ML20247G701
| ML20247G701 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 05/31/1998 |
| From: | Christopher Boyd, Wilson I WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP. |
| To: | |
| Shared Package | |
| ML20247G683 | List: |
| References | |
| WCAP-15054, NUDOCS 9805200320 | |
| Download: ML20247G701 (135) | |
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- METALLURGICAL INVESTIGATIONtAND
^
R;OOTCAUSE ASSESSMENT
- OF PART LENGTH
- CRDM' ,
iHOUSING: MOTOR TUBE
[ '
LCRACKINGLAT PRAIRIE ISLAND NUCLEAR
+
GENERATING.PLANTL -
UNIT 2
'W e~ s. t i n g h' o u s' e ' Energy Systems IQ A
e b 'bO 82 P PDR
1 f WESENGHOUSE NON-PROPRIETARY CLASS 3 WCAP-15054 l
l MetallurgicalInvestigation and Root Cause Assessment i
of Part Length CRDM Housing Motor Tube Cracking at Prairie Island Nuclear Generating Plant Unit 2 Gutti V. Rao i Engineering and Materials Technology
)
l May 1998 Reviewer:
I. L'W. Wilson, Advisory Engineer Approved: .
C. H. Boyd, Manageh Engineering and Materials Technology Westinghouse Electric Company Energy Systems i P.O. Box 355 Pittsburgh, PA 15230-0355 C1998 Westinghouse Electric Company AllRights Reserved I o:\4177-non:1b-051398 I
All TABLE OF CONTENTS -
EXECUTIVE
SUMMARY
.................. . ........ .. ...... ... ................ .. ....... .... .......... ........... ......... xiii 1
INTRODU CTION .... . ... . . . ... . . ...... .... .. . ... .. ..... . . . . ..... . .... . .. ...... .. ...... . .. ... . ........ .. ... . ...... .... 1 -
1.1 B AC KG ROUND . . . . ... . . ...... ... .. . ... . ....... ....... ........ .. ......... .. .......... ....... .... . ..... . . .. .. . 1 -1 1.2 PART LENGTH CRDM DESIGN ASPECTS.... ... ... ............................................ . 1-2 1.3 INDUSTRY ACTIVITIES AND INSPECTIONS.. ............. .. ............................... 1-3 1.4 METALLURGICAL INVESTIGATION................. ........... ........................1-4 2 EXAMINATIONS AND TES15............................ .................... . ....... ................. ...... .... .. .. 2-1 2.1 SURFACE AND NDE EXAMINATIONS........ .................. . .. ............................... 2-1 2.2 DIMENSIONAL MEASUREMENTS AND SAMFLE SECTIONING............... .. 2-1 2.3 METALLOGRAPHIC EXAMINATIONS.... .......................... . ...... ....................... 2-2 2.4 FRACTOGRAPHIC EXAMINATIONS.......................... ...........................2-2 2.5 CHEMISTRY EVALUATIONS ......... ...... .. ............. ......... ...... ....................... ........... 2-2 2.6 MECHANICAL PROPERTY AND RESIDUAL STRESS MEASUREMENTS .. . 2-3 2.7 I REVIEW OF DESIGN AND FABRICATION RECORDS..... .......... ............. ... 2-3
-l 2.8- MECHANISTIC AND ROOT CAUSE ASSESSMENTS ..... . ................................ 2-3 3 RESULTS AND DISCUSSION............. ...................................................................3-1 4 CONCLUSIONS............................................................................................................4-1 S LIST OF REFERENCES . . . . . . . . . .. .. ........ .. .. ........ .. ... . . . . . .. .... .. .... ...... .. ....... ............... .. .. ...... . . . 5-1 1-i 1
I o:\4177-non. doc 1b-051398 May 1998 I
l L. -
i v
LIST OF TABLES Table 1-1 Part Length CRDM Housing Issue Plant Action Summary as of
~ April 1998.............................................................................................................1-5 Table 2-1 - Summary of Dimensional Measurements ......... .... .......................................... ...... 2-4
. Table 2-2' G-9A Weld Metallography Sample Identification Summary................................ 2-5 Table 2-3 _ Summary of Electron Microprobe Quantitative Chemistry Chemistry Analysis Results of the Base and Weld Materials in G-9A Weld...... 2-6 2
Table 2-4 . Bulk Chemistry Analysis Results of the Base and Weld Materials of
-)
G-9AWeld..........................................................................................................2-7 l Table 2-5 Summary of Residual Stmss Measumment Results .................................... .......... 2-9 Table 3-1 Domestic Plants Where Royal Industries PLCRDMs are Currently Installed . . .. . . . . . . .. ... ..... . . ... . .. .. . . . .. ..... .... . . .. .... . .. .-. ....................................3-11 1
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o:\4177-non. doc 1b-051398 May 1998
1 I
vii I
LIST OF FIGURES l Figum 1-1 Section of Prairie Island Unit 2 G-9 Motor Tube Weld Received at the Hot Cell for Examina tion .......................... ... .. .................. ................. ........................... 1-6 Figure 1-2 Schematic of Weld Cc nfiguration for Part I.ength CRDMs, Showing the Location of the Crack in the Prairie Island Unit 2 G9-A Weld .............................1-7
. Figum 1-3 Arrangement of Part Length CRDMs Showing the Location of Pmssure Boundary Welds . . . .
