DCL-25-034, Diablo Canyon, Unit 2, ASME Section XI Inservice Inspection Program Request for Alternative NDE-U2-RPV-20-Year

Diablo Canyon, Unit 2, ASME Section XI Inservice Inspection Program Request for Alternative NDE-U2-RPV-20-Year

ML25142A381
Person / Time
Site:	Diablo Canyon
Issue date:	05/22/2025
From:	Rogers J Pacific Gas & Electric Co
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
DCL-25-034
Download: ML25142A381 (1)

Text

A member of the STARS Alliance Callaway

• Diablo Canyon

• Palo Verde

• Wolf Creek PG&E Letter DCL-25-034 Justin E. Rogers Station Director Diablo Canyon Power Plant Mail code 104/5/502 P.O. Box 56 Avila Beach, CA 93424 805.545.3088 Justin.Rogers@pge.com U.S. Nuclear Regulatory Commission 10 CFR 50.55a(z)(1)

ATTN: Document Control Desk Washington, DC 20555-0001 Diablo Canyon Unit 2 Docket No. 50-323, OL-DPR-82 ASME Section XI Inservice Inspection Program Request for Alternative NDE-U2-RPV-20-Year for Diablo Canyon Power Plant (DCPP) Unit 2

Dear Commissioners and Staff:

Pursuant to 10 CFR 50.55a(z)(1), Pacific Gas and Electric Company (PG&E) hereby requests Nuclear Regulatory Commission (NRC) approval of Inservice Inspection Program Request for Alternative NDE-U2-RPV-20-Year for DCPP Unit 2. This request proposes NRC approval of an alternative to the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code,Section XI, Paragraph IWB-2411, requirement for volumetric examination of essentially 100 percent of reactor vessel pressure-retaining welds identified in Table IWB-2500-1 once each 10-year interval. The proposed alternative is a one-time extension of the examination frequency, which is consistent with an industry standard approach that has been approved and implemented at numerous utilities. The proposed alternative and associated basis are provided in the Enclosure to this letter.

PG&E requests NRC authorization of the proposed alternative by September 4, 2025, to support preparations for Unit 2s Twenty-Fifth Refueling Outage (2R25),

which is currently scheduled to begin on October 5, 2025. PG&E makes no new or revised regulatory commitments (as defined by NEI 99-04) in this letter. If you have any questions regarding this information, please contact Mr. James Morris, Manager, Regulatory Services, at (805) 545-4609.

Sincerely, Justin E. Rogers Date 5/22/2025 m

Pacific Gas and Electric Company*

A member of the STARS Alliance Callaway

• Diablo Canyon

• Palo Verde

• Wolf Creek Document Control Desk PG&E Letter DCL-25-034 Page 2 rntt/51275261 Enclosure cc:

Diablo Distribution cc/enc:

Mahdi O. Hayes, NRC Senior Resident Inspector Samson S. Lee, NRC Senior Project Manager John D. Monninger, NRC Region IV Administrator State of California, Pressure Vessel Unit

Enclosure PG&E Letter DCL-25-034 Pacific Gas and Electric Company Diablo Canyon Power Plant, Unit 2 ASME Section XI Inservice Inspection Program Request for Alternative NDE-U2-RPV-20-Year to Extend the Reactor Vessel Weld Inservice Inspection Interval for Diablo Canyon Power Plant (DCPP) Unit 2 In Accordance with 10 CFR 50.55a(z)(1)

-- Alternative Provides Acceptable Level of Quality and Safety --

Enclosure PG&E Letter DCL-25-034 Page 1 of 10 Diablo Canyon Power Plant Unit 2 10 CFR 50.55a Request for Alternative NDE-U2-RPV-20-Year in Accordance with 10 CFR 50.55a(z)(1)

--Alternative provides an acceptable level of quality and safety--

Table of Contents

1. American Society of Mechanical Engineers (ASME) Code Component(s) Affected

5. Proposed Alternative and Basis for Use

6. Duration of Proposed Alternative

7. Precedents

8. References

Enclosure PG&E Letter DCL-25-034 Page 2 of 10 Diablo Canyon Power Plant Unit 2 10 CFR 50.55a Request for Alternative NDE-U2-RPV-20-Year in Accordance with 10 CFR 50.55a(z)(1)

--Alternative provides an acceptable level of quality and safety--

1.

