ML17059B590
ML17059B590 | |
Person / Time | |
---|---|
Site: | Nine Mile Point |
Issue date: | 06/30/1997 |
From: | Delwiche D, Horn R, Rodabaugh J NIAGARA MOHAWK POWER CORP. |
To: | |
Shared Package | |
ML17059B591 | List: |
References | |
GENE-B13-01739, GENE-B13-01739-44-R0, GENE-B13-1739, GENE-B13-1739-44-R, NUDOCS 9706200179 | |
Download: ML17059B590 (140) | |
Text
GENE B13-01739-44 Revision 0 June 1997 Metallurgical Evaluation ofFailed Shroud Tie Rod Lower Spring Contact Wedge Latches Nine MilePoint Unit 1, RF014 D. E. Delwiche, Program Manager Metallurgical Evaluations Plant Materials Technology
// ~~ yg Reviewed by:
R.. Horn, Engineering Fellow Materials Technology Approved by:
J. F. Rodabaugh, Mission Mana r
In Vessel Repairs Reactor Modification Services 970b200i79 970bis PDR ADQCK 05000220 P
GENE B13-01739-44 Revision 0 June 1997 REVISION STATUS SHEET Revision Draft A
roval D. E.
Dehviche Date 5/5/97 Draft Issue Descri tion DraftA D. E.
Delwiche 5/13/97 Draft Issue, with editorial Changes Draft B D. E.
Delwiche 6/6/97 Draft Issue, with addition ofMetallurgical Evaluation Results for 90 and 166 degree wedge latches.
Rev. 0 D. E.
Delwiche 6/10/97 Resolution ofReview Comments and MinorEditorial Chan es
GENE B13-01739-44 Revision 0 June 1997 IMPORTANTNOTICE REGARDING CONTENTS OF THIS REPORT Please read carefully The only undertakings of the General Electric Company (GE) respecting information in this document are contained in the contract between Niagara Mohawk Power Corporation and GE, as identified in PO 15247, and nothing contained in this document shall be construed as changing the contract.
The use of this information by anyone other than Niagara Mohawk Power Corporation, or for any purpose other than that for which it is intended is not authorized; and with respect to any unauthorized
- use, GE makes no representation or warranty, express or implied, and assumes no liability as to the completeness, accuracy, or usefulness of the information contained in this document, or that its use may not infringe upon privately owned rights.
GENE B13-01739-44 Revision 0
, June 1997 EXECUTIVE
SUMMARY
During the Spring 1997 refueling outage of Nine Mile Point Unit 1, the nuclear core shroud repair assemblies, installed during the 1995 outage, were found to be degraded.
The degradation consisted of loose tie rods and failed lower spring contact wedge latches (retainer clips). This report describes the results of the metallurgical evaluation performed at GE's Vallecitos Nuclear Center laboratories to validate the root cause of the retainer clip failures.
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The root cause of the contact w'edge latch failure was determined to be high sustained loads applied to the underside of the latch nose (due to unacceptable movement of the shroud repair assemblies during plant operation) resulting in an intergranular stress corrosion (SCC) crack fracture of the contact wedge latch.
Crack initiation and growth of the SCC fracture occurred within one cycle of plant operation.
Such crack growth is consistent with laboratory predictions of SCC propagation rates of AlloyX-750 in the BWR environment under high sustained loads.
GENE B13-01739-44 DraftRevision 0 June 1997 CONTENTS 1.0 INTRODUCTION................................................................................................5 2.0
SUMMARY
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3.0 BACKGROUND
....................................................................................................7 3.1 DESIGN DESCRIPTION 3.2 FIELD INSPECTIONS...................................
3.3 INITIALASSESSMENT OF FAILURECAUSE....
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10 4.1 RECEIPT EXAMINATION
....10 4.1.1 RADIOLOGICALSURVEY 4.1.2 VISUALEXAMINATION.
4.2 SCANNINGELECTRON MICROSCOPIC (SEM) FRACTOGRAPHY
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...12 4.2.1 350 DEGREE LATCHSEM FRACTOGRAPHY.
4.2.2 90 DEGREE LATCHSEM FRACTOGRAPHY 4.3 OPTICALMICROSCOPY
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....13 4.3.1 350 DEGREE LATCHMICROSCOPY.
4.3.2 90 DEGREE LATCHMICROSCOPY..
4.3.3 166 DEGREE LATCHMICROSCOPY.
4.4 MATERIALSPROPERTIES VERIFICATION
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.15 4.4.1 MATERIALCOMPOSITIONALANALYSIS 4.4.2 MICROHARDNESS TRAVERSE..............
4.4.3 MICROSTRUCTURALASSESSMENT
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....16 5.0 ANALYSISOF RESULTS..................................................................
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5.2 CRACKGROWTH CONSIDERATIONS....
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.18 6.0 ROOT CAUSE OF FAILURE...................................
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7.0 REFERENCES
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GENE B13-01739-44 Revision 0 June 1997 I.O INTRODUCTION During the Spring 1997 refueling outage at Nine Mile Point Unit 1 (NMP1), anomalies were found with the shroud repair hardware. In particular, irregularities were found with the lower spring contact wedge latches (also referred to in this report and elsewhere as "retainer clips"). The shroud repair hardware was in service for approximately two years. The anomalies consisted ofloose tie rods and failed lower spring contact wedge latches. This report describes the metallurgical evaluations of the failed contact wedge latches, and the results ofthose evaluations. In addition, a non-failed wedge latch was included in the evaluation.
The anomalies were found during planned visual inspections of the shroud repair hardware and during the planned replacement ofa shroud repair assembly at 270 degrees.
GENE B13-01739-44 Revision 0 June 1997 2.0
SUMMARY
Allfour shroud repair assemblies were found to have lost vertical preload and three of the wedge latches that prevent relative motion between the lower spring and the wedge were damaged.
One latch had failed in service(the 90 degree latch), another failed during the removal process(the 350 degree latch), and a third had visual evidence, ofdamage(the 270 degree latch). The fourth(166 degree latch) had no evidence ofdamage.
