L-2022-157, Response to Request for Additional Information Regarding Seabrook Steam Generator Tube Inspection Report
| ML22272A531 | |
| Person / Time | |
|---|---|
| Site: | Seabrook  |
| Issue date: | 09/28/2022 |
| From: | Strand D NextEra Energy Seabrook |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| L-2022-157 | |
| Download: ML22272A531 (5) | |
Text
NEXTera ENERGY@
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U.S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555-0001 RE:
Seabrook Station Docket No. 50-443 September 28, 2022 L-2022-157 10 CFR 50.36 Response to Request for Additional Information Regarding Seabrook Steam Generator Tube Inspection Report
References:
- 1.
NextEra Energy Seabrook, LLC letter SBK-L-22036, "Steam Generator Tube Inspection Report",
April 25, 2022 (ADAMS Accession No. ML22115A158)
- 2.
NRG electronic memorandum, "Request for Additional Information RE: Seabrook Steam Generator Tube Inspection Report Review", August 15, 2022 (ADAMS Accession No.
M L22228A055)
In Reference 1, NextEra Energy Seabrook, LLC (NextEra) submitted the fall 2021 Steam Generator Tube Inspection Report required by Seabrook Technical Specification (TS) 6.8.1.7. The report summarized the steam generator tube inspections conducted during Refueling Outage (RFO) 21 in the Fall of 2021.
In Reference 2, the NRG requested additional information to complete their evaluation of the information provided in Reference 1. The enclosure to this letter provides NextEra's response to the NRC's request for additional information (RAI).
This letter contains no new or revised regulatory commitments.
Should you have any questions regarding this submittal, please contact Mr. Kenneth Mack, Fleet Licensing Manager at 561-904-3635.
Sincerely, Dianne Strand General Manager, Regulatory Affairs Enclosure cc:
USNRC Region I Administrator USNRC Project Manager USNRC Senior Resident Inspector NextEra Energy Seabrook, LLC, P.O. Box 300, Lafayette Road, Seabrook, NH 03874
Seabrook Station Docket No. 50-443 REQUEST FOR ADDITIONAL INFORMATION FALL 2021 STEAM GENERATOR TUBE INSPECTION REPORT SEABROOK STATION. UNIT. NO 1 DOCKET NO. 50-443 L-2022-157 Enclosure Page 1of4 By letter dated April 25, 2022 (Agencywide Documents Access and Management Systems Accession No. ML22115A158), NextEra Energy Seabrook LLC (the licensee) submitted information summarizing the results of the fall 2021 steam generator (SG) tube inspections performed at Seabrook Station, Unit No. 1 (Seabrook) during Refueling Outage 21 in the Fall of 2021.
All pressurized water reactors have Technical Specifications (TS) according to§ 50.36 of Title 10 of the Code of Federal Regulations that include a SG Program with specific criteria for the structural and leakage integrity, repair, and inspection of SG tubes. Seabrook TS 6.8.1. 7 requires that a report be submitted within 180 days after the initial entry into hot shutdown following SG inspections performed in accordance with TS 6.7.6.k, which requires that an SG Program be established and implemented to ensure SG tube integrity is maintained.
To complete its evaluation of the inspection report, the U.S. Nuclear Regulatory Commission staff requests the following information:
- 1. Please provide the following information about the SG tube support plate (TSP) deposits and the two indications of axial outside diameter stress corrosion cracking (ODSCC) reported in SG-B, Row 8, Column 116, on the hot leg at the uppermost tube support plate (TSP 08H):
a) The estimated chemical composition of the deposits and a qualitative or approximate effect on tube temperature.
b) The extent of the deposition in each SG, estimated extent of TSP flow-hole blockage, and plans for deposit removal.
c) A description of how the presence of the deposits is being incorporated into the operational assessment (OA), degradation assessment (DA), and future inspection plans.
d) With respect to the DA, are the tubes in all four steam generators considered susceptible to cracking due to TSP deposits?
Seabrook Station Response to RAI-1:
- a. The chemical composition of the deposits in the quatrefoil openings of the subject tube is not known since the location is inaccessible. The SGs at Seabrook Station do not have inspection ports at the uppermost TSPs, and the use of a tool (in the past) for performing upper bundle in-bundle (UBIB) exams was discontinued at the start of the OR19 exams due to communication issues between the tool and the control station once the tool entered the secondary-side.
However, samples of the OR21 sludge deposits were obtained and characterized. Deposit samples were composed primarily of magnetite (71.1 wt.% iron) with small amounts of carbon (0.23 wt.%). Hematite was also detected; nickel, titanium and manganese concentrations were also detected but were lower than in the OR19 samples. Aluminum and silicon were present in trace amounts, and the average copper concentration in the OR21 sample remained unchanged from the OR19 average of 0.4 wt.%.
