ML050560035

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Evaluation of Relief for Use of Mechanical Nozzle Assemblies as an Alternate to the American Society of Mechanical Engineers Code Repairs
ML050560035
Person / Time
Site: San Onofre  Southern California Edison icon.png
Issue date: 03/09/2005
From: Gramm R
NRC/NRR/DLPM/LPD4
To: Ray H
Southern California Edison Co
Pham B, NRR/DLPM, 415-8450
References
TAC MC0334, TAC MC0335
Download: ML050560035 (20)


Text

March 9, 2005 Mr. Harold B. Ray Executive Vice President Southern California Edison Company San Onofre Nuclear Generating Station P.O. Box 128 San Clemente, CA 92674-0128

SUBJECT:

SAN ONOFRE NUCLEAR GENERATING STATION, UNITS 2 AND 3 -

EVALUATION OF RELIEF FOR USE OF MECHANICAL NOZZLE ASSEMBLIES AS AN ALTERNATE TO THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS (ASME) CODE REPAIRS (TAC NOS. MC0334 AND MC0335)

Dear Mr. Ray:

By letter dated July 2, 2003, as supplemented by letters dated January 22 and September 15, 2004, you requested relief in accordance with 50.55a of Title 10 of the Code of Federal Regulations (10 CFR), associated with the third 10-year interval inservice inspection (ISI) program for San Onofre Nuclear Generating Station (SONGS), Units 2 and 3. Specifically, relief request ISI-3-7 of your July 2, 2003, submittal requested to use a mechanical nozzle seal assembly (MNSA) model, labeled MNSA-2. These MNSAs are intended to be used on a contingency basis to provide an alternative to ASME Section XI repair requirements of bottom head pressurizer heater sleeves found to be leaking, as a result of primary water stress corrosion cracking.

The NRC has reviewed your request for authorization to use the MNSA-2 design, and reports its findings in the enclosed NRC Safety Evaluation. Pursuant to 10 CFR 50.55a(a)(3)(i) the staff authorizes the requested temporary application of the MNSA-2 model on a contingency basis as an alternative to ASME Section XI requirements, of leaking pressurizer heater sleeves at SONGS, Unit 2, for a period not to exceed Cycle 13 operation (estimated to end November 2005), at which time, the licensee intends to modify all Alloy 600 heater sleeves in SONGS, Unit 2, with Alloy 690 heater sleeves as a preemptive measure to preclude future heater sleeve leaks as indicated in its supplemental letter dated September 15, 2004. Such modification was completed in SONGS, Unit 3 during its Cycle 13 refueling outage of October to December 2004, and therefore, further request to use the MNSA-2 is no longer needed for Unit 3.

Sincerely,

/RA/

Robert A. Gramm, Chief, Section 2 Project Directorate IV Division of Licensing Project Management Office of Nuclear Reactor Regulation Docket Nos. 50-361 and 50-362

Enclosure:

Safety Evaluation cc w/encls: See next page

March 9, 2005 Mr. Harold B. Ray Executive Vice President Southern California Edison Company San Onofre Nuclear Generating Station P.O. Box 128 San Clemente, CA 92674-0128

SUBJECT:

SAN ONOFRE NUCLEAR GENERATING STATION, UNITS 2 AND 3 -

EVALUATION OF RELIEF FOR USE OF MECHANICAL NOZZLE ASSEMBLIES AS AN ALTERNATE TO THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS (ASME) CODE REPAIRS (TAC NOS. MC0334 AND MC0335)

Dear Mr. Ray:

By letter dated July 2, 2003, as supplemented by letters dated January 22 and September 15, 2004, you requested relief in accordance with 50.55a of Title 10 of the Code of Federal Regulations (10 CFR), associated with the third 10-year interval inservice inspection (ISI) program for San Onofre Nuclear Generating Station (SONGS), Units 2 and 3. Specifically, relief request ISI-3-7 of your July 2, 2003, submittal requested to use a mechanical nozzle seal assembly (MNSA) model, labeled MNSA-2. These MNSAs are intended to be used on a contingency basis to provide an alternative to ASME Section XI repair requirements of bottom head pressurizer heater sleeves found to be leaking, as a result of primary water stress corrosion cracking.

The NRC has reviewed your request for authorization to use the MNSA-2 design, and reports its findings in the enclosed NRC Safety Evaluation. Pursuant to 10 CFR 50.55a(a)(3)(i) the staff authorizes the requested temporary application of the MNSA-2 model on a contingency basis as an alternative to ASME Section XI requirements, of leaking pressurizer heater sleeves at SONGS, Unit 2, for a period not to exceed Cycle 13 operation (estimated to end November 2005), at which time, the licensee intends to modify all Alloy 600 heater sleeves in SONGS, Unit 2, with Alloy 690 heater sleeves as a preemptive measure to preclude future heater sleeve leaks as indicated in its supplemental letter dated September 15, 2004. Such modification was completed in SONGS, Unit 3 during its Cycle 13 refueling outage of October to December 2004, and therefore, further request to use the MNSA-2 is no longer needed for Unit 3.

