ML080630679
| ML080630679 | |
| Person / Time | |
|---|---|
| Site: | Watts Bar  |
| Issue date: | 02/29/2008 |
| From: | Raghavan L Plant Licensing Branch III-2 |
| To: | Campbell W Tennessee Valley Authority |
| Chernoff M, NRR/DORL, 415-4041 | |
| References | |
| TAC MD4448, TAC MD4449 | |
| Download: ML080630679 (11) | |
Text
February 29, 2008 Mr. William R. Campbell, Jr.
Chief Nuclear Officer and Executive Vice President Tennessee Valley Authority 6A Lookout Place 1101 Market Street Chattanooga, TN 37402-2801
SUBJECT:
WATTS BAR NUCLEAR PLANT, UNIT 1 - SAFETY EVALUATION OF RELIEF REQUESTS PDI-2 AND PDI-4 FOR THE SECOND 10-YEAR INSERVICE INSPECTION PROGRAM FOR REACTOR PRESSURE VESSEL WELD EXAMINATIONS (TAC NOS. MD4448 AND MD4449)
Dear Mr. Campbell:
By letter dated February 7, 2007, as supplemented by letters dated July 25 and December 14, 2007, Tennessee Valley Authority (the licensee) submitted Relief Requests PDI-2 and PDI-4 for the Watts Bar Nuclear Plant (WBN), Unit 1, second 10-year inservice inspection (ISI) interval.
Pursuant to Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Section 50.55a(a)(3)(i),
Relief Request PDI-2 requested relief from certain American Society of Mechanical Engineers (ASME) Code requirements pertaining to the examination of reactor vessel nozzle to vessel welds.
The U.S. Nuclear Regulatory Commission (NRC) staff has reviewed the request and determined that the proposed alternative to use a reduced examination area provides an acceptable level of quality and safety; and the proposed alternative is authorized. The NRC staff further determined that the requirements for essentially 100 percent examination coverage of the weld volume are impractical, and relief is granted pursuant to 10 CFR 50.55a(g)(6)(i). These conclusions are applicable for the second 10-year ISI interval at WBN, Unit 1, which began on May 27, 2007, and ends on May 26, 2017.
Pursuant to 10 CFR Part 50, Section 50.55a(a)(3)(i), Relief Request PDI-4 requested relief from certain ASME Code requirements pertaining to reactor pressure vessel upper vessel shell to flange welds. The NRC staff has reviewed the request and concludes that the proposed alternative provides an acceptable level of quality and safety. Therefore, pursuant to 10 CFR 50.55a(a)(3)(i), the proposed alternative is authorized for the second 10-year ISI interval at WBN, Unit 1, which began on May 27, 2007, and ends on May 26, 2017.
W. Campbell, Jr. Further details on the bases for the NRC staffs conclusions are contained in the enclosed safety evaluation. If you have any questions regarding this issue, please feel free to contact the WBN, Unit 1 Project Manager, Margaret Chernoff, at (301) 415-4041.
Sincerely, L. Raghavan, Chief Watts Bar Special Projects Branch Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-390
Enclosure:
Safety Evaluation cc w/enclosure: See next page
ML080630679 NRR-028 OFFICE WBSP/PM WBSP/LA CVIB/BC OGC/NLO WBSP/BC NAME MChernoff BClayton MMitchell DRoth LRaghavan DATE 02/29/08 02/28/08 By memo dated 02/29/08 02/29/08 01/17/08
William R. Campbell, Jr.