Similar wedge latches on the mid-supports and on the upper springs were found to be normal. The lower spring latches are similar in physical features to the'upper spring and mid-support latches but have different applied loadings.
The root cause oF the latch failure and the tie rod looseness is related to the design assumption ofsliding on the vessel surface.
Refer to report GENE B13-01739-40 (Reference
- 1) for a full discussion ofshroud repair anomalies. This report describes the results of the metallurgical evaluation performed at GE's Vallecitos Nuclear Center laboratories to validate the root cause of the retainer clip failures.
3.0 BACKGROUND
GENE B13-01739-44 Draft Revision 0 June 1997 The as found condition, design description, and field inspection results are discussed in this section. Also included is the initial assessment of the cause oflatch failure.
3.1 Design Description The shroud repair was designed to structurally replace the circumferential welds in the core shroud. Four assemblies are placed approximately uniformlyaround the shroud (azimuths 90, 166, 270, and 350 degree).
Each assembly functions to vertically hold the shroud to the shroud support cone and to horizontally support the shroud at the top guide and core plate elevations. In addition, there are other horizontal supports that would prevent unacceptable horizontal movement ofany shroud cylindrical segment that could be produced by failure of the horizontal shroud welds.
Figure 1 shows an elevation view ofone set ofshroud repair assemblies.
There are four such sets at azimuths 90, 166, 270, and 350 degrees around the core shroud. The tie rod is the main component for reacting axial loads. The lower spring is the linear spring for supporting the shroud at the core plate elevation. The lower wedge is a component that was machined based on actual site measurement to fitbetween the RPV and the lower spring with a small compression of the lower spring at room temperature.
The latch is a wishbone shaped piece, intended to prevent relative motion between the lower wedge and the lower spring. Figure 2 provides detail of the lower spring wedge latch within the shroud repair assembly.
Similar latches are also used to prevent relative motion at the mid-support and at the upper spring. The lower support is an assembly that connects the shroud repair hardware to the shroud support cone. The tie rod nut is at the top ofthe tie rod and is used to tighten the assembly.
During installation, the tie rod nut was torqued to preload the assemblies to assure minimal tightness ofcomponents.
The mid-support is used to limit relative motion between the middle of the shroud and the RPV. The upper spring is a linear spring for supporting the shroud at the top guide elevation. For more extensive description, see Reference l.
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GENE B13-01739-44 Revision 0 June 1997
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3.2 Field Inspections The lower support wedge latch at 90 degrees was found broken and separated during the visual inspection. The "nose" piece of the latch was missing and later found on the lower support cone at approximately azimuth 330 degrees.
Figure 3 is a photograph of the broken 90 degree latch. Based on an examination ofphotographs of the fracture surface taken at the NMP-1 site, and IVVIvideo tapes, the failure was judged to be not consistent with a fatigue mechanism.
In addition, there was no visible evidence ofplastic deformation, which would be necessary for a single event overload type offailure. The failure surface appeared to be consistent with a stress corrosion failure under high stress.
Based solely on the visual information, a stress corrosion fracture was believed more likely than an overload fracture.
Video tape inspection of the other three lower wedge latches showed them all to be intact, but the 350 degree latch appeared to be "bent". In addition, the lower spring wedges had evidence oflocal hard contact with the wedge latch, due to vertical loads within the tie rod assembly. Since the latches are AlloyX-750 and the lower spring wedges are Type 316 low carbon stainless steel, the lower spring wedges willshow surface imprint before the latches.
The similar latches used in each mid-support assembly and two similar latches are used in each upper spring assembly all had been visually examined and all appeared normal.
Because ofdesign differences, these other latches can not be loaded as severely as the lower wedge latches. The contact force between the RPV and the shroud repair is much smaller at these locations as compared to the contact force at the lower wedge. In addition, these latches are not loaded during plant heat-up.
3,3 InitialAssessment of Failure Cause During normal plant operation there are only a few sources ofloads on the shroud repair.
These are installation, differential thermal and pressure expansion, fluidflowand dead weight. The dead weight, fluid flow, and installation stresses are low. The main forces on the shroud repair are due to differential thermal expansion between the shroud, RPV, and shroud repair, which both are in the vertical and horizontal directions.
Based on the initialIVVIobservations of the loose tie rod at 270 degrees and the failure of the latch at 90 degrees, different potential causes were postulated.
These causes were possible vibration leading to yielding of the tie rods, fatigue of the latch, or other unexpected displacements causing a single event failure. Evaluations solely by IVVItechniques and photo
GENE B13-01739-44 DraftRevision 0 June 1997 macrographs ofthe fracture face of the latch, are ofcourse insufficient evidence to establish the actual cause oflatch fracture.
A review of the stress analyses showed that the tie rods could not have been overloaded to yield, and the failure surface did not show visible evidence ofplastic deformation typical ofa single event overload failure. The jagged, irregular failure surface of the broken 90 degree latch tends to rule out fatigue as a possible failure mechanism.'However, the evidence obtained by macroscopic field observation strongly suggests that the latch fracture was due to a stress corrosion mechanism rather than a fatigue or mechanical overload failure. The surface has the irregular features with characteristics ofsecondary cracking, suggestive of stress corrosion under high stress.
The only known source ofhigh stress is due to restraint ofdifferential vertical motion between the RPV and the lower spring wedge. Ifthe lower spring wedge did not slide vertically along the RPV, then the differential displacement must occur between the lower spring and the lower wedge.
Such movement willcause high stress in the latch. Sources of such differential displacement are the vertical looseness of the tie rods and the differential displacements discussed in Reference
- 1. Therefore, the root cause of the latch failure and the tie rod looseness is related to the design assumption of the contact wedge sliding on vessel surface.
GENE B13-D1739-44 Revision D June 1997 4.'0 LABORATORYCHARACTERIZATIONOF CRACKS Details of the metallographic examination performed on the wedge latches are presented in this section. The focus of the evaluation was to perform a reasonable amount ofwork to provide a high confidence, technically supportable understanding of the cause ofwedge latch failure. Three lower contact wedge latches were examined at the General Electric Vallecitos Nuclear facilityin Pleasanton, California. Two failed latches and one undamaged latches were examinaed.
