ML20249C850

Proposed Alternative NDE-RCS-SE-2R22 to the Requirements of the ASME Code

ML20249C850
Person / Time
Site:	Diablo Canyon
Issue date:	10/22/2020
From:	Jennifer Dixon-Herrity NRC/NRR/DORL/LPL4
To:	Welsch J Pacific Gas & Electric Co
Lee S, NRR/DORL/LPL1, 415-3168
References
EPID L-2019-LLR-0101
Download: ML20249C850 (11)

Text

October 22, 2020 Mr. James M. Welsch Senior Vice President, Generation and Chief Nuclear Officer Pacific Gas and Electric Company Diablo Canyon Power Plant P.O. Box 56, Mail Code 104/6 Avila Beach, CA 93424

SUBJECT:

DIABLO CANYON NUCLEAR POWER PLANT, UNIT 2 - PROPOSED ALTERNATIVE NDE-RCS-SE-2R22 TO THE REQUIREMENTS OF THE ASME CODE (EPID L-2019-LLR-0101)

Dear Mr. Welsch:

By letter dated October 31, 2019, Pacific Gas and Electric Company (PG&E, the licensee) submitted a request to the U.S. Nuclear Regulatory Commission (NRC) for the use of an alternative to certain American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPV Code),Section XI requirements at the Diablo Canyon Nuclear Power Plant, (Diablo Canyon), Unit 2.

Specifically, pursuant to Title 10 of the Code of Federal Regulations (10 CFR) 50.55a(z)(1), the licensee requested to use proposed alternative NDE-RCS-SE-2R22 on the basis that the alternative provides an acceptable level of quality and safety.

The NRC staff has reviewed the subject request and concludes, as set forth in the enclosed safety evaluation, that PG&E has adequately addressed all of the regulatory requirements set forth in 10 CFR 50.55a(z)(1).

All other ASME BPV Code,Section XI requirements for which relief was not specifically requested and approved remain applicable, including third-party review by the Authorized Nuclear Inservice Inspector.

J. Welsch If you have any questions, please contact the Diablo Canyon project manager, Samson Lee, at 301-415-3168 or Samson.Lee@nrc.gov.

Sincerely,

/RA/

Jennifer L. Dixon-Herrity, Branch Chief Plant Licensing Branch IV Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-323

Enclosure:

Safety Evaluation cc: Listserv

SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION PROPOSED ALTERNATIVE REQUEST NDE-RCS-SE-2R22 PACIFIC GAS AND ELECTRIC COMPANY DIABLO CANYON NUCLEAR POWER PLANT, UNIT 2 DOCKET NO. 50-323

1.0 INTRODUCTION

By letter dated October 31, 2019 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML19304B277), Pacific Gas and Electric (PG&E, the licensee),

submitted proposed alternative request NDE-RCSSE-2R22 for the fourth 10-year inservice inspection (ISI) interval at Diablo Canyon Nuclear Power Plant (Diablo Canyon), Unit 2. The licensees request describes alternative requirements to the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (Code),Section XI, Rules for Inservice Inspection of Nuclear Power Plant Components, Appendix VIII, Supplements 2 and 10, as modified by ASME Code Case N-695, Qualification Requirements for Dissimilar Metal Piping Welds, and ASME Code Case N-696, Qualification Requirements for Mandatory Appendix VIII Piping Examinations Conducted From the Inside Surface.

Specifically, pursuant to Title 10 of the Code of Federal Regulations (10 CFR) 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

Adherence to Section XI of the ASME Code is mandated by 10 CFR 50.55a(g)(4), which states in part, that ASME Code Class 1, 2, and 3 components will meet the requirements, except the design and access provisions and the preservice inspection requirements, set forth in the ASME Code,Section XI.

Paragraph (z) of 10 CFR 50.55a states, in part, that alternatives to the requirements of 10 CFR 50.55a(b)-(h) may be used, when authorized by the U.S. Nuclear Regulatory Commission (NRC or Commission) 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.