Sincerely,

/RA/

Robert A. Gramm, Chief, Section 2 Project Directorate IV Division of Licensing Project Management Office of Nuclear Reactor Regulation Docket Nos. 50-361 and 50-362

Enclosure:

Safety Evaluation cc w/encls: See next page DISTRIBUTION:

PUBLIC JDixon-Harrity RidsNrrDlpmLpdiv WKoo RidsNrrLADBaxley PDIV-2 r/f RidsNrrDlpmLpdiv-2 RidsAcrsAcnwMailCenter RidsOgcRp TChan RidsRgn4MailCenter KManoly MHartzman Accession No. ML050560035 NRR-106 OFFICE PDIV-2/PM PDIV-1/LA EMEB EMCB OGC w/Nlo PDIV-2/SC NAME BPham DBaxley KManoly TChan RHoefling RGramm DATE 3/9/05 3/4/05 1/25/05 2/8/05 02/14/05 3/9/05 OFFICIAL RECORD COPY

San Onofre Nuclear Generating Station, Units 2 and 3 cc:

Mr. Daniel P. Breig, Plant Manager Mr. Ed Bailey, Chief Nuclear Generation Radiologic Health Branch Southern California Edison Company State Department of Health Services San Onofre Nuclear Generating Station Post Office Box 997414 (MS7610)

P. O. Box 128 Sacramento, CA 95899-7414 San Clemente, CA 92674-0128 Resident Inspector/San Onofre NPS Mr. Douglas K. Porter c/o U.S. Nuclear Regulatory Commission Southern California Edison Company Post Office Box 4329 2244 Walnut Grove Avenue San Clemente, CA 92674 Rosemead, CA 91770 Mayor Mr. David Spath, Chief City of San Clemente Division of Drinking Water and 100 Avenida Presidio Environmental Management San Clemente, CA 92672 P. O. Box 942732 Sacramento, CA 94234-7320 Mr. Dwight E. Nunn, Vice President Southern California Edison Company Chairman, Board of Supervisors San Onofre Nuclear Generating Station County of San Diego P.O. Box 128 1600 Pacific Highway, Room 335 San Clemente, CA 92674-0128 San Diego, CA 92101 Mr. James D. Boyd, Commissioner Eileen M. Teichert, Esq. California Energy Commission Supervising Deputy City Attorney 1516 Ninth Street (MS 31)

City of Riverside Sacramento, CA 95814 3900 Main Street Riverside, CA 92522 Mr. Ray Waldo, Vice President Southern California Edison Company Mr. Gary L. Nolff San Onofre Nuclear Generating Station Power Projects/Contracts Manager P.O. Box 128 Riverside Public Utilities San Clemente, CA 92764-0128 2911 Adams Street Riverside, CA 92504 Mr. Brian Katz Vice President, Nuclear Oversight and Regional Administrator, Region IV Regulatory Affairs.

U.S. Nuclear Regulatory Commission San Onofre Nuclear Generating Station 611 Ryan Plaza Drive, Suite 400 P.O. Box 128 Arlington, TX 76011-8064 San Clemente, CA 92764-0128 Mr. Michael Olson Mr. Steve Hsu San Diego Gas & Electric Company Department of Health Services P.O. Box 1831 Radiologic Health Branch San Diego, CA 92112-4150 MS 7610, P.O. Box 997414 Sacramento, CA 95899

SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO RELIEF FOR USE OF MECHANICAL NOZZLE ASSEMBLIES AS AN ALTERNATIVE TO THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS (ASME)

BOILER AND PRESSURE VESSEL CODE (CODE) REPAIRS AT SAN ONOFRE NUCLEAR GENERATING STATION (SONGS), UNITS 2 AND 3 SOUTHERN CALIFORNIA EDISON COMPANY DOCKET NOS. 50-361 AND 50-362

1.0 INTRODUCTION

By letter dated July 2, 2003, as supplemented by letters dated January 22 and September 15, 2004, Southern California Edison (SCE, the licensee) submitted requests to install on a temporary basis a new design of mechanical nozzle seal assemblies (labeled MNSA-2) as an alternative to the requirements of ASME Code Section XI, pursuant to 10 CFR 50.55a(a)(3)(i).

This design is an improvement of an older model MNSA (labeled MNSA-1) that had been previously installed on a temporary basis at SONGS, Units 2 and 3. SCE intends to install, on a contingency basis, MNSA-2s on heater sleeves located at the bottom head of the pressurizers which are found during inspection to be leaking, primarily as a result of primary water stress corrosion cracking (PWSCC). At the time of its original submittal, SCE had requested authorization to install MNSA-2 at both Units 2 and 3 for Cycle 13 operation, in the event that heater sleeves on the pressurizers are found to be leaking. During the NRC staffs review of this request, however, modifications were completed at Unit 3 to replace Alloy 600 heater sleeves with Alloy 690, therefore, this safety evaluation only applies to Unit 2, despite having both units mentioned in various sections.

2.0 BACKGROUND

The heater sleeves are welded to the pressurizer bottom head with internal J-groove welds.

These welds have been found to be susceptible to PWSCC. The typical permanent repair of these sleeves consists of installing a half nozzle or heater sleeve replacement with an external weld repair, in accordance with ASME Section XI requirements. In the past, the NRC has accepted MNSA repairs on a temporary basis for two operating cycles. By letter to licensees dated December 5, 2003, the NRC staff provided additional guidance regarding information needed to justify the use of MNSAs as a means for permanent repair.

Currently there are no MNSA-2s installed at either Unit 2 or Unit 3. SCE is currently planning to modify all Alloy 600 heater sleeves with Alloy 690 heater sleeves as a preemptive measure to preclude future heater sleeve leaks. This modification has been completed in SONGS Unit 3 during its Cycle 13 refueling outage (RFO) of October to December 2004, and is planned to occur during the Unit 2 Cycle 14 RFO (currently scheduled to begin in November of 2005).

Until the Alloy 600 heater sleeves are modified during the Cycle 14 RFO for SONGS, Unit 2, SCE considers the MNSA-2 repair, without flaw characterization, to be a prudent alternative to the half nozzle repair because it precludes the need to enter an unscheduled reduced inventory condition. Therefore, the licensee requests to use MNSA-2s during the remainder of Cycle 13 operation for SONGS, Unit 2.

