L-2025-110, Supplement to Seabrook OR23 Steam Generator Tube Inspection Report
| ML25155A139 | |
| Person / Time | |
|---|---|
| Site: | Seabrook  |
| Issue date: | 06/04/2025 |
| From: | Mack K NextEra Energy Seabrook |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| L-2025-110 | |
| Download: ML25155A139 (1) | |
Text
NEXTeraM ENERGY~
SEABROOK U.S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, DC 20555-0001 Re:
Seabrook Station Docket No. 50-443 Renewed Facility Operating License NPF-86 Supplement to Seabrook OR23 Steam Generator Tube Inspection Report
Reference:
June 4, 2025 L-2025-11 O 10 CFR 50.36
- 1. Letter L-2025-101 "Seabrook OR23 Steam Generator Tube Inspection Report," dated May 5, 2025 (ADAMS Accession No. ML25125A317)
In Reference 1, NextEra Energy submitted the Seabrook Station Cycle 23 Refueling Outage (OR23) Steam Generator Tube Inspection in accordance with Seabrook Station Technical Specification (TS) 6.8.1.7. As noted in the transmittal letter for Reference 1, the previous report addressed the requirements of TS 6.8.1.7, except for the analysis of stress corrosion cracking mechanisms aspect required by subsection "d" of TS 6.8.1.7. This analysis is provided in the Enclosure to this letter.
This letter contains no new regulatory commitments.
Should you have any questions regarding this submission, please contact Maribel Valdez, Fleet Licensing Manager, at 561-904-5164.
I declare under penalty of perjury that the foregoing is true and correct.
Executed on June 4, 2025.
Sincerely, egulatory Compliance
Enclosure:
Supplement to Seabrook Station OR23 Steam Generator Tube Inspection Report cc:
USNRC Regional Administrator, Region I USNRC Project Manager, Seabrook Station Nuclear Plant USNRC Senior Resident Inspector, Seabrook Station Nuclear Plant NextEra Energy Seabrook, LLC P.O. Box 300, Lafayette Road, NH 03874
Enclosure to L-2025-110 Supplement to Seabrook Station OR23 Steam Generator Tube Inspection Report (5 Pages Follow)
Seabrook Station Docket No. 50-443 Enclosure L-2025-110 Page 1 of 5 Supplement to Seabrook Station OR23 Steam Generator Tube Inspection Report This Enclosure provides supplemental information to section D of the Seabrook OR23 SG Tube Inspection Report (Ref. NRC ADAMS Accession No. ML25125A317).
D. An analysis summary of the tube integrity conditions predicted to exist at the next scheduled inspection (the forward-looking tube integrity assessment) relative to the applicable performance criteria, including the analysis methodology, inputs, and results Stress Corrosion Cracking (SCC) Mechanisms:
A forward-looking tube integrity operational assessment (OA) was performed for existing SCC degradation mechanisms to demonstrate there is reasonable assurance that the structural and leakage integrity performance criteria per plant Technical Specifications (TS 6.7.6.k.b) will be met until the next planned inspection of the SGs in OR25. A 2-cycle inspection interval duration of 2.8 EFPY is assumed in analyses. Framatome's probabilistic analysis program, MUL Tl FRAM, was utilized for evaluation of axial SCC. Specifically, MUL Tl FRAM was used to assess axial ODSCC at TSPs, at DNGs/DNTs and at expansion transitions (EXT), as well as axial PWSCC at TTS expansion transitions.
Axial ODSCC/PWSCC: A fully probabilistic, multi-cycle OA evaluation of axial ODSCC at TSP locations (per the SGMP Integrity Assessment Guidelines) was performed using the Monte Carlo analysis package within MULTIFRAM. The inputs to the OA model consist of the inspection POD curve, distributions of crack growth rates and EOC crack lengths, the relationship between crack maximum depth and structural depth, and flaw initiation parameters describing the past and projected progression of degradation. For crack length and depth growth rate distributions for all existing mechanisms, a combination of the EPRI default growth rates and industry A600TT growth rate data was used in the OA evaluations.
The POD curve was based on ETSS 120402.1 and was developed from the limiting plant-specific array noise at TSP edges, and from the MAPOD methodology (through the EPRI transfer function). OA results for axial ODSCC at TSP locations demonstrated that the projected worst-case (lower 95/50) degraded tube burst pressures satisfy the SIPC limit of 0.05 POB. The projected (upper 95/50) leakage under limiting accident conditions was also determined to be zero. Therefore, SIPC and AILPC are projected to be met at OR25 for axial ODSCC at TSP locations.