4.1 Receipt Inspection Three lower wedge latches (90 degree, 166 degree, and 350 degree) were packaged in a 55 gal drum prepared as a Type A radioactive shipment container at the Nine Mile Point Unit 1 site, and transported to General Electric's Vallecitos Nuclear Center (GF;VNC) metallurgical laboratory for failure analysis. The shipping container was provided by GE-VNC.
4.1.1 Radiological Survey Upon receipt at the metallurgical laboratory, the latches were unpackaged in the GE-VNC-RHO(Remote Handling Operation) controlled corrider area. A radiological survey was performed.
Results indicated gamma+ beta activity to be in the range (approx 2R/hr contact, and 300 mR at 18 inches) allowing hands-on evaluation procedures to be used.
Following radiological survey, the latches were decontaminated by ultrasonic cleaning in a mild, diluted CORPEX cleaning solution, followed by deionized water rinsing and air dry.
4.1.2 Visual Examination Visual examination and documentation of the surface condition was performed on each latch. Particular attention was applied to evidence ofplastic deformation, and condition of the fracture, surfaces.
Direct low magnification photographs were prepared to document condition. The results are provided in Figures 3 through 6, and 8 through 11.
Figures 3 and 4 are photo-macrographs of90 degree lower spring contact wedge latch, found in the broken and separated condition. The nose piece of the latch was found in the annulus region of the reactor. This is the latch fracture face photographed during site IVVIactivities, used to make the preliminary assessment that the fracture was probably due to a stress 10
GENE B13-01739-44 DraftRevision 0 June 1997 corrosion mechanism.
The irregular surface condition was suggestive of a corrosion related mechanism. In Figure 4 it is noted that the fracture surface is uniformly colored with an oxidation characteristic of that produced by exposure to the BWR environment. There was no evidence ofplastic deformation, suggesting mechanical overload was not a causitive factor.
In addition, itwas noted that the underside of the latch "nose" had an oxide pattern suggestive ofsustained contact during service.
Figures 5 and 6 are close-up photographs of the 166 degree latch. This latch was found intact and without damage.
Figure 6 is an enlarged view of the underside of the nose region of the latch. In contrast to that observed on the 90 degree latch documented in Figure 4, an absence of oxide patterning is found on this latch suggesting no sustained 'contact during service. Since the 166 degree latch was not obviously cracked, the nose portion of the latch was cut from the remainder of the latch and examined under the SEM at relatively low magnification. This technique is effective in locating small or tight cracks ifthey are present.
The results are provided in Figure 7. Figure 7 has typical views of the 'inside'orner region.
The examination revealed no cracking in the area ofcrack initiation as found on the other wedge latches.
Figures 8 through llare close-up photographs of the 350 degree latch. This latch was reported as "bent" in the IVVIreport, the "bend" being located at the underside of the nose.
During removal, the latch broke, separating into two pieces.
Figure 8 is an enlarged view of the 350 degree latch, showing the location ofseparation. The separation is the same location as the fracture of the 90 degree latch, with the implication that the latch was nearly through-wall cracked during service and removal handling resulted in the final separation.
Figure 9 shows a side view of the broken segments of the 350 latch, showing an absence ofplastic deformation. The IVVIindicated a possible deformation - now known to be caused by a yawning open ofa nearly through wall crack. Figure 10 is an enlarged view of the 350 degree latch fracture faces. Note the gradation ofoxide coloration, ranging from darkest brown in the region ofcrack initiation (oldest crack surface) to a light brown in the region of the crack tip (recent crack growth in the BWR environment). The unoxidized band at the lower edge was caused by room temperature ductile fracture during latch removal. Figure ll is an enlarged view of the fracture face and the underside of the latch nose of the 350 degree latch. Note the gray band at the upper edge of the underside surface. This band was caused by surface contact during service.
Compare this condition with photos of the 166 degree latch in Figures 5 and 6, which show no pattern ofcontact on the oxide surface.
GENE B13-01739-44 Revision 0 June 1997 4.2 Scanning Electron Microscopic (SEM) Fractography 4.2.1 350 Degree Latch SEM Fractography Scanning Electron Microscopic Fractography (SEM) was performed on the fracture face removed from the 350 degree and 90 degree-latches.
The 350 degree latch was selected for a more comprehensive evaluation because the BWR service crack was not through wall, and therefore contained a crack tip. In the region adjacent to the crack tip, the surface was nearly free ofoxide, allowing clear imagery offractographic features.
I'igures 12 through 19 provide the results. The 90 degree latch fracture face was studied to confirm the mechanism of fracture was the same.
Figure 12 is an SEM view at 12 X of the fracture face ofthe 350 degree latch. This low magnification macroscopic view showed secondary cracking associated with the primary fracture. This is characteristic of SCC growth. This figure also provides location information for the higher magnification views ofFigures 13-16.
Figure 13 is a 300 X magnification view of the fracture face in the region of crack initiation.
See arrow location 4 in Figure 12. Note the intergranular nature of the fracture, with minor plastic deformation and moderate oxide build-up. Figure 14 is the same fracture face at a magnification of 200 X in the mid-fracture region. Note the intergranular characteristic of the surface, with moderate oxide build-up. This is location 3 in Figure 12. Figures 15 a k, b (Location 2 in Figure 12), are fracture face views taken at 200 and 300 X of the region near the crack tip. Note the slight build-up ofoxide, indicating recent growth in the BWR environment.
Figure 16 contains views of the fracture face (300 X and 1000 X) at location 5 in Figure 12. This is the region ofductile rupture which occurred at room temperature during latch removal.
The central region of the 350 degree latch fracture face is shown in Figure 17. The left hand edge of the fracture (arrow location 1 in the Figure) was the region oflow temperature fracture during latch removal. This figure provides location information for the higher magnification views offigure 18. Figure 18 contains 200 X and 300 Xviews of the fracture surface shown in Figure 17 at location 1. Note the IGSCC character, and the only very slight oxide. This region is near the crack tip. Location 2 has appearance the same as Figure 14.
