ML23048A320

Response to NRC Requests for Additional Information Regarding 2022 Unit 1 Steam Generator Tube Inspections

ML23048A320
Person / Time
Site:	Palo Verde
Issue date:	02/17/2023
From:	Cox M Arizona Public Service Co
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
102-0857-MSC/DHD
Download: ML23048A320 (1)

Text

Technical Specification 5.6.8 102-0857-MSC/DHD Palo Verde February 17, 2023 Nuclear Generating Station P.O. Box 52034 Phoenix, AZ 85072 Mail Station 7605 U.S. Nuclear Regulatory Commission Tel 623 393 5753 ATTN: Document Control Desk Washington, DC 20555-0001

Subject:

Palo Verde Nuclear Generating Station Unit 1 Docket No. 50-528 Renewed Operating License Number NPF-41 Response to NRC Requests for Additional Information Regarding 2022 Unit 1 Steam Generator Tube Inspections By letter dated November 4, 2022 [Agencywide Documents Access and Management System (ADAMS) Accession No. ML22308A166], Arizona Public Service Company (APS) submitted information summarizing the results of the Spring 2022 steam generator (SG) tube inspections at Palo Verde Nuclear Generating Station (PVNGS)

Unit 1.

The Nuclear Regulatory Commission (NRC) staff reviewed the information provided by APS and determined that additional information was needed to complete its review of the steam generator tube inspections. A clarifying call was held between the NRC staff and APS on January 12, 2023, to discuss the additional information needed. The APS response to the final NRC staff requests for additional information

[ADAMS Accession No. ML23013A063] dated January 13, 2023, is provided in the enclosure to this letter.

No commitments are being made to the NRC by this letter. Should you need further information regarding this response, please contact me at (623) 393-5753.

Sincerely, Cox, Matthew Digitally signed by Cox, Matthew S(Z05628)

S(Z05628) Date: 2023.02.17 12:50:10 -07'00' Matthew S. Cox Department Leader, Nuclear Regulatory Affairs MSC/DHD/cr

Enclosure:

Response to Requests for Additional Information for Steam Generator Tube Inspections in Unit 1 cc: S. A. Morris NRC Region IV Regional Administrator D. Galvin NRC NRR Project Manager for PVNGS L. N. Merker NRC Senior Resident Inspector for PVNGS A member of the STARS (Strategic Teaming and Resource Sharing) Alliance Callaway

• Comanche Peak

• Diablo Canyon

• Palo Verde

• Wolf Creek

ENCLOSURE Response to Requests for Additional Information for Steam Generator Tube Inspections in Unit 1

Enclosure Response to Requests for Additional Information for Steam Generator Tube Inspections in Unit 1 Introduction By letter number 102-08524, dated November 4, 2022 [Agencywide Documents Accession No. ML22308A16], Arizona Public Service Company (APS) submitted information summarizing the results of the Spring 2022 steam generator (SG) tube inspections at the Palo Verde Nuclear Generating Station (PVNGS) Unit 1.

All pressurized water reactors have Technical Specifications (TS) according to Section 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. Palo Verde Unit 1 TS 5.6.8 requires that a report be submitted within 180 days after the initial entry into hot shutdown (MODE 4) following completion of an inspection of the SGs performed in accordance with TS Section 5.5.9, which requires that a SG Program be established and implemented to ensure SG tube integrity is maintained.

The Nuclear Regulatory Commission (NRC) staff reviewed the information provided by APS and determined that additional information was needed to complete its review of the SG tube inspections. A clarification call was held on January 12, 2023. The APS response to the final NRC staff requests for additional information [ADAMS Accession No. ML23013A063] dated January 13, 2023, is provided in this enclosure. The NRC requests are provided followed by the APS responses. Request 1 is provided first with each sub-part, followed by re-statement of each sub-part with the APS response.

NRC Request 1 Section 2.0, Scope of Examination Performed, of the spring 2022 SG tube inspection report states that bobbin probe inspections were performed as partial lengths from both the hot leg and cold leg due to black sludge on the inside diameter (ID) of some tubes. Based on Table 1, Examination Summary, it appears that the partial exams were only performed on less than half of the tubes in SG 12. The report also stated that the black sludge caused excessive probe wear and poor data quality when the probe passed around both square ends. Please provide the following information regarding the black sludge:

a. Identify if this is the first time black sludge has been detected on the ID of SG tubes at Palo Verde Unit 1 or when it was first detected.

b. Identify if there is any black sludge present on the ID of tubes in SG 11 or is it just limited to tubes in SG 12.

c. Describe the extent of the black sludge (e.g., number of tubes affected, how much of the ID of the affected tubes are covered, are the affected tubes located throughout the tube bundle or located close together).

d. Identify how the black sludge on the ID of tubes is impacting the eddy current data (e.g., a comparison of the typical signal to noise ratio with and without black sludge).

e. Provide insights on the source of the black sludge on the ID of the SG tubes and its composition, if available.

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Enclosure Response to Requests for Additional Information for Steam Generator Tube Inspections in Unit 1

f. Identify what actions have been taken or are being considered to address the black sludge since it appears to impact eddy current data quality.

NRC Request 1a Identify if this is the first time black sludge has been detected on the ID of SG tubes at Palo Verde Unit 1 or when it was first detected.

APS Response 1a. A similar black sludge was first noted during 1R19 Steam Generator inspections but was not noted during the subsequent 1R21 Steam Generator inspections. This issue has not been identified in the Unit 2 or Unit 3 Steam Generator tubes.

NRC Request 1b Identify if there is any black sludge present on the ID of tubes in SG 11 or is it just limited to tubes in SG 12.