Following similar analyses, SIPC and AILPC were also projected to be met at OR25 for axial ODSCC at smaller DNGs/DNTs ("~2V, <5V) and for axial ODSCC at DNGs/DNTs "?:.5V.
In the analyses of DNGs/DNTs "?:.5V, the POD curve was developed from the manual dent noise from the +Point' probe, and the axial dent length was conservatively used to develop an input for crack length. For this effort, axial/circumferential extents of all DNGs/DNTs "?:. 5V were measured to develop the input. For axial ODSCC at HL/TTS expansion transition (EXT), input parameters to the probabilistic Monte Carlo OA model were similar to the ones mentioned above for axial ODSCC at TSPs, specific for the region of interest (ROI). The POD curve describing array probe detection in this ROI (ETSS 120400.1) was developed from the limiting, site-specific array noise at EXT regions with MAPOD using a modified A-hat model, supplemented with +Point' data. Results confirmed that SIPC and AILPC were projected to be met at OR25 for axial ODSCC at HL/TTS expansion transitions. In the evaluation of axial PWSCC at the tubesheet EXT, the
Seabrook Station Docket No. 50-443 Enclosure L-2025-110 Page 2 of 5 POD curve describing array probe detection in this ROI (ETSS 20501.1) was developed from the limiting, site-specific array noise at EXT regions with the MAPOD technology. OA results from the Monte Carlo analysis package confirmed that SIPC and AILPC are met for this degradation mechanism at OR25.
Potential SCC Mechanisms: Bobbin, Array, and +Point' noise were monitored and reviewed at regions of interest (ROls) for potential sec degradation mechanisms to confirm that PODs were not adversely affected and no additional inspections with +Point' were required in OR23 to support the OA. For axial ODSCC at the FOB (ETSS 120402.1 ), the POD curve was developed from the limiting site-specific array noise in conjunction with MAPOD technology. For circumferential ODSCC at TTS (ETSS 120400.1 ), the POD curve was supplemented with +Point' data and was developed by increasing the limiting noise obtained from a select Seabrook SG of the prior inspection (OR21 ), in conjunction with MAPOD. Structural integrity and leakage criteria were projected to be met at OR25 for all potential sec mechanisms.
OA results summary of the forward-looking tube integrity conditions predicted to exist at the next inspection (OR25) is outlined in the Table below for each of the existing/potential axial ODSCC/PWSCC degradation mechanisms discussed above.
Degradation Mechanism No. of flaws POB POL upper 95/50 leak proj'n
(%)
(%)
at OR25 (gpm)
Existinq Mechanisms Axial ODSCC at TSPs 7
0.51 0.27 0.0 Axial ODSCC at TSP 16 1.58 0.63 0.0 DNTs ('?!2V, <5V)
Axial ODSCC at 26 0.76 2.69 0.036 DNT/DNG '?!5V Axial ODSCC at TIS 9
0.26 0.30 0.0 expansion transition (EXT)
Axial PWSCC at TIS 12 0.20 0.03 0.0 expansion transition (EXT)
Potential Mechanisms Axial ODSCC at FDBs 2
0.87 0.96 0.0 Circ ODSCC at TTS 1
0.01 0.62 0.0 Acceptance Criteria:
- 5%
- 5%
- s;0.3455 gpm1 Note 1: Value is based on AILPC (500 gpd), adjusted for a conservative operational leakage of 1.0 gpd using the H* leakage factor (2.49). [500 - (1.0*2.49)= 497.51 gpd = 0.3455 gpm]
Circumferential ODSCC at a DNT/DNG: As mentioned in the response to sub-section C.2 of the original OR23 SG Tube Inspection Report (ML25125A317), the flaw indication in tube R4C119 (OSC-0.53") of SG-B which had a complex ECT signal and showed attributes of both volumetric wear and circumferential cracking, was conservatively classified as a circumferential ODSCC indication at a DNG/DNT. Since this is the first reported occurrence of this mechanism in A600TT SG tubing, existing data and qualified techniques do not exist for developing the inputs for probabilistic models for initiation, growth, crack length/depth
Seabrook Station Docket No. 50-443 Enclosure L-2025-110 Page 3 of 5 characteristics and POD. As such, a simplistic/deterministic approach was used to evaluate the forward-looking tube integrity assessment for this degradation mechanism.
