ML20216F190
| ML20216F190 | |
| Person / Time | |
|---|---|
| Site: | South Texas  |
| Issue date: | 09/04/1997 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20216F187 | List: |
| References | |
| NUDOCS 9709110182 | |
| Download: ML20216F190 (9) | |
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UNITED STATES
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NUs IAR REGULATORY COMMISSION WASHINGTON. D.C. 30MH001
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NOS.90 AND 77 10 FAtlllTY OPERATING LICENSE NOS. NPF-76 AND NPF-80 HOUSTON LIGHTING & POWER COMPANY CITY PUBLIC SERVICE BOARD OF SAN ANTONIO CENTRAL POWER AND LIGHT COMPANY CITY Of_ AUSTIN. TEXAS DOCKET NOS.60-498 AND 50-499 MUTH TEXAS PROJECT. UNITS 1 AND 2 l
1.0 ]NTRODUCTION 5
By application dated May 17, 1996, as supplemented by letters dated June 14, t
1996. March 17, July 29 and July 30, 1997, the Houston Lighting & Power Company, et al. (the licensee) requested changes to the Technical S)ecifications (Appendix A to facility Operating License Nos. NPF-76 and N)f-80)d changes would revise Technical Specification (TS) Section 3/4.4.5 for the South Texas Project Units 1 and 2 (STP-1 and -2).
The propose Steam Generators, 3/4.4.6 Reactor Coolant System Leakage, and associated Bases by allowing the use of Westinghouse (W) designed laser welded sleeves to re) air defective steam generator (SG) tubes. The June 14, 1996 supplemental su)mittal provided the plant-spectfic W reports for the specific application of laser welded SG tube sleeving at STP discussed in the original submittal.
The March 17, 1997 supplen, ental submittal provided corrected TS pages that include references to the topical sleeve reports ano sleeve inservice inspection requirements. The July 29 and July 30, 1997 submittals provided responses to a Request for Additional Information from the staff dated June 30, 1997.
The supplemental submittals did not affect the staff's initial no significant hazards consideration determination.
The licensee submitted WCAP-13698, Revision 2, " Laser Welded Sleeves for 3/4 Inch Diameter Tube feedring-Type and Westinghouse Preheater Steam Generators,"
dated April 1995 (proprietary), which provides the technical basis for licensing the use of W designed laser welded sleeves to repair degraded SG tubes.
This report summarizes the generic design, structural, and thermal-hydraulic analyses for three distinct types of sleeves, it includes a discussion of the supporting mechanical, leakage and corrosion test results and describes the sleeve installation processes and sleeve inspection methodology.
Revision 2 of WCAP-13698 addresses laser welded sleeves for use j
in Combustion Engineering (CE) feedring-type SGs and for W Models D3, 04, DS, l
9709110182 970904 PDR ADOCK 05000498 P
- El and E2 areheater-type SGs, all of which use 3/4 inch outside diameter SG tubing.
Tae licensee also submitted WCAP-14653, " Specific Application of Laser Welded Sleeves for the South Texas Power Plant Steam Generators," dated June 1996 (proprietary), which provides the technical bases supporting the licensing of W designed laser welded sleeves as described in Revision 2 of WCAP-13698 for use at STP-1 and -2.
The staff previously reviewed identical and closely similar W 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 W 1aser welded sleeves.
Details of prior staff evaluations of W sleeves may be found in the SEs for Byron and Braidwood Nuclear Power Stations, Units 1 and 2, Docket Nos. 50-454, -455, -456 and -457, dated March 8, 1994; Maine Yankee Nuclear Power Plant, Docket No. 50-309, dated May 22, 1995; and Calvert Cliffs Nuclear Power Plant, Units 1 and 2,. Docket Nos. 50-317 and 50-318, dated March 22, 1996.
These evaluations apply to the proposed STP license amendment.
This SE discusses only those issues warranting revision, amplification, or inclusion based upon current experience.
A summary of the principal technical issues regarding the design and use of W 1aser welded sleeves follows.