Pursuant to 10 CFR 50.55a(z)(1), the licensee is proposing an alternative to the requirements of the ASME Code,Section XI, Appendix VIII, Supplements 2 and 10, as modified by ASME Code Case N-695 and Code Case N-696. Based on the above, and subject to the following technical Enclosure

evaluation, the NRC staff finds that regulatory authority exists for the licensee to request, and for the Commission to authorize, the alternative requested by the licensee.

3.0 TECHNICAL EVALUATION

3.1 Proposed Alternative NDE-RCS-SE-2R22 ASME Code Components Affected The reactor vessel outlet (hot leg) nozzle-to-safe end and safe end-to-piping welds are ASME Code Class 1 and are listed in the table below.

Code Cat./Item No.* Description Weld Number Line ID/Nominal Wall N-770-2, A-2 Loop 1 outlet nozzle- WIB-RC-1-1 (SE) 29/2.5 to-safe end R-A, R-1.20 Loop1 outlet safe WIB-RC-1-2 29/2.5 end-to-pipe N-770-2, A-2 Loop 2 outlet nozzle- WIB-RC-2-1 (SE) 29/2.5 to-safe end R-A, R1.20 Loop 2 outlet safe WIB-RC-2-2 29/2.5 end-to-pipe N-770-2, A-2 Loop 3 outlet nozzle- WIB-RC-3-1(SE) 29/2.5 to-safe end R-A, R1.20 Loop 3 outlet safe WIB-RC-3-2 29/2.5 end-to pipe N-770-2, A-2 Loop 4 outlet nozzle- WIB-RC-4-1(SE) 29/2.5 to-safe end R-A, R1.20 Loop 4 outlet safe WIB-RC-4-2 29/2.5 end-to-pipe

• Safe end welds with SE suffix are dissimilar metal welds fabricated with Alloy 182 weld material and are examined in accordance with ASME Code Case N-770-2 Alternative Examination Requirements and Acceptance Standards for Class 1 PWR Piping and Vessel Nozzle Butt Welds Fabricated with UNS N06082 or UNS W86182 Weld Filler Material With or Without the Application of Listed Mitigation Activities,Section XI, Division 1.

Applicable Code Edition and Addenda

The proposed alternative applies to inspections performed during the fourth ISI interval, which commenced on March 13, 2016, and is scheduled to end on August 26, 2025, concurrently with the expiration of the operating license. The Code of record for the fourth Inservice Inspection Interval Program Plan (ISIPP) is based on the ASME Code,Section XI, 2007 Edition with 2008 Addenda.

Applicable Code Requirements The ISIPP is augmented with the requirements of ASME Code Case N-770-2, as modified by 10 CFR 50.55a for the examination of dissimilar metal vessel nozzle butt welds containing Alloy 82/182 material. These welds are identified as Category N-770-2, Item A-2, in the Diablo

Canyon Unit 2 ISIPP. These requirements apply to the Diablo Canyon Unit 2 dissimilar metal welds connecting the reactor nozzles to the reactor coolant system (RCS) piping.

Diablo Canyon risk-informed ISIPP examination Category R-A, Item R.120 (formally Code Category B-F, B5.10 in the 2007 Edition through 2008 Addenda), specifies volumetric examination for the RCS safe end-to-piping welds.

All specified volumetric ultrasonic testing (UT) scheduled to be conducted during the Unit 2 refueling outage 22 (2R22) is required to be performed per ASME Code Section XI, Appendix VIII, Supplement 2 (wrought austenitic welds) and Supplement 10 (dissimilar metal welds).

The Diablo Canyon ISIPP references ASME Code Cases N-695 and N-696, which are unconditionally approved for use in NRC Regulatory Guide (RG) 1.147, Revision 18, Inservice Inspection Code Case Acceptability, ASME Section XI, Division 1 (ADAMS Accession No. Ml16321A336). Updated revisions of ASME Code Cases N-695-1 and N-696-1 are listed as conditionally acceptable in Draft RG DG-1342 (ADAMS Accession No. ML18114A225), the proposed Revision 19 of RG 1.147.