Figure 19 is a high magnification (2000X) view ofthe fracture face of the 350 degree contact wedge latch. The area selected is the central region of the view ofFigure 15a, and was 12
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GENE B13-01739-44 Draft Revision 0 June 1997 selected because itwas relatively clean ofsurface oxide. Features characteristic ofa fatigue fracture were not observed.
An intergranular failure mode was observed.
4.2.2 90 Degree Latch SEM Fractography Scanning Electron Microscopic Fractography (SEM) was also performed on the fracture face removed from the 90 degree latch.'While the 350 degree latch"was selected for a more comprehensive evaluation latch because the BWR service crack was not through wall, the 90 degree latch fracture face was studied to confirm the mechanism offracture was the same.
Figures 20 through 23 provide the results.
Figure 20 is a SEM view (12X) of the fracture face of the 90 degree contact wedge latch. This low magnification macroscopic view shows secondary cracking associated with the primary fracture.
As was found on the 350 degree wedge latch fracture face, this is characteristic of SCC growth. This figure also provides location information for the higher magnification views of the 90 degree wedge latch fracture, Figures 21-23. Figure 21 is a 200X view of the fracture face in the area ofcrack initiation. The cracking is intergranular with heavy oxide buildup characteristic ofIGSCC. Figure 22 is a 200X view ofthe region near the site offinal separation of the 90 degree wedge latch. The cracking is clearly intergranular, with less oxide buildup than seen in Figure 21. Figure 23 is a 800X view of the center offracture face of Figure 22. Note the oxide buildup and obscuring of the fracture face detail.
4.3 Optical Microscopy 4.3.1 350 Degree Latch Microscopy Asingle section was prepared for optical microscopic evaluation of the 350 degree latch fracture. The section was located as indicated in 1'igure 24. This location was selected as it captured secondary cracking as well as the primary surface feature ofcrack initiation, growth, crack tip, and room temperature ductile separation.
The plane ofpolish is perpendicular to the plane of the service fracture. Figure 25 is an as-polished, and etched view of the fracture.
Figure 26 contains optical microscopic views of the latch failure. Photo a. is the region of the crack mouth (initiation). Photo b. is the mid-fracture region, and c. is the region offinal separation characterized by transgranular ductile rupture. The upper left photograph in I'igure 27 shows secondary cracking characteristic of IGSCC. Magnified views of the crack
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GENE B13-01739-44 Draft Revision 0 June 1997 are noted in the lower photo ofFigure 27. This crack morphology is characteristic ofIGSCC.
Figure 28 is a high (250X) view of the fracture in the region of the crack initiation. Evidence ofminor cold work was observed.
Figure 29 is a high magnification (250X) view of the mid-fracture region of the 350 degree wedge latch, clearly showing the IGSCC nature. Figure 80 shows high magnification (250X) view oflatch fracture face in region offinal separation.
The final separation occurred by ductile rupture, and has an associated plastic deformation.
The observation is fullyconsistent with the results ofSEM fractography, as seen in the views of Figure 16, 4.8.2 90 Degree Latch Microscopy Asingle section was prepared for optical microscopic evaluation of the 90 degree latch fracture. The section was located and prepared in manner similar to that used for the 350 degree latch sample. This location was selected as itcaptured secondary cracking as well as the primary surface features of crack initiation, growth and final separation.
The plane of polish is perpendicular to the plane of the service fracture. Figure 31 is an as-polished, and etched view of the fracture.
Figure 32 contains optical microscopic views of the latch failure. Photo a. is the region of the crack mouth (initiation). Photo b. is the mid-fracture region, and c. is the region offinal separation. There was no transgranular ductile rupture associated with this latch fracture.
Figure 33 shows secondary cracking characteristic of IGSCC. Figure 34 is a high (250X) view of the fracture in the region of the crack initiation. Note evidence ofminor cold work, similar to that found on the 350 degree latch. Figure 35 is a high magnification (250X) view of the mid-fracture region of the 90 degree wedge latch, clearly showing the IGSCC nature.
Figure 36 is a view ofa portion of the secondary cracking near the mouth of the secondary crack, with significant oxide buildup. This buildup is characteristic of a crack surface exposed to the BWR environment for some time. By comparison, the 350 degree latch was a more "recent" failure.
4.8.8 166 Degree Latch Microscopy Figure 37 has cross-sectional views of the uncracked 166 degree wedge latch. Note the lack of incipient cracking or plastic deformation in the area of crack initiation on the 90 and 350 degree latches.
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GENE B13-01739-44 Revision 0 June 1997
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4.4 Materials Properties Verification The lower contact wedge latches were fabricated to GE Drawing 112D6560 ofAlloyX750:
ASTM Specification ASTM-B-637 Type 3, Heat HT¹51072-2, as indicated in the Appendix A-
"AlloyX-750 Certificate ofConformance/Compliance/CMTR."
Heat Treatment Procedure was in accordance with GE-SAP-AH-1Revision 2, dated 10/25/1994, and GI':NE Specification, P50YP107/ Attachment 1. These documents indicate an age hardening heat treatment at 1300 degrees F for 20 1/2 hours, followed by a fan air cool. The material of a failed wedge latch was tested by various means to assess the correctness ofmaterial composition and heat treatment.
The results are documented in the followingparagraphs.
4.4.1 Material Compositional Analysis Initialcompositional analysis screening was performed at the time of the SEM fracture surface study. Figure 38 is EDS (Energy Dispersive X-ray Spectroscopy) data used to qualitatively check composition of the latch material. The scan was prepared on a relatively clean (low oxidation) region of the fracture face. The data are consistent with AlloyX-750.