......................................................................................1-8
.i Figure 2-1 Appearance of G-9A Weld in the As-Received Condition............ .. .................... 211 l
i Figum 2-2 OD Surface Appearance of G-9A Motor Tube Lower Weld in the As-Received Condition ..........................................................................................2-12 Figum 2-3 Close Up View of the Canopy Weld and G-9A Motor Tube Transition Weld Showing the Location of Leak (180 ).: ..............................................2-13 l 1
Figure 2-4 OD Surface Appearance of G-9 Tube Upper Weld Tube Sample Following Diametric Sectioning... ............................................................2-14 Figum 2-5 ID Surface Appearance of G-9 Tube Upper Weld 'B' Following Diametric Sectioning .............................. .......................................................2-15 Figure 2-6 As-Received Appearance of the E-7 Motor Tube Lower Weld
. Sample (E-7A) ..................................................................................................2-16 i Figum 2-7 . As-Received Appearance of the E-7 Motor Tube Upper Weld Sample (E-7B) ...............................................................................................2-17 Figure 2-8 As-Received Appearance of the G-5 Motor Tube Lower Weld Sample (G-5A) ....................................................................... ......... 2-18 Figure 2-9 As-Received Appearance of the G-5 Motor Tube Upper Weld Sample (G-5B) .................................. ............................. .. ........... .. . 2- 19 .i l
Figum 2-10 ID Surface Appearance of G-9A Tube Weld Sample After Sectioning and l Removal of 120 Segment.... ........ .................... ............... .................................. ... .... 2-20 )
Figure 2-11 Borescopic Examination Results of the ID Surface of G-9A Motor Tube Weld .......................................................................... ....... 2-21 ;
Figure 2-12 Bomscopic Examination Results of the ID Surface of G-9A Motor !
Tube Weld ..............................................................................................2-22 i Figum 2-13 Light Optical Macrograph Illustrating the Location of Crack at the Weld Interface on the ID Surface of G-9A Tube (180* location)................... .. ... ......... 2-23 Figure 2-14 Light Optical Macrograph Illustrating the Location of Crack at the Weld Interface on the ID Surface of G-9 Tube (180 location)........... ................ .......... 2-24 !
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o:\4177-non. doc:1b-osn9s _ May 1998 i
_ _ _ _ _ _ _ _ _ _ . _ _ _ _ _ _ _ . _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ . _ _ _ _ _ . . _ _ _ _ _ . . . . _ . . . ._ . _ _ _ . _ _ _ _ . . _ _ _ _ _ _____ . _ _ . _ __)
viii -
LIST OF FIGURES (Cont.)
Figum 2-15 Sectioning Procedum Employed for G-9A Motor Tube Weld Sample Illustrating the Identification and Location of Samples Taken for Various Investigations and Tests ~ ................................................................................2-25 Figure 2-16 Metallography Results of the G-9A Weld Showing the Location of Cracking l . in the Buttering, and the Weld Overlay and Weld Repair Regions at 149* Location .......................................................... ................................2-26 l
Figure 2-17 Metallography Results of G-9A Weld Illustrating the Cracking Morphology at149 Location ................................-.................................................................2-27 Figum 2-18 Metallographic Examination Results of the G-9A Tube Lower Weld Illustrating the Intercellular Cracking and Micro-fissuring in the 309 Buttering (149' Location) ...............................................................................................2-28 Figure 2-19 Metallography Results Illustrating the Crack ' Iip Region and Secondary Cracking Along Intercellular Boundaries (149' Location) ... .. ... . . ... . ... .. . ... 2-29 l Figure 2-20'- Metallography Results Illustrating the Microstructum of the Diluted Zone in the 308 Weld Metal (149 Location)...................... . .. .. ..... . . . .... . . ... ... . 2-30 Figum 2-21 Metallography Results Illustrating the Weld Metal Microstructum in the Buttering (149 ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . .. .. .. . .. .... 2-31 Figure 2-22 Metallography Results Illustrating the Grain Size and Inclusions in the 304 Stainless Steel Adapter (149 ) .......................................... .......................2-32 E Figum 2-23 Metallography Results of an Axial Section Through the G-9A Weld at j 90 Location Showing the Weld Overlay Morphology........... ........... .. ........... 2-33 i Figum 2-24 Metallography Results Showing Detailed Microstructure of Region 'C'in Figum 2-23.......................................................................................2-34 Figure 2-25 Metallography on Section at 220 Showing Weld Overlay Morphology )
and Evidence of Weld Repair on the 1D............................ .. ... .. . . . .. .. .. .. 2-35
- Figum 2-26 ' Metallography Results Showing Weld Microstructure on Section at 300 ....... 2-36 Figure 2-27 Metallography Results Illustrating the Morphology of Cracking in the Buttering Near G-9A Weld Interface (90 )........... ... ...................................2-37 Figure 2-28 Metallography Results Illustrating the Morphology of Cracking in the L Buttering Layer Adjacent to the Weld Interface (220 ) ......................... ..... ....... 2-38 l- Figure 2-29 . Cracking Morphology Seen Near the ID Surface on the Metallographic Section G9D3A through the G-9A Weld (260 ) .................... ........................2-39 Figure 2-30 Metallography Results Showing the Cracking Morphology in the L Mixing Zone Near the Weld Interface (260 )............. .......................................2-40 E
o:M177-non. doc-05139s May 1998
LIST OF FIGURES (Cont.)