3.0 EVALUATION OF RELIEF REQUEST - ISI-3-7, MNSA-2 3.1 Component for Which Relief is Requested:

Pressurizer Heater Sleeves, ASME Section III, Class 1.

3.2 Code Requirement:

Section 55a to 10 CFR Part 50, requires in part in paragraph (g), that all inservice examinations and system pressure tests conducted during the first 10-year interval and subsequent intervals on ASME Code Class 1, 2, and 3 components comply with the requirements in the latest edition and addenda of Section XI incorporated by reference in 10 CFR 50.55a(b), on the date 12 months prior to the start of the 10-year interval. By reference to, and implementation of, ASME Code Section XI, paragraphs IWB-3132 or IWB-3142,10 CFR 50.55a also requires that existing flaws in ASME Code Class 1, 2, or 3 components be removed by mechanical means, or else that the components be repaired or replaced to the extent necessary to meet the acceptance standards in ASME Code Section XI, Article IWB-3000. Detection of leaks in the structural portion of an ASME Code Class 1, 2, or 3 component is direct evidence of a flaw in the component.

Paragraph IWA-4170 of Section XI of the ASME Code requires that repairs and the installation of replacements to the reactor coolant pressure boundary be performed and reconciled in accordance with the Owners Design Specifications and Original Code of Construction for the component or system. The pressurizers in SONGS were designed and constructed to the rules of ASME Section III, 1971 Edition with Addenda through Summer 1971.

ASME Code Section III, Paragraph NB-3671.7 Sleeve Coupled and Other Patented Joints, requires that ASME Code Class 1 joints be designed to meet the following criteria:

(1) provisions must be made to prevent separation of the joint under all service loading conditions, (2) the joint must be designed to be accessible for maintenance, removal, and replacement activities, and (3) the joint must either be designed in accordance with the rules of ASME Code,Section III, Subarticle NB-3200, or else be evaluated using a prototype of the joint that will be subjected to additional performance tests in order to determine the safety of the joint under simulated service conditions.

These conditions apply to the design, installation, inspection, and maintenance of MNSAs.

3.3 Licensees Proposed Alternative:

Pursuant to 10 CFR 50.55a(a)(3)(i), SCE requests NRC authorization to use the improved design of the MNSA-2, in applications at the heater sleeves located on the bottom head of the pressurizer vessels at SONGS, Unit 2. SCE makes this request in order to repair leaks attributed to PWSCC.

The typical repair of nozzles or heater sleeves of this type uses a half-nozzle replacement with external weld repair. These repairs would extend high risk reduced inventory reactor coolant system (RCS) drain-down activities and significantly increase worker radiation exposure to perform extensive field machining and temper bead welding activities.

As an alternative, SCE proposes to use the MNSA-2 as a repair to restore pressure boundary integrity and prevent leakage.

3.4 Licensees Basis for Relief (As Stated):

A. Background The pressurizer, including the heater sleeve penetration assemblies, was designed by Combustion Engineering. Combustion Engineering is currently owned by Westinghouse Electric Company.

The pressurizer heater sleeves, thirty (30) on each Unit's pressurizer, are manufactured from Ni-Cr-Fe, SB-167 material (Alloy 600). One heater sleeve at SONGS 3 has a half nozzle welded to the exterior of the vessel. The remaining 30 heater sleeves at SONGS Unit 2 and 29 heater sleeves at SONGS Unit 3 are welded to the internal cladding of the vessel lower head and the heater elements are welded to the lower end of the sleeves. The heater elements are internally supported for seismic loading and vibration by two heater support plates. The outside diameter of the sleeve is approximately 1.660 inches, and the inside bore is approximately 1.273 inches. The length of the sleeves varies from approximately 14 3/8 inches long to approximately 18 3/8 inches long, depending on the location on the bottom head. The upper end of the heater sleeve is provided with a short oversize segment to serve as an anti-ejection device should the sleeve to vessel weld fail completely.

  • Pressurizer Vessel The pressurizer is a low alloy steel vessel with the shell and top head internally clad with 304 austenitic stainless steel and the bottom head with a Ni-Cr-Fe cladding.

The Ni-Cr-Fe heat affected zone of the J-weld has proven to be susceptible to PWSCC. Numerous instances of nozzle cracking have been identified in the industry in recent years. Studies performed by the Westinghouse Owner's Group (Report CENPSD-690-P, Appendix 1, Reference 1 [of Enclosure 7 to the

licensees July 2, 2003, submittal]) have found that the crack growth is predominantly axial. The dominant condition that promotes axial growth rather than circumferential growth is high circumferential stress (hoop stress) compared to the axial stress. The hoop stresses are due to residual stress caused by weld shrinkage that diminishes quickly as the distance from the J-weld increases and operating stresses. The susceptibility to PWSCC is based on several factors that deal with material, stress, and environment.

Inspections required by ASME Section Xl, IWB-2500 for Examination Category B-P are performed during each refueling outage. Additionally, the walkdown inspections performed in response to Generic Letter 88-05, "Boric Acid Corrosion of Carbon Steel Reactor Coolant Pressure Boundary Components" as recommended by the Combustion Engineering Owner's Group are performed during each refueling outage.

B. MNSA-2 Application, Description, and Design

1. Overview The MNSA-2 is a mechanical device designed to replace the function of partial penetration J-groove welds that attach Alloy 600 nozzles or heater sleeves to the pressurizer. MNSA-2 provides a seal against leakage and positively captures the sleeve preventing ejection in the unlikely event of complete 360-degree weld failure. Figure 1 [of Enclosure 7 to the licensees July 2, 2003, submittal] shows a representative drawing of the MNSA-2 for a heater sleeve.