Tennessee Valley Authority WATTS BAR NUCLEAR PLANT cc:
Mr. Gordon P. Arent Mr. Michael A. Purcell New Generation Licensing Manager Senior Licensing Manager Tennessee Valley Authority Nuclear Power Group 5A Lookout Place Tennessee Valley Authority 1101 Market Street 4X Blue Ridge Chattanooga, TN 37402-2801 1101 Market Street Chattanooga, TN 37402-2801 Mr. Ashok S. Bhatnagar Senior Vice President Ms. Beth A. Wetzel, Manager Nuclear Generation Development Corporate Nuclear Licensing and and Construction Industry Affairs Tennessee Valley Authority Tennessee Valley Authority 6A Lookout Place 4X Blue Ridge 1101 Market Street 1101 Market Street Chattanooga, TN 37402-2801 Chattanooga, TN 37402-2801 Mr. James R. Douet Mr. Masoud Bajestani, Vice President Vice President Watts Bar Unit 2 Nuclear Support Watts Bar Nuclear Plant Tennessee Valley Authority Tennessee Valley Authority 3R Lookout Place P.O. Box 2000, EQB 1B 1101 Market Street Spring City, TN 37381 Chattanooga, TN 37402-2801 Mr. Michael K. Brandon, Manager Mr. H. Rick Rogers, Vice President Licensing and Industry Affairs Nuclear Engineering & Technical Services Watts Bar Nuclear Plant Tennessee Valley Authority Tennessee Valley Authority 3R Lookout Place P.O. Box 2000 1101 Market Street Spring City, TN 37381 Chattanooga, TN 37402-2801 Mr. Michael J. Lorek, Plant Manager General Counsel Watts Bar Nuclear Plant Tennessee Valley Authority Tennessee Valley Authority 6A West Tower P.O. Box 2000 400 West Summit Hill Drive Spring City, TN 37381 Knoxville, TN 37902 Senior Resident Inspector Mr. John C. Fornicola, Manager Watts Bar Nuclear Plant Nuclear Assurance U.S. Nuclear Regulatory Commission Tennessee Valley Authority 1260 Nuclear Plant Road 3R Lookout Place Spring City, TN 37381 1101 Market Street Chattanooga, TN 37402-2801 County Mayor P. O. Box 156 Mr. Larry E. Nicholson, General Manager Decatur, TN 37322 Performance Improvement Tennessee Valley Authority Mr. Lawrence E. Nanney, Director 4X Blue Ridge Division of Radiological Health 1101 Market Street Dept. of Environment & Conservation Chattanooga, TN 37402-2801 Third Floor, L and C Annex 401 Church Street County Executive Nashville, TN 37243-1532 375 Church Street Suite 215 Dayton, TN 37321
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELIEF REQUESTS NO. PDI-2 AND PDI-4 TENNESSEE VALLEY AUTHORITY WATTS BAR NUCLEAR PLANT, UNIT 1 DOCKET NO. 50-390
1.0 INTRODUCTION
By letter dated February 7, 2007, Tennessee Valley Authority (TVA, the licensee) submitted the updated inservice inspection (ISI) program for the second 10-year Interval at Watts Bar Nuclear Plant (WBN), Unit 1, and Requests for Relief (RR) Nos. PDI-2 and PDI-4. RR PDI-2 proposes an alternative to American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code section XI 2001 Edition through the 2003 Addenda, Examination Category B-D, Full Penetration Welded Nozzles in Vessels - Inspection Program B, Items No. B3.90, Examination Requirements/Figures IWB-2500-7, for the examination volumes of the Class 1 reactor vessel pressure retaining nozzle-to-vessel welds. RR PDI-4 proposes an alternative to ASME Section XI, paragraph IWA-2232 of the ASME Section XI 2001 Edition through the 2003 Addenda, for use of Appendix VIII and Performance Demonstration Initiative (PDI) reactor pressure vessel shell-to-flange welds in lieu of the requirement of Appendix I and the associated Article 4 of ASME Section V. In response to the staffs requests for additional information (RAI), the licensee supplemented the original request with letters dated July 25 and November 9, 2007. The WBN, Unit 1 second 10-year ISI interval started on May 27, 2007 and is scheduled to be completed by May 26, 2017.
The staff has reviewed and evaluated the information provided by TVA regarding RR Nos. PDI-2 and PDI-4. The staffs conclusions are presented in this safety evaluation.
