ML19023A026

Request for Alternative from Volumetric/Surface Examination Frequency Requirements of ASME Code Case N-729-4

ML19023A026
Person / Time
Site:	Diablo Canyon
Issue date:	01/25/2019
From:	Robert Pascarelli Plant Licensing Branch IV
To:	Welsch J Pacific Gas & Electric Co
Singal B, 301-415-3016
References
EPID L-2018-LLR-0107
Download: ML19023A026 (8)

Text

UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 January 25, 2019 Mr. James M. Welsch Vice President, Nuclear Generation and Chief Nuclear Officer Pacific Gas and Electric Company Diablo Canyon Nuclear Power Plant P.O. Box 56, Mail Code 104/6 Avila Beach, CA 93424

SUBJECT:

DIABLO CANYON NUCLEAR POWER PLANT, UNITS 1 AND 2 - REQUEST FOR ALTERNATIVE FROM VOLUMETRIC/SURFACE EXAMINATION FREQUENCY REQUIREMENTS OF ASME CODE CASE N-729-4 (EPID L-2018-LLR-0107)

Dear Mr. Welsch:

By letter dated August 1, 2018 (Agencywide Documents Access and Management System Accession No. ML18213A375), Pacific Gas and Electric Company (the licensee) submitted alternative request ISI-RXHDVS that provides an alternative from volumetric/surface examination frequency requirements of American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code) Case N-729-4, "Alternative Examination Requirements for PWR [Pressurized-Water Reactor] Reactor Vessel Upper Heads With Nozzles Having Pressure-Retaining Partial-Penetration WeldsSection XI, Division 1," at Diablo Canyon Nuclear Power Plant (DCPP), Units 1 and 2.

ASME Code Case N-729-4 requires volumetric or surface examinations of all primary water stress corrosion cracking-resistant reactor pressure vessel closure head (RVCH) nozzles every inservice inspection interval, and direct visual examinations of the upper head outer surface every three refueling outages or 5 years, whichever is less. The licensee proposed to increase the volumetric/surface examination interval from 10 years to approximately 14 years for DCPP, Unit 1 and approximately 16 years for DCPP, Unit 2. Specifically, pursuant to Title 10 of the Code of Federal Regulations ( 10 CFR), paragraph 50.55a(z)( 1), the licensee requested to use the proposed alternative on the basis that the alternative would provide an acceptable level of quality and safety.

The U.S. Nuclear Regulatory Commission (NRC) staff reviewed the licensee's request and determined that the alternative method proposed by the licensee provides an acceptable level of quality and safety for the examination frequency requirements of the RVCHs. Accordingly, the staff concludes that the licensee has adequately addressed all of the regulatory requirements set forth in 10 CFR 50.55a(z)( 1). Therefore, the staff authorizes the one-time use of the alternative, for the duration up to, and including, the end of the operating license on November 2, 2024, for DCPP, Unit 1 and the end of the operating license on August 26, 2025, for DCPP, Unit 2.

J. Welsch All other requirements of the ASME Code,Section XI, and 10 CFR 50.55a(g)(6)(ii)(D) for which relief was not specifically requested and approved herein by the NRC staff remain applicable, including the third-party review by the Authorized Nuclear lnservice Inspector.

If you have any questions concerning this matter, please contact the Project Manager, Mr. Balwant K. Singal at 301-415-3016 or via e-mail at Balwant.Singal@nrc.gov.

Sincerely, Robert J. Pascarelli, Chief Plant Licensing Branch IV Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket Nos. 50-275 and 50-323

Enclosure:

Safety Evaluation cc: Listserv

UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION PROPOSED ALTERNATIVE ISI-RXHDVS FROM VOLUMETRIC/SURFACE EXAMINATION FREQUENCY REQUIREMENTS OF ASME CODE CASE N-729-4 DIABLO CANYON NUCLEAR POWER PLANT, UNITS 1 AND 2 PACIFIC GAS AND ELECTRIC COMPANY DOCKET NOS. 50-275 AND 50-323

1.0 INTRODUCTION

By letter dated August 1, 2018 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML18213A375), Pacific Gas and Electric Company (PG&E, the licensee), submitted alternative request ISI-RXHDVS that provides an alternative from volumetric/surface examination frequency requirements of American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code) Case N-729-4, "Alternative Examination Requirements for PWR [Pressurized-Water Reactor] Reactor Vessel Upper Heads With Nozzles Having Pressure-Retaining Partial-Penetration WeldsSection XI, Division 1," at Diablo Canyon Nuclear Power Plant (DCPP), Units 1 and 2.