Figum 2-31 Metallography Results Illustrating Cracking Morphology in the Mixed Zone Near the Interface (300 ) .............. ................................ ........ ...... ............. 2-41
- Figure 2-32 - Metallography Results on a Section at Clock Location 60 Illustrating .
the Presence of Martensite Layer and Equiaxed Grains at the Weld Interface........................................................................................................2-42 Figure 2-33 Metallography Results on a Section at Clock Location 60 Illustrating the Presence of Martensite Layer and Equiaxed Grains at the Weld Interface....... 2-43 Figure 2-34 Metallography Results Hlustrating Weld Microstructure in Upper Weld on a Section at 270 Location ............ ................ .. ............. ........................ 2-44
- Figure 2-35 Metallography Results Illustrating Weld Buttering Micmstructure in G-9B Upper Weld on a Section at 270 Location.......... ... .................................... 2-45 Figure 2-36 Metallography Results Illustrating the Weld Microstructure in G-5B Upper Weld at 120 Location ..................................... ................................... 2-46 Figure 2 Metallography Results Hlustrating the Weld Buttering Microstructure in G-5B Upper Weld at 120 Location ..... .................. ......................................... 2-47 Figure 2-38 Metallography Results Hlustrating the Weld Microstructure in G-5B Upper Weld (240 ) .. ..... .............................................................. .................... 2-48 Figure 2-39 Metallography Results Illustrating the Weld Buttering Microstructure in G-5B Upper Weld (240 ) ................... ................................. ............................... 2-49
' Figure 2-40 Metallography Results Hlustrating Weld Overlay Microstructure in G-5A Lower Weld at 0 Location ........................................................ . ................. 2-50 Figure 2-41 Metallography Results Illustrating Weld Buttering Microstructure in G-5A Lower Weld at 0* Location ................ .............................. ...................... .... 2-51
' Figure 2-42 '. Metallography_Results Illustrating the Weld Mict. dructum in G-5 A Lower Weld at 120" Location .. ............................... .... ................................ 2-52 Figure 2-43 Metallography Results Illustrating the Weld Buttering Microstructure in G-5A Lower Weld at 120 Location .................................................................... 2-53 Figure 2-44 Circumferential Layout of Fracture Faces of Freshly Opened Cracks in the G-9 Tube Lower Weld...... ............................... ... ... ........ ..... .... ........................ 2-54 Figure 2-45 Light Optical Fractograph of the Freshly Opened Crack in the G9C1 Tube Segment (Between 150 and 220 Clock Location) Containing the Ima k Loca tion .. .... . ... . ...... . . .... ... ...... .. .... .......... . . . . . . . ... ... ... ..........................2-55 Figure 2-46 Light Optical Fractograph Illustrating the High Temperature Dark Oxide Covered Fracture Morphology in G9C1 (150 -220 ) Tube Segment Containing the leak Loca tion ...................................... ......... . ... .......................... 2-56 i a:\4177-non. doc:1b-051398 May 1998 l
x LIST OF FIGURES (Cont.)
Figure 2 Light Optical Fractograph Illustrating the High Temperature Dark Oxide L
Covered Fracture Morphology in G9C1 (150 -220 ) Tube Segment -
Containing the Leak Location .... ....................... ............ ............................... ...... 2-57
. Figure 2-48 . Light Optical Fractograph Illustrating the High Temperature Dark Oxide Covered Fracture Morphology in G9C1 (150*-220*) Tube Segment Containing the Leak Loca tion .. ..... ................... ............... ..................................... 2-58
' Figure 2-49 Light Optical Fractograph of Freshly Opened Crack in Tube Segment G9B1B1 Between 60 and 90* Locations.............. . .... ............................. ........... 2-59 Figure 2-50 . Light Optical Fractograph of Freshly Opened Crack in Tube Segment G92A Between 220 - and 260* Clocks Locations........................... ...................... ........... 2-60 Figure 2-51 Light Optical Fractograph of Freshly Opened Crack Showing Dark Oxide Covered Fracture Face in Tube Segment G9B1D1 Between 90* and 150 Clock Loca tions (150* side) ....................................... ................... ......... ............... 2-61 Figure 2-52 Light Optical Fractograph of Freshly Opened Crack Showing Dark Oxide Covered Fracture Face in Tube Segment G9B1D1 Between 90 and 150*
Clock Locations (90 Side)..................................... ... ................................2-62 i
Figure 2-53 Light Optical Fractograph of Freshly Opened Crack in G9C1 (150 to 220) !
Segment Containing leak Location. Regions Where Higher Magnification i SEM Fractographs are Taken are Identified .... .. ............ ...................... ........... 2-63 Figure 2-54 Scanning Electron (SEM) Fractograph of Freshly Opened Crack Showing l Dendritic Weld Surface Morphology and Presence of Secondary Cra cks (185* Loca tion) .............. ................ .......... ........ ............... .......................... 2-64 l Figure 2-55 Scanning Electron (SEM) Fractograph of Freshly Opened Crack Showing Dendritic Weld Surface Morphology and Presence of Secondary H Cra cks (185* Loca tion) ................. ........................ .... .... ......... ............... ......... ... 2-65 ~
l Figure 2 Higher Magnification SEM Fractograph Showing Dimpled Rupture Near i OD Surface Adjacent to the Leak Location (185 ) ..... ......................................... 2-66 !