Since circumferential ODSCC at a ONT/ONG has been an existing degradation mechanism for SGs with mill-annealed (MA) Alloy 600 tubing, the analysis uses inspection results and operating experience (OE) from an operating commercial, U.S.-based nuclear unit with A600MA SG tubing. Twelve (12) indications of this mechanism have been reported to-date at this unit (3 affected SGs) over a 16-year period, with a maximum of 2 indications found during any one SG inspection and the degradation mechanism was not found in 3 of twelve SG inspections. This provides confidence that Seabrook (with A600TT tubing) is likely to experience not more than 2 such occurrences of this mechanism per cycle. Key results benchmarked include the circumferential extent of the ONG/ONT and that of the flaw.
Circumferential extent of the ONG/ONT is relevant because cracks have not been known to develop to a circ extent that is larger than its coincident ONG/ONT. As part of the analysis, the circumferential extent of all ONG/ONT '?:.5V in the Seabrook SGs were measured.
Results ranged from 20 to 149 degrees for Seabrook and 38.5 to 112 degrees for the flaws of the A600MA SGs. Comparison of the ONG/ONT distribution for Seabrook with that of the 12 indications of the A600MA SGs shows notable similarities, with the latter having a population shifted towards larger circumferential extents. It should be noted that although Seabrook has a higher operating Thot than the A600MA SGs (621 F vs. 609 F), the latter's material susceptibility to SCC is assumed to have a greater impact on flaw initiation and growth than temperature. For comparison of the flaw circumferential extent, the Seabrook OR23 indication measured 23 degrees; whereas the circ extents of the 12 indications from the A600MA SGs ranged from 16. 7 to 45 degrees, with 10 of the 12 indications having a greater circ extent than the Seabrook flaw. The smaller circumferential extent of the Seabrook flaw is consistent with reduced susceptibility of Alloy 600TT tubing to SCC and is well bounded by the flaws from the A600MA SGs.
The distribution of circumferential extent of ONG/ONT '?:.5V at Seabrook revealed an upper 95th crack angle of 94 degrees. Per the EPRI SG In-Situ Pressure Test (ISPT) Guidelines, if the crack angle of a circumferential crack remains under 180 degrees, CM is met for structural integrity. Using the conservative assumption that the crack that forms at the dent has the same circumferential extent as the dent, there is still significant margin from the upper 95th crack angle (94 degrees) to the limit of 180 degrees (structural limit is 270 degrees with NOE uncertainties). Therefore, with the only observed flaw circ extent of 23 degrees, and the upper 95th percentile circ extent of ONG/ONT '?:.5V having considerable margin to the structural integrity limit, there is no realistic potential for ODSCC at a ONG/ONT to exceed a metallurgical circ extent of 270 degrees and be vulnerable to burst over the next two fuel cycles.
Based on the frequency of occurrence of this degradation mechanism (discussed above) in the A600MA SGs, while it is more likely for Seabrook to experience one such flaw in the span of a 2-cycle inspection interval based on the OR23 results, it is conservatively assumed that Seabrook could experience 4 such indications in a 2-cycle inspection interval.
In addition, while the indications are expected to be bounded by the 45-degree maximum flaw circumferential extent measured for the A600MA SGs, a more conservative range of circ extents is used in the evaluation for leakage. Using leakage calculation equations developed for a 100% TW circ crack (which are based on section 9.4 of the SGMP Integrity Assessment Guidelines and section A.4 of the EPRI SG ISPT Guidelines), leak rates were calculated for varying circumferential crack angles at accident (MSLB) conditions. The 95/50 limits for yield and ultimate strength, Sy and Su, were conservatively calculated and used as inputs in the analyses. The limiting A600TT properties used were based on Table
Seabrook Station Docket No. 50-443 Enclosure L-2025-110 Page 4 of 5 4-1 of the EPRI SG Degradation Specific Management Flaw Handbook Rev 2. Leak rates calculated at 70 deg F were adjusted for SG secondary-side temperature (540 deg F) per Table 9-2 of the EPRI SG ISPT Guidelines. Dividing the crack angle-specific leak rate into the allowable leakage limit (0.3445 gpm) determines the permitted number of 100% TW cracks for varying circ crack extents while remaining below the AILPC limit (assuming no additional sources of leakage). The results of the leakage calculations for MSLB conditions are shown on the left-hand side of the Table below.
