ML20044G590
| ML20044G590 | |
| Person / Time | |
|---|---|
| Issue date: | 06/18/1992 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20044B531 | List: |
| References | |
| NUDOCS 9306030358 | |
| Download: ML20044G590 (4) | |
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UNITED STATES n
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NUCLEAR REGULATORY COMMISSION WASHINGTON, D C. 20555 W...../
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION BABC0CK AND WILC0X TOPICAL REPORT BAW-20450 REVISION 1 ON STEAM GENERATOR TUBE KINETIC SLEEVING PROCEH MATERIALS INTEGRITY SECTION MATERIALS AND CHEMICAL ENGINEERING BRANCH DIVISION OF ENGINEERING TECHNOLOGY INTRODUCTION In a letter of March 31, 1992, Babcock and Wilcox (B&W) submitted Topical Report BAW-2045P, Revision 1, January 1992, " Recirculating Steam Generator Kinetic Sleeve Qualification for 3/4 inch OD Tubes," for the U. S. Nuclear Regulatory Commission (NRC) to review and accept for referencing in licensing actions.
B&W submitted supplemental information in a letter of May 19, 1992.
The B&W sleeving methodology described in the report uses a kinetic welding process to join the sleeve to the tube wall in the tube support plate region and two methods to form a tube sheet joint:
rolling to produce a mechanical seal or kinetically welding to produce a joint.
The design objective of the process is to produce joints with no leakage, but the process is qualified as a leak limiting sleeving process.
In a letter of January 4,1990, from J.
Richardson to J. H. Taylor (B&W), the NRC originally reviewed and approved the B&W kinetically welded sleeve methodology for referencing as a topical report (BAW-2045, Revision 0). The methodology described in this report is the same as previously reviewed by the NRC and used in operating nuclear plants.
Revision 1 of the report expands upon the original report by including design improvements based on field experience and by including a newly designed sleeve to compensate for defects at the support plate intersections.
The topical report includes a technical description of the sleeve design; design verification, including analyses and tests; process qualification; sleeve installation procedures; and nondestructive examinations.
BACKGROUND The tubes in operating pressurized water reactor (PWR) steam generators can be degraded by corrosion phenomena such as wastage, pitting, intergranular attack, stress corrosion cracking, crevice corrosion, and other phenomena such as denting and vibration wear. Tubes that become excessively degraded reduce the integrity of the primary-to-secondary pressure boundary. These tubes are considered defective and must be repaired or removed from service.
Historically, at areas of the steam generator tube wall that have been severely degraded, licensees have installed plugs at the inlet and outlet of 9306030358 920618 PDR TOPRP EMVEW C
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. the steam generator tube when the wall thickness decreased to a calculated value referred to as a " plugging limit." Licensees have performed eddy current testing (ECT) to measure the degradation of the steam generator tube, and the tube plugging limit conservatively accounts for the uncertainties in ECT measurements and contains an additional margin in wall thickness for expected or postulated degrad:ltion during service.
Installing plugs in a steam generator tube removes the heat transfer surface of the plugged tube from service and reduces the primary coolant flow available for core cooling.
Installing sleeves on the steam generator tubes does not greatly affect the heat transfer capability of the tube being sleeved; therefore, a large number of sleeves can be installed without significantly affecting the primary flow rate.
The sleeve spans the section of the original steam generator tube that has to be repaired, and the ends of the sleeve are expanded and sealed to provide acceptable leak resistance and a redundant load-carrying path.
Since 1983, B&W has been installing kinetically welded sleeves in recircu-lating steam generators in Europe and the United States. The sleeves are of three basic designs: (1) kinetic welds in both joints completely within the tube sheet, (2) kinetic or rolled joints within the tube sheet and a freespan weld above the tube sheet and (3) free span kinetic welds on each side of a tube support plate.
The applicant stated that no plant has been shut down because of leakage or failure of a kinetic sleeve.
DISCUSSION A sleeve installed in a steam generator tube must (1) maintain the structural integrity of the steam generator tube under normal operating and postulated accident conditions and (2) limit or prevent leakage if a hole forms through the wall of the steam generator tube.
B&W performed tests and analyses to demonstrate the capability of the sleeve to perform these functions under normal operating and postulated accident conditions.