Figure 2-57 Scannmg Electron Fractograph of Crack Tip Region Showing the Preservice and Lab Fracture Morphologies (150* Location) ...... ........ ................................... 2-67 Figure 2-58 Scanning Electron Fractograph of Crack Tip Region Showing the I Preservice and Lab Fracture Morphologies (150* Location)................ .......... . 2-68 .
Figure 2-59 Scanning Electron Fractograph of Crack Tip Region Showing the Preservice and Lab Fracture Morphologies (150 Location)..................... .. .... 2-69 Figure 2-60 Higher Magnification Scanning Electron Fractograph of Pre-service Crack Showing Oxide Covered Fracture Face and the Presence of Secondary Cracking (150 Location) ............ ........ ...... ..... ............... .. . 2-70 oM177-non. doc-osD98 May 1998
1 i
xi LIST OF FIGURES (Cont.)
Figure 2-61 Higher Magnification SEM Fractograph Illustrating the Laboratory Induced Dimpled Fracture Morphology (150 ) .... ... ................................. ....... 2-71 Figum 2-62 , ' Lower Magnification SEM Fractograph of Freshly Opened Crack at 240 Location Illustrating Locations Where Higher Magnification Photographs are Taken (Reflected Electron Mode)... ..........................2-72 Figure 2-63 SEM Reflected Electron Image Fractograph Illustrating Fracture Morphologies Of The Pre-Service And Laboratory Fractured Regions (240 ) .......................................... ..... ..... . . ............. .... ......... .. 2-73 Figure 2-64 SEM Fractograph Illustrating Fractum Morphologies Of The Pre-Service And Laboratory Fractured Regions (240 )....... ......................................2 Figure 2-65 SEM Fractograph Illustrating The Pre-Service And Laboratory Fracture Regions (300* Loca tion) ............ ... .... .................. ......... ....... ... ........ .... ... ..... ......... 2-75 Figure 2-66 SEM Fractograph Illustrating the Pm-service and Laboratory Fractum Regions (300 Location). ................................ .......................................2-76 Figum 2-67 SEM Fractograph Illustrating the Oxide Covemd Fracture Morphology of the Pre-Service Crack at 300* Location ............................................... .............. 2-77 Figure 2-68 Higher Magnification SEM Fractograph Illustrating the Laboratory Fractund Dimpled Morphology a t 300 Location .................. ........... .................................. 2-78 Figure 2-69 Low Magnification SEM Reflective Image Fractograph Showing the Fracture Appearance Near the Leak Region Close to OD at 185 Location...... 2-79 Figure 2-70 SEM Fractograph Showing the Oxide Covered Fracture Appearance near the OD Surface Where Leak Occurred (185* Location) ....................................... 2-80
' Figure 2-71 SEM Fractograph Showing the Evidence of Smooth Surface Potentially Formed by Mechanical Rubbing or Impact Loading Near the leak Location. 2-81 Figure 2-72 SEM Fractograph Illustrating the Remaining Ligament Measurement Procedure in the Tube Segment G9C1............ ........................ ............................ 2-82 Figure 2-73 SEM Fractograph Illustrating the Remaining Ligament Measurement Proced ure in the Tube Segment G9C1................... .. ............. ............... .............. 2-83 Figure 2-74 . SEM Fractograph Illustrating the Remaining Ligament and " Mixed Zone" Width Measurement Procedure in Tube Segment G9C1.... ............................. .. 2-84 Figure 2-75 SEM Fractograph Illustrating the Remaining Ligament and " Mixed Zone" Width Measurement Procedure in Tube Segment G9C1... . ...................... ... ... 2-85 Figure 2-76 SEM Fractograph Illustrating the Remaining Ligament and " Mixed Zone" Width Measurement Procedure in Tube Segment G9C1..... .............................. 2-86 i
i oM177-non. doc:1b-051398 May 1998 1
_ _ - _ _ _ - _ _ _ _ _ _ _ _ - _ _ _ _ _ _ - _ _ _ _ _ _ = _ _ _ _ . . _ _ _ _ _ - _ _ _ _ -_ .- _ _. _ _ _
h xii L LIST OF FIGURES (Cont.)
