ML20138G053
| ML20138G053 | |
| Person / Time | |
|---|---|
| Site: | Byron, Braidwood  |
| Issue date: | 04/29/1997 |
| From: | Hosmer J COMMONWEALTH EDISON CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9705060151 | |
| Download: ML20138G053 (8) | |
Text
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Commonwealth L.tswn Company I &) Opus Pl.n r Dou ner) Grin e, it 6051 WOI April 29,1997 U.S. Nuclear Regulatory Conunission Washington, D.C. 20555 Attention:
NRC Document Control Desk j
Subject:
Information Related to the Braidwood 1 AIR 06 Steam Generator Inspection and the 3 Volt Interim Flugging Criteria Renewal Technical Specification Amendment Braidwood Nuclear Power Station Unit 1 NRC Docket Numbr: 50-456 Byron Nuclear Power Station Unit 1
)
NRC Docket Number: 50-454
References:
1.
Teleconferences dated April 8th, April 16th, and April 23,1997, between the Commonwealth Edison Company and the Nuclear Regulatory Commission Regarding the AIR 06 Steam Generator Inspection 2.
J.11osmer letter to the Nuclear Regulatory Commission dated August 17, 1996, transmitting Request to Amend the Technical Specification to Renew the 3 Volt Interim Plugging Criteria (IPC) for the Byron and Braidwood Steam Generators 3.
R. Ramm letter to D. Farrar dated March 4,1994, transmitting the Safety Evaluation for the Westinghouse Laser Welded Sleeve During the Referenced teleconferences, the Commonwealth Edison Company -(Comed) and the Nuclear Regulatory Commission (NRC) discussed the steam generator inspection results at Braidwood Unit 1, specifically, the results of the locked tube inspection. Braidwood has repaired the locked tubes with the installation of the laser welded sleeve as discussed in Reference 2. The following details the discussions, and updates the information provided during the teleconferences.
During the current Braidwood Unit 1 Cycle 6 refuel inspection, a sample oflocked tubes were de-plugged and inspected. A number of circumferential indications were found in the locked tubes at the top of tubesheet roll transition. The inspection scope was expanded to 100% of the locked tubes. A total of 49 of 85 locked tubes contained circumferential indications at the top of the tubesheet.
To ensure the design basis of the locked tubes is maintained during the upcoming operating cycle, all locked tubes will be repaired at the top of the tubesheet with a 12 inch Westinghouse elevated laser welded sleeve. This repair process was licensed via Reference 3. The previously expanded tubes and any newly locked tubes will be sleeved and removed from service by tube plugging.
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Eight circumferential indications ( including the largest circumferential indication detected in the j
l locked tubes during the current AIR % outage) were pressure tested and shown to have significant j
margin over the structural integrity necessary to meet the 3.0 Volt IPC design bases.
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Also discussed during the phone calls was the sleeve repair process for the locked tubes. The attachment contains a discussion of: 1) the applicability of WCAP-13698, Revision 1, "" Laser Welded Sleeves for 3/4 inch Diameter Tube Feedring-Type and Westinghouse Preheater Steam j
i Generators" for the repair of the locked tubes, and 2) the installation of the laser welded sleeve i
does not adversely affect the integrity of the sleeve, tube, or the locking expansions. The conclusion that Comed has reached is that the design basis for sleeves in inservice tubes, Reg i
2 Guide 1.121, is more stringent than the design basis for the locked tubes; therefore, the application.
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of a licensed sleeving process is an appropriate repair. The attaciunent documents that the sleeve i
in a locked tube will not have an adverse affect on any portion of the steam generator.
l In Reference 2, Comed submitted a request to amend the Technical Specifications to allow for the renewal of the 3 Volt Interim Plugging Criteria for Braidwood Unit I and Byron Unit 1. To ensure the expedited approval of this request, Comed is requesting that the Byron portion of the request 4
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be separated from the Braidwood package. This should allow completion of the Staffs review on the Braidwood Unit 13.0 Volt IPC renewal package.' Braidwood Unit 1 is currently scheduled to i
enter Mode 4 on May 15th; therefore, Comed is requesting approval of this amendment request by j
May 9th.