ASME Code Component(s) Affected The affected component is the Diablo Canyon Power Plant (DCPP) Unit 2 reactor pressure vessel (RPV). Specifically, the following ASME Boiler and Pressure Vessel (BPV) Code, Section Xl (Reference 8.1), Table IWB-2500-1 examination categories and item numbers are addressed in this request.

Examination Category*

Item No Description B-A B1.10 Shell Welds B-A B1.11 Circumferential Shell Welds B-A B1.12 Longitudinal Shell Welds B-A B1.20 Head Welds B-A B1.21 Circumferential Head Welds B-A B1.22 Meridional Head Welds B-A B1.30 Shell-to-Flange Weld B-D B3.90 Nozzle-to-Vessel Welds B-D B3.100 Nozzle Inner Radius Section

• Examination Category B-A welds are defined as "Pressure Retaining Welds in Reactor Vessel," whereas Examination Category B-D welds are defined as "Full Penetration Welded Nozzles in Vessels." Throughout this request the above examination categories are referred to as "the subject examinations" and the ASME BPV Code, Section Xl, is referred to as "the Code."

2.

Applicable Code Edition and Addenda

ASME Code,Section XI, Rules for Inservice Inspection of Nuclear Power Plant Components, 2007 Edition with 2008 Addenda (Reference 8.1).

Enclosure PG&E Letter DCL-25-034 Page 3 of 10

3.

Applicable Code Requirement

IWB-2411, "Inspection Program," requires volumetric examination of essentially 100 percent of RPV pressure-retaining welds identified in Table IWB-2500-1 once each 10-year interval. The DCPP Unit 2, fourth 10-year inservice inspection (ISI) interval began on March 13, 2016, and is nominally scheduled to end on March 12, 2026. The applicable Code for the fifth 10-year ISI interval will be Section XI, 2019 Edition.

4.

Reason for Request

An alternative is requested from the requirement of IWB-2411 that volumetric examination of essentially 100 percent of RPV pressure-retaining Examination Category B-A and B-D welds be performed once each 10-year interval. A one-time extension of the interval between examinations of Category B-A and B-D welds, from 10 years to up to 20 years will result in a reduction in person-rem exposure consistent with industry precedents listed in Section 7.

5.

Proposed Alternative and Basis for Use Pacific Gas and Electric Co. (PG&E) proposes to perform the ASME Code Examination Category B-A and B-D volumetric examinations during the fifth ISI interval. The proposed inspection extension is consistent with the latest revised implementation plan, OG-10-238 (Reference 8.2).

NOTE: PG&E will perform volumetric examination of the RPV nozzle to hot and cold leg safe end welds from the inside surface per the requirements of ASME Code Case N-770-7 in the 2R25 refueling outage as well as future refueling outages per Code Case N-770-7 or latest approved revision.

In accordance with 10 CFR 50.55a(z)(1), an alternate inspection interval is requested on the basis that the current interval can be revised with negligible change in risk by satisfying the risk criteria specified in Regulatory Guide 1.174, Revision 1 (Reference 8.3).

The methodology used to conduct this analysis is based on that defined in the study WCAP-16168-NP-A, Revision 3, "Risk-Informed Extension of the Reactor Vessel In-Service Inspection Interval" (Reference 8.4). This study focuses on risk assessments of materials within the beltline region and extended beltline regions of the RPV wall. The results of the calculations for DCPP Unit 2 were compared to those obtained from the Westinghouse pilot plant evaluated in the WCAP.

Appendix A of the WCAP identifies the parameters to be compared. This study demonstrated that DCPP Unit 2 qualifies for an ISI interval extension, since the DCPP Unit 2 parameters are bounded by the results of the Westinghouse pilot plant.

Enclosure PG&E Letter DCL-25-034 Page 4 of 10 Table 1 below lists the critical parameters investigated in the WCAP and compares the results of the Westinghouse pilot plant to those of DCPP Unit 2.