To install the MNSA-2, four holes are drilled and tapped (1/2" diameter x 1 1/2" deep) equally spaced around the leaking sleeve. A counter-bore (approximately 1/4" wide x 3/4" deep) is also machined into the surface of the vessel perpendicular to and around the leaking sleeve. Four threaded rod studs are threaded into the pressurizer, a split Grafoil primary seal is installed in the bottom of the counter-bore, and a split compression collar is placed over the sleeve to compress the Grafoil seal. The seal assembly is compressively loaded via the compression collar and the inboard and outboard flange assembly, which is in the annulus region. Hex nuts and Belleville spring washers are used to live load the Grafoil seal to accommodate small changes in load on the seal due to differential expansion or minute relaxation of the seal over time and prevent seal leakage.

To prevent heater sleeve ejection in the unlikely event of a complete sleeve weld failure, an anti-ejection clamp is also installed and secured in place via the tie rods, Belleville spring washers, and hex nuts. The anti-ejection clamp acts as a restraint only if the sleeve or partial penetration weld completely fails.

More specific details of the MNSA-2 design are provided in Section B.2, below.

2. Design The NRC previously authorized use of the MNSA-2 design for nozzles and sleeves at Waterford 3. The NRC has approved similar requests for temporary repair of pressurizer instrument nozzles and heater sleeves by MNSA-1 (the original design) at Southern California Edison's San Onofre Nuclear Generating Station; Entergy Operations Inc.'s Waterford 3; Arizona Public Service Company's Palo Verde Nuclear Generating Station; and at Dominion Nuclear Connecticut's Millstone Nuclear Power Station.

The original MNSA-1 and MNSA-2 use the same materials of construction and the same seal material. They are attached in a similar fashion, and the seal is loaded by tensioning bolts or studs.

The MNSA-2 design, shown in Figure 1, differs from the original MNSA design in three ways:

  • The counter-bore provision that contains the seal
  • The manner in which the seal is live-loaded
  • The means for diverting leakage, should it occur Each is discussed in detail below.

a) Counter-Bore Provision MNSA-2 uses nuclear grade Grafoil as the sealing material. In all cases, regardless of the angle of the surface of the pressurizer relative to the sleeve, a counter-bore is machined perpendicular to the sleeve to receive and contain the seal. The bottom of the counter-bore is perpendicular to the axis of the sleeve, so the angle of the surface of the pressurizer does not affect the leak tightness of the design. When the MNSA-2 seal is compressed, no side loads are introduced, so shoulder bolts used on the original MNSA-1 are not required. The seal designs are simpler than the original MNSA-1 because they involve no variable angles. Therefore, customizing MNSA components for particular slope angles, for other than bolt lengths, is not required.

b) Seal Live-Loading MNSA-2 uses a live-loaded seal that can accommodate small changes in load on the seal due to differential expansion. The live load provision, provided via Belleville washers, also accommodates minute relaxation of the seal over time to prevent leakage. Finally, it allows for re-tightening of the studs and reloading the seal at some point in the future without disassembly, whereas the original MNSA-1 would require a new seal and complete teardown and re-assembly to reenergize a seal. Figure 1 [of Enclosure 7 to the licensees July 2, 2003, submittal] shows the use of Belleville spring washers.

c) Leak-Off Diversion Leakage control in the MNSA-2 design is accomplished by using a compression collar which includes a collection area (similar to a "lantern ring") positioned immediately outboard of the primary seal, as shown in Figure 1. The compression collar has an additional Grafoil seal at the top that is lightly loaded.

The seal blocks leakage from passing up along the outside of the compression collar where it could reach the threaded rods. The path of least resistance is out through the annulus between the compression collar and the sleeve, tending to divert any leakage away from the fasteners and the vessel. The presence of the collection area does not impair the primary seal in any way.

In the review of the original MNSA-1 design, the NRC evaluated potential corrosion effects of boric acid on the MNSA and associated RCS components.

The evaluation considered:

  • Corrosion of the low alloy material with a MNSA-1 installed was determined to be acceptable
  • Boric acid corrosion of the materials of construction for the MNSA-1 was determined to be acceptable based on CE Owner's Group corrosion testing
  • There is no history of galvanic corrosion problems in similar applications with Grafoil contacting low alloy steel.
  • Potential for SCC failures of the SA 453 Grade 660(A-286) bolts was found to be acceptable There are no changes from the original MNSA-1 to MNSA-2 that adversely impact the four conclusions listed above. With regard to the SA 453 Grade 660 (A-286) bolts, the NRC evaluation concluded that the bolts could be exposed to boric acid deposits or slurries, if the MNSA-1 leaks. This evaluation was appropriate because the design did not include provisions for capturing or diverting seal leakage away from bolting materials. Regardless, at the stress levels that exist in the bolts, including a stress concentration factor of four, the bolts would function satisfactorily. In contrast to the original MNSA-1, the MNSA-2 design includes specific provisions to divert potential seal leakage away from the low alloy steel vessel and the bolting as described below.

The sealing qualities of MNSA-2 are enhanced beyond that of the original MNSA-1 by virtue of the controlled geometry (counter-bore), and by maintaining a live load on the seal. The counter-bore design has been used routinely in hundreds of similar applications for sealing fixed in-core detectors to flanges on the reactor head in Combustion Engineering units. A variety of other repairs and permanent flange upgrades have been installed on both Combustion Engineering and Westinghouse units using both static and live-loaded Grafoil seal technology. Therefore, the possibility of a leak past the primary seal is very small. Nevertheless, in the unlikely event of such a leak, MNSA-2 is designed to limit exposure of the SA-453 grade 660 (A-286) bolting material and the carbon steel vessel by providing a leak-off path.

d) Installation The MNSA-2 installation process will be performed such that it will not degrade the existing heater sleeve pressure boundary, and it does not require draining of the pressurizer to install. The tooling is designed to machine the counter-bore without removing the pressure boundary heater element.