Comed plans to meet with the Staff on April 30,1997, to further discuss the Braidwood inspection results and welcomes the opportunity to address any questions concerning this correspondence.
Sincercly, /
//
OfNf8%'l/
V John B. Hosmer Engineering Vice President Attachment D. Lynch, Senior Project Manager-NRR cc:
G. Dick, Byron /Braidwood Project Manager-NRR C. Phillips, Senior Resident inspector-Braidwood 1
S. Burgess, Senior Resident inspector-Byron A.B. Beach, Regional Administrator-Rill Office of Nuclear Safety-IDNS K:nla\\bybwd\\stgen\\ AIR 06R1
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ATTACHMENT A LASER WELDED SLEEVE REPAIR OF 3.0 VOLT IPC L.OCKED TUBES 4
BACKGROUND In support of the bases for the Byron /Braidwood 3.0 volt Interim Plugging Criteria (IPC),
2 21 tubes per SG were locked into selected tube support plates (TSP) and the tubes plugged. The function of the locked tubes is to limit the plate displacement to less than 0.1 inches during a main steam line break (MSLB). The selected tubes were essentially turned into tie rods for added plate support and stiffness. By limiting the TSP displacement, the length of defects at the TSPs that may be exposed to freespan conditions during a MSLB are also limited, thus limiting the amount ofleakage and tube burst probability to within Technical Specification limits. The maximum axial load that is induced on'a locked tube during a MSLB is less than 500 lbs as discussed in WCAP-14273, " Technical Support for Alternate Plugging Criteria with Tube Expansion at Tube Support Plate Intersections for Braidwood-l and Byron-1 Model D-4 Steam Generators, February 1995" To assure the locked tube fulfills its design function, each locked tube is required to be capable of canying 500 lbs axial load to achieve the displacement criteria.
1 Therefore, the 500 lb axial load capability is the design criteria for the repair oflocked tubes with degradation at the top of the tubesheet.
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The August 19,1997, License Amendment Request to renew 3.0 Volt IPC for j
Braidwood-l Cycle 7 and Byron Unit 1 Cycle 9 specified the use of welded sleeves as a repair for degradation at the top of tubesheet roll transition. The sleeve must be capable of maintaining the 500 lb locked tube MSLB loading required by the bases of 3.0 volt IPC.
The basis for concluding that the welded sleeve meets the locked tube design criteria, is that the design basis for an in-service sleeved tube meets the locked tube design requirement with significant margin.
The laser welded sleeve (LWS) installation process used at Braidwood/ Byron is described in WCAP-13698," Laser Welded Sleeves for 3/4 Inch Diameter Tube Feedring-Type and Westinghouse Preheater Steam Generators, Rev.1" that was licensed by the NRC in a March 4,1994, Safety Evaluation Report. In addition, the LWS installation process implemented at Braidwood/ Byron incorporated lessons learned from the major sleeving campaigns at two other PWR units. These process improvements minimize tube deformations and residual stress associated with tubes that are locked at the support plates. These process improvements include:
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- 3. The upperjoint is not installed in the mid-span between the top of tubesheet and the first tube support plate.
- 4. Target heat treatment temperature at lower end of acceptable range.
The LWS was selected as a repair of a locked tube with a defect at the top of the tubesheet because: 1) The LWS is an approved repair for inservice tubes,2) the LWS is capable of carrying a 500 lb axial load, and 3) the installation of the LWS does not adversely afTect the integrity of the sleeve, tube or locking expansions.
PART 1: LWS Load Carrvine Capabilities in a Locked Tuhe The LWS in a tube mechanically locked for 3.0 volt IPC must be capable of carging a 500 lb axial load during a MSLB. Stmetural assessments of the LWS for the lower and upper sleeve joints are discussed in WCAP-13698, Rev.1, and are summarized below.
