ML18334A013
| ML18334A013 | |
| Person / Time | |
|---|---|
| Site: | Wolf Creek  |
| Issue date: | 12/12/2018 |
| From: | Robert Pascarelli Plant Licensing Branch IV |
| To: | Heflin A Wolf Creek |
| Singal B, NRR, 301-415-3016 | |
| References | |
| EPID L-2018-LLR-0101 | |
| Download: ML18334A013 (11) | |
Text
UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 December 12, 2018 Mr. Adam C. Heflin President and Chief Executive Officer Wolf Creek Nuclear Operating Corporation P.O. Box 411 Burlington, KS 66839
SUBJECT:
WOLF CREEK GENERATING STATION, UNIT 1 - REQUEST FOR RELIEF FROM REQUIREMENTS OF THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS BOILER AND PRESSURE VESSEL CODE, CODE CASE N-666-1 (EPID L-2018-LLR-0101)
Dear Mr. Heflin:
By letter dated July 23, 2018, as supplemented by letter dated August 28, 2018, Wolf Creek Nuclear Operating Corporation (the licensee) submitted a relief request to the U.S. Nuclear Regulatory Commission (NRC) to request relief from the requirements of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code), for Wolf Creek Generating Station, Unit 1 (WCGS). The licensee plans to use the NRG-approved ASME Code Case N-666-1, "Weld Overlay of Class 1, 2, and 3 Socket Welded Connections,Section XI, Division 1," for the temporary repair of the leaking 2-inch socket weld on the essential service water drain line connected to the component cooling water heat exchanger to comply with the requirements of ASME Code Section XI, subparagraph IWA-4611.1, "General Requirements,"
with exceptions.
Specifically, pursuant to Title 1O of the Code of Federal Regulations ( 10 CFR)
Section 50.55a(z)(2), the licensee requested relief from the following requirements of the Code Case, on the basis that compliance with the specified ASME Code repair would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety:
- Weld overlay carbon content requirement;
- Required minimum wall thickness of the pipe base material adjacent to the socket weld requiring overlay; and
- Verification requirement that the socket weld failure was a result of vibration fatigue.
The NRC staff has reviewed the subject request and concludes, as set forth in the enclosed safety evaluation, that the proposed alternative provides reasonable assurance of structural integrity of the subject socket weld on the essential service water piping. The staff concludes that complying with the specified ASME Code requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety. Accordingly, the staff concludes that the licensee has adequately addressed all of the regulatory requirements set forth in 10 CFR 50.55a(z)(2).
A Heflin On July 24, 2018, the NRC staff verbally authorized the use of the subject relief request at WCGS, through Refueling Outage 23, scheduled for the fall of 2019. This letter and the enclosed safety evaluation provide the written follow-up of the NRC staff's basis for the verbal authorization.
All other requirements of ASME Code,Section XI, for which relief has not been specifically requested and approved in this relief request remain applicable, including 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 !want.Sin al nrc. ov.
Robert J. Pascarelli, Chief Plant Licensing Branch .IV Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-482 Enclosure Safety Evaluation cc: Listserv
UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION REQUEST FOR RELIEF FROM THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS BOILER AND PRESSURE VESSEL CODE CODE CASE N-666-1 WOLF CREEK NUCLEAR OPERATING CORPORATION WOLF CREEK GENERATING STATION, UNIT 1 DOCKET NO. 50-482
1.0 INTRODUCTION
By letter dated July 23, 2018 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML18215A176), as supplemented by letter dated August 28, 2018 (ADAMS Accession No. ML18247A468), Wolf Creek Nuclear Operating Corporation (the licensee) submitted a relief request to the U.S. Nuclear Regulatory Commission (NRC) to request relief from the requirements of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code), for Wolf Creek Generating Station, Unit 1 (WCGS).
The licensee plans to use the NRG-approved ASME Code Case N-666-1, "Weld Overlay of Class 1, 2, and 3 Socket Welded Connections,Section XI, Division 1," for the temporary repair of the leaking 2-inch socket weld on the essential service water (ESW) drain line connected to the component cooling water (CCW) heat exchanger to comply with the requirements of ASME Code Section XI, subparagraph IWA-4611.1, "General Requirements," with exceptions.
Specifically, pursuant to Title 10 of the Code of Federal Regulations ( 10 CFR)
Section 50.55a(z)(2), the licensee requested relief from the following requirements of the Code Case, on the basis that compliance with the specified ASME Code repair would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety:
- Weld overlay carbon content requirement;
- Required minimum wall thickness of the pipe base material adjacent to the socket weld requiring overlay; and
- Verification requirement that the socket weld failure was a result of vibration fatigue.