l Figure 2 SEM Fractograph Illustrating the Remaining Ligament and " Mixed Zone" Width Measurement Procedure in Tube Segment G9C1.................... ... ..... ..... 2-87 Figure 2-78 Summary of Fractographic Measurement Data Showing Depth of Cracking and the Mixed Zone Width Around the Circumference in
- G-9 Motor Tube Lower Weld Sample G9C1.. ......... ............................ .. 2-88 Figure 2-79 Energy Dispersive X-ray Chemistry Analysis Results of the Crack Deposits on the Freshly Opened Fracture Face in the Tube Segment G-9C2 Between 150' and 220* Clock Locations .......... ..... .............................. . 2-89 Figure 2-80 Energy Dispersive X-ray Chemistry Analysis Resu% the Crack
. Deposits on the Freshly Opened Fracture Face in the Tube Segment G-9C2 Between 150 and 220 Clock Locations ., ......................... ... . ................ 2-91 Figure 2-81 Energy Dispersive X-ray Chemistry Analysis Results of the Crack Deposits on the Freshly Opened Fracture Face in the Tube Segment G-9C2 Between 150 and 220' Clock Locations ............................... .............. ..... 2-92 Figure 2-82 Energy Dispersive X-ray Chemistry Analysis Results of the Crack Deposits on the Freshly Opened Fracture Face in the 'Ibbe Segment G-9C2 Between 150 and 220 Clock Locations .................. ............................................. 2-93 Figure 2-83 Energy Dispersive X-ray Chemistry Analysis Results of the Crack Deposits on the Freshly Opened Fracture Face in the Tbbe Segment G-9C2 Between 150* and 220 Clock Locations ......................................... .......... 2-94 Figure 2-84 Energy Dispersive X-ray Chemistry Analysis Results of the Crack Deposits on the Freshly Opened Fracture Face in the Tube Segment G-9C2 Between 150' and 220 Clock Locations ............................... ........... ........ 2-95 Figure 2-85 Auger Spectroscopy Results Illustrating the Presence of Copper and Sulfur and Boron in the Oxide Layer on the Fracture Face (Sample G9C1B).. . .......... 2-%
Figure 2-86 Electron Spectroscopy Chemical Analysis (ESCA) Results Illustrating !
the Presence of Zinc and Copper in the Oxide Layer on the Fracture Face...... 2-97 i
Figure 2-87 Electron Spectroscopy Chemical Analysis (ESCA) Results of the Oxide Layer .!
on the Fracture Face Showing the Presence of Zinc, Boron and Copper.......... 2-98 Figure 2-88 Electron Microprobe Chemistry Analysis Results Illustrating the Elemental Concentrations Across the Weld Interface.......................................... 2-99 Figure 2-89 Microhardness Survey Results Across the Weld Interface..... .. 2-100 j l
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oA4177-non. doc-051398 May 1998
L.
xiii EXECUTIVE
SUMMARY
In January 1998, Prairie Island Unit 2 plant was brought to a cold shutdown condition due to
- the detection of an unisolable 031 gpm leak in the part length CRDM housing motor tube. The center section lower weld at G-9 motor tube containing the affected weld was shipped to
- Westinghouse hot cell facilities for investigation. This mport summarizes the results of the metallurgical investigation conducted by Westinghouse to establish the mechanism and mot cause of occurmnce of cracking and leakage in the G-9 part length CRDM motor tube weld at
, Prairie. Island Unit 2. The investigation included NDE and surface examinations, metallographic and fractographic examinations, chemistry and mechanical property assessments and mechanistic and root cause considerations. The msults of the investigation showed that the motor tube weld cracking is caused by " solidification cracking" during weld
~ fabrication followed by crack extension and additional cracking during post weld heat tmatment. Based on the overall results of the investigation,it was concluded that the motor tube cracking is originated from weld fabrication with no service related growth. The root cause of cracking was identified as the occurrence of. weld contamination by sulfur, copper, boron, and zinc during weld fabrication. The examination of some 60 similar welds in the industry including five other welds from Prairie Island Unit 2 fabricated during the same time period, showed no evidence of cracking. As a mitigative measum, Northern States Power adopted a modification to the four part length CRDMs in Unit 2 by cutting the exicting canopy seals, mmoving the part length CRDM housing motor tubes and providing threaded head adapter plugs, each sealed with a fillet weld.
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1.
o:\4177-non. doc 1b-051398 May 1998 I - _ _ __ a
xiv ACKNOWLEDGMENTS The author wishes to acknowledge the assistance of R. P. Shogan, C. Deflitch, R. A. Reese, C. M. McGuire and T. J. Mullen of the Westinghouse Science and Technology Center Hot Cell Facilities. Technical discussions and valuable support from John Johnston of Northern States Power during the course of investigation : apreciated.
Helpful discussions with I. L. W. Wilson, and D. E. Boyle of Westinghouse Nuclear Service Division are also acknowledged. The extra hours put in by Kathy Rulis and Coleen Queen of Westinghouse ESBU Word Processing Center, and J. A. Hoffman of Westinghouse Nuclear Service Division in preparing the manuscript is appreciated.
o:\4177-non. doc-051398 May 1998
L
, 1-1
! 11 INTRODUCTION -
1.1 BACKGROUND
On January 24,1998, the Prairie Island Unit 2 Nuclear Power Plant mcorded a 100% incmase in
- the containment particulate radiation monitor and an increase in unidentified leakage from the
- Reactor Coolant System inventory mass balance. Over the pmvious two-day period, the unidentified RCS leak rate increased from less than 0.1 gpm to 0.26 gpm. A containment -
inspection by the Northern States Power (NSP) Plant personnelidentified the source of the leak -
I to be the upper canopy seal weld of one of the Part Length Control Rod Drive Mechanisms (PLCRDM). The unit was brought to a cold shutdown condition as the plant prepared to
- perform a weld overlay repair to the upper canopy seal weld of the PLCRDM corresponding to core location G-9. During the weld overlay mpair upon close examination of the surface,it became appamnt that the leakage was not from the canopy seal weld, but was from the ?