ASME Code Case N-729-4 requires volumetric or surface examinations of all primary water stress corrosion cracking (PWSCC)- resistant reactor pressure vessel closure head (RVCH) nozzles every inservice inspection (ISi) interval, and direct visual examinations of the upper head outer surface every three refueling outages or 5 years, whichever is less. The licensee proposed to increase the volumetric/surface examination interval from 1O years to approximately 14 years for DCPP, Unit 1 and approximately 16 years for DCPP, Unit 2.

Specifically, pursuant to Title 10 of the Code of Federal Regulations ( 10 CFR) paragraph 50.55a(z)(1 ), the licensee requested to use the proposed alternative on the basis that the alternative would provide an acceptable level of quality and safety.

2.0 REGULATORY EVALUATION

The proposed alternative, ISI-RXHDVS, was submitted under 10 CFR 50.55a(z)( 1), which covers requests for alternatives on the basis that the proposed alternative would provide an acceptable level of quality and safety.

The licensee has proposed an alternative to the requirements of ASME Code Case N-729-4 that requires the 10-year inspection interval for volumetric/surface examinations of PWSCC-resistant RVHC nozzles, including the associated partial-penetration welds.

ASME Code Case N-729-4 is incorporated by reference in 10 CFR 50.55a(a){1)(iii)(C). The regulation in 10 CFR 50.55a(g)(6)(ii)(D)(1 ), requires the holders of operating licenses or combined licenses for PWRs to meet the requirements defined in ASME Code Case N-729-4.

The regulation in 10 CFR 50.55a(z) states, in part, that alternatives to the requirements of 10 CFR 50.55a(b) through (h) may be used when authorized by the U.S. Nuclear Regulatory Commission (NRC), if: (1) the proposed alternative would provide an acceptable level of quality and safety; or (2) compliance with the specified requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.

Based on the above, and subject to the following technical evaluation, the NRC staff finds that regulatory authority exists for the licensee to request the use of an alternative and the NRC to authorize the proposed alternative.

3.0 TECHNICAL EVALUATION

3.1 ASME Code Components Affected

The affected components are the ASME Code Class 1 RVCH nozzles and partial penetration welds fabricated from PWSCC-resistant materials. Each unit's RVCH control rod drive mechanism (CROM) nozzles, thermocouple nozzles, vent nozzle, and reactor vessel level indication system (RVLIS) nozzle are fabricated from Alloy 690 material with Alloy 52/152 attachment welds.

3.2 ASME Code Requirement The regulation in 10 CFR 50.55a(g)(6)(ii)(D), "Augmented ISi requirements: Reactor vessel head inspections," requires the holders of operating licenses or combined licenses for PWRs to meet the requirements of ASME Code Case N-729-4.

Specifically, ASME Code Case N-729-4 requires that partial-penetration welded RVCH penetration nozzles made using PWSCC-resistant materials be examined using a volumetric or surface technique every inspection interval (nominally 10 calendar years). ASME Code Case N-729-4 also requires a direct visual examination of the outer surface of the head for evidence of leakage every third refueling outage or 5 calendar years, whichever is less.

3.3 ASME Codes of Record The ASME Code of record for the current fourth 10-year ISi interval isSection XI, 2007 Edition with the 2008 Addenda, which began on May 7, 2015, for DCPP, Unit 1 and March 13, 2016, for DCPP, Unit 2.

3.4 Proposed Alternative Pursuant to 10 CFR 50.55a(z)(1 ), PG&E requests an alternative from performing the required volumetric/surface examinations for the DCPP, Units 1 and 2 RVCH components identified above on a nominal inspection frequency of 10 years as prescribed in Table 1, Item B4.40, of ASME Code Section XI, Code Case N-729-4. Specifically, PG&E requests to extend the inspection interval of the volumetric/surface examination of the RVCH components for three fuel cycles for DCPP, Unit 1 and four fuel cycles for DCPP, Unit 2 beyond the (nominal) 10 calendar years.

