ML17324B344

Response to NRC Request for Additional Information Regarding Request for Approval of Alternative for Application of Full Structural Weld Overlay, REP-RHRSWOL, Units 1 and 2

ML17324B344
Person / Time
Site:	Diablo Canyon
Issue date:	11/20/2017
From:	Welsch J Pacific Gas & Electric Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
DCL-17 -095
Download: ML17324B344 (13)

Text

Pacific Gas and Electric Companyf9 James M. Welsch Diablo Canyon Power Plant Vice President P.O. Box 56 Nuclear Generation and Avila Beach, CA 93424 Chief Nuclear Officer 805.545.3242 E-Mail: JMW1 @pge.com November 20, 2017 PG&E Letter DCL-17 -095 U.S. Nuclear Regulatory Commission 10 CFR 50.55a ATTN: Document Control Desk Washington, DC 20555-0001 Diablo Canyon Units 1 and 2 Docket No. 50-275, OL-DPR-80 Docket No. 50-323, OL-DPR-82 Response to NRC Request for Additional Information Regarding "Request for Approval of Alternative for Application of Full Structural Weld Overlay, REP-RHR-SWOL, Units 1 and 2"

References:

1. PG&E Letter DCL-17-083, "Request for Approval of Alternative for Application of Full Structural Weld Overlay, REP-RHR-SWOL, Units 1 and 2," dated September 26, 2017

2. E-mail from NRC Project Manager Balwant K. Singal, "Request for Additional Information - Request for Approval for Application of Full Weld Overlay REP-RHR-SWOL, Diablo Canyon Power Plant, Units 1 and 2 (EPID No. L-2017-LLR-0092)," dated November 2, 2017

Dear Commissioners and Staff:

In Reference 1, Pacific Gas and Electric Company (PG&E) submitted a request for approval of alternative for application of a full structural weld overlay for the Diablo Canyon Power Plant Units 1 and 2. In Reference 2, the NRC Staff provided a request for additional information (RAI) via an e-mail, dated November 2, 2017. The Enclosure to this letter provides PG&E responses to the RAI.

There are no new or revised regulatory commitments (as defined by NEI 99-04) in this letter. If you have any questions or require additional information, please contact Mr. Hossein Hamzehee at (805) 545-4720.

A member of the STARS Alliance Callaway

• Diablo Canyon

• Palo Verde*

• Wolf Creek

Document Control Desk PG&E Letter DC L-17-095 November 20, 2017 Page 2 rntU4231/50915941-21 Enclosure cc: Diablo Distribution cc/enc: Kriss M. Kennedy, NRC Region IV Administrator Christopher W. Newport, NRC Senior Resident Inspector Gonzalo L. Perez, Branch Chief, California Department of Public Health Balwant K. Singal, NRC Project Manager State of California, Pressure Vessel Unit A member of the STARS Alliance Callaway

• Diablo Canyon

• Palo Verde*

• Wolf Creek

Enclosure PG&E Letter DCL-17 -095 PG&E Response to NRC Request for Additional Information Regarding "Request for Approval of Alternative for Application of Full Structural Weld Overlay, REP-RHR-SWOL, Units 1 and 2"

References:

1. PG&E Letter DCL-17-049, "Flaw Evaluation of Unit 2 Residual Heat Removal Suction Weld Joint," dated May 18, 2017

2. PG&E Letter DCL-17 -070, "Flaw Evaluation of Unit 1 Residual Heat Removal Suction Weld Joint," dated August 17, 2017

3. PG&E Letter DCL-17 -083, "Request for Approval of Alternative for Application of Full Structural Weld Overlay, REP-RHR-SWOL, Units 1 and 2," dated September 26, 2017 RAI#1:

The cover letter states, in part, ((Temperature monitoring of the RHR lines for both Units 1 and 2 indicated that thermal stratification and temperature cycling are present at both of these weld locations. "

Given the existence of the thermal cycles and thermal stratification, discuss whether the guidance in Electric Power Research Institute (EPRI), topical report MRP-146 ((Management of Thermal Fatigue in Normally Stagnant Non-/so/able Reactor Coolant System Branch Lines," (Proprietary Document, Non-Publical/y Available) is applicable to the subject RHR piping. If yes, please describe the actions that will be taken. If no, please provide justification.