Reason for Request

The licensee stated that ASME Code Cases N-695 and N-696 provide alternatives to ASME Code Section XI, Appendix VIII, Supplements 2 and 10, and include criteria for depth sizing accuracy. Key statements for Code Cases N-695 and N-696 are summarized below:

Code Case N-695 paragraph 3.3(c) states: are qualified for depth sizing when the root mean square error (RMSE) of the flaw depth measurements, as compared to the true flaw depths, do not exceed 0.125 in (3mm).

Code Case N-696 paragraph 3.3(d) states: qualified for depth sizing when the flaw depths estimated by UT, as compared with the true depths, do not exceed 0.125 in.

(3mm) root mean square error (RMSE), when they are combined with a successful Supplement 10 qualification.

Code Cases N-695-1 and N-696-1 revise the qualification requirement for depth sizing to 0.250-inch RMSE. Additionally, Code Case N-695 contains an exclusion in scope section that states: This Case is not applicable to piping welds containing supplemental corrosion resistant clad (CRC) applied to mitigate intergranular stress corrosion cracking. This exclusion is carried through N-695-1.

The licensee stated that the requirements for the 0.125-inch RMSE depth sizing accuracy criterial of Code Cases N-695 and N-696 and the Code Case N-695 exclusion for examinations performed through the CRC are impractical for Diablo Canyon Unit 2 to comply with.

Root Mean Square Error The licensee stated that although examination vendors have qualified for detection and length sizing in accordance with the requirements for examinations from the inside diameter (ID), the vendors have not met the established RMSE of 0.125 inch for indication depth sizing. The result indicates that the Code accuracy standard is impractical for use with the UT technology employed in the qualification efforts. ASME Code Cases N-695-1 and N-696-1 were created to resolve this issue by establishing 0.250-inch RMSE as the required standard for depth measurement. The licensee stated that the Diablo Canyon examination contractor is able to meet the 0.250-inch RMSE standard for depth measurement.

In addition, the licensee stated the outside surface configuration of the Diablo Canyon Unit 2 safe end welds are not suitable for examination. Conditioning of the outside surface would require extensive effort and result in extensive personnel exposure.

Examination Through Corrosion-Resistant Clad The licensee stated the Diablo Canyon Unit 2 reactor vessel was fabricated in the 1970 timeframe, prior to implementation of ASME Code Section XI, Appendix VIII qualification requirements. The distinctive Diablo Canyon Unit 2 RCS clad safe end weld configuration is not encompassed by the industrys performance demonstration initiative (PDI) program used to implement ASME Section XI, Appendix VIII requirements. Consequently, no ISI vendor is qualified to examine the Diablo Canyon configuration.

The licensee stated that the Diablo Canyon Unit 2 safe end welds and safe end forgings have a thin (nominal t of 0.090 inch; 0.073 inch to 0.125 inch in thickness) protective clad layer applied to the ID and outside diameter (OD) of the dissimilar metal weld and the safe end forgings. During the Diablo Canyon Unit 2 reactor vessel fabrication process, the stainless steel safe end forgings were welded to the low alloy steel reactor nozzle forgings and heat treated with entire vessel. This method resulted in the stainless steel safe end forging becoming furnace sensitized. The protective clad layer is intended to act as a barrier to isolate the sensitized safe end dissimilar metal weld and stainless steel safe end forging from the surrounding environment.

The licensee stated that removing the protective clad from either the ID or OD of the RCS safe ends in order to create a configuration bounded by the PDI sample sets would result in extensive personnel exposure and potentially reduce the overall structural integrity of the component. The licensee stated that OD machining to remove the overlay and achieve the required surface finish could exceed 20-man hours per nozzle; the total personnel exposure for four nozzles would reach 12-16 rem. In addition, ID machining of these locations would remove the protective layer and any benefit that the protective layer might afford to the underlying materials by isolating them from the surrounding environment.