Two of the quatrefoil openings of the broach TSP had deposits; the other 2 showed no deposits.
Seabrook Station Docket No. 50-443 L-2022-157 Enclosure Page 2 of 4 Results of a thermal hydraulic model of the SG with the level of detail to determine the change in tube temperature if 2 of the four quatrefoil openings of one TSP in the bundle had deposits are not available for Seabrook. The EPRI SG Management Program (SGMP) also does not have guidance in its portfolio for estimating the change in tube temperature under such a scenario. It should be noted that the level of deposits at the 08H support plate in SG-B is not representative of the rest of the tubes (see response to RAl-1 b). Main steam pressure has also not exhibited an adverse trend over the last few cycles, and the unit has not experienced unexplained fluctuations in SG water level over the period. It is very unlikely that deposit accumulation in the bundle is at the point where a noticeable effect on tube temperature exists.
- b. To benchmark the extent of deposition and TSP flow blockage, a review of ECT c-scan images at quatrefoil intersections in the uppermost TSP was performed for 150 tubes (total) at random locations in the hotleg region of the four SGs, using the results of the array probe. Over 80% of these locations exhibited very little or no deposits at the TSP quatrefoil openings. Of the tubes sampled, tube R8C116 with the OR21 ODSCC indications was representative of the ones with the highest deposit loading at the quatrefoil openings. For comparison, during the last completed (OR17) UBIB exams, the deposits observed consisted of some sludge and scale build-up. There was minimal obstruction of the quatrefoils in OR 17 and most of the openings were visible. To get a better extent of deposition in the bundle, Seabrook Station plans to complete a secondary side deposit loading assessment using the ECT data from the OR21 inspection. Depending on the results of the deposit loading assessment, Seabrook will select the appropriate bundle-cleaning measure(s) to be applied in OR23 such as upper bundle flush (UBF), soft chemical cleaning, etc., in addition to sludge lancing. UBF will also be reviewed as part of the SG secondary side integrity assessment plan going forward to proactively manage tube deposit loading.
- c. The reporting of axial ODSCC at the TSP intersection attributed to deposits was incorporated into the full probability full-bundle OA model. The analysis assessed the SG performance criteria for burst and leakage for axial ODSCC flaws at TSP intersections over 2 full cycles of operation.
EPRI ETSS 128413 probability of detection (POD) distribution for axial ODSCC flaws located at quatrefoil TSP intersections was used as input to the full probability analysis model. The number of undetected flaws located at TSP intersections was previously determined through Weibull failure analysis of all axial ODSCC at TSPs reported across the industry in high stress tubes. In developing the OA for OR21, data points for the two axial ODSCC indications attributed to deposit loading at Seabrook as well as the data point from another plant that had previously seen a similar degradation mechanism attributed to deposits (axial ODSCC at TSP intersection on a non-high stress tube), were added to the remaining industry data for axial ODSCC at TSPs on high stress tubes, and used in the Weibull failure analysis to provide a more conservative distribution.
Axial ODSCC at a TSP intersection in a non-high stress tube has been an existing degradation mechanism at Seabrook prior to OR21. Another A600TT industry SG had previously seen this mechanism attributable to the presence of deposits. Although the OR21 ODSCC indications at a TSP were detected during full-length array probe exams, future DAs for Seabrook will include an array probe technique extension for detection of axial ODSCC due to the presence of deposits. In addition, the presence of deposits will be assessed as part of the SG secondary side integrity assessment incorporated into the DAs as required by the latest revision of the SGMP Integrity Assessment Guidelines.
The SG program will continue to monitor SG performance and chemistry parameters for
Seabrook Station Docket No. 50-443 L-2022-157 Enclosure Page 3 of 4 adverse trends in deposit loading. The program will also perform a secondary side deposit loading assessment to monitor/trend accumulation of deposits in the bundle using the ECT data taken during future SG inspection/maintenance activities.
- d. With respect to the DA, the tubing in all 4 SGs are considered susceptible to cracking due to deposits.
- 2. Appendix B of the report describes condition monitoring for the axial ODSCC indication at the expansion transition in SG-C.
a) Please describe the calculations that produced a "lowest 95/50" predicted burst pressure of 4,555 pounds per square inch (psi) and "calculated burst pressure value for the OR21 [refueling outage 21] ODSCC indication" of 6,962 pounds per square inch. For example, does one calculation refer to the as-found indication and another to an OA projection?
b) If the 95/50 burst pressure for this indication is lower than the OA predictions, discuss whether any adjustments to the OA model are needed.