2.0 REGULATORY REQUIREMENTS ISI of the ASME Boiler and Pressure Vessel Code Class 1, 2, and 3 components is performed in accordance with Section XI of the ASME Code and applicable addenda as required by Title 10 of the Code of Federal Regulations (10 CFR) Section 50.55a(g), except where specific relief has been granted by the Nuclear Regulatory Commission (NRC) pursuant to 10 CFR 50.55a(g)(6)(i).
As stated in 10 CFR 50.55a(a)(3), alternatives to the requirements of paragraph (g) may be used, when authorized by the NRC, if: (i) the proposed alternatives would provide an acceptable level of quality and safety or (ii) 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(g)(4), ASME Code Class 1, 2, and 3 components (including supports) shall meet the requirements, except the design and access provisions and the preservice examination requirements, set forth in the ASME Code,Section XI, "Rules for Inservice Inspection of Nuclear Power Plant Components, to the extent practical within the limitations of design,
geometry, and materials of construction of the components. The regulations require that inservice examination of components and system pressure tests conducted during the first 10-year interval and subsequent intervals comply with the requirements in the latest edition and addenda of Section XI of the ASME Code incorporated by reference in 10 CFR 50.55a(b) 12 months prior to the start of the 120-month interval, subject to the limitations and modifications listed therein. The ASME Code of record for the WBN, Unit 1 second 10-year interval ISI program, is the 2001 Edition with addenda through the 2003 Addenda of the ASME Code with amendments specified in 10 CFR 50.55a.
3.0 EVALUATION Request for Relief No. PDI-2 Component Identification RR No. PDI-2 requests relief from the items in ASME Code,Section XI Examination Category B-D, Full Penetration Welded Nozzles in Vessels - Inspection Program B, Item No. B3.90, Reactor Vessel Nozzle-to-Vessel Welds. This inspection and the subject RRs apply specifically to the following locations and ISI weld inspection numbers.
Loop 1 Hot Leg Nozzle-to-Vessel Weld ISI Weld No. N18 Loop 1 Cold Leg Nozzle-to-Vessel Weld ISI Weld No. N14 Loop 2 Hot Leg Nozzle-to-Vessel Weld ISI Weld No. N17 Loop 2 Cold Leg Nozzle-to-Vessel Weld ISI Weld No. N12 Loop 3 Hot Leg Nozzle-to-Vessel Weld ISI Weld No. N16 Loop 3 Cold Leg Nozzle-to-Vessel Weld ISI Weld No. N13 Loop 4 Hot Leg Nozzle-to-Vessel Weld ISI Weld No. N15 Loop 4 Cold Leg Nozzle-to-Vessel Weld ISI Weld No. N11 ASME Code Requirements In accordance with, ASME Code,Section XI, Examination Category B-D, Item No. B3.90 RPV full penetration nozzle welds require volumetric examination of 100 percent of the weld length and volume as defined in figures IWB-2500-7(a) through (b).
ASME Code Case N-460, Alternative Examination Coverage for Class 1 and Class 2 Welds, as an alternative approved for use by the NRC in Regulatory Guide 1.147, Revision 14, Inservice Inspection Code Case Acceptability,Section XI, Division 1, states that a reduction in examination coverage due to part geometry or interference for any ASME Class 1 or 2 weld is acceptable provided that the reduction is less than 10 percent (i.e., greater than 90 percent examination coverage is obtained).
Proposed Alternative TVA will perform the examinations of the reactor pressure vessel (RPV) nozzle-to-vessel welds by performing ultrasonic test (UT) examinations of the RPV nozzle-to-vessel welds in accordance with the requirements of ASME Code,Section XI, Appendix VIII with examination volumes to include regions up to 2 inch beyond the weld boundary.
NRC Staff=s Evaluation The staffs evaluation of RR PDI-2 is presented in two parts: (1) the licensee=s proposed alternative to use the reduced examination area pursuant to 10 CFR 55a(a)(3)(i), and (2) the licensee=s expected less than essentially 100 percent examination coverage of the alternative weld volume pursuant to 10 CFR 50.55a(g)(5)(iii).