'Further analysis was performed by using a direct coupled plasma spectrographic quantitative analysis technique. More accurate than the SEM/EDS method, the results of this analysis was used to compare the composition with the ASTM-B-637-89, Type 3 designated in the GE Purchase Specification, as well as the check analysis provided in the CMTRfor the appropriate latch heat HT¹51072-2. These comparisons are listed in Figure 39. The sample for the spectrographic analysis was prepared from a 1 gram portion of the metallographic specimen removed from the 350 degree wedge latch. To ensure the removal ofsurface contamination, the sample was alternately etched with concentrated hydrochloric and nitric acids until freshly exposed metal was observed on all surfaces. The sample was then rinsed in deionized water, then dried. The clean sample was then weighed, then disolved in approximately 20 mL concentrated nitric acid and 2 mL hydrochloric acid for each gram of metal. The dissolution was carried out by heating to near boiling in a 100 mL Teflon beaker covered with a Teflon watch glass. The solution was then analyzed with a Spectraspan III plasma emission spectrometer equiped with an ADaM data acquisition and control system.
Standard steel materials used to calibrate the spectrometer response were obtained from the National Bureau ofStandards (NBS), now known as the National Institute ofStandards and Technology. The results obtained in this analysis, provided in Figure 39 demonstrate good agreement with the certified values. It is concluded the composition of the wedge latch material is consistent with AlloyX-750.
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GENE B13-01739-44 Revision 0 June 1997 4.4.2 Microhardness Traverses Microhardness measurements were performed on the polished and etched sample of the 350 degree wedge latch originally used for optical microscopic characterization oflatch cracking.
Figure 40 is a photo-macrograph of the microhardness imprints. Measurements were made in the Knoop scale, with a 500 gram load. Average Knoop readings were approximately 373 I&oop, corresponding to a reading ofapproximately 346 on the HB scale. For this material, I&oop readings ranging from 292 to 391 are specified (or 267 to 363 on the HB scale). No hardness gradients, or increases in readings near the fracture edge, were found. These observations demonstrate the fracture failure was not associated with plastic deformation, or ductile overload. These results are consistent with the results of optical microscopy and SEM fractography, and consistent with the heat treatment.
4.4.3 Microstructural Assessment Based on the Heat Treatment record for Ht451072-2, as documented in the CMTR (Appendix A) the material was supplied in the Annealed (1975 +/- 25 F) condition, and age hardened at 1300 F+/- 15 F for 20 1/2 hours, followed by removal from the furnace and a fan air cool. Optical microscopy of a tensile test specimen of this heat, with this anneal condition, shows a grain size of 7.0. High magnification optical microscopic views of samples prepared from 90 and 166 degree wedge latches Figures 41 and 42 show a microstructure correct and appropriate for this material and heat treat condition.
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GENE B13-01739-44 Revision 0 June 1997 5.0 ANALYSISOF RESULTS 5.1 Fracture Mechanism The preceeding sections of this report have documented the key evaluation evidence associated with the microstructural and fractographic features of the latch failure. They are summarized here (and summarized in Table 1):
1.
The wedge latch material is ofcorrect composition (AlloyX-750), and is as specified.
The heat treatment was correct and resulted in appropriate hardness levels.
2.
There is only minor evidence ofplastic deformation (at the point ofcrack initiation),
as shown by optical microscopy, indicating the failure was not due to a single event overload.
This statement is further supported by the absence ofmicrohardness gradients in the fracture region.
3.
The fracture is intergranular, characteristic ofIGSCC found in BWR internal components.
4.
Crack initiation occurred under the latch nose at the transition from a 200 mil thick section to a much thicker "nose" section.
5.
Those latches that failed had an oxide pattern on the underside ofthe nose, indicating contact during service. The uncracked latch (166 degree latch) did not have the oxide contact pattern. Confirmation of the absence ofcracking was provided by both SEM surface imagery as well as by optical microscopy.
With this evidence, it is concluded that the lower spring contact wedge latches failed as a result of Intergranular Stress Corrosion Cracking of a material having a known susceptibility under high stress conditions. The root cause of the failure of the contact wedge latch failure was determined to be high sustained loads applied to the underside of the latch nose (due to unacceptable movement of the shroud repair assemblies during plant operation) resulting in an intergranular stress corrosion crack fracture of the contact wedge latch. Crack initiation and growth of the SCC fracture occurred within one cycle ofplant operation.
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GENE B13-01739-44 Revision 0 June 1997 5.2 Crack Growth Considerations The results presented in Reference 2 (Intergranular Stress Corrosion Cracking Propagation Rates ofAlloyX-750 in the BWR Environment, dated 25 March 1977) indicate that although AlloyX-750 is a suitable structural material for BWR applications, and the fact that the IGSCC failed wedge latches were manufactured from AlloyX-750 with appropriate composition, heat treatment and mechanical properties, demonstrate that AlloyX-750 is'not immune to IGSCC in the BWR environment. The measured (fracture mechanics specimens) and calculated (CBB specimens) AlloyX-750 crack growth rates indicate that at moderate to high stress intensities, AlloyX-750, even heat treated to obtain its highest resistance to IGSCC in the BWR environment, can suffer rapid intergranular crack growth. These measured and calculated AlloyX-750 crack growth rates are readily high enough to result in failure of the 200 mil thick wedge latch within one fuel cycle, as supported by the data ofReference 2.
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GENE B13-01739-44 Draft Revision 0 June 1997 6.0 ROOT CAUSE OF FAILURE The most probable root cause of the lower spring contact wedge latch (retainer clip) failure was determined to be high sustained loads applied to the underside of the latch nose (due to unacceptable movement of the shroud repair assemblies during plant operation) resulting in an intergranular stress corrosion crack fracture of the retainer clip. Crack initiation and growth of the SCC fracture occurred within one cycle ofplant operation.
Such crack growth is consistent with laboratory predictions ofSCC propagation rates ofAlloyX-750 in the BWR environment under high sustained loads.
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GENE B13-01739-44 Revision 0 June 1997
7.0 REFERENCES
7.1 GENE B13-01739-40, "Nine Mile Point 1-Shroud Repair Anomalies", April1997 7.2 "Intergranular Stress Corrosion Cracking Propagation Rates ofAlloyX-750 in the BWR Environment",
25 March 1997. B. M. Gordon, Corrosion Technology Report.
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GENE B13-01739-44 Draft Revision 0 June 1997 OWER SPRING VESSEL LL LATCH 5'EF LOWER WEDGE Fi<nu~e. Shroud Repair Lower Support Configuration 22
GENE B13-01739-44 Draft Revision 0 June 1897 tr l
1 F~ire S.
Photo-macrograph of 90 degree lower spring contact wedge latch, found in the broken and separated condition. The nose piece of the latch Nfas found in the annulus region of the reactor.