For DCPP, Unit 1, this proposed alternative would extend the volumetric/surface examination through the end of the operating license on November 2, 2024. This will increase the time to the inspection from the required 1O years to approximately 14.0 calendar years.

For DCPP, Unit 2, this proposed alternative would extend the volumetric/surface examination through the end of the operating license on August 26, 2025. At that point, the DCPP, Unit 2 RVCH will have been in service for approximately 15.8 calendar years.

The visual examinations and acceptance criteria, as required by Item B4.30 of Table 1 of ASME Code Case N-729-4, are not affected by this request and will continue to be performed on a frequency of every third refueling outage or 5 calendar years, whichever is less.

3.5 Basis for Use The RVCHs were replaced with new RVCHs using Alloy 690/52/152 materials during the refueling outages that ended in November 2010 for DCPP, Unit 1 and November 2009 for DCPP, Unit 2. In accordance with Table 1 of ASME Code Case N-729-4, Item B4.40, as conditioned by 10 CFR 50.55a(g)(6)(ii)(D), the licensee is currently required to perform a volumetric/surface examination of essentially 100 percent of the required volume or equivalent surfaces of the nozzle tubes every inspection interval.

The evaluation performed in Material Reliability Program (MRP)-375 1 considers a simple factor of improvement (FOi) approach applied in a conservative manner to model the increased resistance of Alloys 690/52/152 compared to Alloys 600 and 182 at equivalent temperature and stress conditions. These FOls were estimated for the material improvements of Alloy 690/52/152 materials using an extensive database of test data. Results for both crack initiation and crack growth conclude that there is a substantially improved resistance to PWSCC for Alloy 690 base material and Alloy 52/152 weld materials. A FOi of 20 bounds, most of the data plotted, and a FOi of 10 bounds all of the crack growth rate data. As discussed in MRP-375, laboratory and plant data demonstrate a FOi in excess of 13 in terms of the time to PWSCC initiation. Conservatively, credit was not taken for the improved resistance of Alloys 690/52/152 to PWSCC initiation in the MRP-375 analyses.

Using the FOi method, including a temperature adjustment factor, the required FOi for DCPP, Unit 1 would be 4.72. Similarly, the implied FOi for DCPP, Unit 2 would be 1.65. Consequently,

4. 72 is considered as the bounding FOi for both units.

Additionally, the operating experience to date for Alloys 690/52/152 has shown resistance to PWSCC. This experience includes steam generators, pressurizers, and RVCHs. Operating experience, with Alloys 690/52/152, includes inservice volumetric/surface examinations on replacement RVCHs currently operating in the United States.

3.6 Duration of Proposed Alternative For DCPP, Unit 1, the proposed alternative would extend the volumetric/surface examination through the end of the operating license on November 2, 2024. For DCPP, Unit 2, the proposed 1 "Material Reliability Program: Technical Basis for Reexamination Interval Extension for Alloy 690 PWR Reactor Vessel Top Head Penetration Nozzles (MRP-375)," dated February 2014 (ADAMS Accession No. ML14283A046).

alternative would extend the volumetric/surface examination through the end of the operating license on August 26, 2025.

3. 7 NRC Staff Evaluation The licensee is proposing to extend the volumetric/surface examinations of the PWSCC-resistant CROM nozzles at DCPP, Units 1 and 2, to the Cycle 22 and Cycle 21 refueling outages, respectively. ASME Code Case N-729-4 requires the inspections to be performed every 10 years and this relief request extends this interval to a total of 14.0 and 15.8 years, for DCPP, Units 1 and 2, respectively.

The inspection frequencies specified in ASME Code Case N-729-4 for RVCH penetration nozzles using Alloy 600/182/82 were developed based, in part, on the materials' crack growth rate equations documented in the following reports: (a) MRP-552 and (b) MRP-115 3

• As its primary technical basis for the proposed alternative, the licensee presented crack growth rate data for the more crack-resistant materials, Alloy 690/152/52 to demonstrate a sufficient FOi of these materials versus the older Alloy 600/82/182 materials. This FOi would then provide the basis for the extension of the inspection interval as requested by the licensee in its proposed alternative.