- transition weld region of the center section of the motor tube slightly above the canopy weld in the G-9 PLCRDM. The utility notified the NRC of the leak per 10CRF50.72 and the industry
' was notified via the INPO Nuclear Network. The WOG General Session was briefed about the leak.
The Prairie Island Unit 1 and Unit 2 reactors am each equipped with four Part length CRDMs.
The PLCRDMs are included in an ASME Code Sec. XI ISI program, but are not typically -
- inspected since they are located in non-peripheral locations on the mactor vessel head; RT -
examination of the G-9A' tube weld section by double wall radiography at the site showed two faintly visible indications oriented along the circumferential direction at the weld.- UT examination of the G-9A tube weld at site confirmed the presence of a discontinuous 360 circumferential indication on the ID surface along the bimetallic weld corresponding to the leak
~ location. A schematic representation of the location of cracking in the G-9A weld is illustrated
' in Figures 1-1 and 1-2. A 24-inch long segment of the tube containing the leak region was
. sectioned out (Figure 1-1) and shipped to Westinghouse hot cell facilities for metallurgical investigations Subsequently, five additional tube segments containing similar welds from thme f part length CRDM motor tubes from the unit wem also shipped to Westinghouse for examinations. These included the upper weld in G-9 tube (G-9B) and the lower and upper s welds in G-5 and E-7 tubes (i.e., G-5A, G-5B, E-7A and E-7B). The motor tube is made of Type 1403 martensitic stainless steel material and is joined at either end with Type 304 austenitic
- stainless tube with full penetration bimetallic welds. The bimetallic weld consisted of a 0.25 inch thick Type 309 stainless steel buttering applied to the Type 403 stainless steel tube followed
~
by Type 308L, full penetration weld (Figure 1-2). The motor tube design drawing requirements '
stipulate a postweld heat tmatment following buttering (Ref.1). 'Ihis could not be verified ;
(pending traceability of the original records. Westinghouse conducted a safety analysis of the !
F "Ihe lower and upper welds in the motor tube are designated respectively by 'A' and 'B' for the sake of j
- cunent discussion in this report.
f- - Reference 1: Royal Industries " Motor Tube Center Section Drawing No.121E005-1," August 1968.
l.
.m..
Introduction:
May 1998 c:\4177-non. doc-051398
1-2 part length CRDM failure and concluded that the failure is bounded by the design basis and that the affected plants can continue to operate without adversely affecting public health and safety while the issue is being addressed. As a result of the G-9A tube weld leakage, the four Part Length CRDMs from the Prairie Island Unit 2 have been removed and replaced with a threaded plug that is seal welded to the Reactor Vessel Head Penetration. The current report summarizes the evaluations and the results of the metallurgicalinvestigation conducted to establish the mechanism and root cause of the occurrence of the cracking and leakage of the G-9A Part Length CRDM motor tube weld at Prairie Island Nuclear Generating Plant Unit 2.
Prairie Island Unit I has committed at a minimum, to inspect the four part length CRDMs during the 1999 refueling outage in November.
1.2 PART LENGTH CRDM DESIGN ASPECTS l
}
The geometry and the arrangement of PLCRDMs and the location of cracking is schematically illustrated in Figure 1-3. The part length control rods and corresponding mechanisms were originally intended to provide negative reactivity directed toward Xenon control. The control rod assembly contained a partial length absorber. The PLCRDM is non-scramming and consists of a motor that " rotates" a roller nut tube and assembly while simultaneously moving a " lead screw" style of drive rod axially into and out of the reactor vessel. These part length CRDMs are not needed in view of advances in fuel design and they have been made functionally inoperable via the use of an " anti-rotation" device installed on their drive rods in the fully ;
withdrawn position. The part length control rod assemblies were removed from the core relatively early in plant life. The part length CRDM motor tube housing still functions to retain reactor coolant pressure.
The roller nut tube and drive rod are housed in an RCS pressure boundary that is mounted to a reactor vessel head penetration.
The mechanism pressure boundary is fabricated from Type 403 martinsitic and Type 304 austenitic stainless steel material. A thin cylinder (motor tube assembly)is fabricated from Type 403 stainless steel and is located adjacent to the stator and roller nut rotor tube. The Type 403 stainless steel material was chosen for its magnetic properties, needed for the operation of the drive. The Type 403 stainless steel motor tube contains two "bi-metallic" full penetration welds at its top and bottom end. i l
The PLCRDMs are attached to the reactor vessel through a 6-inch diameter ACME thread I machined onto the top end of a corresponding vessel head penetration which is mounted to the reactor vessel head, and typically seal welded.
j The part length mechanism alignment and support (laterally and axially) is through the vessel I penetration at the lower end and a " seismic" support plate assembly (laterally) at its upper end.
This is shown in Figure 1-3. This support and alignment arrangement is the same as that of the Westinghouse maEnetic jack mechanism or CRDM which is used for standard full length control rod operation. The magnetic jack mechanism does not contain the Type 403 stainless steel material; the full length CRDM contains only Type 304 stainless steel farging material.
1 Introduction May 1998 i o:\4177-non. doc-Os1398 !
1-3
- l. .
