provide regulatory requirements that state the reactor coolant pressure boundary shall have an extremely low probability of abnormal leakage, of rapidly propagating failure, and of gross rupture (GDC 14), shall be designed with sufficient margin (GDCs 15 and 31), shall be of the highest quality standards practical (GDC 30), and shall be designed to permit periodic inspection and testingto assess their structural and leak tight integrity (GDC 32). Section 3.1.2 of the Watts Bar UFSAR addresses conformance with the GDC in Appendix A to 10 CFR Part 50 (ADAMS Accession No. ML19176A129).

INFORMATION REQUESTED In order to complete its review of the GL 95-05 final report and the evaluation of whether the proposed UFSAR changes meet the SG Program requirements described above, the U.S.

Nuclear Regulatory Commission (NRC) staff requests the following information.

1. The scope of the request is not clear to the NRC staff because it does not identify the operating temperature or actual temperature reduction value for Cycle 4a. The effect of the temperature adjustment methodology is presented as an example based on a temperature CNL-21-043 E1-2 of 12 reduction of 4 degrees Fahrenheit (°F). Provide the following requested information to clarify the scope and operating conditions for the requested amendment.

a. On page 40 of 90 of the CMOA report, it states that the hot-leg temperature (Thot) during Cycle 4a is 612°F, compared to 617°F for Cycle 3. Section 3.2, Technical Analysis, of Enclosure 1 of the LAR states than an operating interval extension of 27 calendar days for Cycle 4a was calculated based on applying the temperature adjustment equation described in Enclosure 2 for a 4°F reduction. State the temperature reduction that will be applied to Cycle 4a and the calendar date at which the mid-cycle outage will begin.

b. The Westinghouse report, LTR-CDMP-21-4 NP-Attachment, Enclosure 3 to the LAR, states that Changing Thot at any point would affect the calculation of the temperature adjustment factor which would consequently affect the operating interval calculations. The proposed UFSAR revision states, in part, when operating temperature differences exist from cycle-to-cycle, could be interpreted as one temperature adjustment per cycle. Please confirm if this should be interpreted as only one temperature difference will be applied during Cycle 4a (and possibly a second single temperature difference in Cycle 4b). If multiple temperatures are intended within an operating cycle, please discuss if the proposed UFSAR wording should be revised. In addition, please provide any supporting data (e.g., plant or laboratory) for Alloy 600 that provides the effect of temperature cycling on stress corrosion crack growth.

c. State whether the application of the temperature adjustment equation will be limited to one-degree Fahrenheit increments. If not, justify that a fraction of a degree can be applied in a calculation of a temperature adjustment.

TVA Response 1a.

In accordance with Section 3.2 of Enclosure 1 to the LAR (Reference 1), a conservative temperature reduction of 4°F will be applied to the analysis supporting WBN Unit 2 Cycle 4a1 for the purpose of revising the GL 95-05 OA (Westinghouse report SG-CDMP-21-1-NP, Revision 0, Enclosure to Reference 2). The rationale for the conservative temperature reduction is that the actual operating temperature reduction of WBN Unit 2 for Cycle 4a is greater than 5°F for the hottest loop in the reactor coolant system.

TVA is planning to commence the WBN Unit 2 Cycle 4a SG inspection outage no later than September 11, 2021. The response to RAI 3 discusses the best estimate margin between the planned outage start date and reaching the performance criteria limits contained in GL 95-05.

1 For purposes of the GL 95-05 OA, Cycle 4a is defined as the period of operation from when WBN Unit 2 entered Mode 4 after U2R3 (November 16, 2020) until the commencement of the Unit 2 Cycle 4 mid-cycle SG inspection outage (no later than September 11, 2021). Cycle 4b is defined as the period of operation when WBN Unit 2 enters Mode 4 after completion of the Unit 2 Cycle 4 mid-cycle SG inspection outage until the commencement of the U2R4 outage (scheduled for March 2022).

CNL-21-043 E1-3 of 12 1b.

