ML20114A063
| ML20114A063 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 07/28/1992 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20114A061 | List: |
| References | |
| NUDOCS 9208100097 | |
| Download: ML20114A063 (4) | |
Text
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/p %qIo, UNITED STATES 31 i
NUCLEAR REGULATORY COMMISSION
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WASHINGTON. O C. 20566 o
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SAFETY EVALVATION BY THE OFFICE OF NMG1 EAR REACTOR REGULATION RELATED TO AMENDMENT NO. U1 TO FACILITY OPERATING LICENSE NO. NPF-3 TOLEO U DIS 0N COMPANY LENTERIOR SERVICE COMPANY MQ THE CLEVELAND ELECTRIC ltLUMINATING COMPANY DAVIS-BESSE NUCLEAR POWER STATION. UNIT N0, 1 DOCKET i40. 50-346 1.0 INTP0 DUCT 10N By letter dated August 16, 1991, supplemented February 3, 1992, Toledo Edison Company requested a revision to the Technical Specifications for the Davis-Besse Nuclear Power Station.
The proposed change would revise the surveillance requirements of Technical Specification 4.4.5, " Steam Generators," to permit the option of using the Babcock & Wilcox (B&W) mech 3nical sleeving process for Once-Through Steam Generator (OTSG) tube repair.
The requested Technical Specification change will allow the use of B&W mechanical sleeves for steam generator tube repair as an alternative to pluggiag degraded tubes.
The change references B&W topical report BAW-2120P, "0TSG Mechanical Sleeve Qualification (Alloy 690)," in Technical Specification Section 4.4.5.4.
The topical report was originally submitted to the NRC by a letter dated March 26, 1991.
The staff approved the topical report as being suitable for referencing in a letter to James H. Taylor of B&W from James E.
Richardson of f1RR dated August 1, 1991.
2.0 EVALVATION The B&W mechanical sleeving methodology consists of inserting a tube of smaller diameter (the sleeve) inside the defective original tube, bridging the defett, and forming a new primary-to-secondary pressure boundary. The sleeve is joined to the inside of the original tube wall by mechanically rolled expansion joint > at the free-span end of the sleeve and at the tube sheet end and thereforr it is of a leak limiting design.
The B&W OTSG mechanical sleeve 1
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. methodology was originally reviewed and approved for use in Arkansas Unit 1 in 1984.
The methodology has been used for the installation of over 1400 sleeves in OTSGs. With the exception of the change -in material from the previously used Alloy 600 to the superior thermally treated Alloy 690, the proposed sleeve design is identical to the original sleeve design.
B&W topical report BAW-2120p describes in detail the analytical methods used for design and qualification of the B&W sleeve.
The topical report also contains the results of the sleeve design verification which included analysis and confirmatory-testing to demonstrate the acceptability.of the technique.
A sleeve installed in a steam generator tube (1) must maintain the structural integrity of the steam generator tube under normal operating-and costulated accident conditions, and (2) must limit or prevent leakage if a through-wall crack in the steam generator tube should develop.
B&W performed tests and analyses to demonstrate the capability of the sleeve to perforr ;hese functions under normal operating and postulated accident conditions.
Design analyses were perfor.aed to verify that-the sleeve conformed to Section 111 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."
The structural adequacy of the sleeve (e.g., the minimum sleeve wall thickness) was evaluated in accordance with Section 111 for design-basis loads. A vibration analysis was performed to demonstrate the adequacy of the
'ed tube.
Fatigue loadings used in qualifying the sleeve joints were olished.
For expanded attachments that depend on frictional forces for ineir strength, Section l} } of the ASME Code does not provide design rules and theoretical stress analysis is inadequate.
In such cases,. Appendix 11 to Section I!! of the ASME Code permits the use of experimental stress analysis to substantiate the critical or governing stresses.
The adequacy of the sleeve attachment to withstand cyclic loadings was 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, On the basis-cf the results of the fatigue tests and analyses, B&W found that the fatigue strength of the sleeve was adequate.
The staff's evaluation of the B&W leeving process was based on the following considerations:
The sleeving of s.eam gelerator 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 3
steam generator tube and to maintain the steam' generator tubing primary-to-secondary pressure boundary under normal and accident conditinns.
A successful sleeving system must provide a corrosion-resistant sleeve material and restore the structural integrity of the sleeved tube.