Torquing the MNSA-2 threaded rods into the pressurizer will be performed at temperatures above bounding RTNDT (10EF for the bottom head) to ensure the bolting stress does not create a potential for brittle failure. The stress calculations (References 4 and 6) document the installation torque values of 27 ft-lbs for the threaded rods. Pre-load conditions are addressed in the installation procedures.

3. MNSA-2 Materials The MNSA-2 assembly is fabricated from the same materials as the original MNSA-1, though with different application of some of the components. A detailed assessment of the MNSA-2 metallic components as related to general corrosion, stress corrosion cracking of nozzles and fasteners, galvanic effects, crevice corrosion, and surface pitting of the constituent components is contained in Appendix 1 of this relief request. There are no potential corrosion problems associated with the application of the MNSA-2 to Alloy 600 small diameter heater sleeves.

The stainless steel portions of the MNSA-2 performing an RCS pressure boundary function are manufactured in accordance with material specifications provided in ASME Section III, Subsection NB and the applicable ASME Section III Appendices. Additionally, the material meets the requirements contained in NB-2000 including examination and testing. Materials are supplied to the provisions of ASME Section III, NCA-3800 by suppliers maintaining a valid Quality System Certificate or a Certificate of Authorization with the scope of Material Supply. Metallic pressure boundary material is certified in accordance with ASME Section III, NCA-3800.

The primary Grafoil seal material is Grade GTJ (used in nuclear applications) composed of 99.5% graphite, with the remaining 0.5% made up of ash, halides, and sulfur. The Grafoil seal itself is chemically resistant to attack from organic and inorganic fluids, and is very resistant to borated water. Similar Grafoil material is used as valve packing in valves installed in the RCS with acceptable results. The Grafoil material is provided under the provisions of a Quality Assurance Program meeting 10 CFR 50 Appendix B that has been approved by SCE. Material testing and certification is provided with the material to verify compliance with the engineered features that are required to ensure functionality and compatibility with the pressure boundary materials and environment.

In summary, there are no potential corrosion or material stress issues associated with applying the MNSA-2 to the pressurizer heater sleeves.

4. MNSA-2 Structural Evaluation The component parts of the MNSA-2 for heater sleeve are analyzed, designed, and manufactured in accordance with ASME Section III, Subsection NB, 1989 Edition, which is approved in 10 CFR 50.55a. The SONGS original Construction Code for the pressurizer is ASME Section III, 1971 Edition (Reference 2),

through and including the Summer 1971 Addenda. As required by ASME Section Xl, an addendum to the SONGS Units 2 and 3 Pressurizer Stress Reports CENC 1275 and CENC 1296 (Reference 7) was completed and includes a reconciliation for use of the 1989 Edition of ASME Section III (Reference 1) as it applies to the MNSA-2 and its interface with the pressurizer.

The analysis for the MNSA-2 components addressed:

  • Stresses not to exceed the allowables as stated in the Code.
  • Fatigue to demonstrate that the Code-prescribed cumulative usage factor of 1.0 is not exceeded (NB-3222.4) for any component The stress analysis considered the loads transmitted to the components of the MNSA-2 due to installation pre-load, normal, upset, and faulted loads at pressure and temperature, and impact loads due to the ejection of the heater sleeve in the unlikely event of a complete failure of the J-weld. The results of the stress analysis demonstrate that the applied stresses on each load-bearing component (tie rods, threaded rods, and top plate) are below the applicable Code allowables, thereby providing assurance of structural integrity for the MNSA-2.

Fatigue evaluations of the MNSA-2 clamp components considered a forty year design life. The calculated fatigue usage factors in Reference 6 are less than 1.0 for MNSA-2 components. The primary component of the usage factors is the stress range between heat-up and cooldown conditions. However, for ten years of operation, the expected numbers of heat-up and cooldown cycles are substantially less than those accounted for in the stress analysis for a 40-year design life.

5. Pressurizer Modification and Structural Evaluation The MNSA-2 is attached to the pressurizer with SA-453 Grade 660 threaded rods and hex nuts. To accommodate the threaded rods, four holes are drilled and tapped into the pressurizer in a circular pattern around the sleeve. To provide a seating surface for the Grafoil seal, a counter-bore is machined into the pressurizer extending out approximately 1/4" from the existing sleeve bore and to a depth of 3/4". The addition of the holes in the pressurizer was analyzed and documented in an attachment to the Westinghouse calculation CN-CI-02-73 (Reference 6) for the heater sleeve locations. The analysis is performed to the requirements of ASME Section III, 1971 Edition through and including the Summer 1971 Addenda. The analysis addresses:
  • Stresses not to exceed the allowables as stated in the Code.
  • Fatigue to demonstrate that the Code prescribed cumulative usage factor of 1.0 is not exceeded (NB-3222.4) at any location.
  • Adequate reinforcement in the wall of the pressurizer for the tapped holes and counter-bore exists (NB-3330)

The stress analysis considered all loads evaluated in the pressurizer original design stress report, including all pressure and temperature transients, the differential thermal expansion loads due to the threaded rods in the tapped holes, compression collar loads, and the loads on the existing J-weld at operating and during shutdown conditions. The applied stresses and stress ranges were evaluated at the counter-bore region and at the tapped holes for compliance with Code allowables. The applied stresses on the pressurizer were modified by the appropriate geometry factors for non-radial effects (where applicable) and by additional factors to take into account stress interaction between the tapped holes and the counter-bore as determined by finite element analysis (FEA). The results of the stress analysis, considering the tapped holes and counter-bore in the pressurizer shell, demonstrate applied stresses are below ASME Code allowables and provide assurance of vessel structural integrity.