LWS LowerJoint Structural Assessment:
WCAP-13698, Rev.1, Section 4.0 provides the mechanical test results of the hybrid expansion joint (HEJ) for 3/4 inch diameter sleeves. The pull-out test is the applicable test described in the WC AP that demonstrate the HEJ can cary the required locked tube load.
The Regulatory Guide (RG) 1.121 structural requirement of the inservice HEJ is to resist pull-out at 3 times the normal operating pressure differential (3xNOP) and 1.43 times the limiting faulted pressure differential. The corresponding axialloading requirements for the j
Braidwood/ Byron Unit I steam generator inservice sleeves is 1522 lbs and 1484 lbs for the normal operating and faulted RG 1.121 requirements, respectively. For the tubes j
mechanically locked and plugged for 3.0 volt IPC, the HEJ must be capable of carrying a i
500 lb axialload during MSLB conditions. Therefore, meeting the RG 1.121 requirements for inservice sleeves with the locked tube configuration when corrected for no internal pressure, assures the locked tube design basis is maintained with significant margin.
The 3.0 volt IPC analysis for support plate displacement determined that the axial load the support plates impart on the locked tube occurs within the first 3 seconds of the MSLB event. After 3 seconds, the locked tubes do not carry an axialload. The below structural assessments were performed at SG conditions 3 seconds into the MSLB event. It is also assumed that the parent tube contains a through-wall defect that allows the sleeve / tube annulus to be pressurized to produce a " loosening" effect on the HEJ.
Pull-out tests were performed on the 3/4 inch HEJ test specimens as described in the referenced WCAP. Axialloads were applied to thejoint that had applied internal pressure associated with normal operating differential pressure. The axial force at which initial slip or non-linear load deflection occurred was recorded on Table 4-3 of the WCAP. The initial slip load for the tests ranged from 3400-3480 lbs and 4290-4300 lbs for the room tenperature and 600*F tests, respectively. Correcting the test results for the actual conditions that the HEJ would experience in a locked tube that is plugged when A-2
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l performing its design function (ie. tubesheet bow, no internal pressure, during a MSLB),
the average corrected pull-out test results would be 2325 lbs. The results of these tests confirm that the HEJ will carry the required 500 lb load with a margin factor of 4.7.
Westinghouse performed additional confirmatory qualification testing to support the process improvement installation sequence change to hardrolling after post weld heat l
treatment of the upper sleeve joint. Specifically, pull-out tests were performed for the Doel-4 hardroll before PWHT process and the Maine Yankee hard-roll after PWHT process.
l The Doel-4 qualification test consisted of room temperature pull-out tests of 13 test l
specimens (hardroll before PWHT) with no internal pressure. The average initial slip axial pull load was 4339 lbs. Correcting for the hardroll after PWHT process that is used at Braidwood/ Byron, the Doel-4 ptll-out test results corresponds to an average pull-out force of 4187 lbs. The HEJ in a locked tube can resist a pull-out force of 3276 lbs at MSLB conditions, when corrected for tubesheet bow, differential pressure, and thermal l
growth mis-match loads.
i Analytically, the pull-out force was estimated from the HEJ to tube contact pressure at normal operating and faulted conditions. The as-installed contact pressure, that is determined by testing, is adjusted by the effect of tubesheet bow, differential pressure, and thermal growth mis-match at faulted conditions. For the HEJ in a locked tube that is j
plugged, the calculated pull-out force at faulted conditions is 3117 lbs.
As a summary, the most limiting mechanical test discussed in the original LWS qualification can carry a axial load of 2325 lbs at MSLB conditions in a plugged locked tube. This provides a factor of 4.7 margin over the 500 lb required load carrying capability. The supplemental testing in support of a process improvement demonstrated that the LWS in a locked tube that is plugged is capable of carrying a axial load of 3276 lbs at MSLB conditions. This provides a factor of 6.6 margin over the 500 lb required load carrying capability. Analytically, the calculated pull-out load for a locked tube that is plugged at faulted conditions is 3117 lbs, which provides a factor of 6.2 over of the 500 lb j
I required load carrying capability. These are summarized in the following table.