GENE B13-01739-44 Draft Revision 0 June 1997
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II F~ire 4. Enlarged view of the 90 degree contact wedge latch fracture surfaces.
Note uniform oxide coloration of fracture surface and absence of plastic deformation.
Oxide pattern on underside of latch "nose" indicates contact during service.
~
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GENE B13-01739-44 Draft Revision 0 June 1997 0
Ps., 'tt F~ire 6. Contact wedge latch removed from the lower position of the 166 degree tie rod assembly.
Clip was intact and without irregularity.
25
GENE B13-01739-44 Draft Revision 0 June 1997 s a,)+
--r F~ire 6. Enlarged view of underside of 166 degree contact wedge latch nose.
Note the absence of oxide pattern indicating no contact during service.
Compare with Figure 4.
26
GENE B13-01739-44 DraftRevision 0 June 1997 Corner rc ii Corner I
a I
L 5
~
F~inre 7. Typical views of "inside corner" region of 166 degree contact wedge latch by SEM imagery, performed to identify possible cracking.
The method, used in place of PT (penetrant testing), revealed no cracking in area of crack initiation as found on cracked wedge latches.
27
GENE B13-01739-44 Draft Revision 0 June 1997 F~inre Enlarged view of 350 degree contact wedge latch, showing location of separation.
The nose separated during latch removal.
GENE B13-01739-44 Draft Revision 0 June 1997 l v pv,lQ~ I~>
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~Ft ure Side view ofbroken segments of 350 degree contact wedge latcit, showing the absence of plastic deformation.
The IVVIphoto indicated a possible deformation - now known to be caused by a yawning open of a nearly through-section crack.
29
GENE B13-01739-44 Draft Revision 0 June 1997 IAf'-
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N gradation of oxide coloration, ranging from darkest brown in the region of crack initiation (oldest crack surface) to a light brown in the region of the crack tip (recent crack) in BWR environment.
The unoxidized band of the lower edge was caused by room temperature ductile fracture during latch removal.
30
GENE B13-01739-44 Draft Revision 0 June 1997 I gd i
I'0 f
d d
id 5
d" "5350 degree contact wedge latch. Note the gray band at the upper edge of the underside surface.
This band was caused by surface contact during plant operation.
Compare with Figures 4 and 6.
GENE B13-01739-44 DraftRevision 0 June 1997 l ll l
w
~
8 ~3 FRACTURE FACE AT END "A"
~Fi ure 12.
SEM view (12X) fracture face of $50 degree wedge latch. This low magnification macroscopic view shows secondary cracking associated with the primary fracture.
This is characteristic of SCC growth. This figure also provides location information for the higher magnification views of the following four figures. End "A"is at the left end of the left hand fracture seen in Figure 10.
GENE B13-01739-44 Draft Revision 0 June 1997 e
t ~
- fa aa Future 1$. Fracture face (800x) in region of crack initiation - Arrow location (4) in Figure 12. Note intergranular nature of fracture, with minor plastic deformation, and moderate oxide buildup.
GENE B13-01739-44 Draft Revision 0 June 1997 4&(
55 55
~Fi ure 14. Fracture face (200X) in mid-fracture region of 850 degree wedge latch.
Note intergranular characteristic of surface, with moderate oxide buildup.
Location (8) in Figure 12.
II
GENE B13-01739-44 Draft Revision 0 June 1997 5s' 5
5 8
J s
55 55 I
Fi ure 15 a 8c b. Fracture face views (200 and 300X) of region near crack initiation in the 850 degree wedge latch. Note slight buildup of oxide, indicating recent growth in BWR environment.
(Location (2) in Figure 12.)
GENE B13-01739-44 Draft Revision 0 June 1997 r
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~Fi ure 16. Vietca of fracture face (800 and 1000X) at location (6) in Figure 12. This is the region of ductile rupture which occurred at room temperature during the 850 degree wedge latch removal.
GENE B13-D1739-44 Draft Revision D June 1997 v jt P
~Fi ure i7.
SEM view (12X) of central region of fracture face of 850 degree wedge latch. The left hand edge of the fracture (arrow 1) is region of low temperature fracture during latch removal.
The figure provides location information for the higher magnification views of the following figures.
n H
GENE B13-01739-44 DraftRevision 0 June 1997 f
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F~iure 18.
SEM views (200 and 800X) of fracture surface shown in Figure 17 at location (I). Note IGSCC character, and only very slight oxide, since this location is near crack tip.
GENE B13-D1739-44 DraftRevision D June 1997 g~iure i0. High magnification (2000X) view of fracture face of 850 degree contact wedge latch (Figure 15a).
Features characteristic of a fatigue fracture are not observed.
An intergranular failure mode is indicated.
GENE B13-01739-44 DraftRevision 0 June 1997 Aqke tI cia?P
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I Future 20.
SEM view (12X) of the fracture face of 90 degree contact wedge latch.
This low magnification macroscopic view shows secondary cracking associated with the primary fracture.
As was found on the 850 degree wedge latch fracture face, this is characteristic of SCC growth. This figure also provides location information for the higher magnification views of the following 90 degree wedge latch fracture figures.
40
GENE B13-01739-44 Draft Revision 0 June 1997 h'~iure 2i. Fracture face (200X) in region of crack initiation of the 00 degree wedge latch. Arrow location on Figure 20. Cracking is intergranular, with heavy oxide buildup compare with Figure 13, the comparable view for the 350 degree wedge latch.
41
GENE B13-01739-44 DraftRevision 0 June 1997
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ar v
Future 22. Fracture face view (200X) of region near site of final separation of the 90 degree wedge latch.
(Arrow O~ location on Figure 20) Cracking is clearly Intergranular, with less oxide buildup than seen in Figure 21.
'gjw gsri r
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~r Fi ure 23. 800X view of center of fracture face of Figure 22. Note oxide buildup and obliteration of fracture face detail.