Fatigue evaluations of the pressurizer shell in the vicinity of the tapped holes and counter-bores considered a forty-year design life. The calculated fatigue usage factors in Reference 6 are less than 1.0 in the vicinity of the tapped holes and counterbores for any location subject to MNSA-2 installation. The primary component of the usage factors is the stress range between heat-up and cooldown conditions. However, for ten years of operation, the expected number of heat-up and cooldown cycles are substantially less than those accounted for in the stress analysis for a 40-year design life.

The area reinforcement calculations performed in the original design stress report in accordance with ASME Code Section III NB-3330 were updated to evaluate the removal of pressurizer metal area by machining the tapped holes and counter-bores. The results of the analysis in Reference 6 showed that for each pressurizer heater sleeve location evaluated for possible MNSA-2 installation, the area available for reinforcement is greater than the area required as a result of metal removal.

C. MNSA-2 Design Requirements In accordance with ASME Section Xl, IWA-7200, replacements shall meet the existing design requirements and the original Construction Code. Alternatively, replacements may meet later Editions of the original Construction Code provided:

  • The requirements affecting the design, fabrication, and examination of the item to be used for replacement are reconciled with the Owner's through the Stress Analysis Report, the Design Report, or other suitable method that demonstrates the item is satisfactory for the specified design and operating conditions.
  • Mechanical interfaces, fits, and tolerances that provide satisfactory performance are compatible with the system and component requirements.
  • Materials are compatible with installation and system requirements.

ASME Section III NB-3200 rules are followed for designing and manufacturing the MNSA-2. Specifically, the joints will be designed to meet the following criteria:

1. Provisions must be made to prevent separation of the joint under all service conditions.
2. The joint must be designed to be accessible for maintenance, removal, and replacement activities.
3. The joint must either be designed in accordance with the rules of ASME Section III, Subarticle NB-3200, or be evaluated using a prototype of the joint that will be subjected to additional performance tests in order to determine the safety of the joint under simulated service conditions.

These topics are discussed below.

1. Joint Integrity In addition to the prototype testing discussed below, the MNSA-2 is analyzed to meet the requirements of NB-3200. The MNSA-2 is designed as an ASME Section III, Class 1, safety-related primary pressure boundary in accordance with the rules of NB-3200 to prevent joint separation under service loads. An addendum to Pressurizer Stress Reports CENC-1275 and CENC-1296 for SONGS Units 2 and 3 (Reference 7) demonstrates that stresses under all service conditions do not exceed the Code allowables as stated within Section III and that fatigue limits are not exceeded using the conditions contained in the Design Specification.
2. Maintenance, Removal, and Replacement Typical for mechanical connections, the MNSA-2 will be accessible for maintenance, removal, and replacement after service. The MNSA-2 is manufactured without welding and is bolted in place, so disassembly is a mechanical evolution that requires de-tensioning the installation bolting.
3. Qualification Testing The original MNSA-1 design was qualified by a series of tests and analyses.

Entergy Operations, Inc. in their submittal for MNSA-2 (Reference 8) discussed hydrostatic and thermal cycling qualification tests.

Seismic Qualification Testing:

Seismic qualification was performed for the SONGS MNSA-2 assembly in accordance with the guidelines in IEEE-344. A test specimen representative of an outer heater sleeve MNSA-2 design for SONGS Units 2 and 3 was attached to an adapter plate and mounted to a shaker table. The heater sleeve test specimen was not welded to the mounting fixtures. The MNSA-2 components were assembled and installed onto the simulated heater sleeve mock-up. The seismic testing consisted of subjecting the MNSA-2 test rig to five operating basis earthquake events and one safe shutdown earthquake event. The mounting fixture permitted pressurization to 3,175 psig +/- 50 psig at ambient temperature during the seismic test. This elevated pressure was conservatively used to account for the fact that the seismic testing was performed at ambient temperatures rather than operating temperatures. The test results indicate that no mechanical damage occurred and no leakage was present during or after the test. Information contained in Reference 5 provides a basis for performing the seismic testing using ambient temperatures and concludes that the test results were applicable to hot conditions.

Summary:

The test program, test results, and analyses described in References 4, 5, 6, 7 and 8 have been reviewed and found to adequately represent or bound the conditions for which SCE proposes to install the MNSA-2 at SONGS Units 2 and 3.

The MNSA-2s to be installed at SONGS Units 2 and 3 will be subjected to the conditions described below which are obtained from the Design Specification (Reference 9) and form part of the basis for analysis. As evidenced by design analyses (References 4, 6, & 7), and seismic test report (Reference 5), the design conditions equal or exceed the operating conditions for which the clamps will be exposed.

SONGS 2/3 MNSA-2 Conditions Design Design Pressure 2500 psia 2500 psia Design Temperature (Pressurizer) 700EF 700EF Nominal Operating Pressure 2250 psia Normal Temperature (Pressurizer) 653EF D. Inservice Testing and Inspection

1. ASME Section Xl Preservice ASME Code Section Xl Preservice inspection requirements, applicable to the MNSA-2 during each 10-year inspection (ISI) interval, include a system leak test at the end of each refueling outage and bolting examination, based on the schedule of percentages required. For the MNSA-2 installed on pressurizer heater sleeves, the Bolting B-G-2 examination requirements would allow VT-1 examination to be performed as follows: (a) in place under tension, and (b) when the connection is disassembled or when the bolting is removed. This examination is required once each ten-year interval.
2. ASME Section XI Pressure Tests A VT-2 examination will be performed in conjunction with a system leakage pressure test (per IWA-5000) as part of plant re-start and will be conducted at normal operating pressure and temperature for SONGS Unit 2 and 3 Pressure and Temperature Limits as stated in the applicable Technical Specifications.
3. ASME Section Xl Inservice Inspection The VT-1 inservice inspections required by ASME Section Xl for Examination Category B-G-2 are performed during each refueling over the 10-year interval and would not be performed more frequently than each refueling cycle. The VT-2 inspection required by ASME Section Xl for Examination Category B-P is required to be performed prior to plant startup following each refueling outage.