1 HEJ Pull-out Loads in a Locked Tube Pull-out Load Locked Tube Margin (Ib)
(Pull-out Load /500 lb)
WCAP-13698 Average 2325 4.7 Corrected
- Pull Load Doel-4 Test Corrected
- 3276 6.6 Analytical Pull-out Load for 3117 6.2 Locked Tube Corrected for HEJ in Locked Tube that is plugged at MSLB conditions.
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e The original HEJ qualification as described in WCAP-13698 and the subsequent pull-out testing program for Doel-4 and Maine Yankee both demonstrate that the lower joint of the LWS installed in a locked tube that is plugged is fully capable of carrying the additional locked tube axial load of 500 lbs during MSLB conditions with significant margin.
Therefore, the design bases of the locked tube is met.
LWS Upper Joint Structural Assessment WCAP-13698, Rev.1, Section 3.0 provides the structurai analyses and Section 4.4.2 provides the mechanical testing results of the welded joint. Compressive load tests and tensile load tests were performed on six 7/8" diameter laser welded joint specimens. The test specimens were loaded to failure and the load recorded. The compressive loading tests results ranged from 6610 lbs to 7520 lbs. The tensile loading test results ranged from 9015 lbs to 9380 lbs. Although no mechanical testing was performed as part of the 3/4" diameter LWS qualification, the 3/4" and 7/8" LWS contain similar nominal and minimum weld dimensions and therefore, the 7/8" LWS mechanical testing results apply to the 3/4" LWS.
Tensile load tests were performed in 1994 on two pulled LWS tube / sleeve joints that were removed from the Doel-4 Model E steam generators (3/4 inch diameter tubing). The laser welded joints were installed in crevice packed locked tubes with the hardroll after PWHT installation process. One joint was tensile loaded to 11,000 lbs with no failure and the other joint was loaded to 13,200 lbs with no failure.
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Therefore, the upper joint of the LWS installcd in a locked tube that is fully capable of canying the additional locked tube axial load of 500 lbs during MSLB conditions with significant margin. The design bases of the locked tube is met.
PART 2: Locked Tube Installation Assessment The LWS process implemented at Braidwood and Byron have incorporated the lessons learned from the Maine Yankee and Doel-4 sleeving campaigns. Improvements have been made to the installation process to minimize tube deformation and residual stress associated with tubes that are locked at the support plates. The process improvements include:
- 1. Lower sleevejoint hard rolled after post weld heat treatment of the upperjoint.
- 2. Post weld heat treatment of the upperjoint is not performed with an oscillating heater.
- 3. The upper joint is not installed in the mid-span between the top of tubesheet and the first tube support plate.
- 4. Target heat treatment temperature at lower end of acceptable range.
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l Tube bulging and bowing during LWS installation have been identified at a foreign plant and in subsequent laboratory tests. The deformations have been attributed to tubes locked at the support plates when the lowerjoint is hard rolled prior to post weld heat treatment of the upper joint. In this " fixed-fixed" configuration, the heat treatment temperature imposes a compressive stress induced by restraining thermal growth that may exceed the yield strength of the heated area and cause bulging. When the upperjoint was located in i
the center of the freespan between the upper and lower locks, the heat treatment temperature also would cause the tube to bow as the compressive force exceeded the critical buckling load in the heated area. Both the bulging and bowing phenomena are corrected by eliminating the fixed-fixed condition while heat treating. By heat treating the upper joint prior to hardrolling the lowerjoint, the sleeve is free to thermally displace and reduces the compressive force that can cause bulging or bowing. The Braidwood/ Byron elevated LWS upper joint is installed approximately 4 inches above the tubesheet which is far from the mid-point of the 36 inch span to the first support plate. This also reduces the probability of tube bowing during PWHT.