As noted in the response to RAI 1.a, only one temperature difference will be applied during Cycle 4a. Prior to entering Mode 4 following the WBN Unit 2 Cycle 4a SG inspection, TVA will make available to the NRC any single temperature change that will be applied to the OA during Cycle 4b and its impact on the operating interval for Cycle 4b. Additionally, if TVA does apply a temperature change during Cycle 4b, the effects on the OA would be addressed in the GL 95-05 90-day report following the WBN Unit 2 Cycle 4a SG inspection outage (see the response to RAI 4).

Multiple temperature changes are not intended during either Cycle 4a or Cycle 4b and the response to RAI 2 addresses the changes to the proposed WBN dual-unit UFSAR wording.

The following information discusses the NRC request to provide any supporting data (e.g., plant or laboratory) for Alloy 600 that provides the effect of temperature cycling on stress corrosion crack growth.

The effect of temperature on crack growth rates of mill annealed Alloy 600 is often expressed in technical literature in terms of the Arrhenius equation. For ODSCC in Alloy 600MA tubing, the typical activation energy is 30 kcal/mol and crack growth rates at different power levels and thus temperatures can be adjusted accordingly. However, transitions in power levels also lead to cyclic changes in tube differential pressure.

Cyclic changes in tube stress levels can enhance crack growth rates.

Electric Power Research Institute (EPRI) Technical Report 3002016069 (Reference 3) provides an analysis of the effects of power level cycles on crack growth rates for thermally treated as well as mill annealed Alloy 600 tubing in support of anticipated flexible power operations. Daily small power level cycles (100% to 80% reactor power) are considered as well as less frequent larger power level cycles (100% to 30% reactor power). Stress corrosion crack growth rates are decreased by lower operating temperatures at lower power levels. However, cyclic changes in tube differential pressure from power cycles lead to enhanced crack growth. Data and analyses in NUREG/CR-6721 ANL/01/07, Effects of Alloy Chemistry, Cold Work, and Water Chemistry on Corrosion Fatigue and Stress Corrosion Cracking of Nickel Alloys and Welds, dated April 2001, provided a method to evaluate the effect of cyclic loading on crack growth rates. For relatively frequent and significant power level changes, the effect of temperature changes and cyclic loading essentially offset one another leading to a composite long term crack growth rate bounded by the 100% power level growth rate.

Any change to the power level cycle at WBN Unit 2 during Cycle 4a or Cycle 4b would be much less severe than anticipated flexible power operation scenarios discussed in EPRI Technical Report 3002016069. Tube stresses will cycle by less than the power transients considered in EPRI Technical Report 3002016069. The tube hoop stress is directly proportional to the differential pressure between the SG primary and secondary side. With only a small change in reactor power, the change in differential pressure will be less than the transients described in EPRI Technical Report 3002016069; therefore, no cyclic loading effect is expected.

1c.

The temperature adjustment equation will be limited to 1°F increments.

CNL-21-043 E1-4 of 12 References

1. TVA letter to NRC, CNL-21-011, Expedited Application for Approval to Use a Growth Rate Temperature Adjustment When Implementing the Generic Letter 95-05 Analysis for the Watts Bar Nuclear Plant (WBN), Unit 2 Steam Generators (WBN TS-391-21-002), dated February 25, 2021 (ML21056A623 and ML21056A624)

2. TVA letter to NRC, WBL-21-006, Watts Bar Nuclear Plant (WBN) Unit 2 - Refueling Outage 3 Generic Letter 95-05 Voltage-Based Alternate Repair Criteria Final Report, dated February 11, 2021 (ML21042B342)

3. Assessment of the Effects of Flexible Power Operation on Tube Crack Growth Rate in Steam Generators. EPRI, Palo Alto, CA: 2019. 3002016069, October 2019.