The licensee stated that the Diablo Canyon Unit 2 RCS safe end configuration with the protective ID and OD clad layers applied to the dissimilar metal weld and safe end forgings is unique to a small number of Westinghouse-designed units. In addition, suitable blind test samples are not available to support ASME Code Section XI, Appendix VIII, Supplement 2 and 10 qualifications for the Diablo Canyon Unit 2 configuration.

Proposed Alternative and Basis for Use Root Mean Square Error PG&E proposes to use a vendor qualified for ID detection and length sizing per ASME Code Section XI, Appendix VIII as applicable to the welds similar in configuration and materials (i.e.,

without CRC) to the welds included in this request.

The examination vendor contracted to perform the safe end examination at Diablo Canyon has demonstrated the ability to depth size indications in dissimilar metal welds with an RMSE of 0.189 inch instead of the 0.125-inch RMSE required by ASME Code Section XI, Appendix VIII, Supplement 10, and Code Case N-695. Additionally, the vendor demonstrated the ability to

depth size with a 0.245-inch RMSE when applying combined aspects of ASME Code Section XI, Appendix VIII, Supplements 2 and 10, per Code Case N-696. Both of these qualifications are within the prescribed RMSE of 0.250 inch established in Code Cases N-695-1 and N-696-1.

The licensee stated that if a reportable flaw is detected and determined to be ID surface connected during the examination of the welds in accordance with this relief request, the licensee will provide a flaw evaluation for review and approval, including the measured flaw size as determined by UT examination. Eddy current testing will be used to determine if flaws are surface connected. Additional data, including details of the surrounding ID surface contour in the region of flaw and percentage of the exam area where UT probe liftoff is evident, if any, will be included.

If any flaws requiring depth sizing are detected during the examination of the welds in accordance with this relief request, the following criteria shall be implemented:

Flaws detected and measured as less than 50 percent through-wall in depth shall be measured per Code Cases N-695-1 and N696-1.

Flaws detected and measured as 50 percent through-wall depth or greater and to remain in service with mitigation or repair will be classified as indeterminate depth as indicated in Draft RG DG-1342. The licensee shall submit flaw evaluations for review and approval prior to reactor startup. The flaw evaluation shall include:

o Information concerning the mechanism that caused the flaw o Information concerning the inside surface roughness/profile of the region surrounding the flaw o Information concerning areas where UT probe liftoff is observed The licensee stated that all welds included in this request have been previously examined from the ID with an ASME Code Section XI, Appendix VIII detection process qualified on similar configurations and materials (i.e., without CRC) in the thirteenth, sixteenth, and nineteenth refueling outages. The UT examinations were supplemented by surface profilometry and eddy current testing. Greater than 90 percent coverage of the required exam areas was achieved in all cases. This inspection history confirms that the inside surface profiles of the welds included in this request are suitable for UT examination from the ID in accordance with the referenced requirements as modified by the proposed alternative sizing requirements.

Examination Through Corrosion Resistant Clad The licensee is proposing to use vendor procedures, personnel, and equipment qualified in accordance with the PDI implementation of ASME Code Section XI, Appendix VIII, Supplements 2 and 10, as modified by the requirements of Code Cases N-695-1 and N-696-1, to examine the nozzle-to-safe end dissimilar metal and safe end-to-piping stainless steel welds from the ID through the protective clad layer.

The licensees inspection vendor has performed additional demonstration activities in order to validate the ability to detect flaws, length-size the flaws, and depth-size the flaws through a clad layer and ID weld inlays. Although not identical, the open test samples clad and weld layer thicknesses conservatively encompass the Diablo Canyon dissimilar metal weld configuration.

The test samples include flaws of various depths and lengths oriented in both the axial and circumferential directions in the weld and heat-affected zones for the dissimilar metal weld (which is representative of the nozzle to safe end welds).

When examining the test specimens, the vendor used a PDI qualified (detection and length sizing) ID examination procedure; the current revision of that same procedure will be employed in the 2R22 examinations. The vendor demonstrated the capability to accurately detect, length, and depth-size the test sample flaws through the CRC in each of the samples and configurations examined. The results of these activities verify that the proposed examination technique is appropriate for application to the Diablo Canyon Unit 2 outlet nozzle-to-safe end welds and safe end-to-piping welds, since the vendor procedure is the same for both weld types.