i Although the PWHT temperature range qualified was 1250*F -1600*F, the earlier LWS campaigns in the industry had used a target heat treatment tube temperature of 1500*F to 1550*F. Testing had shown that the tube residual stresses could be significantly reduced by a lower target temperature. The Braidwood/ Byron LWS installation process requires a PWHT target temperature near the minimum temperature required by the sleeve license approval (a minimum of 1400*F for 5 minutes). The earlier LWS campaigns also utilized
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an oscillating heater for greater tube coverage, which was later determined to be a t
contributor to bulging and bowing in the fixed-fixed condition as it induced greater compressive forces than a smaller heated area. The Braidwood/ Byron LWS process uses a shorter stationary heater to limit the heat treating to a smaller region, thus minimizing the compressive stresses.
The tubes at Braidwood/ Byron that were mechanically locked for 3.0 volt IPC will have a tensile stress due to the expansions at the support plates. Welding into a tube that is in tension will not have an adverse affect on the weld, sleeve or parent tube integrity during welding as the sleeve is free to move relative to the tube (hardroll oflower joint after PWHT). This has been confirmed by the originallaser weld qualification testing where welds were installed in tubes that contained 20 ksi tensile stress. During the PWHT the tensile stresses are offset by the expansion of the tube and the tube goes into compression.
Upon cooling, the tube will have a tensile stress that will be less than initial tensile stress.
Corrosion testing of the 3/4" LWS with applied axial stress had previously been performed to evaluate the corrosion resistance of a sleeve installed in a locked tube. Doped steam corrosion tests were performed with applied axial stress of 15.8 ksi and 26 ksi through-out i
the duration of the test. The test results are documented in WCAP-14596, Rev. O, which was used to support the Model F LWS license. The corrosion test results with applied axial stresses of 15.8 ksi and 26 ksi both demonstrate that for a LWS installed in a tube locked for 3.0 volt IPC, the service life would be expected to be more than 11 times l
longer than a roll transition.
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As confirmation that the sleeving process does not adversely affect the locked tube or sleeve, Comed performed an additional inspection of 21 sleeves in locked tubes following the completion of the sleeving process. The focus of this inspection was to identify any abnormalities or deformations that may have been caused by the sleeving process. Bobbin profilometry was performed to measure the sleeve inside diameter in area that bounded the heat treated area. No indication of abnormalities or deformation were identified by this inspection. Additional ultrasonic examinations (UT) of 20 sleeves in the IB t'G were performed following PWHT to determine if PWHT had any adverse affect on the sleeve or tube. Seven (7) of the sleeves inspected were installed in locked tubes. The post-PWHT UT data was compared to the pre-PWHT data taken as part of the installation process.
No changes were identified.
The occurrence ofinward bulging / denting (collapse) have been identified in several laser welded sleeves at two domestic plants. The inward bulging is described as a localized inward bulge or dent of approximately 60 mils located above the lowerjoint expanded region in the unexpanded portion of the sleeve. The cause of the inward bulging was attributed to a flow diode effect in which throughwall cracks in the parent tube at lower temperatures allow secondary water to enter the annulus between the sleeve and the tube.
At higher temperatures, the cracks close due to differential thermal expansion and do not allow the water to escape. Consequently, the pressure builds as temperature increases to where the sleeve yields inward. The inward bulging is located in the unexpanded region of the sleeve and sufliciently far from the lower and upper joints to not cause significant additionalloading to those joints. Therefore, no structural concern exists for a LWS that contains an inward bulge CONCLUSION Structural assessments of the upper and lower sleeve joints discussed in WCAP-13698, Rev.1 and supplemented by additional testing, indicate the LWS in a locked tube is fully capable of carrying the necessary MSLB loads required by the 3.0 volt IPC design bases with significant margin.
For the reasons discussed above, Comed has concluded that the use of a LWS as a repair of a 3.0 volt IPC locked tube is acceptable and would not adversely affect the function of the sleeve or tube.
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