NRC RAI

2. The proposed UFSAR Section 5.5.2.4 language states that, This same temperature adjustment methodology will be used to modify the average growth rate used to determine the upper voltage repair limits. The justification for applying the temperature adjustment to the upper voltage repair limit is not clear to the NRC staff. Generic Letter 95-05, Section 2.a.2, Determination of the Upper Voltage Repair Limit for [Tube Support Plate]

TSP Intersections, notes that the method for determining the flaw growth allowance is discussed in Section 2.b.2(2) and should be a plant-specific average growth rate or 30 percent per effective full power year, whichever is larger. Section 2.b.2(2) states, in part, that, If both of the two previous inspections employed similar guidelines, the most limiting of the two previous growth rate indications should be used to estimate the voltage growth for the next inspection cycle. In addition, it is not clear to the staff that applying a temperature reduced growth to the upper voltage repair criteria would be conservative if subsequent plant operation occurs above the current reduced power Thot value. Justify why it is appropriate to apply the temperature adjustment to the upper voltage repair limit.

TVA Response TVA will not apply the temperature adjustment methodology to determine the upper voltage repair limits. The proposed change to WBN UFSAR Section 5.5.2.4 is shown below (changes to the proposed UFSAR in the LAR are shown in strikeouts and in bold italics).

Also, when normal operating temperature differences exist from either cycle-to-cycle, or within a cycle, an exception to the GL 95-05 analysis in the form of a temperature adjustment to the growth rate calculation in accordance with Section 10.5.6.1.6 of Reference 27 will be applied. The temperature adjustment methodology will be used to determine the End of Cycle voltage distribution of axial indications for comparison to the conditional probability of tube burst of less than or equal to 1 x 10-2 and to determine the total primary-to-secondary leak rate from an affected SG during a postulated main steam line break event. This same temperature adjustment methodology will be used to modify the average growth rate used to determine the upper voltage repair limits. The upper voltage repair limit will be determined using the guidance of GL 95-05 and the plant-specific average growth rate will correspond to the temperature applicable to 100% reactor power operation. This exception applies until the Unit 2 Steam Generators are replaced(28).

provides the revision to the WBN UFSAR marked up pages that were provided in the LAR (Reference 1 to the TVA Response to RAI 1). Enclosure 3 provides the revision to the CNL-21-043 E1-5 of 12 WBN UFSAR retyped pages that were provided in the LAR. Enclosures 2 and 3 supersede the revisions to the WBN UFSAR pages that were provided in the LAR.

NRC RAI

3. The Watts Bar, Unit 2, GL 95-05 90-day report (ADAMS Accession No. ML21042B342) states in Section 6.4, Cycle Operating Period, that 285 days (Thot temperature not specified) is calculated to be the maximum number of days SG-3 could operate into the current cycle (Cycle 4) and meet the acceptance criterion for conditional burst probability.

Section 3.2, Technical Analysis, of Enclosure 1 of the LAR states that accounting for a 4°F temperature adjustment enables an operating interval extension of 27 calendar days. This suggests that the leakage and burst criteria can be met for operation 312 days into Cycle 4.

However, the Westinghouse CMOA Section 4.3, Stress Corrosion Cracking, when discussing a 5°F Thot temperature reduction states that a mid-cycle outage is scheduled to begin on September 15, 2021, corresponding to 303 calendar days of operation. Clarify the differences between the GL 95-05 report and CMOA calendar days of operation. Discuss the best estimate margin in calendar days between the planned outage start date and reaching the performance criteria limits contained in GL 95-05.

TVA Response As noted in the response to RAI 1a, TVA plans to start the WBN Unit 2 Cycle 4a outage no later than September 11, 2021, which is 299 days from Mode 4 startup from the U2R3 refueling outage on November 16, 2020.

The CMOA (Westinghouse report SG-CDMP-20-23-P, Revision 2), which was enclosed to TVA letter CNL-21-040, dated March 23, 2021, provides the allowed inspection interval for the existing degradation mechanisms excluding axial ODSCC at TSPs where GL 95-05 is applied.

Even operating at full power conditions until the mid-cycle inspection, there is a probability margin of greater than 3.5% to the structural and leakage integrity performance criteria for all degradation mechanisms evaluated in the CMOA. Regardless of this available margin, TVA plans to perform an inspection for all SG tube degradation mechanisms at the scheduled mid-cycle outage. The outage date of September 15, 2021, used in SG-CDMP-20-23 P, is an estimate that bounds the planned outage start date of no later than September 11, 2021, while still providing a projection of degradation for comparison against the mid-cycle inspection results as recommended in the EPRI SG Integrity Assessment Guidelines (Reference 1).

