ML20129G723
| ML20129G723 | |
| Person / Time | |
|---|---|
| Site: | Callaway  |
| Issue date: | 10/01/1996 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20129G714 | List: |
| References | |
| NUDOCS 9610070274 | |
| Download: ML20129G723 (8) | |
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UNITED STATES
,j NUCLEAR REGULATORY COMMISSION 2
WASHINGTON. D.C. 30085 4001
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION i
RELATED TO AMENDMENT NO.116 TO FACILITY OPERATING LICENSt NO. NPF-30 f
UNION ELECTRIC COMPANY CALLAWAY PLANT. UNIT 1 DOCKET NO. 50-483
1.0 INTRODUCTION
By letter dated April 12, 1996, as supplemented by letters dated August 2, 1996, August 19, 1996, and September 5,1996, Union Electric Company (UE),
requested changes to the Technical Specifications (Appendix A to Facility Operating License No. NPF-30) for the Callaway Plant, Unit 1.
The proposed amendment would revise Technical Specification (TS) 3/4.4 and,the associated j
Bases to address the installation of laser welded tube sleeves designed by Westinghouse Electric Corporation (Westinghouse) in the Callaway Plant steam generators (SG).
f The August 2, 1996, August 19, 1996, and September 5,1996, supplemental letters provided only clarifying information and did not change the original no significant hazards consideration determination published in the Federal Reaister on May 8, 1996 (61 FR 20857).
The sleeve type is an elevated.tubesheet sleeve.
It is designed to repair tubes with degradation in the expansion transition zone at the top of the tubesheet. The sleeve is inserted inside the tube and held in position by hydraulically expanding the ends of the sleeve. This hydraulic expansion also brings the sleeve ends into contact with the parent tube in preparation for 1
subsequent welding or rolling. The structural attachment of the sleeve to the i
tube is accomplished by means of two different joint types:
a rolled joint (mechanirslly expanded) in the (lower) tubesheet end and an autogenous laser weld at ea (upper) freespan end. The material of construction for the sleeves is a nickel-iron-chromium alloy,. alloy 690, a Code-approved material (ASME SB-163), incorporated in ASME Code Case N-20.
Extensive analyses and testing were performed on the Westinghouse sleeve and sleeve / tube joints to demonstrate that Regulatory and Code design criteria were satisfied under normal operating and postulated accident conditions. The i
details af the sleeve qualifications are discussed in report WCAP-14596, l
" Laser Welded Elevated Tubesheet Sleeves for Westinghouse Model F Steam i
Generators," dated March 1996 (proprietary). This generic sleeving report presents the technical bases. supporting the licensing of laser welded sleeves for use in ll/16-inch diameter SG tubes such as those at Callaway.
(The bounding assumptions in generic WCAP-14596 were verified by the licensee staff to bound all of the appropriate Callaway Plant conditions).
9610070274 961001 PDR ADOCK 05000483 P
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The staff previously reviewed closely similar Westinghouse documents supporting requests for changes to the TS at other plants. The bulk of the technical and regulatory issues for the present request are identical to those reviewed in previous safety evaluations (SEs) concerning the use of Westinghouse laser welded sleeves. This SE discusses only those issues warranting revision, amplification, or inclusion based upon current experience. A summary of the principle technical issues regarding the design and use of Westinghouse laser weldeo sleeves follows. Details of the prior i
staff evaluation of Westinghouse sleeves may be found in SEs for Calvert Cliffs Nuclear Power Plant Units 1 and 2, Docket Nos. 50-317 and 50-318, dated March 22, 1996; DC Cook Nuclear Power Plant Unit 1, Docket No. 50-315, dated January 4,1996; Maine Yankee Nuclear Power Plant, Docket No. 50-309, dated May 22, 1995; and Joseph M. Farley Nuclear Power Plant, Units 1 and 2, Docket Nos. 50-348 and 50-364, dated October 22, 1990. These evaluations are germane i
to the proposed Callaway license amendment.
2.0 BACKGROUND
A sleeve is a tube slightly smaller in diameter than an SG tube that is inserted into an SG tube to bridge a degraded or susceptible section of tube.
j The length of a sleeve is selected according to the individual installation circumstance. Generally, they vary in length between one and three feet. The sleeve becomes the pressure boundary and thereby restores the structural integrity of a degraded or potentially degraded portion of the original SG tube.