A bounding flaw evaluation will be performed for all pressurizer heater sleeves in accordance with the 1989 ASME section Xl Code, IWB-3600. Existing site specific bounding flaw evaluations will be updated to address the installation of a MNSA-2 clamp. Flaw growth due to fatigue and stress corrosion cracking has been previously considered and will be reevaluated for potential MNSA-2 effects.

E. Additional Outage Inspections After a MNSA-2 is installed, it will be included in the SONGS Boric Acid Leakage Program and Alloy 600 Inspection Program. These programs were recently discussed in detail in the SCE response to a request for additional information regarding Bulletin 2002-02 (Reference 10).

Additionally, SCE will visually inspect for leakage the counterbore/annulus region of each installed MNSA-2 device during each refueling outage.

3.5 Evaluation

3.5.1 Mechanical Evaluation 10 CFR 50.55a(a)(3) allows licensees to use alternatives to the requirements of the ASME Code when authorized by the Director of the Office of Nuclear Reactor Regulation. The licensee must demonstrate that, pursuant to the requirements of 10 CFR 50.55a(a)(3)(i), the alternatives will provide an acceptable level of quality and safety in lieu of meeting the ASME Code requirements, or that, pursuant to the requirements of 10 CFR 50.55a(a)(3)(ii), complying with the Code requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.

The licensee requests the use of MNSA-2s pursuant to 10 CFR 50.55a(a)(3)(i), stating that this alternative provides an acceptable level of quality and safety. To determine if MNSA-2s provide an acceptable level of quality and safety, the staff compared the MNSA-2 design and operational characteristics to the applicable ASME Code requirements and evaluated the licensees commitments associated with the use of MNSA-2s.

A MNSA is designed, fabricated, and constructed in accordance with ASME Section III, 1989 Edition, no Addenda, paragraph NB-3200, using approved ASME Code materials (except for the Grafoil gasket, which is a non-Code material), in accordance with applicable ASME Section III, Subsection NB rules. The assembly is designed to prevent separation of the device from the wall under all service loadings, and thus acts as a complete replacement of the J weld between the Alloy 600 sleeve and the pressurizer. It therefore replaces both the sealing and the structural integrity of the J welds, and becomes part of the reactor pressure boundary.

Leakage from a MNSA should therefore be treated as pressure boundary leakage. The design is supported by the Westinghouse technical analysis and tests that meet the design criteria specified in ASME Code Section III, Subsection NB. The technical analysis includes the required ASME Section XI, IWA-4170(b), reconciliation of the construction codes for the use of a component built to a later edition of the Code, which the staff finds acceptable. Additionally, MNSA installations are accessible for maintenance, removal, and replacement. The provisions of NB-3671.7 are therefore satisfied.

MNSAs of the MNSA-1 type have been approved for installation on a temporary basis at other nuclear plants (e.g., Palo Verde Nuclear Generating Station). The acceptance was based on industry experience which demonstrated that the structural integrity and leak tightness of the MNSAs, and the structural integrity of the components to which the MNSAs are attached, was maintained for at least through one or two cycles. The staff has also reviewed qualifying

seismic and other structural tests performed by the manufacturer that demonstrate the structural integrity of the MNSA-2s. SCE has also provided revised ASME Section III Class 1 fatigue analyses of the pressurizer, modified to account for the presence of the MNSA-2 counterbore and bolt holes, to demonstrate that the fatigue requirement of NB-3222.4(e) is met for the life of the plant. This requirement is that the cumulative usage factor (CUF) not exceed 1.0 under the licensing design conditions. Based on examination of the calculations submitted by the licensee, the staff considers the probability of exceeding the ASME Section III Class 1 CUF limit in the short-term operation of the one cycle requested by the licensee as very low.

3.5.2 Materials Evaluation The MNSA-2 uses the following materials for its components:

(1) Nuclear grade (grade GTJ) Grafoil as the sealing material (2) Stainless steel A286 (SA-453 grade 660) for fasteners (threaded rod, tie rod, and hex nut)

(3) 17-7 PH stainless steel for Belleville spring washers (4) Stainless steel type 340 (SA-479) for compression collar, upper flange, and top plate Except for Grafoil, all materials used for MNSA-2 are Code-approved materials. The nuclear grade Grafoil seal materials consist of 99.5 percent graphite with the remaining 0.5 percent made up of ash, halides, and sulfur and is very resistant to borated water. The licensee stated that Grafoil material is provided under the provisions of a Quality Assurance Program meeting 10 CFR Part 50 Appendix B that has been approved by SCE. Similar Grafoil material is used as valve packing in valves installed in the RCS with acceptable results. The Grafoil seal was also used in a variety of other repairs and permanent flange upgrades on both Combustion Engineering and Westinghouse units. Based on the service experience of the Grafoil materials in similar applications, the use of Nuclear Grade Grafoil as seal materials in the MNSA-2 design is acceptable.