The GL 95-05 90-day report (Westinghouse report SG-CDMP-21-1-NP, Revision 0) determines the inspection interval for axial ODSCC at TSPs at locations where GL 95-05 is applicable.

SG-CDMP-21-1-NP, Section 7.2, justifies an inspection interval of 285 days before the performance criteria of GL 95-05 are reached. As noted in Section 3.2 of the LAR (Reference 2), NRC approval of the LAR is expected to result in a 27-day extension to the inspection interval and a final allowable interval of 312 days. Therefore, the best estimate margin between the planned outage start date and reaching the performance criteria limits contained in GL 95-05 is 13 calendar days (312 calendar days minus 299 calendar days).

CNL-21-043 E1-6 of 12 References

1. Steam Generator Management Program: Steam Generator Integrity Assessment Guidelines: Revision 4. EPRI, Palo Alto, CA: 2016. 3002007571 dated June 2016.

2. TVA letter to NRC, CNL-21-011, Expedited Application for Approval to Use a Growth Rate Temperature Adjustment When Implementing the Generic Letter 95-05 Analysis for the Watts Bar Nuclear Plant (WBN), Unit 2 Steam Generators (WBN TS-391-21-002), dated February 25, 2021 (ML21056A623 and ML21056A624)

NRC RAI

4. The Watts Bar, Unit 2, CMOA Section 4, Operational Assessment, states that, This OA performs an evaluation for degradation mechanisms not covered under GL 95-05 voltage-based alternate repair criteria for axial ODSCC at TSP intersections. The GL 95-05 evaluation is documented in Reference 22 and Reference 29. Although CMOA Section 4.3, Stress Corrosion Cracking, discusses how the Cycle 4a temperature reduction mitigates growth of axial ODSCC at TSP intersections evaluated through GL 95-05, it does not provide tube integrity results for the temperature adjusted operating duration provided.

Therefore, it is not clear to the NRC staff which document will serve as the updated design basis if this LAR is approved. If the LAR is approved, discuss how the operational assessment for axial ODSCC at TSP intersections covered under GL 95-05 will be updated for the remainder of Cycle 4a and Cycle 4b.

TVA Response This LAR is a license and program change only with no change to the design basis for the WBN Unit 2 SGs.

For WBN Unit 2 Cycle 4a, the GL 95-05 90-day report (Westinghouse report SG-CDMP-21-1-NP, Revision 0) justifies an inspection interval of 285 days for axial ODSCC at TSP intersections covered under GL 95-05, which equates to August 28, 2021. Following NRC approval of the LAR, the temperature adjustment to the growth rate calculation methodology will be implemented in a revision to SG-CDMP-21-1-NP that will support the OA for operation of WBN Unit 2 until the Cycle 4a outage start date of no later than September 11, 2021. TVA will submit the revision to GL 95-05 90-day OA (SG-CDMP-21-1-NP) to the NRC by August 1, 2021. contains the new commitment associated with this RAI response.

For WBN Unit 2 Cycle 4b, TVA will submit a new GL 95-05 OA report within 90 days following initial entry into Mode 4 from the WBN Unit 2 Cycle 4a mid-cycle SG inspection outage in accordance with WBN Unit 2 TS 5.9.9, "Steam Generator Tube Inspection Report.

Additionally, TVA performed a causal analysis regarding the progression of ODSCC, particularly in WBN Unit 2 SG3, which was detected during the U2R3 SG inspections. TVA requested that Westinghouse review the causal analysis and determine whether the current GL 95-05 OA is affected by the conclusions regarding the SG3 chemistry and historical lead (Pb) concentrations in the secondary side sludge samples. Subsequently, Westinghouse evaluated the results of the causal analysis and determined that there was no impact to the current GL 95-05 OA report based on the current chemistry trends. The causal analysis has been entered into the TVA corrective action program and is available for NRC inspection.

