WBL-25-048, Unit 2 - Cycle 6 Steam Generator Tube Inspection Report
| ML25295A538 | |
| Person / Time | |
|---|---|
| Site: | Watts Bar  |
| Issue date: | 10/22/2025 |
| From: | Reneau W Tennessee Valley Authority |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| WBL-25-048 | |
| Download: ML25295A538 (1) | |
Text
1V4 TENNESSEE VALLEY AUTHORITY Post Office Box 2000, Spring City, Tennessee 37381 WBL-25-048 October 22, 2025 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001 Watts Bar Nuclear Plant, Unit 2 Facility Operating License No. NPF-96 NRC Docket No. 50-391 10 CFR 50.4
Subject:
Watts Bar Nuclear Plant (WBN) Unit 2 - Cycle 6 Steam Generator Tube Inspection Report In accordance with the requirements of WBN Unit 2 Technical Specification (TS) 5.9.9, "Steam Generator Tube Inspection Report", the Enclosure provides the 180 Day Steam Generator Inspection Report for the Unit 2 Cycle 6 (U2R6) outage. This report is required to be submitted within 180 days after the initial entry into MODE 4 following the completion of an inspection performed in accordance with TS 5. 7.2.12, "Steam Generator (SG) Program." The report provides the complete results of the tube inspections.
There are no new regulatory commitments contained in this letter. Please direct any questions concerning this matter to Jonathan Johnson, WBN Site Regulatory Compliance Manager, at jtjohnson0@tva.gov.
Respectfully,
~a~
William C. Ren au Site Vice President Watts Bar Nuclear Plant WBN-25-048
WBN-25-048 U.S. Nuclear Regulatory Commission WBL-25-048 Page 2 October 22, 2025
Enclosure:
Tennessee Valley Authority - Watts Bar Nuclear Plant - Unit 2 - Watts Bar U2R6 - 180 Day Steam Generator Tube Inspection Report cc: (w/ enclosure):
NRC Regional Administrator - Region II NRC Senior Resident Inspector - Watts Bar Nuclear Plant NRC Project Manager - Watts Bar Nuclear Plant
WBN-25-048 E1 of 1 ENCLOSURE Tennessee Valley Authority Watts Bar Nuclear Plant Unit 2 Watts Bar U2R6 180 Day Steam Generator Tube Inspection Report
1 Watts Bar U2R6 180 Day Steam Generator Tube Inspection Report Author:
Connor Rigsby SG Program Engineer Signature / Date Reviewers Robert Himmelspach NDE Eddy Current Level III Signature / Date Maxwell Trent SG Program Manager Signature / Date b
Digitally signed by Rigsby, Rigs y Connor Connor 1
Date: 2025.10.14 18:41 :21 -04'00' Himmelspach, Robert John Digitally signed by Himmelspach, Robert John Date: 2025.10.15 04:57:10 -04'00' Trent Maxwel I Digitally signed by Trent, Maxwell Date: 2025.10.15 09:52: 15 -04'00'
2 Introduction In accordance with WBN Unit 2 Technical Specification Section 5.7.2.12 Steam Generator Program and Technical Specification Section 5.9.9 Steam Generator Tube Inspection Report, this report documents the scope and results of the Watts Bar Nuclear (WBN) Unit 2 Refueling Outage 6 (U2R6) replacement steam generator (RSG) inspections. In addition, there are fourteen specific reporting requirements associated with the 180 Day Report in the Electrical Power Research Institute (EPRI) Steam Generator Management Program (SGMP): Steam Generator Integrity Assessment Guideline, Revision 5, Attachment G. Each numbered reporting requirement listed below is followed with the associated information based on the inspections performed during WBN U2R6.
The WBN U2R6 inspection, during the spring of 2025, was the second in-service inspection (ISI) since steam generator (SG) replacement in 2022. The U2R6 inspection included only primary side inspections.
The primary side inspections included 100% eddy current testing of all open SG tubing and visual inspections.
1.
Design and Operating Parameters.