B&W performed design analyses to verify that the sleeve conformed to Section III or XI of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code) and Regulatory Guide 1.121, " Bases for Plugging Degraded PWR Steam Generator Tubes." B&W evaluated the structural adequacy of the sleeve (the r
minimum sleeve wall thickness) in accordance with Section III for design-basis loads.
B&W performed a vibration analysis to demonstrate the adequacy of the sleeved tube and established fatigue loadings for qualifying the sleeve joints.
Section III of the ASME Code does not provide design rules and theoretical stress analysis is inadequate for expanded attachments that depend on frictional forces for their strength.
In such cases, Appendix II to Section III of the ASME Code requires the use of experimental stress analysis to substantiate the critical or governing stresses. The adequacy of the sleeve attachment to withstand cyclic loadings may be demonstrated by means of a fatigue test. The fatigue analysis considered bounding values of temperatures and pressures associated with transients, and calculations included load ranges based on conservative assumptions.
B&W received the results of the fatigue tests and analyses and found that the fatigue strength of the sleeve was adequate.
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. EVALUATION The staff considered the following in evaluating the sleeving process proposed by B&W. The sleeving of steam generator tubes is a repair technique that is an alternative to removing defective or degraded tubes from service by plugging.
Sleeves are designed to span a defective or degraded region of a steam generator tube and to maintain the primary-to-secondary pressure boundary of the steam generator tube under normal and accident conditions. A successful sleeving system must have a corrosion-resistant pressure boundary material and provide for the structural integrity and adequate leak resistance of the sleeved tube.
The B&W sleeve is manufactured from thermally treated nickel Alloy 690.
Worldwide corrosion studies reported in the open literature and work at the B&W 1aboratories indicate that thermally treated Alloy 690 in primary and secondary water is more resistant to corrosion than is the nickel Alloy 600 used for steam generator tubes.
The higher chromium content of Alloy 690 is believed to be the major factor contributing to its enhanced corrosion resistance. Alloy 690 is a Code-approved material (ASME 58-163), covered by ASME Code Case N-20-3, for the use as tubes in condensers and heat exchangers, and is acceptable to the NRC staff under Revision 28 of Regulatory Guide 1.85,
" Design and Fabrication Code Class Applicability-ASME Section III, Division 1," April 1992.
The NRC staff has approved thermally treated Alloy 690 tubing for use in replacement steam generators to be placed in service and for use in other types of sleeves.
Corrosion test results presented in BAW-2045P confirm that the sleeving methodology (kinetic welding) used by B&W does not promote stress corrosion cracking in the sleeve / tube assemblies used in the tests.
The staff concurs that thermally treated Alloy 690 used for sleeves is an improvement over the Alloy 600 used in the original steam generator tubing.
Upon reviewing the analytical results, structural tests, and metallurgical evaluations provided by B&W for its sleeving methodology, the staff found that (1) the sleeve-to-tube joints have an acceptable leak resistance, (2) the structural strength of the sleeve and tube-to-sleeve joints under normal and accident conditions and the fatigue strength under transient loads are adequate, and (3) the stress-relief heat treatment of the kinetic weld joints does not degrade the integrity of the joint.
Following the guidelines in Regulatory Guide 1.121 for tube degradation limits, B&W established a plugging limit of 40 percent of the original sleeve wall.
Eddy current techniques are available to perform necessary sleeve / tube inspections as needed to detect defects and to verify proper installatien of the sleeve.
The report has shown that available techniques are capable of providing 20 percent defect sensitivity in the required areas of the tube / sleeve pressure boundary.
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-CONCLUSIONS The staff reviewed BAW-2045P, Revision 1, and previously reviewed and arproved BAW-2045P, Revision 0.
Licensees referencing this topical report as a basis for re' vising their technical specification to allow sleeving must reference all pertinent sections of the topical report in the text of the technical specifications; that is, they should state that defective tubes must be removed from service by plugging or must be repaired by sleeving in accordance with B&W Topical Report BAW-2045P, Revision 1.
The sleeves change the thickness and diameter of the wall, reducing the sensitivity of eddy current testing. Therefore, the licensee must use special inspection methods. B&W described a proprietary method in the topical report with supporting validation data that demonstrate the inspectability of the sleeve and underlying tube.
Licensees referencing the topical report must provide a commitment to validate the adequacy of any system that is used for periodic inservice inspections as well as commit to improve testing methods as better methods are developed and validated for commercial use.
The staff finds that B&W Topical Report BAW-2045P, Revision 1, is suitable for referencing in license applications.
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