2.0 BACKGROUND
A sleeve is a tube slightly smaller in diameter than a SG tube that is inserted into a SG tube to bridge a degraded or susceptible section. 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 develo) ment of sleeve technology, licensees removed defer *,ive SG tubes from service )y plugging. However, this reduced the heat transfer area.
The reduction in heat transfer (or other thermal-hydraulic oper, ting parameters) can be tolerated up to a point before other system omyJences of the reduced SG performance become limiting.
Beyond this limit, a utility had
-to make operational changes resulting in reduced electrical generating capacity of the affected unit.
Because sleeves have minimal effect upon the thermal-hydraulics of a SG, their use is essentially unrestricted. This means a licensee may restore 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 skeves in previously unprecedented numbers, up to nearly 100 percent of the SG aces on a single unit.
. The licensee's proposal addressed the use of three basic sleeve designs:
a full length tubesheet sleeve (FLTS), an elevated tubesheet sleeve (ETS) and a tube support sleeve bottom surface of the(TSS).
The FLTS spans from the end of the tube, at the tubesheet, tc a point above the secondary side surface of the tubesheet.
The ETS spans from a location within the tubesheet, approximately 14 inches above the tube end, to a point above the secondary side surface of the tubesheet. The TSS is installed centered approximately on a tube support intersection or in a freespan section of SG tube. All sleeve types are first secured by hydraulically expanding the upaer and lower portions of the sleeve. The hydraulic expansion brings tie sleeve ends into contact with the parent tube in pre)aration for subsequent welding or rolling.
The FLTS and the ETS are installed )y means of two different joint types:
an autogenous laser weld at the freespan end of the sleeve (the upper joint) and a rolled joint (mechanically expanded) at the tubesheet end of the sleeve (the lowerjoint).
The TSS is laser welded to the SG tube at each freespan end of the sleeve. The material of construction for the sleeve is a nickel-iron-chromium alloy, alloy 690, a Code approved material (ASME SB-163),
incorporated in ASME Code Case N-20.
3.0
SUMMARY
OF PREVIOUS REVIEWS Previous staff evaluations of W 1aser welded sleeves addressed the technical adequacy of the sleeves in the principal areas of pressure retaining component design:
structural requirements, material of construction, welding and post weld heat treatment, and sleeve alugging limits.
The staff found the analyses and tests submitted to address t1ese areas of c a ponent design to be acceptable as summarized below, 3.1 Structural Reauirements The sleeves function to restore the structural integrity of the tube pressure boundary. Consequently, W performed structural analyses for a variety of loadings including design pressure, operating transients, and other parameters selected to envelope loads imposed during normal operating, upset, and accident conditions at STP-1 and -2.
The stress analyses of sleeved tube assemblies documented in Revision 2 of WCAP-13698 were performed in accordance with the requirements of the ASME Boiler and Pressure Vessel Code, Section 111. W cites these analyses, along with the results of qualification testing and previous plant operating experience, to demonstrate the capability of the sleeved tube assembly of restoring SG 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 by 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 690 over that of alloy 600. Accelerated stress corrosion tests in caustic and aqueous chloride solutions also indicated alloy
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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 (RG) 1.85,
" Materials Code Case Acceptability, ASME Section Ill, Divislon 1."
The NRC staff has approved use of_ alloy 690 tubing in replacement SGs as well as sleeving applications.
3.3 Weldina and Post Weld Heat Treatment W employs automatic autogenous laser welding to join the sleeve to the parent tube in the freespan regions. W specifically qualified and demonstrated the application of this process to the sleeve design 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.
Accelerated corrosion tests confirm a post weld heat treatment (PWHT) be significantly improves the IGSCC resistance of the alloy 600 parent tu 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 princiaal contributor to IGSCC in alloy 600.
Performance of a PWHT greatly reduces tie 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.)
In its July 29, 1997 submittal, the licensee committed to performing PWHT of the laser welded joint in accordance with the W topical sleeve reports.
This commitment is reflected in the proposed TSs.
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 1GSCC.
Stress relief of these joints is j
unwarranted.