The CCW heat exchanger and the drain line are designed to ASME Code, Section Ill, Class 3.
By e-mail dated July 24, 2018 (ADAMS Accession No. ML18207A433), the licensee provided a response to the NRC staff request for additional information. By letter dated August 28, 2018, the licensee incorporated the information provided by e-mail dated July 24, 2018 into its original request. No new information was provided by the letter dated August 28, 2018.
Enclosure
On July 24, 2018 (ADAMS Accession No. ML18207A436), the NRC verbally authorized the use of the relief request, at WCGS, through Refueling Outage 23, scheduled for the fall of 2019.
This safety evaluation documents the NRC staffs detailed technical basis for the verbal authorization.
2.0 REGULATORY EVALUATION
Adherence to Section XI of the ASME Code is mandated by 10 CFR 50.55a(g)(4), "lnservice inspection standards requirement for operating plants," 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 examination requirements, set forth in ASME Code,Section XI.
The regulation at 10 CFR 50.55a(z), "Alternatives to codes and standards requirements," states, in part:
Alternatives to the requirements of paragraphs (b) through (h) of [10 CFR 50.55a]
or portions thereof may be used when authorized by the Director, Office of Nuclear Reactor Regulation .... A proposed alternative must be submitted and authorized prior to implementation. The applicant or licensee must demonstrate that: (1) "Acceptable level of quality and safety." The proposed alternative would provide an acceptable level of quality and safety; or (2) "Hardship without a compensating increase in quality and safety." Compliance with the specified requirements of this section 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 Component(s) Affected The affected components are the CCW heat exchanger, a nominal pipe size 2 inch (NPS 2) drain line, and the ESW system. The CCW heat exchanger and NPS 2 drain line are designed and constructed to the ASME Code, Section Ill, Class 3. The heat exchanger is designed to the 1974 with Summer 1975 Addenda and the piping is designed to the 1974 Edition with Summer 1975 Edition Addenda.
3.2 Applicable Code Edition and Addenda ASME Code,Section XI, 2007 Edition with 2008 Addenda.
3.3 Applicable ASME Code Requirements ASME Code Section XI, subparagraph IWA-4611.1(a) states:
Defects shall be removed in accordance with IWA-4422.1. A defect is considered removed when it has been reduced to an acceptable size.
The use of ASME Code Case N-666-1 provides the requirements for the application of weld overlay to socket welds with through wall leaks. This Code Case is conditionally approved by the NRC in Regulatory Guide (RG) 1.147, Revision 18, "lnservice Inspection Code Case Acceptability, ASME Section XI, Division 1," dated March 2017 (ADAMS Accession No. ML16321A336) and is applicable to WCGS. The licensee stated that they will take exceptions to the following paragraphs of ASME Code Case N-666-1.
ASME Code Case N-666-1, paragraph 1(b) states:
Use of this Case is limited to Class 1, 2, or 3, NPS 2 ... and smaller socket welded connections with base material of P-No. 1 [Carbon Manganese Steels]
Group 1 and P-No. 1 Group 2, or P-No. 8. For water backed welding, the carbon content of P-No. 1 Group 2 materials shall be limited to 0.30% maximum.
ASME Code Case N-666-1, paragraph 1(d) states, in part:
Prior to performing a repair, the Owner shall verify that the pipe base material adjacent to the socket weld requiring overlay meets the required minimum wall thickness.
ASME Code Case N-666-1, paragraph 2(a) states, in part:
The Owner shall verify that the socket weld failure is a result of vibration fatigue.
3.4 Reason for Request The licensee attempted to isolate Train A of the ESW from the Train A CCW heat exchanger utilizing isolation valves near the heat exchanger to repair the socket weld connection but was not successful. The licensee stated that in order to remove water from the tube side of the heat exchanger, Train A of the ESW would have to be drained to an elevation which would render Train A of the ESW Train completely non-functional. The licensee noted that with Train A of the ESW completely non-functional, WCGS would be placed into an elevated and undesirable risk condition.
The licensee stated that an alternative is to place the plant in cold shutdown (Mode 5) prior to draining the ESW system, but maneuvering the power plant between 100 percent power and Mode 5, has inherent risk. In addition, the licensee asserted that only one Train of residual heat removal (RHR) would be available for shutdown cooling if Train A of the ESW system was drained. A single Train of RHR for shutdown cooling is not desirable in terms of defense-in-depth. The licensee explained that establishing the condition of ESW drained to complete the weld overlay will result in a hardship without a compensating increase in quality and safety.