Tables 2 and 3 provide additional information requested by the NRC and included in Appendix A of the WCAP.

Table 1:

Critical Parameters for the Application of Bounding Analysis for DCPP Unit 2 Parameter Pilot Plant Basis Plant-Specific Basis Additional Evaluation Required?

Dominant Pressurized Thermal Shock (PTS) Transients in the Risk Study are Applicable NRC PTS Risk Study (Reference 8.5)

PTS Generalization Study (Reference 8.6)

No TWCF 1.76E-08 Events per year (Reference 8.4) 2.56E-09 Events per year (Calculated per Reference 8.4)

No Frequency and Severity of Design Basis Transients 7 heatup/cooldown cycles per year (Reference 8.4)

Bounded by 7 heatup/

cooldown cycles per year No Cladding Layers (Single/Multiple)

Single Layer (Reference 8.4)

Single Layer No

Enclosure PG&E Letter DCL-25-034 Page 5 of 10 Table 2 below provides a summary of the latest RPV inspection for DCPP Unit 2 and an evaluation of the recorded indications. This information confirms that satisfactory examinations have been performed on the RPV of DCPP Unit 2.

Table 2:

Additional Information Pertaining to RPV Inspection for DCPP Unit 2 Inspection Methodology:

The latest RPV inspection for DCPP Unit 2 was conducted in accordance with the requirements of Appendix VIII of the ASME Code,Section XI, 2001 Edition through the 2003 Addenda as modified by 10CFR 50.55a(b)(2)(xiv, xv, and xvi). Evaluation of recordable indications was performed to the acceptance standards of Section XI, 2001 Edition with 2003 Addenda. Future inservice inspections will be performed to ASME Section XI, Appendix VIII requirements.

Number of past RPV inspections:

Three 10-year inservice inspections have been performed.

Number of indications found:

The indications identified in the beltline and extended beltline regions during the most recently completed insevice inspection are acceptable per Table IWB-3510-1 of Section XI of the ASME Code. There are two indications within the inner 1/10th or inner 1 of the reactor vessel wall thickness. One indication was located in the weld material and one indication was located in the plate material. These two indications required further evaluation per the requirements of the Alternate PTS Rule, 10 CFR 50.61a (Reference 8.7). The evaluation concluded that these two indications are acceptable. The disposition of these two indications is shown in the sub-tables below:

Disposition for the indication that was located in the weld material:

TWE (in.)

Allowable scaled maximum number of occurrences per 1,664 square-inches per requirements of 10 CFR 50.61a Number of DCPP Unit 2 indications Evaluated (Axial/Circ.)

TWEMIN TWEMAX 0

0.075 No Limit 0

0.075 0.475 277 1 (0/01) 0.125 0.475 151 1 (0/01) 0.175 0.475 37 0

0.225 0.475 14 0

0.275 0.475 6

0 0.325 0.475 5

0 0.375 0.475 2

0 0.425 0.475 1

0 0.475 Infinite 0

0

Enclosure PG&E Letter DCL-25-034 Page 6 of 10 Table 2 continued:

Additional Information Pertaining to RPV Inspection for DCPP Unit 2 Disposition for the indication that was located in the plate material:

The plant-specific total length (1,664 inches) of RPV beltline welds that were volumetrically inspected and the plant-specific total surface area (14,560 square-inches) of RPV beltline plates that were volumetrically inspected are comprised of the upper-to-intermediate shell circumferential weld, the intermediate-to-lower shell circumferential weld, three longitudinal welds in the intermediate shell, and three longitudinal welds in the lower shell.

While the three upper shell longitudinal welds were inspected and evaluated, the length and area associated with these welds are conservatively excluded from the total inspected length and area. This is conservative because the scaled maximum allowable number of occurrences (presented in column 3 in sub-tables above) would be even larger if these welds were included in the calculation of the volumetrically inspected weld length and plate area.

Weld length and area were also adjusted based on examination coverage of each weld as this is considered a conservative approach.