CNL-21-043 E1-7 of 12 NRC RAI

5. Section 3.2, Voltage Growth Rates, of the GL 95-05 final report describes how the voltage growth rates were determined in both the preliminary and GL 95-05 report operational assessments (OAs) using indications identified in successive inspections. It states, For the U2R3 preliminary GL 95-05 OA evaluation, there were a total of 155 growth data points used for all four SGs combined. According to Tables 3-11 through 3-14, 964 indications were identified in successive inspections and used to determine growth rates.

a. Describe the historical data review (lookback) processes performed in the GL 95-05 final report to determine when the second refueling outage (U2R2) indication was present and how the voltage was determined to calculate a growth rate.

b. Explain the large difference in the number of repeat indications from U2R2 to U2R3 between the GL 95-05 final report and the preliminary OA.

c. During U2R2, a total of 193 distorted support indications (DSIs) were reported.

During U2R3, using the GL 95-05 methodology, a total of 1240 DSIs were reported, including indications exceeding the upper voltage repair limit (DSVs), with 1041 indications returned to service. Table 7-2 Operational Assessment Leak and Burst Results for EOC-4a, projects a total of 1854 indications. Discuss how the number of new indications during the current operating cycle (Cycle 4a) was projected for each SG.

TVA Response 5a.

For all bobbin DSIs reported at U2R3, the corresponding signals from the U2R2 inspections were reviewed to determine if any signal precursors were apparent. The lookbacks were performed by two Level III eddy current analysts in order to provide an additional level of confidence in the results. Each analyst recorded the U2R2 signal voltage, if evident, and an average was determined from the two analysts measurements. To ensure consistency in the approach, the bobbin lookback data was used in lieu of U2R2 recorded voltages of reported DSIs in the U2R2 eddy current database. However, a comparison did not find an appreciable difference between the U2R2 DSI voltages and the lookback measurements on the same indications.

The point-to-point growth for each indication was initially calculated in two different ways. First, the point-to-point growth was calculated by subtracting the average lookback voltage from the U2R3 measurement, and second, by subtracting the minimum lookback voltage from the U2R3 measurement to maximize the difference.

Sensitivity cases found that the difference between using a growth distribution based on average lookback voltage or minimum lookback voltage was insignificant. Due to the insignificance in terms of the results, the average lookback voltage was used for the final growth rate distribution calculation.

Not all of the indications demonstrated a historical precursor signal in the U2R2 inspection data. For example, the DSIs that exceeded the upper repair voltage limit (DSVs) were found to grow from no detectable degradation or from a small voltage (e.g., 0.2 volt (V)). This means that the growth points in the upper end of the growth distribution were essentially not reduced from the voltage growth distribution used in the preliminary operational assessment.

CNL-21-043 E1-8 of 12 This process was performed for each SG, with the bounding growth from SG3 applied in the OA to all SGs.

5b.

The reason for the difference in the number of repeat indications between the GL 95-05 preliminary OA and the GL 95-05 final report is that the historical lookback data was not included in the preliminary OA point-to-point growth calculations. In the preliminary OA report, a total of 155 instances of repeat DSIs existed based upon the U2R2 eddy current database. When performing lookbacks on U2R2 raw data, non-zero voltage signals were measured by eddy current analysts in 1,133 of the 1,240 U2R3 DSI indications resulting in a total of 964 growth data points.

Figure 1 displays the distribution of the U2R2 bobbin lookbacks of the 1,240 DSIs compared to the DSI voltage distribution reported at U2R3. As shown in Figure 1, the U2R3 voltage distribution reflects a growth offset from the complete set of lookback data. Also as shown in Figure 1, the high voltage points in the tail of the U2R3 voltage distribution contribute significantly to the U2R3 growth curve because there are no U2R2 lookback values near that voltage range.

In summary, the lookback process provided a point-to-point growth value for each DSI reported in U2R3 because many of the reported indications, or precursors to the indications, existed during U2R2. The growth rates using the lookback data were only applied to the GL 95-05 final report (SG-CDMP-21-1-NP) and were not applied to the preliminary OA, resulting in the difference in the number of repeat indications considered for the growth rates.

Figure 1 CNL-21-043 E1-9 of 12 5c.