3.5 Proposed Alternative The licensee proposed to apply ASME Code Case N-666-1, with the exceptions listed in Section 1.0 of this safety evaluation, to repair the leaking 2-inch socket weld on ESW piping in lieu of repair/replacement activities in accordance with the ASME Code,Section XI.
3.6 Basis for Use The licensee stated that it will follow all the requirements of ASME Code Case N-666-1 with three exceptions listed in Section 1.0 of this safety evaluation and discussed in the subsections below. The NRC staff's evaluation followed the organization structure of the Code Case to evaluate the licensee's request against the requirements specified in the Code Case.
3.6.1 General Requirements ASME Code Case N-666-1, Section 1, "General Requirements," (c) limits its application to certain base materials and welding processes; (d) provides temperature limitations while performing the overlay and states that the base material adjacent to the socket weld requiring overlay shall meet the required minimum wall thickness; and (e) states that a socket weld may not be overlaid more than one time.
Paragraph 1(b) of Section 1 limits the carbon content of P-No. 1 Group 2 materials to 0.30 percent for water-backed welding. The licensee requested to use P-No. 1 Group 2 materials with a carbon content of 0.32 percent for water-backed welding. In addition, paragraph 1(d) of Section 1 requires that the pipe base material adjacent to the socket weld requiring overlay meets the required minimum wall thickness. The licensee could not verify that the pipe base material within the socket weld fitting requiring overlay meets the required minimum wall thickness. Therefore, the licensee requested relief from the carbon content limits and wall thickness requirements of paragraphs 1(b) and 1(d), respectively.
In regards to the requested relief from paragraph 1(b ), the licensee stated that the potential concerns associated with welding on the P-No. 1 Group 2 material, with a carbon content of 0.32 percent, are brittle microstructure and hydrogen induced cracking (HIC). The licensee asserted that brittle microstructure is not believed to be an issue because (a) a summary of the hardness of martensite as a function of carbon content found that a shift of carbon content of 0.30 to 0.32 percent shifts the peak hardness from approximately 500 to 540 Vickers, which is a relatively minor shift; (b) consideration should also be taken that welding on the coupling and drain line will not be an ideal quench as the water inside is not ambient; therefore, a hardness level of 540 Vickers is not expected; (c) the Code Case specifies a minimum of 2 layers to be deposited, which will have a tempering effect on the heat affected zone microstructure; and (d) the material with a carbon content of 0.32 percent, and associated carbon equivalent of 0.44, will follow similar low hardenability behavior as shown in other P-No. 1 Group 2 materials with higher carbon equivalent values. In addition, the licensee asserted that HIC is not an issue because (a) low hydrogen consumables and practices will be used; and (b) multiple weld layers will have a tempering effect on the heat affected zone.
In regards to the requested relief from paragraph 1(d), the licensee stated that the pipe wall thickness where the overlay will be installed is greater than the minimum required wall thickness; however, the portion of the piping within the socket could not be examined. The licensee further stated that due to the design of the overlay, the load carrying capacity is transferred from the pipe through the overlay to the coupling. As a result, the licensee asserted that the load carrying capacity of the connection is maintained.
3.6.2 Evaluation ASME Code Case N-666-1, Section 2, "Evaluation," limits the applicability of the Code Case to a specific degradation mechanism and provides corrective action provisions for continued
service beyond the next refueling outage. Paragraph 2(a) of Section 2 limits the use of the Code Case to instances where the socket weld failure is a result of vibration fatigue.
The licensee performed radiography testing (RT} and vibration reading in order to characterize the flaw and determine the degradation mechanism. The licensee stated that the RT shows general wall loss on the piping inside diameter and in the location of the socket weld. The licensee also performed ultrasonic examination on the piping starting at the toe of the socket weld and extending to the downstream valve EGV0029. The licensee reported that the as-found wall thickness is a minimum of 0.157 inches with the calculated minimum required wall thickness of 0.143 inches. The piping location where the weld overlay will be installed is greater than the minimum required wall thickness.
The licensee explained that reduced wall thickness may be attributed to corrosion due to the carbon steel material in a raw water system. In addition, the vibration readings show that during normal operations, there is an element of fatigue. Based on the RT and the vibration readings, it is most likely that the corrosion initiated the flaw and reduced the wall thickness to a point where fatigue propagated the flaw through wall.
In the letter dated August 28, 2018, the licensee stated that although corrosion may have been the initiator of the flaw, vibration is present. The licensee further stated that the failure mechanism resulting from vibration fatigue (as required by the Code Case) could not be verified; therefore, requiring relief from the vibration fatigue degradation mechanism requirement of paragraph 2(a). The licensee explained that it is reasonable that the likelihood of a leak to occur in this location, due to a corrosion failure mechanism, is low. The repair will be removed from service prior to the end of Refueling Outage 23 and the socket weld connection will be reworked/repaired to meet the requirements of the ASME Code. In addition, the licensee stated that walkdowns will be completed at least once every 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> to identify potential leakage post repair.