RR PDI-2, Part 1 The ASME Code requires a volumetric examination of 100 percent of the weld length and volume as defined in ASME Code.Section XI, Figures IWB-2500-7(a) through (b) for full penetration welds of RPV inlet and outlet nozzles. The licensee has proposed as an alternative to the ASME Code to perform the ASME Code examination on a reduced examination volume of 2 inch beyond the widest part of the boundary of the deposited weld material in lieu of the requirements of ASME Code,Section XI, Figures IWB-2500-7(a) and IWB-2500-7(b).
The licensee provided sketches showing the configuration of the nozzle-to-vessel weld and the revised examination volume. The specific weld configurations and revised examination volumes are depicted in sketches of Figures 1, 2, 3, and 4 attached to the licensee=s request for additional information response letter dated July 25, 2007 (ML072070252). The revised examination volumes depicted in these sketches extends to 2 inch from each side of the widest part of the nozzle-to-vessel weld and are, therefore, consistent with licensee's request for the reduced examination volume. All other aspects of the examination volumes for RPV welds remain unchanged in the licensee's request.
The acceptability of the reduced examination volume is based on prior full volumetric examinations of the welds and base metal, as well as the internal stress distribution near the weld.
Prior full volumetric examinations of the nozzle-to-vessel welds included within the scope of this relief request cover the full volume of base metal, extending to a distance equal to 2 the through-wall thickness from each side of the widest part of the nozzle-to-vessel weld, as required by the ASME Code. This base metal region included in the original ASME Code volume was extensively examined during construction, preservice inspection, and prior ISIs. These examinations all show the ASME Code volume to be free of unacceptable flaws. The creation of flaws during plant service in the volume excluded from the proposed reduced examination volume is unlikely because of the low stress in the base metal away from the weld. The stresses caused by welding are concentrated at, or near, the weld. Cracks, should they initiate, occur in the highly-stressed area of the weld. The highly-stressed areas are within the volume included in the reduced examination volume proposed by the licensee. The prior full volume examinations of the base metal in addition to the examinations of the highly-stressed areas of the weld provide an acceptable level of quality and safety.
The weld volume and the adjacent base metal volume will be examined in accordance with the provided sketches. The examinations will consist of techniques and procedures qualified in accordance with the ASME Code,Section XI, Appendix VIII, Supplements 4, 6, and 7. The weld and base metal volumes will be interrogated from the nozzle bore using techniques and procedures specifically qualified to inspect the nozzle-to-vessel weld from the nozzle bore. These procedures were qualified in January 2003 in accordance with ASME Code,Section XI, Appendix VIII, Supplement 7, as administered by the Performance Demonstration Initiative.
The nozzle-to-vessel examination volume is accessible from the vessel inside diameter (ID) surface and will be examined in four orthogonal directions for the first 15 percent of weld thickness with respect to the vessel ID surface using ASME Code,Section XI, Appendix VIII, Supplement 4 qualified techniques. The remaining 85 percent of weld volume accessible from the vessel ID surface will be examined in two opposing circumferential scanning directions using ASME Code,Section XI, Appendix VIII, Supplement 6, qualified techniques to interrogate for transverse defects.
To ensure the extremities of the weld are included in the examination volume, a margin of 1/2 inch will be conservatively added to the scanning path of all transducers in all directions as allowed by component geometry. This is standard practice for nozzle-to-shell, shell welds, and weld examinations. The sketches included in the licensee's RR reflect this additional conservatism. The licensee also noted that no weld repairs are encapsulated within the existing nozzle-to-vessel welds. Therefore, the staff determined that since there are no repairs in the area to be examined that could extend past the original weld boundaries, the examination will encompass the entire weld and the licensees proposed alternative examination will provide an acceptable level of quality and safety.