Proposed RPV inspection schedule for balance of plant life:

The fourth RPV inspection is currently scheduled for 2025. This RPV inspection is proposed to be performed during the fifth ISI interval. The proposed RPV inspection extension is consistent with the latest revised implementation plan, OG-10-238 (Reference 8.2).

TWE (in.)

Allowable scaled maximum number of occurrences per 14,560 square-inches per requirements of 10 CFR 50.61a Number of DCPP Unit 2 indications Evaluated (Axial/Circ.)

TWEMIN TWEMAX 0

0.075 No Limit 0

0.075 0.375 117 1 (0/01) 0.125 0.375 45 1 (0/01) 0.175 0.375 12 0

0.225 0.375 4

0 0.275 0.375 1

0 0.325 0.375 0

0 0.375 Infinite 0

0

Table 3 summarizes the inputs and outputs for the calculation of TWCF.

Enclosure PG&E Letter DCL-25-034 Table 3:

Details of TWCF Calculation for DCPP Unit 2, at 54 Effective Full-Power Years (EFPY)

Inputs(1)

Inter. & Lower Shell Twall [inches]:

8.625 Upper Shell Twall [inches]:

10.75 No.

Region and Component Description Material Heat No.

Identification Copper

[weight %]

Nickel

[weight %]

R.G.

1.99 Position Chemistry Factor

[°F]

RTNDT(u)

[°F]

Fluence

[Neutron/cm2, E > 1.0 MeV]

1 Upper Shell Plate B5453-1 C5162-1 0.11 0.60 1.1 74.0 28 4.31E+17 2

Upper Shell Plate B5453-3 C5162-2 0.11 0.60 1.1 74.0 5

3 Upper Shell Plate B5011-1R C4377-1 0.11 0.65 1.1 74.8 0

4 Intermediate Shell Plate B5454-1 C5161-1 0.14 0.65 1.1 102.7 52 2.45E+19 5

Intermediate Shell Plate B5454-2 C5168-2 0.14 0.59 1.1 99.6 67 6

Intermediate Shell Plate B5454-3 C5161-2 0.15 0.62 1.1 110.5 33 7

Lower Shell Plate B5455-1 C5175-1 0.14 0.56 1.1 98.2

-15 2.44E+19 8

Lower Shell Plate B5455-2 C5175-2 0.14 0.56 1.1 98.2 0

9 Lower Shell Plate B5455-3 C5176-1 0.10 0.62 1.1 65.2 15 10 Upper Shell to Intermediate Shell Circumferential Weld 8-201 21935 0.183 0.704 1.1 172.2

-56 4.92E+17 11 Intermediate Shell to Lower Shell Circumferential Weld 9-201 10120 0.046 0.082 1.1 34.0

-56 2.39E+19 12 Upper Shell Axial Welds 1-201 A 21935/12008 0.22 0.87 1.1 211.2

-50 4.73E+17 13 Upper Shell Axial Welds 1-201 B 21935/12008 0.22 0.87 1.1 211.2

-50 3.01E+17 14 Upper Shell Axial Welds 1-201 C 21935/12008 0.22 0.87 1.1 211.2

-50 2.45E+17 15 Intermediate Shell Axial Weld 2-201 A 21935/12008 0.22 0.87 1.1 211.2

-50 1.34E+19 16 Intermediate Shell Axial Weld 2-201 B 21935/12008 0.22 0.87 1.1 211.2

-50 1.72E+19 17 Intermediate Shell Axial Weld 2-201 C 21935/12008 0.22 0.87 1.1 211.2

-50 1.60E+19 18 Lower Shell Axial Welds 3-201 A 33A277 0.258 0.165 1.1 126.3

-56 1.59E+19 19 Lower Shell Axial Welds 3-201 B 33A277 0.258 0.165 1.1 126.3

-56 1.34E+19 20 Lower Shell Axial Welds 3-201 C 33A277 0.258 0.165 1.1 126.3

-56 1.73E+19 Outputs Methodology Used to Calculate T30: Regulatory Guide 1.99, Revision 2 (Reference 8.8)

Controlling Material Region No.