The WBN Unit 2 RSGs were installed during the Unit 2 Cycle 4 refueling outage (U2R4 in June of 2022).
The WBN Unit 2 RSGs are of a similar design to the original steam generators (OSG) which had a vertical shell and continuous bend U-tubes with an integral preheater. WBN Unit 2 is a Westinghouse four-loop plant with Westinghouse Model 68 Axial Extent Preheater (AXP) RSGs.
Table 1-1 Steam Generator Design and Operating Parameters SG Model / Tube Material / # SGs per Unit Westinghouse Model 68AXP / Alloy 690TT/ 4
- of tubes per SG / Nominal Tube Diameter / tube thickness 5128 / 0.75 in. / 0.043 in.
Support Plate Style / Material Advance Tube Support Grid / 409 stainless Steel Last Inspection Date 2023 (U2R5)
EFPM Since Last Inspection 16.13 EFPM (1.34 Effective Full Power Years (EFPY))
Total Cumulative SG EFPM 32.04 EFPM (2.67 EFPY)
Mode 4 Initial Entry 5/10/2025 Observed Primary-to-Secondary Leak Rate Below Detection Nominal Thot at Full Power Operation 617ºF Loose Parts Strainer The Model 68AXP has a feedwater flow distribution box with small diameter holes (0.29) acting as strainers to prevent the introduction of significant foreign objects into the SGs Degradation Mechanism Sub-Population The U2R6 condition monitoring and operational assessment (CMOA) considered the perimeter tubes wear rate as different from the interior tubes wear rate.
SG program guideline deviations since last Inspection None Steam Generator Schematic See Figure 1-1 and Figure 1-2 below.
3 Figure 1-1: Schematic Design for Watts Bar Unit 2 Model 68AXP Replacement Steam Generators
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! I 2X 5EC0tf;l:ARY l,!Atj\¥1,'r'S INSPCC'Tl°" POOT 2X UPilER l'R\!\.'\IONS
4 Figure 1-2: Tube Support Structure for Watts Bar Unit 2 Model 68AXP Replacement Steam Generators All U-Bend Support Strap are 2" Wide s
AXIAL FLOW
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C12 C11 C1O CO9 COB CO7 CO6 CO5 CO4 CO3 CO2 CO1 r-TWO-PHASE EXIT REGION CROSS FLOW Tub e Supports are ed by 2" and 1" attice Bars form L
(Se e Figure 5.5) v--
COLD SIDE RECIRCULATING ENTRANCE CROSS FLOW E-FEEDWATER ENTRANCE CROSS FLOW
5 2.
The Scope of Inspections Performed on each SG The U2R6 inspection program addressed the known eddy current signals observed in the Watts Bar Unit 2 RSGs from the past inspections and potential RSG tube degradation mechanisms relevant to the design and material of the WBN U2 RSG tubing. The inspections were performed with qualified non-destructive examination (NDE) techniques for each potential mechanism. The defined primary side base scope that was implemented in all four SGs included:
100% Bobbin coil inspection of all open tubes in all four RSGs full length.
Array or rotating probe coil (RPC) for all tubes identified in (Reference 1) within the high fluid velocity or transition fluid velocity zones, plus a minimum of three periphery tubes deep:
o HL up to the first support o CL up to the seventh support 100% Array or +Point to inspect previously reported dings (DNG)/dents (DNT)/distorted dent signals (DDS) > 2 volts plus any newly reported DNG/DNT/dent with indication (DNI) > 2 volts in all RSGs.
100% Array or +Point of new and previous bulges (BLG), manufacture burnish marks (MBM),
over expansions (OXP), and absolute drift indications (ADI) in all RSGs.
+Point or Array Special Interest inspection of new and previous wear indications detected with Bobbin.
+Point or Array Special Interest inspections of tube locations with non-resolved Bobbin signals from the base scope inspection to characterize the underlying condition.
In addition to the eddy current inspections, visual inspections were performed on the primary side. Primary side inspections included the entire divider plate to channel head weld and all clad surfaces in both the HL and CL of all four SGs. A total of 114 plugs (57 plugged tubes) were installed prior to U2R6. All 114 plugs received a visual inspection with no leakage or anomalies identified.