H, I
u
GENE B13-01739-44 Draft Revision 0 June 1997 e
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4 FRACTURE FACE MOUNT, POLISH AND ETCH THIS FACE F~inre 24. Sketch of sectioning location for optical microscopy 850 degree
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wedge latch.
GENE B13-01739-44 Draft Revision 0 June 1997 I
<p'2 t.>> Isla 2 t
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FINAL SEPARATION INITIATION
~Fi nre 5.
As-polished and etched views (20X) of 850 degree latch section.
Plane of polish is perpendicular to fracture face at location indicated in Figure 24.
GENE B13-01739-44 DraftRevision 0 June 1997 S
t@~ tt<<\\+ps 6 tft.i Io REGION OF CRACK MOUTH (INITIATION)128X Df'-
MID FRACTURE REGION - NOTE SECONDARY IGSCC CRACKS.
128X c.
REGION OF FINALSEPARATION TRANSGRANULARDUCTILE RUPTURE. 128X
~Fi ure 6.
Views of fracture by optical microscopy 650 degree wedge latch
GENE B13-01739-44 Revision 0 June 1997 selected because itwas relatively clean ofsurface oxide. Features characteristic ofa fatigue fracture were not observed.
An intergranular failure mode was observed.
4.2.2 90 Degree Latch SEM Fractography Scanning Electron Microscopic Fractography (SEM) was also performed on the fracture face removed from the 90 degree latch.-'While the 350 degree latch was selected for a more comprehensive evaluation latch because the BWR service crack was not through wall, the 90 degree latch fracture face was studied to confirm the mechanism offracture was the same.
Figures 20 through 23 provide the results.
Figure 20 is a SEM view (12X) of the fracture face of the 90 degree contact wedge latch. This low magnification macroscopic view shows secondary cracking associated with the primary fracture. As was found on the 350 degree wedge latch fracture face, this is characteristic of SCC growth. This figure also provides location information for the higher magnification views of the 90 degree wedge latch fracture, Figures 21-23. Figure 21 is a 200X view of the fracture face in the area ofcrack initiation. The cracking is intergranular with heavy oxide buildup characteristic ofIGSCC. Figure 22 is a 200X view of the region near the site offinal separation of the 90 degree wedge latch. The cracking is clearly intergranular, with less oxide buildup than seen in Figure 21. Figure 23 is a 800X view of the center offracture face of Figure 22. Note the oxide buildup and obscuring of the fracture face detail.
4.8 Optical Microscopy 4.8.1 850 Degree Latch Microscopy Asingle section was prepared for optical microscopic evaluation of the 350 degree latch fracture. The section was located as indicated in Figure 24. This location was selected as it captured secondary cracking as well as the primary surface feature ofcrack initiation, growth, crack tip, and room temperature ductile separation.
The plane ofpolish is perpendicular to the plane of the service fracture. Figure 25 is an as-polished, and etched view of the fracture.
Figure 26 contains optical microscopic views of the latch failure. Photo a. is the region of the crack mouth (initiation). Photo b. is the mid-fracture region, and c. is the region offinal separation characterized-by transgranular ductile rupture. The upper left photograph in Figure 27 shows secondary cracking characteristic of IGSCC. Magnified views of the crack
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GENE B13-01739-44 DraftRevision 0 June 1997 rt t
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~Fi nre 27.
Secondary cracking - IGSCC. Magnified views of crack noted in Figure 25 (upper photo). This crack morphology is characteristic of IGSCC.
0
GENE B13-01739-44 Draft Revision 0 June 1997 j
0 8I
~Fi ure 28. High magnification (250X) view of fracture in region of crack initiation 850 de ree wed e latch. Note evidence of minor cold work.
5 R,
~Fi ure 29. High magnification (250X) views of mid fracture region of 550 degree wedge latch, clearly shows IGSCC nature.
GENE B13-01739-44 Draft Revision 0 June 1997 v
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~Fi ure $0.
High magnification (250X) view of 850 degree latch fracture edge in region of final separation.
The final separation occurred by ductile rupture, and has an associated plastic deformation.
The observation is fullyconsistent with the results of SEM fractography, as especially seen in the views of Figure 16.
GENE B13-01739-44
'raft Revision 0 June 1997 a
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Plane of polish is perpendicular to fracture face indicated in Figure 20. Note secondary branch cracking, and compare with view of 850 degree latch section of Figure 25.
49
GENE B13-01739-44 DraftRevision 0 June 1997
- a. REGION OF CRACK MOUTH (INITIATION)128 X
- b. MID-FRACTUREREGION-NOTE SECONDARY IGSCC CRACKS. 128 X
- c. REGION OF FINALSEPARATION-IGSCC, WITHOUTDUCTILE RUPTURE. 128 X Figure~S. Viewof fracture by optical microscopy 90 degree wedge latch.
128K 50
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GENE B13-01739-44 DraftRevision 0 June 1997 128 X
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250 X F~ire 99. Secondary cracking in fracture of 90 degree contact wedge latch IGSCC nature is same as that observed in 350 degree latch. Compare with Figure 27.
51
0
GENE B13-01739-44 Draft Revision 0 June 1997 jfi
~Hre 54. High magnification view (250X) of fracture in region of crack initiation of 90
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degree wedge latch. Note evidence ofminor cold work.
I v
~Fi re 55. High magnification (250X) view of mitt-fracture region, clearly showing IGSCC nature.
52
e GENE B13-01739-44 Draft Revision 0 June 1997 re e
e =
~Fi re 56. Secondary cracking in region 5 of Figure 81, with oxide buildup, characteristic of exposure to the BWR environment.
GENE B13-01739-44 Draft Revision 0 June 1997 h
128 X 20 X
~Fi re 87. Cross. sectional views of nncracked 166 degree wedge latch Note.lack of incipient cracking or plastic deformation in area of crack initiation on the 90 degree and 850 degree latches.