XX RTMAX-XX

[°R]

Fluence

[Neutron/cm2, E >1.0 MeV]

FF (Fluence Factor)

T30 [ºF]

TWCF95-XX Limiting Axial Weld - AW 16 2.3357 652.38 1.72E+19 1.1492 242.71 1.020E-09 Limiting Plate - PL 5

2.3483 650.29 2.45E+19 1.2411 123.62 7.521E-11 Limiting Circumferential Weld - CW 5

2.3520 649.67 2.39E+19 1.2349 123.00 4.956E-15 Limiting Forging - FO n/a TWCF95-TOTAL = (AWTWCF95-AW + PLTWCF95-PL + CWTWCF95-CW + FOTWCF95-FO):

2.56E-09 Note 1: Material properties and fluence inputs are based on WCAP-18924-NP (Reference 8.9) and WCAP-18853-NP (Reference 8.10).

Page 7 of 10

Enclosure PG&E Letter DCL-25-034 Page 8 of 10

6.

Duration of Proposed Alternative This request is applicable to the DCPP Unit 2 ISI program for the fourth and fifth 10-year inspection intervals.

7.

Precedents

1. Diablo Canyon Power Plant, Unit No. 1 - Request for Alternative RPV-U1-Extension to Allow Use of Alternate Reactor Inspection Interval Requirements (TAC No. MF4678), dated June 19, 2015, Agencywide Document Access and Management System (ADAMS) Accession Number ML15168A024.

2. Surry Power Station Units 1 and 2 - Relief Implementing Extended Reactor Vessel Inspection Interval (TAC Nos. ME8573 and ME8574), dated April 30, 2013, ADAMS Accession Number ML13106A140.

3. Vogtle Electric Generating Plant, Units 1 and 2 - Request for Alternatives VEGP-ISI-ALT-05 and VEGP-ISI-ALT-06 (TAC Nos. MF2596 and MF2597),

dated March 20, 2014, ADAMS Accession Number ML14030A570.

4. Catawba Nuclear Station Units 1 and 2: Proposed Relief Request 13-CN-003, Request for Alternative to the Requirement of IWB-2500, Table IWB-2500-1, Category B-A and Category B-D for Reactor Pressure Vessel Welds (TAC Nos.

MF1922 and MF1923), dated March 26, 2014, ADAMS Accession Number ML14079A546.

5. Sequoyah Nuclear Plant, Units 1 and 2 - Requests for Alternatives 13-ISI-1 and 13-ISI-2 to Extend the Reactor Vessel Weld Inservice Inspection Interval (TAC Nos. MF2900 and MF2901), dated August 1, 2014, ADAMS Accession Number ML14188B920.

6. Byron Station, Unit No. 1 - Relief from the Requirements of the ASME Code to Extend the Reactor Vessel Inservice Inspection Interval (TAC No. MF3596),

dated December 10, 2014, ADAMS Accession Number ML14303A506.

7. Wolf Creek Generating Station - Request for Relief Nos. I3R-08 and I3R-09 for the Third 10-Year Inservice Inspection Program Interval (TAC Nos. MF3321 and MF3322), dated December 10, 2014, ADAMS Accession Number ML14321A864.

8. Callaway Plant, Unit 1 - Request for Relief I3R-17, Alternative to ASME Code Requirements Which Extends the Reactor Vessel Inspection Interval from 10 to 20 Years (TAC No. MF3876), dated February 10, 2015, ADAMS Accession Number ML15035A148.

9. Braidwood Station, Units 1 and 2 - Request for Relief from the Requirements of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code) (CAC Nos. MF8191 and MF8192), dated March 15, 2017, ADAMS Accession Number ML17054C255.

10. South Texas Project, Units 1 and 2 - Relief from the Requirements of the ASME Code Regarding the Third 10-Year Inservice Inspection Program Interval

Enclosure PG&E Letter DCL-25-034 Page 9 of 10 (EPID L-2018-LLR-0010), dated July 24, 2018, ADAMS Accession Number ML18177A425.

11. Donald C. Cook Nuclear Plant, Unit No. 1 - Approval of Alternative to the ASME Code Regarding Reactor Vessel Weld Examination - Relief Request ISIR-4-08 (EPID: L-2018-LLR-0106), dated October 26, 2018, ADAMS Accession Number ML18284A310.