Structural welds such as those used in the PLCRDMs Part length Mechanisms are not utilized on the magnetic jack mechanism.
1.3 INDUSTRY ACTIVITIES AND INSPECTIONS
' The Prairie Island Unit 2 motor tube leakage issue raised industry wide concern relative to the integrity of similar welds in the part length CRDM motor tube housings in plants that am l
curmntly in service. Some non-Westinghouse plants have full length CRDMs with similar welds. Several activities have been initiated by the Westinghouse, B&W and CE owners groups in an effort to alleviate the NRC concerns and mitigate the issue.
On February 27,1998 the Westinghouse Owners Group (WOG) held a meeting with the NRC whem Westinghouse and NSP presented pmliminary assessment results of the Prairie Island motor tube issue. Based on the Preliminary hot cell examination results Westinghouse reported that the G-9 motor tube leakage issue is caused by a fabrication related defect that could be isolated and that based on the Westinghouse safety assessment results, the motor tube failure is bounded by design basis and is not a safety concern. While the NRC concurred with these conclusions,it identified some amas where additional work was requimd to be done and raised specific questions that need to be responded to by the owners group. On March 6,1998 WOG provided a written response to the NRC questions, including a conditional assessment mport on the part length motor tube issue. A subgmup was formed by the WOG materials subcommittee to address the issue. The subgroup is currently active in developing mitigative procedums (such as " cut and cap" or inspect) and in preparing to respond to the NRC concerns.
To date, several WOG plants have taken inniative to either inspect or (cut and cap) modify the housings during scheduled outages. A summary of actions taken by the WOG plants to date is provided in Table 1-2. A subgroup meeting was held with the NRC on May 7,1998.
The basic design manufacturing process of B&W and CE bi-metallic full length CRDM housings is similar to the Westinghouse part length CRDM housings manufactured by Royal Industries.
Three basic designs were adopted for the CRDMs in B&W plants. These am: type A design by Royal Industries, type 'C' design by Diamond Power Specialty (DPSC) and a type 'B' which is a hybrid with type 'C' mechanism fitted in type 'A' motor tube. The CE and B&W owners groups have initiated significant activities in the areas of inspection and fabrication record review. On May 13,1998 they submitted their respective status summary reports including future plans to the NRC. The full length CRDM housing motor tube welds in these plants are subject to pressure boundary ASME Section XI code inspection requirements. Tia CE and B&W owners groups met with NRC on April 23 to appraise them of their mitigative plans on the issue.
On March 31,1998 an Industry information meeting on the issue was conducted by Westinghouse and NSP at the Westinghouse facilities in Pittsburgh. The objectives of the meeting were:
- 1. To disseminate the Westinghouse failure analysis results
- 2. To share information on the existing inspection and fabrication history data.
Introduction May 1998 o:\4177-non. doc-051398
l 1-4 l
l
- 3. To initiate the development of a white paper based on the available data and information to convince the industry that the Prairie Island motor tube issue is an isolated one.
The meeting was attended by the three owner's groups and EPRI personnel. It was felt that there is a large population of inspection data in the industry and when processed and put in proper perspective, the data could go a long way to support that the Prairie Island tube issue is an isolated event.
1.4 METALLURGICAL INVESTIGATION This report summarizes the examinations, evaluations and findings of the metahurgical investigation of cracking and leakage of the G-9A motor tube lower bimetallic weld from Prairie Island Nuclear Generating Plant Unit 2. The investigation was centemd on the 24 inch motor tube segment containing the leaking weld (G-9A) from the G-9 motor tube.
Examinations also included the remaining five other welds from G-9, E-7 and G-5 part length CRDM motor tubes from the unit. The metallurgical evaluations, conducted at the Westinghouse hot cell facilities (mferred to elsewhere in this report as Hot Cell), included the following major tasks:
+ Surface Examinations e Dimensional Measurements
- Sample Sectioning
- Metallographic Examinations
. Fractographic Examinations i
e Chemistry Evaluations
)
e Mecl.anical Property Measurements e Review of Available Fabrication History Records e Residual Stress Measumments The overall purposes of the investigation were to establish the mechanism and cause of the motor tube leakage and further to develop information that would be helpful in developing corrective actions. A summary of the evaluations and results are presented in the sections to follow.
(
i introduction May 1998 c:\4177-non.dco051398
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l Mechanistic and Root Cause Considerations May 1998 tx\41W-non doc-051398 P. ~ l
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l Figure 1-2 Schematic of Weld Configuration for Part Length CRDMS, Showing the Location of the Crack in the Prairie Island Unit 2 G9-A Weld i Mechanistic and Root Cause Considerations May1998 c:\417/-non. doc-051396 t:
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) Arrangement of Part Length CRDMs Showing the Location of Pressure Boundary Welds Mechamstic and Root Cause Considerations May 1998 o \4177-non. doc-051398 i
1 u______ . _ _ _ _ _ _
- 2-1 I
i L
2 EXAMINATIONS AND TESTS !
l l 2.1 SURFACE AND NDE EXAMINATIONS l The "as-received" surface condition of the G-9A tube sample was examined visually and by light optical microscopy techniques for evidence of cracking, deposits, surface attack, and/or other mechanical distress marks. The examinations were carried out on the inside diameter (ID) and outside diameter (OD) surfaces and the results were documented photographically by l a digital mcording system. The results of the surface examinations are illustrated in Figums 2-1 through 2-14. !