PG&E Response:

EPRI Report Material Reliability Program (MRP)-146 is applicable to the subject residual heat removal (RHR) piping . EPRI Report MRP-146, Revision 0 was issued in 2005. In 2007, Pacific Gas and Electric Company (PG&E) implemented the requirements of MRP-146 Revision 0 under NEI 03-08 protocol, "Guideline for the Management of Materials Issues," by performing an assessment of non-isolable normally stagnant branch lines in the reactor coolant system. The subject RHR piping was included in this assessment. The subject RHR piping was screened out for thermal fatigue using MRP-170 software (EPRI Thermal Fatigue Evaluation per MRP-146, Version 1.0), which applied MRP-146 screening criteria. An MRP-146 supplemental examination of the subject RHR piping was also not required because the subject RHR piping was screened out. EPRI Report MRP-146 was revised in June 2011 (Revision 1) and September 2016 (Revision 2), and the applicable screening criteria for the subject RHR piping has not been revised in these revisions.

The operating experience (OE) from the Unit 2 nineteenth refueling outage and OE from the Unit 1 twentieth refueling outage were reported to EPRI MRP and industry members via conference calls in June 2016 and May 2017, respectively following the MRP-146 protocols. The OE is being evaluated by EPRI MRP

Enclosure PG&E Letter DCL-17 -095 industry members for industry guidance and actions. PG&E will implement any industry guidance and actions from EPRI, per NEI 03-08 and MRP-146 protocols.

RAI#2:

Page 7, Item (c)(1), states that inside diameter weld repairs shall be assumed in the weld residual analyses to conservatively bound any actual weld repairs that may have occurred during original construction. Item (c)(1) also stated that no repairs were performed on the inside diameter of the subject welds during construction. Please clarify whether the weld residual stress analyses includes an inside diameter repair.

PG&E Response:

PG&E confirms that the weld residual stress analysis does include a postulated inside diameter (I D) weld repair as recommended by the USNRC Safety Evaluation Report (SER) [ML101660468] for EPRI Report MRP-169, Revision 1A, "Technical Basis for Preemptive Weld Overlays for Alloy 82/182 Butt Welds in PWRs." This postulated ID weld repair follows the guidance of the NRC SER for MRP-169, Revision 1A, Section 3.2.2 of the SER, paragraph 3, which states,

{(The residual stress analysis assumes a highly unfavorable, pre-overlay residual stress condition which would result from an inside diameter surface weld repair during construction."

RAI#3:

In its request the licensee cites DCPP, Units 1 and 2, RHR suction weld joint flaw evaluation reports (References 71 and 82) in support of its evaluation. The NRC staff is currently reviewing these reports as separate licensing actions. The NRC staff believes that the referenced reports provide basis for operation for one cycle as opposed to the time period currently proposed to the end of plant life in the relief request. The referenced flaw evaluations provide an effective period of 35 months (without the weld overlay) whereas the proposed weld overlay is designed to the end of plant life. Please describe how the weld overlay can support the longer period of operation proposed in the submittal as compared to the effective period in the flaw evaluations (e.g., weld overlay will reduce the flaw growth).

PG&E Response:

1 PG&E letter DCL-17-049, "Flaw Evaluation of Unit 2 Residual Heat Removal Suction Weld Joint," dated May 18, 2017 (ADAMS Accession No. ML171388138) 2 PG&E letter DCL-17-070, "Flaw Evaluation of Unit 1 Residual Heat Removal Suction Weld Joint," dated August 17, 2017 (ADAMS Accession No. ML172298617) 2

Enclosure PG&E Letter DCL-17 -095 In References 1 and 2, PG&E evaluated the respective as-found conditions for a minimum of one additional cycle of operation. However, in those evaluations, PG&E did not presume that full structural weld overlays (SWOLs) had been installed.

The longer period of operation proposed in Reference 3 will be supported by the imposition of the compressive weld residual stresses resulting from the SWOL installation. The longer time period proposed in Reference 3 commences with the successful installation of the SWOLs.

The proposed SWOLs are currently being designed to completely replace the structural element of the piping at the subject location. In addition, the SWOLs will impart compressive residual stresses in the remaining ligament of the subject piping, such that the flaw growth will be halted or curtailed significantly, and the integrity of the piping will be maintained. The design and flaw growth evaluations (which assume that the SWOLs are installed) are in progress. The requirements for design and flaw growth evaluations that are specified in Code Case N-7 40-2 will be met.