12. R. E. Ginna Nuclear Power Plant - Issuance of Relief Request ISI-18 Regarding Fifth 10-year Inservice Inspection Program Interval (EPID L-2018-LLR-0104), dated April 22, 2019, ADAMS Accession Number ML19100A004.

13. Point Beach Nuclear Plant, Units 1 and 2 - Approval of Relief Requests 1-RR-13 and 2-RR-13 Regarding Extension of Inspection Interval for Reactor Pressure Welds from 10 to 20 years (EPID L-2019-LLR-0060), dated March 4, 2020, ADAMS Accession Number ML20036F261.

14. St. Lucie Plant, Unit 2 - Authorization of RR#15 Regarding Extension of ASME Requirements Related to Reactor Pressure Vessel Weld Examinations from 10 to 20 Years (EPID L-2020-LLR-0283), dated September 30, 2021, ADAMS Accession Number ML21236A131.

15. Oconee Nuclear Station, Units 1, 2, and 3 - Authorization and Safety Evaluation for Alternative Reactor Pressure Vessel Inservice Inspection Intervals (EPID L-2021-LLR-0004), dated November 19, 2021, ADAMS Accession Number ML21281A141.

16. Turkey Point Nuclear Generating Unit Nos. 3 and 4 - Authorization of Relief Request Nos. 8 and 9 Regarding Extension of Inspection Interval for Reactor Pressure Vessel Welds (EPID L-2021-LLR-0038), dated May 10, 2022, ADAMS Accession Number ML22123A192.

8.

References

1. American Society of Mechanical Engineers Boiler and Pressure Vessel Code,Section XI (ASME Section XI), 2007 Edition with 2008 Addenda.

2. PWROG Letter OG-10-238, Revision to the Revised Plan for Plant Specific Implementation of Extended Inservice Inspection Interval per WCAP-16168-NP, Revision 1, Risk-Informed Extension of the Reactor Vessel In-Service Inspection Interval. PA-MSC-0120, July 12, 2010 (ADAMS Accession Number ML11153A033).

3. NRC Regulatory Guide 1.174, Revision 1, "An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis," U.S. Nuclear Regulatory Commission, November 2002, (ADAMS Accession Number ML023240437).

4. Westinghouse Report, WCAP-16168-NP-A, Revision 3, "Risk-Informed Extension of the Reactor Vessel In-service Inspection Interval," October 2011 (ADAMS Accession Number ML11306A084).

5. NUREG-1874, "Recommended Screening Limits for Pressurized Thermal Shock (PTS)," U.S. Nuclear Regulatory Commission, March 2010, (ADAMS Accession Number ML15222A848).

Enclosure PG&E Letter DCL-25-034 Page 10 of 10

6. NRC Letter Report, "Generalization of Plant-Specific Pressurized Thermal Shock (PTS) Risk Results to Additional Plants," U.S. Nuclear Regulatory Commission, December 14, 2004 (ADAMS Accession Number ML042880482).

7. Code of Federal Regulations, 10CFR Part 50.61a, "Alternate Fracture Toughness Requirements for Protection Against Pressurized Thermal Shock Events," U.S. Nuclear Regulatory Commission, Washington D.C., Federal Register, Volume 75, No. 1, dated January 4, 2010, with corrections to paragraph (g) dated February 3, 2010, March 8, 2010, and November 26, 2010.

8. NRC Regulatory Guide 1.99, Revision 2, Radiation Embrittlement of Reactor Vessel Materials, U.S. Nuclear Regulatory Commission, May 1988 (ADAMS Accession Number ML003740284).

9. Westinghouse Report, WCAP-18924-NP, Revision 1, Diablo Canyon Units 1 and 2 Initial License Renewal: Time-Limited Aging Analysis (TLAAs) on Reactor Vessel Integrity (RVI), November 2024.

10. Westinghouse Report, WCAP-18853-NP, Revision 0, Diablo Canyon Unit 2 License Renewal: Reactor Pressure Vessel Beltline and Extended Beltline Radiation Exposure Projections, April 2024.