The number of indications at end-of-cycle (EOC) is projected using the following formula from GL 95-05.

N1 = (1/POD)(Nd) - Nr where:

N1 = assumed frequency distribution of bobbin indications Nd = frequency distribution of indications actually detected Nr = frequency distribution of repaired indications POD = probability of detection of ODSCC flaws The U2R3 flaws are separated into 0.1V increment bins up to the largest size indication detected. For each voltage bin, the number of detected flaws is the basis for the projection of the number of beginning of cycle (BOC) flaws assumed in the analysis. The formula above is applied to each voltage bin with the associated POD for that voltage. A POD value of 0.6 is applied for every voltage bin up to 3.2V, a POD of 0.9 is applied between 3.2V and 6.0V, and a POD of 0.95 is applied at 6.0V and greater. The flaws in tubes plugged during U2R3 are subtracted from the total in each voltage bin.

As an example, SG3 had 60 detected indications measured in the 0.6V bin, which was applied as an input to the GL 95-05 evaluation. Five of these indications were in tubes that were plugged. The POD applied for the 0.6V bin is 0.6 meaning that 100 indications [(1/0.6)*60] are projected to exist at the BOC from the 60 indications that were detected in this voltage range at U2R3. After subtracting the five indications that were in plugged tubes, there are 95 assumed BOC indications.

The sum of these calculations for each voltage bin for each SG yields the total number of BOC indications. The BOC quantity includes returned to service flaws, undetected flaws, and new indications that initiate during the cycle. Therefore, this sum represents the number of EOC indications. The SG specific growth rate is applied to the BOC distribution for the projected cycle duration, resulting in the EOC flaw voltage distribution.

NRC RAI

6. Section 2.b.2(2) of GL 95-05 states that voltage growth rates should only be evaluated for those intersections at which bobbin indications can be identified at two successive inspections, except if an indication changes from non-detectable to a relatively high voltage (e.g., 2.0 volts). Table 3-16, Figure 3-6, and Figure 3-7 of the GL 95-05 final report indicate newly detected indications with relatively high voltage were used in the growth rate distributions, but this is not stated in the description in Section 3.2 of how growth rates were determined. Clarify if the Table 3-16 indications shown as 0.00 Vpp (volts peak-to-peak) in U2R2 were used in determining voltage growth rates and if there were any exceptions taken to the high voltage growth indications.

TVA Response No exceptions were taken to any growth point in the GL 95-05 assessment. The growth points from Table 3-16 of the GL 95-05 final report (SG-CDMP-21-1-NP), including those that grew from 0.00V during U2R2 were used in generating the growth distributions. As noted in Table 3-16 of SG-CDMP-21-1-NP, most of the high growth points occurred in SG3, which was CNL-21-043 E1-10 of 12 the SG with the bounding growth distribution. As such, the limiting SG3 growth distribution was applied to each SG in the OA.

NRC RAI

7. Describe the strategy used in supplemental testing of bobbin probe DSIs with a +Point' rotating probe compared to the guidance in Section 3.b of GL 95-05. In addition, identify any exceptions to the guidance in Section 3.b of GL 95-05.

TVA Response Sections 3.2 and 3.4.1 of the NRC Safety Evaluation (SE) for the approval of the implementation of the GL 95-05 voltage based alternate repair criteria for WBN Unit 2 (WBN Unit 2 License Amendment 28, ML19063B721) discusses the use of +Point'2 probe or qualified eddy current inspection techniques for confirmation of axial ODSCC at WBN2. During WBN U2R3, the guidance in Section 3.b of GL 95-05 was followed with the exception of one location based on the use of a qualified technique. A 13.39 volt Dent (DNT) in SG4 was inspected with a qualified array probe in Row 43 Column 77 at cold leg tube support C02. This was an exception to the guidance from GL 95-05 Section 3.b, which states, All intersections with dent signals greater than 5 volts should be inspected with RPC. However, the inspection of this location was in accordance with the NRC SE for WBN Unit 2 License Amendment 28 because the array probe technique has been qualified for this purpose on a site-specific basis.