Staff Conclusions RR PDI-2, Part 1 The staff has reviewed the licensee's submittal and concludes that the proposed alternative to reduce the UT volume to 2 inch from the widest part of the nozzle-to-vessel weld on each side of the weld crown, in lieu of 2 the through-wall thickness from the widest part of the nozzle-to-vessel weld on each side of the weld crown provides an acceptable level of quality and safety. Therefore, pursuant to 10 CFR 50.55a(a)(3)(i), the proposed alternative is authorized for the subject ASME Code,Section XI, Class 1, RPV welds for the second 10-year ISI interval at WBN, Unit 1.
All other ASME Code,Section XI requirements for which relief was not specifically requested and approved in the subject RRs for relief remain applicable, including third-party review by the authorized Nuclear Inservice Inspector.
RR PDI-2, Part 2 ASME Code,Section XI, Examination Category B-D, Item No. B3.90, RPV Full Penetration Welds of Nozzles requires volumetric examination of 100 percent of the weld length and volume as defined in Figures IWB-2500-7(a) through (b) (i.e., to a distance of 2 of the RPV shell thickness on either side of the weld (2 t)). For RPV Outlet Nozzles Welds N18, N17, N16, and N15, the licensee expects to obtain less-than-essentially-100-percent volumetric coverage of the alternative reduced examination area as described in Part 1 of this RR PDI-2. The licensee is requesting relief from the ASME Code-required examination coverage for examinations of the reduced examination area to be performed in the second 10-year ISI interval for WBN, Unit 1. The licensee compared the volumetric coverage obtained in first 10-year ISI interval for WBN, Unit 1 using the ASME Code-required volume as described in ASME Code,Section XI, Figures IWB-2500-7(a) through (b) to the alternative reduced examination area in Part 1 of this RR.
The licensee noted that the previous examinations indicated that the maximum achievable coverage for the subject outlet nozzle welds is approximately 76 percent. However, the achievable coverage may be greater from that of the previous examinations due to improvements using new volumetric NDE technology and techniques. For the second 10-year ISI interval, the reduced volume examination eliminates a portion of the excess base material from the examination area. However, the nozzle and vessel base materials are less susceptible to cracking than the weld area and heat affected zone, and the proposed reduction in examination volume should not reduce the effectiveness of the volumetric examinations.
In comparing the original required examination volume dimensions verses the proposed alternative examination area, the licensee determined that it will still encounter the same limitation of transducer lift-off during the examination of the reduced examination area due to the design of the subject nozzles. The licensee=s contractor identified that the same limitation is also documented in the pre-service examination data package for nozzles N15, N16, N17, and N18, when examined using the full specified volumes of the ASME Code requirements.
Therefore, based on the information and sketches provided by the licensee, the staff determined that the limitation of transducer lift-off due to the configuration of the subject nozzles will also be encountered during performance of the reduced volume examination and will not adversely impact the quality of the examination as compared to when encountering the same limitation transducer lift-off using the full ASME Code examination volume of 2 t. In addition, the examination of the reduced area as compared to ASME Code examination volume of 2 t should detect any significant service-induced degradation that occurs in the subject welds, and there is reasonable assurance that evidence of it will be detected by the examination. The staff determined that in order for the licensee to perform a volumetric examination the subject nozzles would have to be redesigned. Therefore, the staff determined that the ASME Code-required essentially 100 percent volumetric coverage is impractical on the reduced examination area and that the proposed volumetric examination coverage of 76 percent provides reasonable assurance of structural integrity of the subject nozzles.
Staff Conclusions RR PDI-2, Part 2 For RR PDI-2, Part 2, the staff has reviewed the licensee's submittal and concludes that the ASME Code requirements are impractical and that the licensee=s proposed alternative provides reasonable assurance of structural integrity of the subject nozzles and the examinations should detect any significant service-induced degradation that occurs in the subject welds. Therefore, for RR PDI-2, Part 2, the staff concludes that relief is granted pursuant to 10 CFR 50.55a(g)(6)(i) for the second 10-year ISI interval for WBN, Unit 1. The staff has determined that granting relief pursuant to 10 CFR 50.55a(g)(6)(i) is authorized by law and will not endanger life or property, or the common defense and security and is otherwise in the public interest giving due consideration to the burden upon the licensee that could result if the requirements were imposed on the facility.