Seabrook Station Response to RAl-2:
- a. For condition monitoring (CM), the ODSCC indication in tube R25C51 at the hotleg expansion transition in SG-C was profiled using the guidance of ETSS 128431 and assessed regarding the need for in-situ pressure testing. The depth profile information was used along with the tube material flow stress as input to a model (Weak Link) which is a function of the Single Flaw Model (vendor proprietary) computer program. The Single Flaw model determined the structural/burst equivalent lengths and depths, which are used as inputs to a Monte Carlo simulation of burst pressure.
The Weak Link model resulted in a calculated burst pressure of 6,962 psi for the OR21 (as-tound) ODSCC indication. To clarify the information in Appendix B of the OR21 SG Tube Inspection Report (ML22115A158), the 4,555-psi value was the specific CM value of the indication for burst effective depth and length of 65.31%TW and 0.176 inches, respectively, utilizing ETSS 128431 and Equation 5-11 in the SG Degradation Specific Management Flaw Handbook Rev. 2.
- b. Since the previous (OR20) OA prediction determined a 95/50 burst pressure of 4,974 psi (which was less than the 95/50 burst pressure (6,962 psi) of the OR21 indication), no adjustment to the OA model was required.
- 3. Table 3c of the report lists newly reported TSP wear indications, including four indications sized at 24 to 30 percent through-wall. The report also states that the indications were identified in lookbacks at previous inspection data and have not grown over several cycles. A previous response on this topic (ML21140A307) stated that the residual eddy current signal from the TSP makes it difficult to detect low-level wear. Discuss what factors (e.g., deposits) have been assessed as potential causes of residual signals that affect detection at TSPs.
Seabrook Station Response to RAl-3:
In performing enhanced probe inspections using the X-probe (array/bobbin combo probe) in OR21, the array coils in the X-probe allow enhanced detection of low-level wear at structures whereas the bobbin coil may be influenced by the mix residual signal. Three of the 4 indications in question were
Seabrook Station Docket No. 50-443 L-2022-157 Enclosure Page 4 of 4 affected substantially by the TSP mix residual and were reported by the array coils but not the bobbin coils; the fourth indication (R44C100 in SG-D) was reported by both the bobbin and array coils. All 4 indications produced very small low-amplitude signals. Several factors are known to potentially cause mix residual signals that affect detection at TSPs: the two frequencies chosen for the bobbin coil mix, tube noise at the TSP location, deposits on the OD of the tube or in the open broach holes, the material properties of the TSPs in the SG or the TSP ring used in the calibration standard, to name a few. The new (OR21) TSP wear indications are very small signals. It should be noted that the X-probe was not used at the 4 locations in question in OR20; hence, the low-level signals went undetected until the X-probe was used at these locations in OR21. Very few 2-frequency bobbin coil mix channels completely cancel the TSP signal. The mix residual signals encountered at TSPs at Seabrook are not expected to mask significant signals of interest.
- 4. Section D of the report, in the section on volumetric wear indications at or above the top of tubesheet, lists the areas at which wear indications are considered to originate from foreign objects (FOs). Comparison with the previous (spring 2020) inspection report show the wear source was changed for some indications in the 2021 inspection. For example, an indication in SG-A, Row 11, Column 76, was changed from a new FO wear indication in 2020 to a TSP wear indication in 2021, and an indication in SG-D, Row 5, Column 100, was changed from a TSP wear indication in 2020 to an FO wear indication in 2021. Section D of the 2021 report also states there were no newly reported FO volumetric indications. Please discuss the criteria for judging whether an indication is wear from structures or loose parts, and discuss how these criteria were applied to the examples noted above.
Seabrook Station Response to RAI-4:
There were no newly reported FO wear volumetric indications during the OR21 inspections. The indication in SG-A (R11 C76) was attributed to FO wear and sized with ETSS 21998.1 for both the 2020 and 2021 inspections. The indication was sized in 2021 at 21 % TW (rather than 17% TW), and inadvertently listed as TSP wear (Table 3c) instead of FO wear (Table 3b) in the OR21 report (ML22115A158). No change in criteria was made for judging whether an indication represents wear from structures or loose parts. To implement the above change, delete the row entry for tube R11 C76 in SG-A from the TSP wear listing (Table 3c), and insert the following row entry in the FO wear listing (Table 3b):
SG Row Col Location
%TW A
11 76 06C-0.8" 21%
The indication in SG-D (R5C100) was attributed to TSP wear for both the 2020 and 2021 inspections, and as listed in Table 4b of the OR20 report (ML20295A551) and Table 3c of the OR21 report (ML22115A158). In the OR21 report, the row entry provided below was inadvertently entered in Table 3b and should be omitted from the report. No change in criteria was made for judging whether an indication represents wear from structures or loose parts.
SG Row Col Location
%TW D
5 100 06H+0.71" 33%