The NRC staffs review relied upon Alloy 690/152/52 crack growth rate data from two NRC contractors: Pacific Northwest National Laboratory (PNNL) and Argonne National Laboratory (ANL). This data is documented in an internal publicly available NRC staff memorandum, "Transmittal of Preliminary Water Stress Corrosion Cracking Data for Alloys 690, 52, and 152," dated October 30, 2014 (ADAMS Accession No. ML14322A587). The NRC's confirmatory research generally supports the assertion that the crack growth rate of Alloy 690/52/152 is more crack-resistant, but differs from the MRP-375 data in some respects.

The licensee calculated that an extension of the intervals to 15.8 years would require an FOi of 6.6. The NRC staff concludes that the licensee's calculated FOi of 4. 72, to support an extension of the ASME Code Case N-729-4 inspection frequencies, is acceptable by the NRC staff's independent calculation. The staff also concludes that the application of an FOi of 4. 72 to the 75th percentile curves in MRP-55 and MRP-115 bounded essentially all of the NRC data included in the PNNL and ANL data summary report. Therefore, the NRC staff concludes that this analysis supports the assertion that volumetric inspection intervals for DCPP, Units 1 and 2 RVCHs of not more than 15.8 years does not pose a higher risk than that associated with an Alloy 600/182/82 RVCH inspected at intervals of 2.25 reinspection years.

The NRC staff concludes that the past bare metal visual examinations on the RVCHs is a reasonable means to demonstrate the absence of leakage through the nozzle/J-groove weld prior to the time the examination was conducted. The NRC staff also concludes that performance of future bare metal visual examinations in accordance with the code case is adequate to demonstrate the absence of leakage at or prior to the time the examinations are conducted. Finally, the NRC staff concludes that the proposed alternative's frequency for bare 2

"Materials Reliability Program (MRP) Crack Growth Rates for Evaluating Primary Water Stress Corrosion Cracking (PWSCC) of Thick-Wall Alloy 600 Material (MRP-55)," dated July 2002 (ADAMS Accession No. ML023010510).

3 "Materials Reliability Program Crack Growth Rates for Evaluating Primary Water Stress Corrosion Cracking (PWSCC) of Alloy 82, 182, and 132 Welds (MRP-115NP)," dated November 2004 (ADAMS Accession No. ML051450555).

metal visual examinations in conjunction with the new frequency of volumetric examinations is sufficient to provide reasonable assurance of the structural integrity of the RVCH.

Based on the above, the NRC staff concludes that the licensee's technical basis for the proposed alternative is acceptable, and that the proposed alternative provides an acceptable level of quality and safety as required by 10 CFR 50.55a(z)( 1).

4.0 CONCLUSION

As set forth above, the NRC staff has determined that the alternative method proposed by the licensee in the proposed relief request provides an acceptable level of quality and safety for the examination frequency requirements of the RVCHs. Accordingly, the NRC staff concludes that the licensee has adequately addressed all of the regulatory requirements set forth in 10 CFR 50.55a(z)( 1). Therefore, the staff authorizes the one-time use of alternative request, ISI-RXHDVS, at DCPP, Units 1 and Unit 2 for the duration up to, and including, the end of the operating license on November 2, 2024, for DCPP, Unit 1 and the end of the operating license on August 26, 2025, for DCPP, Unit 2.

All other requirements of the ASME Code,Section XI, and 10 CFR 50.55a(g)(6)(ii)(D) for which relief was not specifically requested and approved herein by the NRC staff remain applicable, including the third-party review by the Authorized Nuclear lnservice Inspector.

Principal Contributor: S. Cumblidge Da~: January 25, 2019

SUBJECT:

DIABLO CANYON NUCLEAR POWER PLANT, UNITS 1 AND 2 - REQUEST FOR ALTERNATIVE FROM VOLUMETRIC/SURFACE EXAMINATION FREQUENCY REQUIREMENTS OF ASME CODE CASE N-729-4 (EPID L-2018-LLR-0107) DATED JANUARY 25, 2019 DISTRIBUTION:

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. ema1*1 dae t d January 17 2019 OFFICE NRR/D0RL/LPL4/PM NRR/D0RL/LPL4/LA NRR/DMLR/MPHB/BC* NRR/D0RL/LPL4/BC NAME BSingal PBlechman SRuffin RPascarelli DATE 1/24/2019 1/24/2019 1/17/19 1/25/2019 OFFICIAL RECORD COPY