Prior to the development of sleeve technology, a defective SG tube was removed from service by plugging. However, this reduces the heat transfer area. The reduction in heat transfer (or other thermal-hydraulic operating parametert; can be tolerated up to a point before other system consequences of the reduced SG performance became limiting.
Beyond this limit, a utility has to make operational changes resulting in reduced electrical generating capacity of the affected unit.
Because sleeves have minimal effect upon the thermal-hydraulics of an SG, their use is essentially unrestricted. This means a licensee may restore l
degraded sections of SG tubes to like new condition without experiencing a serious penalty with regard to unit generating capacity. This has led to increased use of sleeves versus plugs where practical.
Recently, some foreign and domestic plants have installed sleeves in previously unprecedented numbers, up to nearly 100 percent of the SG tubes on a single unit.
About 29,000 Westinghouse laser welded sleeves have been installed in foreign and domestic plants since 1988. Over eight years of operating experience with Westinghouse sleeves has shown the technology to be highly reliable. No operationally induced degradation or leakage has occurred in any Westinghouse j
laser welded sleeves.
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,, 3.0
SUMMARY
OF PREVIOUS REVIEWS Previous staff evaluations of Westinghouse sleeves addressed the technical adequacy of the sleeves in the principal areas of pressure retaining component design:
structural requirements, material of construction, welding, nondestructive examination, and sleeve plugging limit. The staff found the analyses and tests submitted to address these areas of component design and inspection to be acceptable and are summarized below:
3.1 Structural Requirements The function of sleeves is to restore the structural integrity of the tube pressure boundary.
Consequently, structural analyses were performed for a variety of loadings including design pressure, operat,ing transients, and other parameters ulected to envelope loads imposed during normal operating, upset, and accident conditions.
Stress analyses of sleeved tube assemblies were performed in accordance with the requirements of the ASME Boiler and Pressure Vessel Code,Section III. These analyses, along with the results of qualification testing and previous plant operating experience, were cited to demonstrate the sleeved tube assembly is capable of restoring steam generator tube structural integrity.
3.2 Material of Construction The sleeves are fabricated from thermally treated alloy 690, a Code approved material (ASME SB-163) covered b3 ASME Code Case N-20.
The staff found the use of alloy 690 is an improvement over the alloy 600 material used in the original SG tubing.
Corrosion tests conducted under Electric Power Research Institute (EPRI) sponsorship confirmed test results regarding the improved corrosion resistance of alloy 590 over that of alloy 600. Accelerated stress corrosion tests in caustic and aqueous chloride solutions also indicated alloy 690 resists general corrosion in aggressive environments.
Isothermal tests in high purity water have shown that, at normal stress levels, alloy 690 has high resistance to intergranular stress corrosion cracking (IGSCC) in extended high temperature exposure. The NRC concluded, as a result of these laboratory corrosion tests, that alloy 690 is acceptable for use in nuclear power plants.
The NRC endorsed the use of Code Case N-20 in Regulatory Guide 1.85,
" Materials Code Case Acceptability, ASME Section III, Division 1."
The NRC staff has approved use of ' alloy 690 tubing in replacement steam generators as well as sleeving applications.
3.3 Welding and Post Weld Heat Treatment Automatic autogenous laser welding is employed to join the sleeve to the parent tube in the freespan regions. The application of this process to the W sleeve design was specifically qualified and demonstrated during laboratory tests employing full scale sleeve / tube mockups. Qualification of the welding procedures and welding equipment operators was performed in accordance with the requirements of the ASME Code,Section IX.
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,. Accelerated corrosion tests have confirmed that a postweld heat treatment (PWHT) significantly improves the IGSCC resistance of the alloy 600 parent tube material in the weld zone. A PWHT reduces the residual stresses resulting from welding. Residual stresses from forming operations (such as bending, welding, etc.) are known to be a principal contributor to IGSCC in alloy 600.
Performance of a PWHT greatly reduces the residual stresses from welding thereby enhancing the IGSCC resistance of the alloy 600 portion of the weld zone. The alloy 690 sleeve material is highly resistant to IGSCC either with or without PWHT. All laser welded joints will be heat treated in accordance with the Westinghouse generic sleeving report and the NRR staff position.
The rolled joint used to join the sleeve to the tube within the tubesheet effectively isolates the alloy 600 of the parent tube from the environment and thus is not susceptible to IGSCC. Stress relief of these joints is unwarranted.