3.6.3 Design ASME Code Case N-666-1, Section 3, "Design," provides dimensional requirements for the completed weld overlay and provisions for allowable filler metals. The proposed alternative does not take exception to Section 3 of ASME Code Case N-666-1.
3.6.4 Procedure ASME Code Case N-666-1, Section 4, "Procedure," provides provisions for (a) determining the extent of cracking; (b) seal welding and peening of the crack; (c) visual examination of the seal weld, remaining socket weld, and adjacent base material; and (d) performing the structural weld overlay. The proposed alternative does not take exception to Section 4 of ASME Code Case N-666-1. In the letters dated July 24, and August 28, 2018, the licensee stated that weld overlay will be removed from service prior to the end of Refueling Outage 23, scheduled for the fall of 2019. The socket weld connection will be reworked/repaired to meet the requirements of the ASME Code.
3.6.5 Final Examination and Testing ASME Code Case N-666-1, Section 5, "Final Examination and Testing," provides provisions for (a) visual and nondestructive examination of the final structural weld overlay; (b) dimensional inspection of the completed weld overlay; and (c) performing a system leakage test. The
proposed alternative does not take exception to Section 5 of ASME Code Case N-666-1. In addition, the licensee will follow the condition imposed on Code Case N-666-1 in RG 1.147, which requires a surface examination after installation of the weld overlay. In the letters dated July 24 and August 28, 2018, the licensee stated that walkdowns will be completed at least once every 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> to identify potential leakage post repair.
3.6.6 Hardship The licensee stated that performing an ASME Code repair could have a detrimental impact on the overall risk by requiring a plant shutdown. The licensee stated that the application of ASME Code Case N-666-1, along with the exceptions stated above, will maintain acceptable structural and leakage integrity while minimizing plant risk and personnel exposure by minimizing the number of plant transients that could be incurred, if degradation is required to be repaired based on the requirements of the ASME Code,Section XI. The licensee concluded that compliance with the current ASME Code requirements results in a hardship without a compensating increase in the level of quality and safety.
3.7 Duration of Proposed Alternative The licensee stated that the weld overlay will be removed from service prior to the end of Refueling Outage 23, scheduled for the fall of 2019. The licensee further stated that the socket weld connection will be reworked/repaired to meet the requirements of the ASME Code.
3.8 NRC Staff Evaluation The NRC staff evaluated the licensee's proposed alternative with respect to the provisions of ASME Code Case N-666-1, which has been conditionally approved for use by the NRC.
3.8.1 General Requirements In the letter dated August 28, 2018, the licensee stated that the proposed alternative satisfies the requirements of Section 1 of ASME Code Case N-666-1, except paragraphs 1(b) and 1(d).
The NRC staff evaluated the exceptions to paragraphs 1(b) and 1(d) as follows:
- Paragraph 1(b) of ASME Code Case N-666-1 limits the carbon content of P-No. 1 Group 2 materials to 0.30 percent for water-backed welding. To justify the use of this Code Case on P-No. 1 Group 2 material with a carbon content of 0.32 percent, the licensee has demonstrated in its letter dated August 28, 2018, that (a) a carbon content of 0.02 percent greater than allowable by the Code Case will have a negligible impact on the hardness of the material; (b) the multiple weld passes, as specified in the Code Case, will have a tempering effect to decrease the hardness of the material; and (c) the use of low hydrogen electrodes and multiple weld passes will limit the potential for HIC.
Based on the technical basis for welding on pipe material having a carbon content of 0.32 percent, the NRC staff finds the proposed alternative is acceptable with respect to paragraph 1(b ).
- Paragraph 1(d) of ASME Code Case N-666-1 states that "[p]riorto performing a repair, the Owner shall verify that the pipe base material adjacent to the socket weld requiring overlay meets the required minimum wall thickness." The licensee stated that the pipe wall thickness where the overlay will be installed is greater than the minimum required wall thickness; however, the portion of piping within the socket could not be examined
through ultrasonic testing. To justify the use of this Code Case without verifying that the portion of the piping within the socket meeting the required minimum wall thickness, the licensee stated that due to the design of the overlay, the load carrying capacity is transferred from the pipe through the overlay and to the coupling. Based on the proposed weld overlay being designed to support piping loads without crediting the existing pipe base material, the NRC staff finds the proposed alternative is acceptable with respect to paragraph 1(d).