All other ASME Code,Section XI requirements for which relief was not specifically requested and approved in the subject requests for relief remain applicable, including third-party review by the authorized Nuclear Inservice Inspector.
Request for Relief No. PDI-4 Component Identification ASME Code Class 1 Reactor Pressure Vessel (RPV) Upper Vessel Shell-to-Flange Welds, Table IWB-2500-1 Category B-A, Item Number B1.30.
ASME Code Requirements In accordance with ASME Code,Section XI, Table IWB-2500-1 Category B-A, Item Number B1.30 upper vessel shell-to-flange welds require a volumetric examination of essentially 100 percent of the weld length. In addition, ASME Code,Section XI, Paragraph IWA-2232 states that:
"Ultrasonic examinations shall be conducted in accordance with Appendix I." Further, ASME Code,Section XI, Appendix I, Paragraph 1-2110 (b) states that: AUltrasonic examination of reactor vessel-to-flange welds, closure head-to-flange welds, and integral attachment welds shall be conducted in accordance with Article 4 of Section V, except that alternative examination beam angles may be used.@
Licensee=s Proposed Alternative Examination TVAs proposed alternative is to use the procedures, personnel, and equipment qualified to meet the requirements of ASME [Code],Section XI, Appendix VIII, Supplements 4 and 6 as administered by the Electric Power Research Institute's Performance Demonstration Initiative (PDI) to conduct the required vessel-to-flange weld examinations.
NRC Staff Evaluation
The licensee has proposed to examine the RPV flange-to-upper shell weld from the reactor vessel inside surface by using procedures, equipment, and personnel qualified by PDI. The licensee will use remote mechanized equipment to examine the reactor vessel flange-to-shell weld from the inside surface in accordance with the PDI requirements. In September 1999, 10 CFR 50.55a was issued and required an expedited implementation of the 1995 Edition with 1996 Addenda, ASME Code,Section XI, Appendix VIII supplements in accordance with specific dates and specific components to be examined. The RPV flange-to-upper shell weld was not included in the September 1999 10 CFR 50.55a PDI requirements.
PDI qualified procedures are technically superior to the previous ASME Code,Section V, Article 4 methodologies that are amplitude based. The PDI examinations are more sensitive to identifying flaws, because the examination sensitivity levels, detailed procedure criteria, and blind demonstrations enhance or verify their effectiveness. Prior to implementing PDI examinations volumetric examinations in accordance with ASME Code,Section V were conducted at the less sensitive level of 50 percent distance amplitude correction curve (DAC) for flaws located in the outer 80 percent of the material thickness and 20 percent DAC for flaws located from the clad-base metal interface to a depth of about 20 percent of the material thickness. In addition the PDI qualified procedures provide a higher level of flaw probability of detection because all signals are interpreted as flaws and are not dependent on minimum amplitude response. All indications will be measured and assessed in accordance with the applicable criteria.
The staff determined that the licensee=s proposed alternative to use the ASME Code,Section XI, Appendix VIII examination requirements in the performance of the reactor vessel inside surface volumetric examination surpasses the quality of the examination techniques specified by the 1989 Edition of Section V of the ASME Code. Therefore, the licensee=s proposed alternative provides reasonable assurance of quality and safety of the RPV flange-to-upper shell weld.
Staff Conclusions For RR PDI-4, the staff has reviewed the licensee's submittal and concludes that the licensee=s proposed alternative provides reasonable assurance of quality and safety and is authorized pursuant to 10 CFR 50.55a(a)(3)(i) for the second 10-year ISI interval at WBN, Unit 1. All other ASME Code,Section XI requirements for which relief was not specifically requested and approved in the subject requests for relief remain applicable, including third-party review by the authorized Nuclear Inservice Inspector.
Principle Contributor: Thomas K. McLellan Date: February 29, 2008