3.4 Nondestructive Examination The baseline nondestructive examination of sleeved tubes is conducted using ultrasonic testing (UT) and eddy-current testing (ECT). UT is performed after welding to confirm the laser welds are consistent with critical process dimensions and are of acceptable weld quality. Westinghouse presented data on a UT system demonstrating post weld examinations of the sleeve / tube assembly will be adequate. Standards which included undersized welds were used in the qualification of the UT technique. The results of the qualification tests demonstrate the system can confirm there is a continuous metallurgical bond between the sleeve and tube and that the weld size (width) meets minimum acceptable dimensions.
ECT is then used to establish baseline inspection data for every installed sleeve / tube.
In performing the inspection, the licensee will use Electric i
Power Research Institute (EPRI) "PWR Steam Generator Tube Examination Guidelines" Appendix G qualified personnel and Appendix H qualified ECT techniques.
For future sleeve / tube inspections, the licensee committed to following Revision 4 of the EPRI guidelines.
Future revisions of the guidelines will be evaluated and adopted as appropriate. The licensee also modified the TS to incorporate sleeve / tube inspection scope and expansion criteria.
3.5 Tube and Sleeve Plugging / Repair Limits
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i The tube and sleeve minimum acceptable wall thickness is determined using the criteria of Regulatory Guide 1.121, " Bases for Plugging Degraded PWR Steam Generator Tubes," and ASME Code Section III allowable stress values and pressure stress equations. According to RG 1.121 criteria, an allowance for nondestructive evaluation (NDE) uncertainty and postulated operational growth of tube or sleeve wall degradation must be accounted for when using NDE to determine plugging or repair limits. Therefore, a conservative tube or sleeve wall thickness allowance for postulated degradation growth and eddy current uncertainty of 20 percent through wall per cycle were assumed for the purpose
! of determining the tube or sleeve plugging /' repair limit (tubes may be plugged or, if feasible, sleeved, and sleeves would be plugged only).
In accordance with the guidance of NUREG-1431, " Standard Technical Specifications for Westinghouse Plants," the licensee is reducing the tube plugging / repair limit from 48 percent to 40 percent through wall of nominal tube wall thickness (including the 20 percent NDE uncertainty). The sleeve i
l plugging limit, calculated based upon the most limiting of normal, upset, or i
faulted conditions for the Callaway Plant steam generators, was determined to be 39 percent of the sleeve nominal wall thickness, including the 20 percent NDE uncertainty, Plugging or repair of tubes and plugging of sleeves when i
degradation reaches the plugging / repair limit provides assurance the minimum acceptable wall thickness will not be violated during the next subsequent cycle of operation.
4.0 DISCUSSION Experience with all types of SG tube sleeves has led to several areas of l
concern outside the scope of basic sleeve design and qualification discussed l
above. These include instances of cracking in sleeved SG tubes, service life predictions for sleeved tubes, application of PWHT and the effect of tube i
lockup, and primary-to-secondary leakage limits.,
4.1 Cracking in Sleeved SG Tubes Recent experiences at two U.S. plants indicated the freespan joint of a sleeved alloy 600 steam generator tube may be susceptible to IGSCC. The affected joints are of the mechanically expanded type. These employ a hydraulic expansion followed by a hard roll in the center of the hydraulically expanded region. The hard roll forms the structural joint and leak limiting seal.
Inner diameter initiated cracks have been detected in the alloy 600 parent tube material at the lower hard roll transition and lower hydraulic transition of freespan joints. The cracks were detected after four to seven years of service. Since a number of sleeved tubes with this joint type have operated up to 14 years in one of the affected units, it is clear that not all such sleeved tubes are likely to develop cracks after a given service interval.
Accelerated corrosion tests of laser welded sleeve joints have shown the hydraulic transition to have little or no susceptibility to IGSCC. Service times exceeding eight years have been achieved for sleeved tubes with laser welded joints at U.S. plants. No instances of service induced IGSCC have occurred in any of these joints. The staff is monitoring these developments for potential impact on welded sleeve installations.
4.2 Service Life Predictions for Sleeved SG Tubes l
The staff position on sleeving considers the method unable to assure an unlimited service life for a repaired tube. The conservative view is sleeving creates new locations in the parent tube which may be susceptible to IGSCC after new incubation times are expended.
Incubation times are not quantified.