NRC RAI

8. According to Section 3.1, U2R3 Inspection Results, all DSIs with a bobbin probe voltage amplitude greater than or equal to 0.75 volts were tested with a +Point' probe. Tables 3-2 through 3-5 show that additional +Point' inspections were performed on DSIs with bobbin probe voltage less than 0.75 volts. Clarify the criteria used to select DSI indications less than 0.75 volts for +Point' probe inspection.

TVA Response For any tube where at least one DSI greater than or equal to 0.75V was detected, all DSI indications within that tube (including DSI indications less than 0.75V) were tested with +PointTM probe in order to generate a larger database of +PointTM confirmation results.

NRC RAI

9. During the November 17, 2020, public meeting (ADAMS Accession No. ML20337A040) discussing the Watts Bar 2 ODSCC at tube support plates, TVA stated that preliminary results showed that Unit 2 could operate for 240 days if a probability of detection (POD) of 1 was applied to all ODSCC indications in the scope of GL 95-05 equal to or greater than 3.2 volts. Using a POD of 0.95 (indications greater than or equal to 6 volts) and 0.9 (for indications between 3.2 and 6.0 volts), the GL 95-05 final report (Table 7-2) indicates that operation for 285 days will meet the acceptance criteria. Discuss any differences in the preliminary and final evaluations that resulted in the different calculated operating times.

2 +POINT is a trademark or registered trademark of Zetec, Inc. Other names may be trademarks of their respective owners.

CNL-21-043 E1-11 of 12 TVA Response The appreciable differences between the preliminary OA model and the final OA model for the GL 95-05 evaluation for axial ODSCC at TSP locations are related to the improved POD for large voltage indications and growth distribution. The POD for the final OA reflects the alternate POD approved by the NRC with a step change to 0.9 at 3.2V and to 0.95 at 6V, which is different from the POD function used in the preliminary OA. Additionally, a more robust growth distribution was developed for the final OA that was based on bobbin lookback data for U2R2.

The performance of the lookbacks and resulting updated growth distribution added more than a month to the OA duration. The dataset of U2R3 DSIs that were also recorded as DSIs in U2R2 only consisted of 155 total points from the four SGs, which served as the basis of the growth distribution in the preliminary OA. GL 95-05 states that growth rate distributions consisting of fewer than 200 indications should apply a bounding probability distribution function of growth rates based on consideration of experience to date at similarly designed and operated units.

For the preliminary OA, the growth distribution based on the 155 instances of point-to-point growth from the SGs was applied with the addition of all new indications greater than 2.0V, which were assumed to grow from 0.0V. The smaller dataset magnifies the effect of the higher growth points in the dataset on the EOC predictions. This is evident from a review of Figure 2 below, which shows the growth distribution by SG as well as the cumulative growth distribution applied during the preliminary OA evaluation. The preliminary OA growth curve is not smooth due to lack of data, most noticeably in the 0.85-0.95 region of the cumulative distribution function (CDF). The growth jumps from ~0.3V at 0.85 to ~3V at 0.95, whereas that same approximate voltage growth difference spans 0.53 to 0.98 CDF in the SG3 growth distribution in the final OA.

The growth CDF used in the preliminary OA includes only the large new growth points from the SGs (>2.0V as recommended in GL 95-05), and there was insufficient existing data for the rest of the range of DSI voltages resulting in a biased CDF due to the inclusion of only the large new indications and insufficient data in the lower voltage range. The use of bobbin lookbacks provides a reasonable method to supplement the U2R2 data by using precursor signals as the initial measurement for point-to-point growth calculations. Historical Alloy 600MA growth data from other plants was not used because WBN Unit 2 DSI indications were initiating at an earlier point in the operating life.