No primary side scope expansion was required for the U2R6 SG inspections. All primary side inspections were completed satisfactory with no unexpected degradation or anomalies reported. During U2R6, secondary side top-of-tubesheet (TTS) cleaning and visual inspections were not conducted. However, visual inspection of all accessible possible loose parts (PLPs) identified through eddy current testing was available as a contingency. No PLPs were detected in U2R6; therefore, no secondary side inspections were required.
3.
Nondestructive Examinations Techniques Utilized for Tubes with Increased Degradation Susceptibility Due to the high fluid gap velocity between the periphery tubes, an Array inspection was performed in each of the four SGs to identify foreign object wear and PLPs. The inspection was performed as listed in Section 2 with no foreign object wear or PLPs detected.
- 4.
Nondestructive Examinations Techniques Utilized for Each Degradation Mechanism Found Table 4-1 below provides the NDE techniques that were used for the detection of each degradation mechanism considered to be existing for the U2R6 inspection.
6 Table 4-1: NDE Techniques for Each Existing Degradation Mechanism for U2R6 Degradation Mechanism Detection Probe Type Detection Technique ETSS Sizing Probe Type Sizing Technique ETSS Wear at U-Bend Support Structures Bobbin I96044.3 Rev 0 Bobbin 96004.1 Rev 14 Array I17908.5 Rev 1 Array 17908.2 Rev 0 I17908.5 Rev 1 Wear at Horizontal ATSGs Bobbin I96041.1 Rev 8 Bobbin 96004.1 Rev 14 Array I17908.5 Rev 1 Array 17908.2 Rev 0 I17908.5 Rev 1
- 5.
Location, Orientation (if linear), Measured Size (if available), and Voltage Response for Each Indication.
There were no new degradation mechanisms reported following the WBN U2R6 inspection. The two existing degradation mechanisms are the following:
- 1. Wear at U-Bend Support Structures
- 2. Wear at Horizontal Advanced Tube Support Grids (ATSGs)
Table 5-1: Wear Indications <20% Through Wall (TW)
SG1 SG2 SG3 SG4 Total 215 521 234 145 1,115 Table 5-2: Watts Bar U2R6 Indications
- All SGs SG Row Col Loc Inch Volts Indic
%TW Characterization Resolution 1
100 83 C04 0.74 1.13 PCT 21 ATSG Wear Return to Service 2
96 83 C03
-0.95 1.22 PCT 21 ATSG Wear Return to Service 3
93 36 C06
-0.76 1.08 PCT 20 ATSG Wear Return to Service 3
101 78 C03
-1.00 1.04 PCT 20 ATSG Wear Return to Service 3
103 72 C03
-1.00 1.06 PCT 20 ATSG Wear Return to Service 6.
A Description of the Condition Monitoring Assessment and Inspection Results for Each Degradation Mechanism Found Condition Monitoring (CM) Assessment Summary Based on the 100% eddy current inspection, two existing degradation mechanisms were found: mechanical wear at U-Bend Support Structures and mechanical wear at ATSGs. No tubes exceeded the CM limits or required in-situ pressure testing to support the CM assessment.
During U2R6, a total of 18 wear indications at U-Bend Support Structures in 13 tubes were identified. The deepest indication was measured to be 16% TW in SG4 R102 / C65 at support VS4. A bobbin probe was used to detect the indication and it was sized using ETSS 96004.1. The depth of indications ranged from 10% TW to 16% TW. Flaw lengths were measured with the array probe and sized with ETSS 17908.2 and ETSS I17908.5. The maximum flaw length recorded was 0.35 inches while the average length was 0.22 inches. All U-Bend Support Structure wear indications were conservatively assumed to be flat. None of the indications exceeded the 50.2% TW CM limit which contains material, burst relation, and NDE measurement uncertainties at 95% probability and 0.50 confidence level (95/50). None of the U-Bend Support Structure wear indication lengths exceeded the CM bounding limit of 2.5 inches. Since all wear
~20%TW
7 indications were less than 50.2% TW and 2.5 inches in length, structural performance criteria has been satisfied.