Based on the above evaluation, the NRC staff finds that the proposed alternative satisfies the requirements of Section 1 of ASME Code Case N-666-1, except paragraphs 1(b) and 1(d).
However, the licensee has provided adequate justifications for the deviations from paragraphs 1(b) and 1(d).
3.8.2 Evaluation The licensee stated that the proposed alternative satisfies the requirements of Section 2 of ASME Code Case N-666-1, except paragraph 2(a). The NRC staff evaluation of the exception to paragraph 2(a) follows:
- Paragraph 2{a) of ASME Code Case N-666-1 limits the use of the Code Case to socket weld failure as a result of vibration fatigue. To justify the use of this Code Case without verifying that the socket weld failure was a result of vibration fatigue and the failure may have been initiated by corrosion, the licensee has (a) demonstrated that the likelihood of a leak to occur in this location due to a corrosion prior to the next refueling outage is low; and (b) stated that the temporary repair will be monitored on walkdowns at least once every 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. Based on the low likelihood of a leak occurring and the frequency of the walkdowns to identify leakage, the NRC staff finds the proposed alternative is acceptable with respect to paragraph 2(a).
Based on the above evaluation, the NRC staff finds that the proposed alternative satisfies the requirements of Section 2 of ASME Code Case N-666-1, except paragraph 2(a). However, the NRC staff has determined that the licensee has provided adequate justification for the deviation from paragraph 2(a).
3.8.3 Design The NRC staff finds that the proposed alternative will follow and does not take exception to Section 3 of ASME Code Case N-666-1. Therefore, the proposed alternative is acceptable with respect to Section 3 of ASME Code Case N-666-1.
3.8.4 Procedure The NRC staff finds that the proposed alternative will follow and does not take exception to Section 4 of ASME Code Case N-666-1. Therefore, the proposed alternative is acceptable with respect to Section 4 of ASME Code Case N-666-1.
3.8.5 Final Examination and Testing The NRC staff finds that the proposed alternative will follow and does not take exception to Section 5 of ASME Code Case N-666-1. Therefore, the proposed alternative is acceptable with respect to Section 5 of ASME Code Case N-666-1. In addition, the NRC staff notes that the
licensee (a) will follow the condition imposed on Code Case N-666-1 in RG 1.147, which requires a surface examination after installation of the weld overlay, and (b) will perform walkdowns at least once every 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> to identify potential leakage post repair.
3.8.6 Hardship Justification The NRC staff evaluated the technical basis of this request against the criteria contained in 10 CFR 50.55a(z)(2). The NRC staff notes that performing the specified ASME Code compliant repairs will require a plant shutdown, which will lead to unnecessary plant transients and additional radiation dose. The plant shutdown is undesirable in terms of plant safety because it increases loads on the systems and components. The NRC staff finds that the ASME Code compliant repair of the subject piping would not increase plant quality or safety significantly.
The NRC staff, therefore, concludes that requiring an ASME Code compliant repair is a hardship or unusual difficulty without a compensating increase in plant quality or safety.
In summary, the NRC staff concludes that the proposed alternative will provide reasonable assurance of the structural integrity of the subject socket weld on the ESW piping until the next refueling outage because the licensee will follow (a) the requirements of ASME Code Case N-666-1 with exceptions for which the licensee has provided appropriate justifications; and (b) comply with the condition for surface examination after installation of the weld overlay imposed on Code Case N-666-1 in RG 1.147, Revision 18.
4.0 CONCLUSION
As set forth above, the NRC staff determines that the proposed alternative provides reasonable assurance of structural integrity of the subject socket weld on the ESW piping. The NRC staff concludes that complying with the specified ASME Code requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.
Accordingly, the NRC staff concludes that the licensee has adequately addressed all of the regulatory requirements set forth in 10 CFR 50.55a(z)(2). Therefore, the NRC staff authorizes the use of the relief request as documented in letters dated July 23, 2018, and August 28, 2018, through Refueling Outage 23, which is scheduled for the fall of 2019.
All other ASME Code,Section XI, requirements for which relief was not specifically requested and authorized by the NRC staff will remain applicable including third-party review by the Authorized Nuclear lnservice Inspector.
Principal Contributors: B. Allik, NRR J. Tsao, NRR Date: December 1 2 , 201 8
ML18334A013 *Memo dated 11/14/18 OFFICE NRR/D0RL/LPL4/PM NRR/D0RL/LPL4/LA NRR/DMLR/MPHB/BC* NRR/D0RL/LPL4/BC NAME BSingal PBlechman SRuffin RPascarelli (LRegner for)
DATE 12/05/18 11/30/18 11/14/18 12/12/18 w/comments