The licensee stated that the Diablo Canyon ISI vendor has created a technical justification, WDI-TJ-1044, Revision 1, Demonstration Report/Technical Basis Document: Ultrasonic Examination of Diablo Canyon Unit 2 Reactor Pressure Vessel Nozzle to Safe End Welds from the ID Surface Through A Welded Protective Layer, to document the process and results of the additional demonstration activities (ADAMS Accession No. ML102350297).

Duration of Proposed Alternative The licensee is requesting approval of this proposed alternative for the safe end weld examinations for the remainder of the Diablo Canyon Unit 2 fourth ISI interval, which commenced on March 13, 2016, and is scheduled to end on August 26, 2025, concurrently with the expiration of the operating license.

3.2 NRC Staff Evaluation The NRC staff has evaluated proposed alternative request NDE-RCS-SE-2R16 pursuant to 10 CFR 50.55a(z)(1) to determine if the proposed alternative provides an acceptable level of quality and safety. As described above, the licensee is proposing an alternative to the requirements of the ASME Code,Section XI, Appendix VIII, Supplements 2 and 10 as modified by ASME Code Case N-695 and Code Case N-696. These Code Cases require that procedures used to inspect welds from the ID be qualified by performance demonstration. The acceptance criterion established by the Code Cases is an RMSE of not greater than 0.125 inches. To date, examination vendors have qualified for detection and length sizing in accordance with the requirements for examinations from the ID, but the vendors have not met the established RMSE of 0.125 inch for indication depth sizing. The licensee is proposing to use Code Case N-695-1 to satisfy the requirements of ASME Section XI, Appendix VIII, Supplement 10, and Code Case N-696-1 to satisfy the requirements of ASME Section XI, Appendix VIII, Supplement 2. ASME Code Cases N-695-1 and N-696-1 were created to resolve this issue by establishing 0.250-inch RMSE as the required standard for depth measurement.

Use of the revised Code Cases N-695-1 and N-696-1, are listed as Conditionally Acceptable in proposed NRC Draft RG DG-1342, the proposed Revision 19 of RG 1.147, would establish 0.250 inch RMSE as the qualification standard and is met by the examination vendor for existing ASME Code Section XI, Appendix VIII qualification specimens without corrosion resistant cladding. The Diablo Canyon examination contractor is able to meet the 0.250-inch RMSE standard for depth measurement.

The licensee stated that if a reportable flaw is detected and determined to be ID surface connected during the examination of the welds in accordance with this relief request, the licensee will provide a flaw evaluation for review and approval, including the measured flaw size as determined by UT examination. Eddy current (EC) testing will be used to determine if flaws are surface connected. Additional data including details of the surrounding ID surface contour in the region of flaw and percentage of the exam area where UT probe lift-off is evident, if any,

will be included. The staff finds this approach to be acceptable because it will determine if the flaw can stay in service without any repair being performed.

If any flaws requiring depth sizing are detected during the examination of the welds in accordance with this relief request, the following criterial shall be implemented:

Flaws detected and measured as less than 50 percent through-wall in depth shall be measured per Code Cases N-695-1 and N-696-1.

Flaws detected and measured as 50 percent through-wall depth or greater and to remain in-service with mitigation or repair will be classified as indeterminate depth as indicated in Draft RG DG-1342. The licensee shall submit flaw evaluations for review and approval prior to reactor startup. The flaw evaluation shall include:

o Information concerning the mechanism that caused the flaw o Information concerning the inside surface roughness/profile of the region surrounding the flaw o Information concerning areas where UT probe lift-off is observed As stated by the licensee, all welds included in this request have been previously examined from the ID with a qualified ASME Code Section XI, Appendix VIII detection process in the thirteenth, sixteenth, and nineteenth refueling outages. Profilometry and EC testing were also performed and greater than 90 percent coverage of the required exam was achieved. This inspection confirms that the inside surface profiles of the welds included in this request are suitable for UT examination from the ID as proposed in the relief request.