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The tubesheet and free span joints are mechanical _ seals produced by roll expanding the sleeves into the tube. The structural integrity of the joints L
was demonstrated by subjecting sleeve / tube specimens to a series of tests i
representing service conditions which included leak tests, thermal cycling, fatigue, and tensile loading.
5 BAW-2120P contains the retults of the sleeve design verification which included analysis and confirmatory testing to demonstrate the acceptability of the steam generator sleeving technique for defective tubes.
The licensee has confirmed that the design and operating conditiots (including transient conditions and cycles! specified for tha sleeve in the topical report bound 4
the Davis-Besse Nuclear D wer Station steam generator design conditions.
The 7
inpical report includes r technical description of the sleeve design; design verification, including analyses and tests; process qualification; sleeve 4
installation p*ocedures; and nondestructive examinations.
i The B&W mechanical sleeve is manufactured from thermally treated Inconel Ailoy 690.
Worldwide corrosion studies reported in the open literature and work at the S&W laboratories indicate that the corrosion resistance in primary and i
secondary water of thermally treated Alloy 690 is superior to that of the Alloy 600 that was used for steam generator tubes.
The higher chromium 1
content of Alloy 690 is believed to be the major factor in regard to its i
enhanced corresion resistance.
Alloy 690 is,a Code-apprcved. material (ASME i
SB-163), covered by ASME Code Case N-20-3, for use as tubing in condensers i
and heat exchangers and is acceptable to the NRC staf f under Regulatory Guide l
1.85 (Rev. 28, April 1992.)
The NRC staff has approved the use of thermally treated Alloy 690 tubing in replacement steam generators.
Corrosion test results presented in BAW-2120P confirm that the sleeving methodology used by E&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. A calculation to determine the minimum allowable wall thickness in accordance_ with Regulatory Guide 1.121 guidelines gave an allowable wall degradation limit of 70% of wall thickness, but a conservative plugging _ limit of 40% has been 4
established based on staff positions concerning the need for an additional allowance for operational degradation and for eddy-current testing uncertainty.
Eddy current techniques are available to perform necessary sleeve / tube i
inspections for defect detection and to verify proper installation of the i
As described in topical report BAW-2120P, degradation as small as-20%
through wall can be detected in all areas of the tube sleeve except for the roll expansion and the sleeve end, where the limit of detectability is 40%
through wall.
This is the current state-of-the-art capability.
The licensee I
has stated that it will evaluate and. implement better testing methods when they are developed and validated for commercial use to enable detection of degradation as small as 20% through wall in the roll expanded area and the.
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, sleeve ends.
Until better methods are available for inspection of these areas, the licensee will compare inservice inspection results with those obtained during the baseline sleeved tube inspection.
This is acceptable to the NRC staff.
On the basis of its review of the analytical results, structural tests, and metallurgical evaluations provided by B&W for its sleeving methodology, the NRC staff has found that (1) the sleeve-to-tube joints have an acceptable leak resistance, (2) the structural strength of the sleeve and tube / sleeve joints under normal and a".ident conditions and the fatigue strength under transient loads are adequate, and (3) the use of thermally treated Alloy 690 should pro-vide additional assurance against the possibility of stress corrosion cracking.
l The above conclusions are based on the NRC staff's previous approval 'for referencing of the topical report BAW-2120P with associated supplemental information and commitments provided by the licensee The NRC staff has also concluded that the design and operating conditions for the Davis-Besse steam generators are appropriately bounded by those discussed in the approved topical report.
Therefore, the NRC staff has concluded that-the issuance of this amendment is acceptable.
3.0 STATE CONSULTATIQN in accordance with the Commission's regulations, the Ohio State official was notified of the proposed issuance of the amendment.
The State official had no comments.
4,0 ENVIRONMENTAL CONSIDERATION Pursuant to 10 CFR 51.21, 51.32, and 51.35, an environmental assessment and finding of no significant impact has been prepared and published in the Federal Reaister on June 11, 1992 (57 FR 24832).. Accordingly, based upon the environmental assessment, the Commission has determined that the issuance of this amendment will not have a significant effect on the quality of the human environment.
5.0 CONCLUSION
The staff has concluded, based on the considerations discussed above, that:
(1) there is reasonable assurance that the health and safety of -the public will not be endangered by operation in the proposed manner, (2) such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of this amendment will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributor: H. Conrad Data:
July 28,1992
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