54
GENE B13-01739-44 Draft Revision 0 June 1997 SSQ:
>TI 0 TN-5588 GE-VNC Cui-soi-: 8.888keV
= 8 ROI TUE 22-RPR-97 14:56 C8> I.888: i.918 C
.. N ""'L'"'"i I
NNP S58 tilllilt R
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/ I Nonnali2ed Eternentat Wt%
OAO 1.55 3.05 16.09 9.35 67.56 ASM Specifications tnconei X-750 0.7 1.0 g.5 15.5 7.0 73.0 Future 88.
EDS (Energy Dispersive X-ray Spectrum) data use te qualitatively check composition of latch material.
Scan was prepared on a relatively clean (low oxidization) region of the fracture face.
The data are consistent with AlloyX-750.
GENE B13-01739-44 Draft Revision 0 June 1997 Cr X.750 Nominal Composition 15.5 GE-Purchase Specification ASTM-B-'637-89 Type 3 14.0-17.0 CMTR HT N51 072-2 15.40 EDS (SEM)
Composition Analysis 18.09 Direct Coupled Plasma
'uantitative Analysis 15.1 Ni 73.0 70.00 min 71.63 67.56 71.5 Co 0.07 Mo 1.0 0.7-1.20 0.94 1.55 1.0 2.5 2.25-2.75 2.40 3.05 2.4 Al Fe 0.7 7.0 0.04 0.4-1.00 5.00-9.00 0.08 max 0.73 8.14 0.053 0.4 9.35 N/A 1.1 'within spec on check analysis 8.9 N/A Cu 0.25 max 0.50 max 0.02 F~ire $9. X750 Contact Wedge Latch Material Compositional Analysis Wt %
56
t
GENE B13-D1739-44 Draft Revision D June 1997 3P.
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<<s~, j 20 X F~ire 40. Etched view of 050 degree latch section (same view as Figure 20) showing microhardness imprints. Measurements in Knoop hardness scale, with 500g load.
Avg.
878 Knoop (292 - 891 Knoop specified) 846 HB (267 - 868 HB specified)
GENE B13-01739-44 Draft Revision 0 June 1997 1
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~Ft re 41. High magnification (1000X) view of 90'broken) wedge latch microatrnctnre.
The structure is typical of a correctly heat treated X750 wedge latch material.
58
GENE B13-01739-44 Draft Revision 0 June 1997 v
i
~pi re 42. High magnification optical microscopic view of uncracked 166 degree wedge latch.
GENE B13-01739-44 DraftRevision 0 June 1997 Test/Wedge Latch Field visual inspection Lab-visual examination Optical microscopy SEM fractography Material properties Ij Hdntt transverse Micro-structure Failure mode 90 Degree Nose piece "missing" Nose separated Load contact pattern on underside of nose IGSCC nature No cold work Intergranular heavy oxide Heat ¹ HT ¹51072-2 As expected for specified heat treatment IGSCC initiation to final separation 16fl Degree Un-damaged No "load " contact pattern, Uncracked by SEM Not performed (no cracking found)
Not performed (no cracking found)
Heat ¹ HT ¹51072-2 Un-cracked 350 Degree Nose piece "bent" Nose separated (during removal)
"Load" contact pattern on underside of nose IGSCC nature Minor cold work at final separation Intergranular light-to-moderate oxide Heat ¹ HT ¹51072-2 Consistent with heat treatment Nominal properties No gradients IGSCC with ductile final separation Table 1. Summary ofAnalytical Test Results - NMP - Tie rod contact wedge latch failures (270 degree wedge latch - not examined) 60
0
Appendix A AlloyX-750 Certificate ofConformance/Compiiance/CMTR
I IP4% w&
H I imam-cv-v i inv vc av rii luui.in@ Qri."vliiLLQlb lriv rtN Nv. iIlc baal iIvbU I. UC SF'ECIPJ TY ALLOYPROCESSING COMPANY, INC.
PO Box 44006 i PNsdvtgh, PA 15239
~ (442) 327-3838
~ FAX (412) 32T<716 INDUSTRIALHEATTREATING METALPROCESSING CERTIFICATION Date:
Customer.
Attn."
De ember10, 1994 Tooling Specialists, inc.
PO Box 828 Latrobe, PA 15650 Kim Parabaugh Customer Order Number:
MateriaL XTGO: Alii-B-637Type 3 Our Order Number.4956-3 Lot ~ 3 TS 31739-7-51-6839
{Gi=PO 529-947749AT)
Quantify 8 Part: 5 spring re~er dwg.11206555 HT'1071-3 24 spring retainer
.dwg.112D6551 HT&1074-I 24 screw top supt dwc.112D6558 lt 4 HT&2286 1D washer jack bolt.
dwg.112D6553 HT&2794 5 screw de. i 1i2D6558 lT 1 HTW2794 5 bolt jack dwg.112D6552 1T 1HTP32286 5 latch Qwc. 1 12D6560 lT 1 HT451072-2 15 latch dwg.51206560 lT 2 HT~51072-2 GOscrew'wg.1 i2D6558 lt 6 hi~32794 Treatment:
Heat Treat Procedure{Gc-SAP-AH-1 REV2),dated10/25/94 8 GE-NE. SPEC. PGOYP107/Attachm nt 1:
Age nardened 13;).'oi={-:/-15oF) 20 1/2 hrs.
Remove and fan ah moL 10CFR21 applies Date ofTreatment i2/9/94/ 5) 12/10/94 Specialty Alloy Proceming Cern pany, inc.
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II'fY NNGARh HU %NARK NR RRESPONDEN E APPROVAL FORM Document:
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'pplicability:
References:
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Other Prepared:
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Develo mentsl Review Ucensing Engineering 1 s~s.>M/4 Aa ssst 4 ttarg )ee.~
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NIR Meeting No.:
NIR Meeting No.:
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Engineenng Manager I:-grR 0
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N/R Meeting No.:
NIR Meeting No.:
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Engineenng Manager 0
Generation:
Operations Manager Tech Support Manager Maintenance Manager Gf System Attorney ra Ucarslnp Manager Rant Manager 0
Other mrs >~sr itrrr FSAR Change:
NCTS Commitments:
LDCR t C3 Attached
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