RAI#4:

Page 5, third paragraph, of the relief request states, in part, "The provisions of ASME Code Case N-740-2 3, Appendix I {Temper Bead) are not required."

Please discuss the welding technique that will be used to install the overlay and cite the relevant ASME Code Sections and articles that the welding will follow.

PG&E Response:

A Welding Procedure Specification (WPS), qualified for groove welding in accordance with ASME Section Ill, Article NB-4000, using ERNiCrFe-7A with a minimum Cr content of 28 percent, will be used per Section 1.2 of Code Case N-740-2. This WPS will also be qualified for deposition of the ER308L, ER309L, or ER316L filler metal to be used for the buffer layer.

PG&E's vendor will use machine gas tungsten arc welding (GTAW) for installation of the overlay. The weld processes to be used for any repairs of the existing pipe base metal surface or the SWOL during installation have not been proposed by the vendor at this time. If the vendor desires to use additional welding processes (such as manual GTAW or shielded metal arc welding) for any work scope involved with installation of the SWOL, PG&E will inform the NRC senior resident or inspectors prior to use.

3 Code Case N-740-2, "Full Structural Metal Dissimilar Weld Overlay for Repair or Mitigation of Class 1, 2, and 3 ItemsSection XI, Division 1.

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Enclosure PG&E Letter DCL-17-095 RAI#5:

On page 6, last paragraph, of the relief request the licensee provided information concerning postulated and measured flaws. The values provided do not appear to precisely match the requirements of Code Case N-740-2. Please confirm that the requirements for measured and postulated flaws contained in the code case will be met or justify alternate requirement.

PG&E Response:

PG&E confirms that the requirements for measured and postulated flaws contained in the Code Case N-740-2 will be met. PG&E proposes to restate the last paragraph on page 6 of Reference 3 as shown below:

Crack growth calculations as required by Code Case N-740-2 will be performed as pari of a design package. Qualified ultrasonic examinations will be conducted prior to the installation of the weld overlay and the flaw size will be measured, per Code Case N-740-2. However, ~flaw characterization and evaluation requirements are will be based on the as found flav1, or a postulated flaw depth of 75 percent through of original wall-thickness axial and circumferential fla~~l, ~~;hichever is greater for two postulated flaws: a postulated axial flaw, and a postulated circumferential flaw. For the postulated axial flaws, the axial flaw length shall be 1.5 inches (38 mm). For the postulated circumferential flaw, + the circumferential flaw length shall be assumed to be 360 degrees. The size of all flaws will be projected to the end of the design life of the overlay, which will be a minimum of 10 years, which exceeds the remaining operating license duration for each Unit.

The existing flaw depth is predicted to grow to approximately 0.6 inches at the end of the current operating cycle [References 1 and 2]. The actions that are specified in the response to RAI #12 will be followed, if measured flaw depth exceeds the predicted flaw depth at the time of the proposed SWOL installation.

RAI#6:

Page 7, last paragraph, of the relief request states, in pari, "The allowable flaw size will be determined in accordance with ASME Section XI, Appendix C for the structural weld ovelay geometry. This time period will meet or exceed the design requirement of the weld overlay (i.e., continued operation of both the units until the expiration of the current operating license)." Given the proposed weld overlay, and the assumption that the crack grows by 1) fatigue, 2) stress corrosion cracking, or 3) a combination of both, please provide the maximum allowable flaw depth permissible at the time of the installation of the weld overlay.

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Enclosure PG&E Letter DCL-17 -095 PG&E Response:

PG&E has not calculated the maximum allowable flaw depth because of the number of calculations and analysis iterations that would need to be performed .

As discussed in the response to RAI #5, the design of the SWOL is based on a specific postulated flaw depth, consistent with the requirements of Code Case N-740-2. The postulated flaw depth is conservative when compared to the predicted flaw depth at the time of the SWOL installation.

The subject welds will be reexamined prior to the SWOL installation to measure the flaw depth. The existing flaw depth is predicted to grow to approximately 0.6 inches at the end of the current operating cycle [References 1 and 2]. If the measured flaw depth exceeds the predicted flaw depth, the actions that are specified in the response to RAI #12 will be followed.