During U2R6, a total of 1,102 wear indications at ATSGs in 760 tubes were identified. The maximum depth of the deepest indication was measured to be 21% TW and was measured in two locations: SG1 R100
/ C83 at support C04 and SG2 R96 / C83 at support C03. A bobbin probe was used to detect the indications and it was sized using ETSS 96004.1. The depth of ATSG indications ranged from 7% TW to 21% TW.
Flaw lengths were measured with the array probe and sized with ETSS 17908.2 and ETSS I17908.5. The maximum flaw length recorded was 0.37 inches while the average length was 0.195 inches. All ATSG wear indications were conservatively assumed to be flat. None of the indications exceeded the 61.5% TW CM limit which contains material, burst relation, and NDE measurement uncertainties at (95/50). None of the ATSG wear indication lengths exceeded the CM bounding limit of 0.37 inches. Since all wear indications were less than 61.5% TW and 0.37 inches in length, structural performance criteria has been satisfied.
Satisfaction of structural integrity implies satisfaction of leakage integrity at accident conditions since steam line break accident condition pressure differential for pop-through is smaller than NOP for pressure-only loading of volumetric flaws. Therefore, CM has been satisfied for degradation associated with ATSG wear and U-Bend Support Structure wear indications at the Watts Bar U2R6 inspection.
Projected and As-Found Inspection Results The forward looking operational assessment (OA) was developed following U2R5 (2023). The OA utilized a deterministic model to determine the max depth indication that could be returned to service while maintaining structural performance criteria until the next scheduled inspection. The projected deterministic-based U2R5 OA results are summarized in Table 6-1.
Table 6-1: Prior OA Deterministic-Based Wear Projections Degradation Mechanism U2R5 Projected EFPY U2R6 Observed EFPY U2R5 Largest Projected Flaw (% TW)
U2R6 Largest Observed Flaw (% TW)
U-Bend Support Wear (Returned to Service) 1.35 1.34 38.58 16 U-Bend Support Wear (Undetected) 1.35 1.34 37.89 16 ASTG Wear (Returned to Service) (1) 1.35 1.34 43.79 21 Notes:
(1) Projected results for undetected ATSG wear flaws from U2R5 OA are not shown because the POD(0.95) value of 12% TW were bounded by the largest returned to service flaw.
- 7.
The number of Tubes Plugged During the Inspection Outage for Each Degradation Mechanism Found Zero tubes were plugged at U2R6.
- 8.
The Repair Methods Utilized, and the Number of Tubes Repaired by Each Repair Method No repair methods were utilized during WBN U2R6.
8 9.
Analysis Summary of the Tube Integrity Conditions Predicted to Exist at the Next Scheduled Inspection Relative to the Applicable Performance Criteria Including Analysis Methodology, Inputs, and Results A simplified deterministic OA method was used to predict tube integrity conditions at the next scheduled inspection. In addition, a fully probabilistic method was performed to ensure accuracy and conservatism of the simplified deterministic method. These methods were utilized in order to provide confidence that the SG structural and leakage performance criteria will be maintained until the next scheduled inspection. The deterministic model evaluated all SGs while the fully probabilistic model only determined probability of burst (POB) and probability of leakage (POL) for the limiting steam generator. In both models, a three cycle interval was justified before the next inspection at U2R9 with conservatism in place. The flaw population in all SGs meets the structural integrity performance criteria for a NOP of 3750 psid with an applied conservative growth rate for at least 4.178 EFPY (three cycles) of operation. For pressure-only loading of volumetric flaws, satisfaction of the structural integrity implies satisfaction of leakage integrity at accident conditions since steam line break accident condition pressure differential for pop-through is smaller than NOP.