The licensee has performed a detailed assessment of the MNSA-2 components regarding the corrosion of MNSA-2 materials. The licensee concluded that there are no potential corrosion (carbon and low alloy steel), galvanic corrosion (Grafoil to carbon and low alloy steel) or stress corrosion cracking issues associated with applying the MNSA-2 to the pressurizer heater sleeves. The staff notes that except for the seal materials, all MNSA-2 components are made of various types of stainless steel materials. In addition, the MNSA-2 assembly does not have any weld joints, therefore, the stainless steel components under normal operation condition will not be susceptible to any corrosion-related degradation because there are no sensitized materials in the assembly. The MNSA-2 repair does not require the removal of the cracks and, therefore, the through-wall cracks will be left as-is in the components. The staff also notes that the cracking in the heater sleeve will not be characterized during the MNSA-2 repair and, consequently, the extent and location of the cracks will not be known. Without a detailed flaw characterization, it is necessary to assume that the cracks are present in the attachment weld and in the heater sleeve adjacent to the attachment weld. In addition, due to the leakage from the heater sleeve, the crevice area between the attachment weld and the primary seal of the MNSA-2 assembly will be filled with borated water. The low alloy carbon steel of the pressurizer in the crevice area is not cladded and is susceptible to corrosion when exposed to

primary coolant environment (borated water). Therefore, there are three areas of concern resulting from a repair of the leaking heater sleeve by the MNSA-2: (1) the extent of corrosion of low alloy carbon steel due to exposure to borated primary water in the crevice area surrounding the heater sleeve, (2) the potential for the cracks left in the attachment weld to propagate into the adjacent low alloy carbon steel by means of fatigue crack growth mechanism and (3) the potential for the continued growth of the cracks left in the heater sleeve.

Based on the available laboratory test results, the corrosion rate of the low alloy carbon steel in the borated primary water environment is expected to be low during normal operating condition.

The potential for general corrosion of the ferritic material was evaluated in Westinghouse Topical Report, WCAP-15973-P, Revision 01, "Low-Alloy Steel Component Corrosion Analysis Supporting Small-Diameter Alloy 600/690 Nozzle Repair/Replacement Programs." This report was submitted for NRC review to support the use of MNSA and half-nozzle for repair of small-diameter Alloy 600 nozzle on a permanent basis. The general corrosion was estimated to be about 1.53 mils per year. Based on this rate, the degradation of the ferritic material will not exceed the code allowable at the end of the plant life. Accelerated boric acid corrosion is not considered in this report because free oxygen does not exist in the closed environment and there is no known mechanism for concentrating boric acid in the crevice region surrounded by the thermal sleeve and the pressure vessel. Although the subject report is still under review, the staff has determined that the available laboratory test results have provided adequate safety margin to support short-term operation of MNSA-2 repairs at SONGS, Unit 2.

For the cracks left in the attachment weld, it is conservative to assume that the cracks will continue to grow into the pressurizer materials (low alloy carbon steel). Based on operating experience, crack growth due to stress corrosion cracking in the low alloy carbon steel is not expected in the primary water environment during normal operating condition. However, cracks can be expected to grow by means of fatigue mechanism. The staff finds that the licensee did not evaluate the potential of fatigue crack growth in the low alloy carbon steel of the pressurizer. In the Westinghouse Topical Report, WCAP-15973-P, Revision 01, an evaluation of fatigue crack growth in the low alloy carbon steel was performed for the Combustion Engineering plants. The results of the evaluation showed that the cracks left in the attachment weld after repair are acceptable because the final flaw sizes at the end of plant life will not exceed the code allowable. The staff has determined that there is enough safety margin in the Westinghouse evaluation to support the use of MNSA-2 repairs of the SONGS, Unit 2, pressurizers for a short term operation not to exceed two operating cycles.

Cracks left in the Alloy 600 heater sleeve will continue to grow because the root cause for PWSCC has not been eliminated. The growth of the crack in the circumferential direction may lead to a total failure of the heater sleeve. However, the MNSA-2 assembly has implemented a clamp which is designed to prevent the ejection of the failed heater sleeve. Therefore, the growth of a crack in the circumferential direction will not create any significant concern in the safe operation of the plant. However, the growth of the crack in the axial direction may cause leakage from the primary pressure boundary when it extends beyond the primary seal of the MNSA-2 assembly. Growth of the axial crack in the heater sleeve is expected to propagate at a slow rate because of diminishing welding residual stresses when the crack propagates away from the attachment weld. Therefore, for a short operating period, crack growth in the axial direction in the heater sleeve will not be significant enough to cause any safety concern.

As discussed above, for short-term operation, the potential corrosion and fatigue crack growth in the carbon and low alloy steel and the potential crack growth in the heater sleeves are not expected to be significant. However, for longer term operation, the extent of the degradation needs to be closely monitored by nondestructive examination to ensure confirmed safe operation of the plant.

In view of significant corrosion of the reactor vessel head at the Davis-Besse plant, the staff is evaluating several issues associated with the long-term implementation of the half-nozzle and MNSA repairs, such as the effect of water chemistry on crack growth and the frequencies of nondestructive examinations, to ensure that there is no occurrence of significant corrosion and crack growth in the affected components. The applicable conditions to support the implementation of MNSA repairs on a permanent basis will be partially provided in the staffs safety evaluation of the Westinghouse Topical Report, WCAP-15973-P, Revision 01, which is expected to be issued in the near future.

4.0 CONCLUSION

Section 55a(a)(3) of 10 CFR Part 50 states that 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 of this section would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety. The staff concludes that, pursuant to 10 CFR 50.55a(a)(3)(i), the use of MNSA-2s as an alternative to an ASME Section XI Code repair on any leaking pressurizer heater sleeves at SONGS, Unit 2, may be authorized for a period not to exceed Cycle 13 operation, since it is found to provide an acceptable level of quality and safety for this time interval. Authorization for the use of MNSA-2s at SONGS, Unit 3, per the licensees original request, is no longer needed, as the licensee has already implemented long-term repairs for the pressurizer heater sleeves in Unit 3.

Principal contributor: M. Hartzman W. Koo Date: March 9, 2005