RAI#7:

Page 9 of the relief request contains language concerning the volume to be inspected and actions to be taken if full coverage is not achieved. This language appears to differ from Code Case N-740-2. Please confirm that inspections will be conducted in accordance with N-740-2 or provide the differences proposed and justification for those differences.

PG&E Response:

PG&E confirms that examinations will be conducted in accordance with Code Case N-7 40-2 and Section XI, Appendix VIII as implemented by the performance demonstration initiative (POl).

PG&E proposes to restate the last paragraph on page 9 of Reference 3 as shown below:

Ultrasonic examination will be petformed to the maximum extent achiev-abl-e practicable, for axial and circumferential flaws, and the total volume vAll be greater than 90 percent of the required examination

'ri-Oiume in accordance with Code Case N-740-2. Schematic representation of the required ultrasonic examination volume is shown in Figure 5.

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Enclosure PG&E Letter DCL-17 -095 RAI#B:

Page 10 of the relief request states, in part, "Two examinations will be conducted, after installation of the weld overlay: the acceptance examination of the overlay and the preservice inspection. The purpose of the acceptance examination is to assure a quality overlay was installed. The purpose of the preservice inspection is to locate and size any indications and to provide a baseline for future examination. " The NRC staff notes that the volume of the acceptance examination is different from that of the pre-service examination as shown in Code Case N-740-2. The relief request needs to make that distinction.

Please confirm that (a) the examination volume for the acceptance examination will follow Figure 1 of Code Case N-740-2, and (b) the examination volume for the preservice and inservice examinations will follow Figure 2 of Code Case N-740-2.

PG&E Response:

PG&E confirms that (a) the examination volume for the acceptance examination will follow Figure 1 of Code Case N-740-2, and (b) the examination volume for the preservice and inservice examinations will follow Figure 2 of Code Case N-7 40-2.

RAI#9:

Page 9 of the relief request states, in part, "Schematic representation of the required ultrasonic examination volume is shown in Figure 5 [of the Enclosure to letter dated September 26, 2017]." The required volume for the acceptance examination is not identified in Figure 5. Please confirm that the solid red area in Figure 5 is the volume that is required to be ultrasonically examined for the acceptance examination.

PG&E Response:

PG&E confirms that the solid red area shown in Figure 5 of the Enclosure to Reference 3 is the volume that is required to be ultrasonically examined for the acceptance examination (the upper schematic representation in Figure 5 is similar to Figure 1 of Code Case N-740-2).

A revised Figure 5 is provided in Attachment 1, which adds labelling of the acceptance examination volume (A-B-C-D) in the upper schematic representation. Preservice and inservice examination volume is also labelled by A-B-C-D in the lower schematic representation. This revised Figure 5 replaces Figure 5 from Reference 3.

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Enclosure PG&E Letter DCL-17 -095 RAI #10:

Code Case N-740-2, Section 3(c), provides provisions for in service examination requirements. Because flaws exist in both welds, please discuss the ultrasonic examination schedules for each weld from the overlay installation to the end of operating license for both units.

PG&E Response:

PG&E will perform the ultrasonic examinations in accordance with Code Case N-740-2. The schedule for ultrasonic examinations is presented below:

Schedule for ultrasonic examinations of DCPP Unit 2 weld WIB-245 SWOL, during upcoming refueling outages:

• Unit 2 twentieth refueling outage (February 2018): Acceptance examination and preservice examination

• Unit 2 twenty-first refueling outage (2R21) (circa October 2019) OR Unit 2 twenty-second refueling outage (2R22) (circa May 2021): lnservice examination (NOTE)

• Unit 2 twenty-fourth refueling outage (circa October 2024): lnservice examination per Code Case N-740-2 paragraph 3(c)(6) if required by 2R21 OR 2R22 exam result

• End of license August 26, 2025 Schedule for ultrasonic examinations of Unit 1 weld WIB-228 SWOL, during upcoming refueling outages:

• Unit 1 twenty-first refueling outage (circa February 2019): Acceptance examination and preservice examination

• Unit 1 twenty-second refueling outage (circa October 2020) OR Unit 1 twenty-third refueling outage (circa May 2022): lnservice examination(NOTE)

• End of license November 2, 2024 NOTE: This inservice examination will be confirmed and resources will be assigned when the associated refueling outage schedule is finalized. This flexibility in lnservice examination schedule is in accordance with Code Case N-740-2.