There were 13 paired data points in the Watts Bar Unit 2 U-Bend support wear population which grew over the 1.3448 EFPY U2R5-U2R6 cycle. The maximum growth rate obtained was 4.46% TW/EFPY. However, the OA uses a conservative growth rate developed from a combination of indications at U2R5 with the newly reported indications at U2R6 which results in a limiting 95th percentile growth rate of 12.78%
TW/EFPY. The bounding growth rate of 12.78 %TW/EFPY is used in the deterministic OA projection for newly initiated flaws.
Regarding ATSG wear, several subpopulations were evaluated and compared to determine the limiting growth rates for the OA. The 95th percentile paired flaw growth rate for all SGs from U2R5 to U2R6 was 5.21% TW/EFPY, and the maximum growth rate for all SGs was 8.93% TW/EFPY. A bounding growth rate was developed for the U2R6 OA using the limiting perimeter tube sub-populations for each SG to ensure that the deterministic and fully probabilistic evaluations are conservative. The bounding 95th percentile paired flaw growth rate for perimeter tubes was 6.7% TW/EFPY in SG3. In addition, newly reported wear indications at U2R6 were evaluated and determined to be a limiting subpopulation. There were 149 new flaws in the perimeter tubes in all SGs at U2R6, with a bounding 95th percentile growth rate of 14.69% TW/EFPY and a maximum growth rate of 15.43% TW/EFPY, both in SG3. Since the U2R5 95th percentile growth rate of 15.12% TW/EFPY bounds the 95th percentile U2R6 growth rate of 14.69%
TW/EFPY for newly initiated flaws, the U2R5 growth rate of 15.12% TW/EFPY is used in the deterministic OA calculations for new flaw initiation.
Deterministic Method Table 9-1 provides maximum depths predicted at the next scheduled inspection for the worst case degraded tubing for both U-Bend Support Structure and ATSG degradation mechanisms using the previously discussed bounding growth rates.
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9 Table 9-1: Watts Bar U2R6 Deterministic Operational Assessment Summary Degradation Mechanism (Wear)
Maximum Depth Predicted at Next Inspection (%TW)
EOC Structural Limit (%TW) (1)
OA Margin to EOC Structural Limit
(%TW)
OA Interval (EFPY)
U-Bend Support (Return to Service) 48.1 56.5 8.4 4.5 U-Bend Support (Undetected) 46.5 56.5 10 4.5 U-Bend Support (Newly Initiated Flaw) 53.4 56.5 3.1 4.178 ATSG (Return to Service) (2) 62.1 67.41 5.31 4.5 ATSG (Newly Initiated Flaw) 63.2 67.41 4.21 4.178 Notes:
(1)
Structural Limit is based on an assumed maximum length of 2.5 inches for U-Bend wear and 0.37 inches for ATSG wear.
(2)
This projection bounds the assumed undetected flaw size of 15% TW on a POD(0.95) from the U2R6 inspection noise for ATSG wear.
Supplemental Methods The OA for ATSG and U-Bend Support Structure wear degradation mechanisms was performed using fully probabilistic methods through application of a Westinghouse developed software package referred to as Full Bundle Model (FBM). The assumed quantity and depth distribution of undetected ATSG wear is determined by postulating a total (detected and undetected) flaw depth distribution which is simulated with the probability of detection (POD) function to produce both detected flaw and undetected flaw size distributions. Both the perimeter and interior sub-populations were simulated and separate undetected flaw size distributions and number of flaw inputs were developed for each sub-population. The fully probabilistic model conservatively uses the bounding perimeter undetected flaw log-normal model.
A fully probabilistic method was used to predict the conditions at the next inspection for SG2 which is the limiting SG. The number of indications and depth distribution in SG2 bound the remaining SGs. For the fully probabilistic method, the limiting degradation mechanism is ATSG wear. The total number of returned to service flaws in SG2 was 513. The fully probabilistic OA for ATSG wear demonstrates that the structural and leakage integrity performance criteria will be satisfied at the end of a three-cycle (4.178 EFPY) inspection interval until U2R9 as shown in Table 9-2.