Using Code Case N-695-1 and N-696-1 for the examination through CRC, the licensee proposes to use vendor procedures, personnel and equipment qualified in accordance with the PDI implementation of ASME Code Section XI, Appendix VIII, Supplements 10 and 2 as modified by the requirements of Code Cases N-695-1 and 696-1 to examine the nozzle-to-safe end dissimilar metal and safe end-to-piping stainless steel welds for the ID through the protective clad layer.

The licensees inspection vendor has performed additional demonstration activities in order to validate the ability to detect flaws, length-size the flaws, and depth-size the flaws through a clad layer and ID weld inlays. The open test samples clad and weld layer thicknesses conservatively and encompass the Diablo Canyon dissimilar metal weld configuration. The test samples include flaws of various depths and lengths oriented in both the axial and circumferential directions in the weld and heat-affected zones for the dissimilar metal weld, which is representative of the nozzle to safe end welds.

As stated above, the DCPP ISI vendor created a technical justification, WDI-TJ-1044 Revision 1, Demonstration Report/Technical Basis Document: Ultrasonic Examination of Diablo Canyon Unit 2 Reactor Pressure Vessel Nozzle-to-Safe End Welds for the ID Surface Through a Welded Protective Layer to document the process and results of the additional demonstration activities.

The licensee also provided information that in the event the proposed alternate examination process grossly mischaracterizes a significant planar flaw, the potential resulting failure of one of the RCS loop outlet welds could result in a loss-of-coolant accident. Depending on the size of the postulated break, the specific consequences would vary. At the smallest end of the break size spectrum, the charging system would be capable of maintaining RCS pressure through normal makeup. Larger break sizes would result in depressurization of the RCS, reactor trip,

and a safety injection. The worst-case consequence would occur if one of the nozzle-to-pipe welds were to suffer 360° through-wall circumferential fracture. In this case, the break size is bounded by the line ID, which is less than the break size used in the large break loss-of-coolant accident design-basis analysis.

ASME has determined that for flaw depths less than 50 percent wall thickness, reasonable assurance that a flaw will be appropriately depth-sized may be obtained using a procedure qualified to 0.250-inch RSME. This standard, incorporated into Code Cases N-695-1 and N-696-1, is included in the NRC Draft RG DG-1342, the proposed Revision 19 of RG 1.147.

The proposed alternative for correcting measured flaw depths for flaws less than 50 percent wall thickness and performing and submitting a flaw specific analysis for flaws equal to or greater than 50 percent wall thickness is unlikely to result in gross flaw mischaracterization and any potential for resulting component failure.

4.0 CONCLUSION

As set forth above, the NRC staff concludes that the licensees proposed alternative to use 0.250 inch RMSE through a protective clad layer per Code Cases N-695-1 and N-696-1 provides an acceptable level of quality and safety in accordance with 10 CFR 50.55a(z)(1).

Therefore, the NRC staff authorizes the use of proposed alternative NDE-RCS-SE-2R22 for the remainder of the Diablo Canyon Unit 2 fourth ISI interval. The NRC staff notes that the approval of proposed alternative NDE-RCS-SE-2R22 does not imply or infer the NRC approval of ASME Code Cases N-695-1 and N-696-1 for generic use.

All other requirements of the ASME Code for which relief was not specifically requested remain applicable, including third party review by the Authorized Nuclear Inservice Inspector.

Principal Contributor: E. Reichelt Date: October 22, 2020

ML20249C850 *via e-mail OFFICE NRR/DORL/LPL4/PM NRR/DORL/LPL4/LA* NRR/DNRL/NPHP/BC* NRR/DORL/LPL4/BC*

NAME SLee PBlechman (LRonewicz for) MMitchell JDixon-Herrity DATE 9/08/2020 9/05/2020 9/01/2020 10/22/2020