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Enclosure PG&E Letter DCL-17-095 RAI #11:

Paragraph 3(c)(4) of Code Case N-740-2 requires, in pari, that if a planar flaws is detected in the outer 25 percent of the base material, weld overlay thickness shall meet the design analysis requirements of Section 2 of the code case. Any indication characterized as stress corrosion cracking in the weld overlay material is unacceptable. Paragraph 3(c)(6) requires, in part, that if inservice examinations reveal planar flaw growth, or new planar flaws, the weld overlay examination volume shall be reexamined during the first or second refueling outage following discovery of the growth or new flaws. Paragraph 3(c)(7) requires that for weld overlay examination volumes with unacceptable indications in accordance with paragraph 3(c)(4), the weld overlay and original defective weld shall be removed. A repair/replacement activity shall be performed in accordance with IWA-4000.

(a) Please confirm that the licensee will follow the requirements of above three paragraphs of the code case and other provisions of Section 3 of the code case (b) Paragraph 3(c)(4) of the code case requires that any indication characterized as stress corrosion cracking in the weld overlay material is unacceptable.

Because the licensee has identified thermal fatigue as a key degradation mechanism, please discuss whether any indication propagated into the weld overlay material by thermal fatigue should also be considerable unacceptable.

If not, please provide justification.

PG&E Response:

a) PG&E confirms that the requirements of the above three paragraphs of the Code Case N-740-2 (Paragraphs 3(c)(4), 3(c)(6) and 3(c)(7)) and other provisions of Section 3 of the Code Case N-740-2 will be followed.

b) PG&E confirms that any indication propagating into the weld overlay material, either by thermal fatigue or stress corrosion cracking, will be considered unacceptable.

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Enclosure PG&E Letter DCL-17 -095 RAI #12:

The NRC staff notes that a pre-overlay examination will be conducted prior to making the overlay. The licensee will be able to use this examination to obtain the current cycle's flaw growth rate. The licensee has assumed that the flaw degradation mechanism is thermal stratification and thermal cycling. If the current cycle's flaw growth rate exceeds the predicted flaw growth by thermal stratification and thermal cycling, discuss actions that the licensee will take to ensure the adequacy of the designed full structural weld overlay that will continue to mitigate this weld. Discuss at what growth rate would there need to be a modification to the weld overlay design and potential change of the re-inspection schedule for the weld repaired.

PG&E Response:

The flaw growth analyses presented in References 1 and 2 show that flaw depth

-vs- time is non-linear. A real-time flaw growth rate cannot be determined with only two measurements (i.e., only two data points will be available at the time of the SWOL installation). As a result, the measured flaw depth will be used to determine whether a modification to the SWOL design is warranted.

As discussed in response to RAI #5, the planned SWOL design is based on conservative assumptions in accordance with Code Case N-740-2, and the predicted flaw depth to be confirmed using UT examination prior to the SWOL installation.

If the measured flaw depth exceeds the predicted flaw depth prior to SWOL installation, the following action will be taken:

• PG&E will re-perform the design calculations to ensure the designed SWOL will continue to mitigate the weld.

PG&E has not identified a specific flaw growth rate (or flaw depth upper bound) that would result in a modification to the SWOL design. However, based on the analytical results from the re-performed design calculations, PG&E will determine whether a modification to the SWOL design is warranted. The SWOL design will meet the requirements of Code Case N-740-2. In addition, PG&E will evaluate if a change in inspection schedule is appropriate in addition to those required per Code Case N-740-2, as discussed in the response to RAI #1 0, if the measured flaw depth exceeds the predicted flaw depth.

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Attachment PG&E Letter DCL-17 -095 Attachment (Revised) Figure 5: Schematic Representation of the Ultrasonic Examination Volume

Attachment PG&E Letter DCL-17-095 Figure 5: Schematic Representation of the Ultrasonic Examination Volume A B A B SWOL Acceptance Examination Volume (A-B-C-D)

A B SWOL Preservice and lnservice Examination Volume (A-B-C-D) 2