Circ crack MSLB NOPD angle 70 F 540 F 70 F 540 F (deg)
Leak Rate (gpm) # of flaws allowed Leak Rate (gpm) # of flaws allowed 20 0.0019 0.0026 119.94 0
0 N/A 30 0.0056 0.0075 41.12 0.001 0.0013 77.39 40 0.0128 0.0171 18.13 0.002 0.0027 38.70 50 0.0249 0.0333 9.29 0.003 0.0040 25.80 60 0.0440 0.0589 5.26 0.006 0.0080 12.90 65 0.0569 0.0761 4.07 0.008 0.0107 9.67 80 0.1133 0.1515 2.04 0.015 0.0201 5.16 90 0.1699 0.2271 1.36 0.023 0.0308 3.37 100 0.2463 0.3292 0.94 0.032 0.0428 2.42 120 0.4781 0.6393 0.48 0.06 0.0802 1.29 140 0.8550 1.1431 0.27 0.104 0.1391 0.74 160 1.4343 1.9177 0.16 0.169 0.2260 0.46 180 2.2852 3.0553 0.10 0.262 0.3503 0.30 Although the OR23 indication measured 23-degree circ extent, the forward-looking tube integrity assessment (OA) conservatively assumed circ crack angles in the range of 50-65 degrees. This range bounds the maximum crack angle measured for the A600MA SGs and is larger than the Seabrook flaw crack angle by a factor 2. For accident (MSLB) conditions, the analysis shows that 4-9 flaws are permitted for the range of circ crack angles evaluated in the OA.
If a 100% TW circ crack were to develop, it would leak at NOPD. Like the calculations described above for MSLB conditions, leak rates were also calculated at NOPD conditions.
Dividing the crack angle-specific leak rate at NOPD into the allowable operational leakage limit (OLPC) of 150 gpd (0.104 gpm), determines the permitted number of 100% TW cracks for varying circ crack extents while remaining below the OLPC limit (conservatively adjusted by 1.0 gpd for current operational leakage), and assuming no additional sources of leakage.
The results of the leakage calculations for NOPD conditions are shown on the right-hand side of the Table above. The results at NOPD conditions illustrate that if a 100% TW circumferential crack were to develop during normal operation, there would be sufficient leakage that it would be detectable, allowing the plant to be shut down in an orderly manner without exceeding the OLPC.
In summary, based on data collected from Seabrook and the U.S.-based nuclear unit with A600MA SG tubing, operation until OR25 is justified for circumferential ODSCC at a DNG/DNT. All postulated flaws over a 2-cycle interval, using bounding depths and circumferential extents, would result in accident-induced leakage less than allowable, with no realistic potential for failure of structural integrity.
Seabrook Station Docket No. 50-443 Enclosure L-2025-110 Page 5 of 5 APPENDIX A - Additional Information Definitions:
Ding (ONG): A signal caused by a reduction of the nominal tube diameter, and occurs in the free span of the tube. This is typically the result of mechanical impact or kinking during fabrication.
Dent (ONT): A signal caused by a reduction of the nominal tube diameter, and occurs at tube support structures or at the top of tubesheet. A dent can be considered a ding located at a tube support structure.
Abbreviations and Acronyms:
AILPC Accident-induced leak pert. crit.
NOE Non-destructive evaluation CL Cold Leg NOPD Normal operating pressure differential CM Condition Monitoring OA Operational Assessment DA Degradation Assessment OD Outside Diameter OBA Design Bases Accident ODSCC OD Stress Corrosion Cracking ONG Ding OLPC Operational Leakage Performance Criteria ONT Dent POB Probability of Burst ECT Eddy Current Testing POD Probability of Detection EFPY Effective Full Power Years POL Probability of Leakage EOC End-Of-Cycle PWSCC Primary Water SCC EPRI Electric Power Research Institute ROI Region of Interest ETSS Exam Technique Spec Sheet sec Stress Corrosion Cracking EXT Expansion Transition SCI Single Circumferential Indication FOB Flow Distribution Baffle SG Steam Generator GPO (gpd)
Gallons per Day SGMP SG Management Program GPM (gpm) Gallons per Minute SIPC Structural Integrity Pert. Criteria HL Hot Leg TS Tubesheet ISPT In-situ pressure test TSP Tube Support Plate MAPOD Model-assisted probability of detection TTS Top of Tube Sheet MSLB Main steam line break TW Through Wall