3,4 Sleeve Pluaaina limits The licensee determined the sleeve minimum acceptable wall thickness using the criteria of RG 1.121, " Bases for Plugging Degraded PWR Steam Generator Tubes,"
i and ASME Code Section 111 allowable stress values and pressure stress equations. According to RG 1.121 criteria, an allowance for nondestructive l-evaluation (NDE) uncertainty and postulated operational growth of tube wall degradation within the sleeve must be accounted for when using NDE to determine sleeve plugging limits.
Therefore, the licensee assumed a conservative. tube wall combined allowance for postulated degradation growth i
and eddy current uncertainty of 20% through wall per cycle for the purpose of determining the sleeve 31ugging limit. The sleeve-plugging limit calculated 4
for STP-1 and -2 used tie most limiting of normal, upset, or faulted conditions to determine a plugging criterion of 62% of the sleeve nominal wall l
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The licensee proposes to use a value of 40% of the sleeve nominal wall thickness as the TS plugging limit. Removal of tubes and/or sleeves from service when degradation reaches the plugging limit provides assurance the minimum acceptable wall thickness will not be violated during the next subsequent cycle of operation.
4.0 DISCUSSibN Experience with all types of SG tube sleeves has led to several areas of concern outside the scope of basic sleeve design and cualification discussed above. These include instances of cracking in sleevec SG tubes, service life predictions for sleeved SG tubes, application of PWHT and the effect of tube 1
lockup, nondestructive examination of sleeves, and primary-to-secondary 1
leakage limits.
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4.1 (rackina in Sleeved SG Tubes Recent experiences at two U.S. plants indicate the alloy 600 SG tube may be j
susceptible to IGSCC at the sleeve freespan joint of a tubesheet sleeve.
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 the freespan joints.
The cracks were detected after 4 to 7 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 with no such degradation, 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 8 years have been achieved for sleeved tubes with laser welded joints at U.S. plants.
The staff is monitoring these developments for potential impact on welded sleeve installations.
4.2 Service Life Predictions for Sleeved SG Tubes The staff considers the sleeving methods 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 conclusion 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
4 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 Lockun Recent field ex)erience with the installation of welded sleeves with PWHT indicated SG tuses may be constrained (" tube lockup") in their tube support plates. 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 hsve 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 i
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 support plates, W modified their sleeve installation procedore on the assumption that all tubes are locked. The modified installation procedure, described in WCAP-14653, minimizes the residual stress of PWHT regardless of tube condition.
4.4 Nondestructive Examination of Sleeves The licensee proposes using ultrasonic testing (UT) and oddy current testing (ECT) as part of the nondestructive examination (NDE) of sleeved tubes prior to service. UT is performed after welding to confirm the laser welds are consistent with critical process dimensions and are of acceptable weld quality. W 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 demonstrate presence of upper and lower hydraulic expansions, demonstrate presence of lower rolled joint, verify proper location of weld, verify post weld heat treatment, verify lack of process anomalies such as blow holes or weld cracking, and establish the baseline inspection data for future inspections.
In performing ECT inspections, the licensee follows the Electric Power Research Institute's "PWR Steam Generator Tube Examination Guidelines," in that Appendix G qualified personnel and Appendix H qualified ECT techniques will be used.
For the pre-service examination of the sleeve welds, South Texas will use the Plus Point coil in the eddy current probe to verify lack of weld process anomalies. The staff finds these examination methods adequate to monitor SG tube integrity.
. for future sleeve inservice inspections, the licensee will be following the most current revision of the EP11 guidelines in terms of inspection scope and expansion criteria as well as personnel and technique qualifications.
The licensee proposes modifying the TS to incorporate sleeve / tube inspection scope and expansion criteria that meet staff expectations. The staff finds this acceptable.
4.5 Enjsry-to-Secondary leakaae Limits 4
While a laser weld should be inherently leak-tight, the lower (rolled) joint of a tubesheet sleeve may not be leak tight. The topical sleeve reports describe leakage test results and plant-specific confirmatory test results that demonstrate adequate leakage integrity of the lower rolled joints.
These results apply directly to STP-1 which has SG tubes hard rolled within the tubesheet. A confirmatory test program will be performed at STP-2 prior to sleeve installation to verify the applicability of these test results to a plant with SG tubes hydraulically expanded within the tubesheet, as described in the licensee's July 30, 1997 submittal.