APS Response 1b. Both Steam Generators contained the black sludge within the tubes, however, there was a greater impact to the inspection efforts of SG 12.

NRC Request 1c Describe the extent of the black sludge (e.g., number of tubes affected, how much of the ID of the affected tubes are covered, are the affected tubes located throughout the tube bundle or located close together).

APS Response 1c. The black sludge encountered during 1R23 Steam Generator inspection activities was not limited to any group or region of tubes. While the sludge was encountered in both generators, it was assumed that more was present within the tube bundle of SG 12. There was no direct correlation noted between regions of the bundle (i.e.,

bundle elevations, perimeter vs. center, etc.). The sludge was present throughout the inspection campaign.

NRC Request 1d Identify how the black sludge on the ID of tubes is impacting the eddy current data (e.g., a comparison of the typical signal to noise ratio with and without black sludge).

APS Response 1d. The black sludge was widespread in SG12. Signal noise was screened throughout the acquisition/analysis process and data that contained unacceptably large noise values was re-acquired. The noise and data quality issues that the report referred to can be attributed to the probe friction and wobbling when tubes were acquired full length, because passing over the second square bend adds significant friction. The decision was made to acquire partial lengths to preserve probe life, and also to cut 2

Enclosure Response to Requests for Additional Information for Steam Generator Tube Inspections in Unit 1 down on the number of re-tests. Because an accurate comparison of the typical signal to noise ratio for tubes with and without sludge could not be determined, noise monitoring values were compared from Unit 1R21 outage and Unit 1R23 outage across SG11 and SG12. These noise values confirm that there was no increase in noise due to the presence of the black sludge.

NRC Request 1e Provide insights on the source of the black sludge on the ID of the SG tubes and its composition, if available.

APS Response 1e. The source of the black sludge is currently assumed to be corrosion and wear products that are common to the RCS of most PWR plants. These products are largely comprised of metal particles and compounds that include iron, nickel, chromium, etc. These particles often deposit onto in-flux regions and other RCS surfaces during operation. EPRI Report 1016766 High Activity CRUD Burst Impacts and Responses suggest that these deposits are released after shutdown when peroxide is injected into the RCS.

Palo Verde specific experience suggests that these deposits are released upon reactor shutdown due to mechanical agitation of RCS components and prior to peroxide injections. The particulates released back into the RCS are captured by filtering the suspended solids and capturing dissolvable products via ion exchangers while circulating the RCS fluid. However, settling, surface adhesion, and/or redeposition may occur in areas of low flow prior to filtration taking full effect. The details of the origin and mitigation of this sludge remain under investigation.

A sample was taken of the sludge in 1R19 which concluded that the deposits were primarily composed of chromium, nickel, and iron. Further examination and analysis noted a crystal structure and suggested that much of the deposits existed in the form of nickel iron chromium spinel that is found in RCS deposits (Westinghouse Report TR-CCOE-16-05). Dry samples of the sludge deposits from 1R19 were powder-like when dry, however could act as an abrasive on eddy current probes 3

Enclosure Response to Requests for Additional Information for Steam Generator Tube Inspections in Unit 1 due to the hardness and structure of the spinel. Another sample was collected in 1R23 for additional off-site analysis however the sample was inadvertently mistaken for rad waste and was discard, which was entered in the site corrective action program.

NRC Request 1f Identify what actions have been taken or are being considered to address the black sludge since it appears to impact eddy current data quality.

APS Response 1f. As explained further in response to part 1d, the impacts of the black sludge on data quality were reconciled during the acquisition process and the data that was analyzed did not suffer from impacts of the black sludge. Because of this impact on acquisition, contingencies such as running partial length, increasing probe stock, and possibly adjusting inspection plans are being considered for future inspections across all 3 units. Additionally, a high impact team is being established to further develop strategies to mitigate impacts from the black sludge.

NRC Request 2 Table 2, Indication Summary, of the spring 2022 SG tube inspection report states that 138 and 140 total tubes have been plugged in SGs 11 and 12, respectively.

However, Appendix E, Plug Maps, states that there were 184 and 183 plugged tubes in SGs 11 and 12, respectively, prior to U1R23. The staff notes that the number of plugged tubes identified on the plug maps are consistent with the information reported in the Palo Verde, Unit 1 spring 2019 SG tube inspection report, dated October 18, 2019 (ML19291F576), and the associated NRC review, dated January 16, 2020 (ML20015A012). Please provide the total number of tubes plugged in SGs 11 and 12.

APS Response The values provided in Table 2 (specifically 138 and 140) are outdated information that carried over from a previous report template and were not updated. The values and images provided in Appendix E reflect the accurate and updated plugged tube configuration for each Unit 1 Steam Generator as follows:

• SG11 - 184 existing tubes plugged, plus 12 additional tubes plugged during 1R23 = 196 total tubes plugged.

• SG12 - 183 existing tubes plugged, plus 10 additional tubes plugged during 1R23 = 193 total tubes plugged.

The existing values prior to U1R23 are documented in Engineering Evaluation 21-14101-002, as well as the associated tube sheet plugging maps referenced therein. The number of tubes plugged in each Steam Generator during 1R23 is accurately and consistently documented in the 1R23 Condition Monitoring Evaluation, the U1C24/25/26 Steam Generator Operational Assessment, and the Final Plugging Lists as documented in the respective tube plugging work orders. Engineering Evaluation 22-04978-003 is in progress to update the current number of plugged tubes (post 1R23) for future plugging allowances and will be issued prior to the next U1 Steam Generator inspection and maintenance campaign.

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