Table 9-2: Watts Bar U2R6 SG2 Three-Cycle Fully Probabilistic ATSG OA Results POB
(%)
POL
(%)
Burst Pressure (psi)
Leak Rate (gpm)
ATSG Wear Existing Flaws 2.330 0.021 3957 0
ATSG Wear New Flaws, Perimeter Region 0.000 0.002 5592 0
ATSG Wear New Flaws, Interior Region 0.111 0.000 4249 0
ATSG Wear Combined 2.438 0.023 3957 0
Acceptance Criteria
< 5.000
< 5.000 min. 3750 Acceptable for Three Cycles?
Yes Yes Yes Yes As shown in Table 9-2, the projected POB and POL were all within the 5% limit for five cycles of operation.
All calculated burst pressures were greater than the minimum NOP of 3750 psid. Thus, the OA supports the next inspection to be performed at the end of 3 operating cycles during U2R9.
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10 10.
The Number and Percentage of Tubes Plugged to Date and the Effective Plugging Percentage in Each SG Table 10-1 below provides the number of tubes plugged and plugging percentage as of the end of U2R6.
No sleeves have been installed and therefore there is no correction for effective plugging percentage.
Table 10-1: Number and Percentage of Tubes Plugged at Completion of U2R6 SG 1 SG 2 SG 3 SG 4 Total Tubes Plugged prior to U2R6 5
46 6
0 57 Tubes Plugged at U2R6 0
0 0
0 0
Total Plugged Tubes to Date 5
46 6
0 57 Percentage Plugged to Date 0.0975%
0.897%
0.117%
0.0%
0.278%
Allowable Percent Tubes Plugged 12%
12%
12%
12%
12%
- 11.
The Results of Any SG Secondary Side Inspections During U2R6, secondary TTS cleaning and visual inspections were not conducted. However, visual inspection of all accessible PLPs or TTS wear indicative of FOs identified through eddy current testing was available as a contingency. No PLPs or TTS wear was detected in this outage; therefore, no secondary side inspections were performed. Table 11-1 displays the known WBN U2 foreign objects remaining in the SGs.
The 3 objects remaining in the SGs were reevaluated at U2R6 and were found to be acceptable to remain in the SGs for a minimum of four cycles (U2R5 through U2R9) before reinspection.
Table 11-1: Remaining WBN U2 Foreign Objects
- 12.
The Scope, Method, and Results of Secondary Side Cleaning Performed in Each SG No secondary side cleanings were performed at U2R6.
- 13.
The Results of Primary Side Component Visual Inspections Performed in Each SG SG Channel Head Bowl Visual Inspections Visual inspections were performed in the channel head bowl on both the hot and cold leg channels during WBN U2R6. Visual inspections of the SG divider plate, inclusive of the entire divider plate-to-channel head weld and all visible clad surfaces, were performed in accordance with Westinghouse NSAL-12-1.
This inspection was performed using the SG manway channel head bowl cameras. Satisfactory inspection results were observed in all SGs with no indications of cladding surface degradation. No breaches in the SG/FO ID Retrieved?
FO Description Leg Row/Col New/Legacy Dimensions (inch.)
1001 No Wire Bristle CL 3/24 New 0.375 x 0.01 3002 No Gasket Material HL 21/76 New 0.5 x 0.075 x 0.125 4001 No Fiber Material HL 6/39 New 0.2 x 0.8 x 0.02
11 cladding or cracking in the divider-to-channel head weld were identified. These inspections are driven by industry operating experience and EPRI guideline recommendations.
Tube Plug Inspections WBN Unit 2 had 114 plugs installed prior to U2R6. All 114 previously installed plugs were visually inspected with no leakage or degradation detected.
14.
Any Plant-Specific Reporting Requirements, if Applicable.
There are no plant-specific requirements to report.
References
- 1. Westinghouse Report, SG-CECO-21-003, Revision 1, Foreign Object Limits Analysis for the Watts Bar Unit 1 and Unit 2 Replacement Steam Generators, November, 2023
- 2. Steam Generator Management Program: Pressurized Water Reactor Steam Generator Examination Guidelines: Revision 8, EPRI, Palo Alto, CA: June 2016. 3002007572.