. They are observed to vary between individual steam generators and the various tubes within, based upon prior experiences with U-bend and roll transition cracking.
This staff position that sleeving has limited service life is due to the circumstances of the sleeving processes.
Sleeve installation methods can enhance one or two of the conditions necessary for IGSCC. The primary contributor is the residual stress resulting from the various joining methods.
Secondarily, the local environment of the tube may be altered as a result of the formation of a wetted crevice between the tube and sleeve.
Remediation of these contributors would benefit sleeved tube life. Of the two, stress relieving may be the most beneficial given the underlying causes of IGSCC and present sleeve designs. As discussed earlier, the sleeve installation procedure includes a PWHT of the weld joints to increase the resistance to IGSCC.
4.3 PWHT and Tube Lockup Recent field experience with the installation of welded sleeves with PWHT indicated SG tubes may be constrained (" tube lockup") in their tube supports.
The result of such tube locking is distortion of the tube (bowing or bulging) during the PWHT. After the heat treatment is completed, the bow or bulge remains. Measurements of the bowing and bulging have shown them to be of negligible values.
These distortions have been analyzed and found to be immaterial to the examination, operation, and safety of the sleeved tubes.
Along with the observed distortion (bowing or bulging) is a residual stress remaining after the heat treatment is completed.
Strain gage measurements of this residual stress have shown it to be moderate compared to that resulting from welding without subsequent PWHT. This issue was the subject of additional testing and analysis related to the use of laser welded sleeves at the Maine Yankee facility during a sleeve installation project.
Based upon the finding that many tubes are fixed in the tube supports, Westinghouse modified their sleeve installation procedure on the assumption that all tubes are locked. The modified installation procedure thereby minimizes the residual stress of PWHT regardless of tube condition.
4.4 Primary-to-Secondary Leakage Limits While a laser weld should be inherently leak-tight, the lower (rolled) joint of a tubesheet sleeve may not be leak tight. Westinghouse analyzed the effects of an abnormal lower joint seal. The analysis shows that even under extreme postulated conditions, it will have satisfactory leakage integrity.
The licensee is adopting a change to its TS incorporating a 150 gpd per SG leakage limit and a 600 gpd total primary-to-secondary leakage through all SGs not isolated from the reactor coolant system. This is consistent with the staff's position regarding primary-to-secondary leakage limits for SGs with sleeved tubes.
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. 4.5 Results of Review Based on the preceding analysis, the NRC staff concludes the repair of SG tubes at Callaway Plant using laser welded tubesheet sleeves designed and installed by Westinghouse in accordance with the methods of WCAP-14596 is acceptable.
Additionally, for the reasons discussed in 3.0 and 4.0 above, the staff finds acceptable these additional changes to the plant TS:
a.
The associated sleeve wall depth based plugging limit value of 39 percent (of nominal sleeve wall thickness) and inspection requirements.
(See Section 3.5) i b.
Reduction of the tube plugging / repair limit from 48 percent to 40 percent i
through wall (of the nominal tube wall thickness) to be consistent with NUREG-1431, " Standard Technical Specifications for Westinghouse Plants."
j (See Section 3.5) c.
The reduction of the primary to secondary normal operational leakage limit from 500 to 150 gpd per steam generator in accordance with staff position for units with TS amendments to allow tube repair by sleeving.
(See Section 4.4) d.
The addition of sleeve / tube inspection scope and criteria for expansion of sleeve / tube inspections.
(See Section 3.4)
5.0 STATE CONSULTATION
In accordance with the Commission's regulations, the Missouri State official was notified of the proposed issuance of the amendment. The State official had no comments.
6.0- ENVIRONMENTAL CONSIDERATION l
The amendment changes a requirement with respect to the installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20 and changes surveillance requirements. The NRC staff has determined that the amendment involves no significant increase in the amounts, and no significant change in the types, of any effluents that may be released offsite, and that there is no significant increase in individual or cumulative 3
occupational radiation exposure. The Commission.has previously issued a proposed finding that the amendment involves no significant hazards j
consideration, and there has been no public comment on such finding (61 FR 20857). Accordingly, the amendment meets the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9).
Pursuant to 10 CFR i
51.22(b) no environmental impact statement or environmental assessment need be prepared in connection with the issuance of the amendment.
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7.0 CONCLUSION
l The Commission 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 the amendment will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributor: Geoff Hornseth Date:
October 1, 1996 i
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