Degraded tubes restored to operation as a result of sleeving are susceptible to additional degradation in the same SG environment. The sleeve is designed to extend 3ast the upper weld joint and into the tubing.
In the event a sleeved tuae failed at the weld, the sleeve extension restricts tube movement and leakage.
Leakage monitoring devices alert plant personnel to implement the ap)ropriate procedures.
However, based on experience with various causes of lea < age through SG tubes including experience related to tubes repaired by sleeving, the staff has required licensees amend the TS to reflect a primary-to-secondary leakage limit of 150 gallons per day (gpd).
With respect to the staff position regarding primary-to-secondary leakage limits for sleeving amendments, the licensee already implemented a change to the TS adopting a 150 gpd per SG 1eakage limit for STP-1. As part of this TS request, the licensee plans to adopt a 150 gpd per SG 1eakage limit for STP-2.
The staff finds this acceptable.
5.0 CHANGES TO THE TECHNICAL SPECIFICATIONS The licensee's proposal would revise TS Sections 3/4.4.5 and 3/4.6.2 as follows:
TS 4.4.5.2 would be revised to change the inservice inspection sample size for repaired SG tubes in all steam generators to 20% and to reflect the addition of TS Table 4.4-3 which will be used to classify the inspection results for repaired SG tubes.
TS 4.4.5.4 would be revised to' add a definition of tubing or tube and modify the definition of plugging limit or repair limit by specifying plugging or repair limit imperfection depths specified in percentage of the nominal wall thickness. This change would be added to specify that any Westinghouse laser welded sleeve which upon inspection exhibits imperfection exceeding 40% of nominal wall thickness, it must be plugged prior to returning the SG-to service.
- J
. TS 4.4.5.4, Note 2 would be revised to add tube repair (sleeving) in accordance with WCAP-13698, Revision 2, " Laser Welded Sleeves for 3/4 Inch Diameter Tube feedring-Type and Westinghouse Preheater Steam Generators,'
A)ril 1995, and WCAP-14653, " Specific Application of Laser Welded Sleeves for tie South Texas Project Power Plant Steam Generators," June 1996, including the pos; weld heat treatment, as acceptable methods for SG tube repair.
TS Table 4.4-2 would be revised to specify its use for nonrepaired SG tubes only.
TS Table 4.4-3 would be added to specify the inspection scope expansion criteria for repaired SG tubes.
TS 3.4.6.2 would be revised to reflect a primary-to-secondary leakage limit of 150 gpd through any one SG.
The TS Bases and the Table of Contents would also be revised consistent with the changes described above.
The staff reviewed the TS changes discussed above and found they consistently incorporate the W 1aser welded sleeving methodology discussed in this SE and will provide adequate assurance of SG tube integrity.
Therefore, the proposed changes are acceptable.
6.0
SUMMARY
The NRC staff concludes the proposed sleeving repair, as described in the H topical sleeve reports WCAP-13698 and WCAP-14653, will produce sleeved tubes with acceptable metallurgical properties, structural and leakage integrity and corrosion resistance. The NRC staff also finds acceptable the proposed preservice and future inspections of the d eeved SG tubes.
7.0 STATE CONSULTATION
in accordance with the Commission's regulations, the Texas State official was notified of the proposed issuance of the amendments.
The State official had no comments.
8.0 ENVIRONMENTAL CONSIDERATION
The amendments change a requirement with respect to installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20 and change a surveillance requirement.
The NRC staff has determined that the amendments involve 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 occupational radiation exposure.
The Commission has previously issued proposed findings that the amendments involve no significant hazards consideration, and there has been no public comment on such findings (61 FR 26938 and 62 FR 17235).
Accordingly, the amendment meets the eligibility
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b criteria for cetegorical exclusion set forth in 10 CFR Sl.22(c)(9).
Pursuant to 10 CFR 51.22(b) no environmental impact statement or environmental assessment need be prepared in connection with the issuance of the amendments.
9.0- CONCLUSION 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 amendments will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributors:
S. M. Coffin J. L. Kennedy Date:
September 4, 1997 b