CNL-21-043 E1-12 of 12 Figure 2 CNL-21-043 Revised Proposed UFSAR Changes (Markups) for WBN Unit 2

5.5-20 WBN Steam Generator Tubing voltage-based Alternate Repair Criteria (ARC) for Axial Outside Diameter Stress Corrosion Cracking (ODSCC) at tube support plate intersections was approved by NRC (23). Implementation of ODSCC ARC using GL 95-05 (24) as guidance is in accordance with Technical Specification inservice examination requirements and Reference 25. As an alternative to the probability of detection of 0.6 required by GL 95-05, a probability of detection (POD) of 0.9 will be applied to indications of axial 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 Steam Generators are replaced(26). A POD of 0.6, in accordance with GL 95-05, will be used for indications less than 3.2 volts. Also, when normal operating temperature differences exist from either cycle-to-cycle, or within a cycle, an exception to the GL 95-05 analysis in the form of a temperature adjustment to the growth rate calculation in accordance with Section 10.5.6.1.6 of Reference 27 will be applied. The temperature adjustment methodology will be used to determine the End of Cycle voltage distribution of axial indications for comparison to the conditional probability of tube burst of less than or equal to 1 x 10-2 and to determine the total primary-to-secondary leak rate from an affected SG during a postulated main steam line break event. The upper voltage repair limit will be determined using the guidance of GL 95-05 and the plant-specific average growth rate will correspond to the temperature applicable to 100% reactor power operation.

This exception applies until the Unit 2 Steam Generators are replaced(28).

5.5.3 Reactor Coolant Piping 5.5.3.1 Design Bases The RCS piping is designed and fabricated to accommodate the system pressures and temperatures attained under all expected modes of plant operation or anticipated system interactions. Stresses are maintained within the limits of Section III of the ASME Nuclear Power Plant Components Code. Code and material requirements are provided in Section 5.2.

Materials of construction are specified to minimize corrosion/erosion and ensure compatibility with the operating environment.

The piping in the RCS is Safety Class 1 and is designed and fabricated in accordance with ASME Section III, Class 1 requirements.

Stainless steel pipe conforms to ANSI B36.19 for sizes 1/2-inch through 12 inches and wall thickness Schedules 40S through 80S. Stainless steel pipe outside of the scope of ANSI B36.19 conforms to ANSI B36.10.

The minimum wall thicknesses of the loop pipe and fittings are not less than that calculated using the ASME III Class 1 formula of Paragraph NB-3641.1 (3), with an allowable stress value of 17,550 psi. The pipe wall thickness for the pressurizer surge line is Schedule 160. The minimum pipe bend radius is 5 nominal pipe diameters; ovalness does not exceed 6%.

Butt welds, branch connection nozzle welds, and boss welds are of a full-penetration design.

Processing and minimization of sensitization are discussed in Sections 5.2.3 and 5.2.5.

Flanges conform to ANSI B16.5.

WBN-3 5.5-52 23.

NRC Safety Evaluation for Watts Bar Nuclear Plant Unit 2, Amendment 28, for Steam Generator Tubing Voltage Based Alternate Repair Criteria for Outside Diameter Stress Corrosion Cracking (ODSCC) dated June 3, 2019.

24.

NRC Generic Letter 95-05, Voltage-Based Repair Criteria for Westinghouse Steam Generator Tubes Affected by Outside Diameter Stress Corrosion Cracking, dated August 3, 1995.

25.

TVA Letter to NRC Application to Revise Watts Bar Nuclear Plant Unit 2 Technical Specifications for Use of voltage-based Alternate Repair Criteria in Accordance with Generic Letter 95-05 (391-WBN2-TS-17-30) dated May 14, 2018 and as supplemented by letter CNL-18-128 dated November 8, 2018.

26.

NRC letter to TVA, WATTS BAR NUCLEAR PLANT, UNIT 2 - Issuance of Amendment No. 48 Regarding Use of Alternate Probability of Detection Values for Beginning of Cycle In Support of Operational Assessment (EPID L-2020-LLA-0273), dated February 9, 2021 (ML21027A167).

27.

EPRI Topical Report 1018047, "Steam Generator Tubing Outside Diameter Stress Corrosion Cracking at Tube Support Plates Database for Alternate Repair Limits:

Addendum 7," dated September 2008 28.

NRC letter to TVA, XXXXX (TAC No. XXXX), dated MM/DD/YY (MLXXXX) (Thot LAR)

CNL-21-043 Revised Proposed UFSAR Changes (Final Typed) for WBN Unit 2