Borescopic examinations were carried out on the ID surface prior to any sectioning to identify l
the presence and location of the cracking. UT examinations were conducted (by ABB personnel) employing procedures similar to the ones followed for the on-site tube inspections.
For the purpose of identification of surface locations and samples, a consistent clockwise ;
orientation location procedure, looking down on the motor tube from above the vessel head, j was established. A site marking noted on the tube identifying ' North' direction was designated
'zero' degree clock location for this purpose. Based on the results of the surface examinations and the NDE examinations, initial sectioning of the tube was carried out to facilitate a more detailed ID surface examination of the tube in the leak region. The initial sectioning consisted of a transverse cut right above the lower canopy seal weld and axial sectioning to remove a l 120 circumferential segment of the remaining tube diametrically across from the leak region.
This corresponded to axial cuts at 60 and 300 clock locations as illustrated in Figure 2-10. The leak location in the tube corresponded to the region between 185 to 190 clock locations.
Closer ID surface examinations were carried out by light optical microscopy after initial sectioning to identify the presence and location of cracking more precisely and to define locations for additional sectioning. The results were documented by digital photography. The results of the ID surface examinations of the G-9A weld sample are illustrated in Figures 2-10 through 2-14.
Surface examinations were also carried out on the G-9 tube upper weld sample (designated G-9B) and G-5 and E-7 upper (G-5B and E-7B) and lower (G-5A and E-7A) samples for evidence of cracking,if any. These results are illustrated in Figums 2-4 and 2-9.
2.2 DIMENSIONAL MEASUREMENTS AND SAMPLE SECTIONING Prior to and following the initial sectioning, dimensional measurements were performed on the G-9A tube weld sample for comparison with the design specifications and further to identify any post-sectioning displacements following sectioning that would provide a measure of locked-in residual stresses. An axial sectioning procedure of the tube was then followed based on the results of the surface (and NDE) examinations. The sectioning procedure is illustrated in Figure 2-15. Six axial strips, each approximately 0.5 inch wide, wem removed at 30 to 60 intervals for metallographic examination. The tube circumferential segments connecting the axial strips containing the crack wem opened in the laboratory and were employed in fractographic examinations with the exception of two samples. The segments extending Examinations and Tests May 1998 oT4177a-non. doc-051398 u_____________________________________________________.__________________________ _ _ _ _ _ _ _ . . _ . _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ . _ _ _ _
2-2 l
! between 0* and 60 location and the segment between 250* and 300 clock location were saved
! for the purposes of utilizing in the NDE calibration and to accommodate mechanical test l
samples respectively, i
[ The G-9B and the G-5A, G-5B and E-7A, E-7B motor tube welds wem sectioned axially at 0 ,
120 ,240 clock locations and were examined metallographically for evidence of cracking. The dimensional measurements are listed in Table 2-1. The dimensional measurement results showed no appreciable change in dimensions due to sectioning.
2.3 METALLOGRAPHIC EXAMINATIONS Light optical metallographic examinations of the G-9A weld and other welds wem performed on the axial strips taken transverse to the weld at various circumferential locations as illustrated in Figure 2-15. A summary of met mount ID numbers cormsponding to each of the clock locations is given in Table 2-2. The metallographic examinations were carried out in the 'as-polished' and ' polished and etched' conditions. The purpose of the metallographic examinations was to identify the presence, location and depth of cracking, material microstructure, the cracking morphology, and its relation to the local microstructure. The metallographic examinations also established the buttering and weld deposit morphologies as well as the presence of any weld repairs. The results of the metallographic examinations are j illustrated in Figures 2-16 through 2-43. The results are discussed in Section 3. '
2.4 FRACTOGRAPHIC EXAMINATIONS i The circumferential segments of the G-9A weld containing the crack were opened carefully for ,
fractographic examinations. A circumferential layout of the fractographic samples is provided j in Figure 2-44. Fractographic examinations of the freshly opened circumferential cracks from i the G-9A tube weld were conducted by light optical (LOM) and scanning electron microscopy (SEM) techniques to establish crack initiation sites, propagation directions and the general !
fracture morphology. The results of the light optical fractographic exanunations are illustrated in Figums 2-45 through 2-53. The results of the scanning electron fractographic examinations are illustrated in Figures 2-54 through 2-78. The results are discussed in Section 3.
2.S CHEMISTRY EVALUATIONS Chemistry evaluation of the ID surface deposits from G-9A tube in the as-received condition were analyzed by energy dispersive x-ray (Edax) analysis to examine for evidence of coolant l contammants that may have played a role in the cracking process. Chemistry analysis of crack deposits at the crack-tip locations was also conducted by energy dispersive x-ray analysis to identify the role of any contaminants in the cracking process.
Auger (AES) and electron spectroscopy chemical analyses (ESCA) were performed on the oxide deposits of the freshly opened cracks to assess the composition, stoichiometry, kinetics and the temperature of oxide formation. Elemental chemistry profiles across the crack in the weld dilution region and spot chemistry analysis of the weld and basic materials were established by Examinations and Tests May 1998 a:\4177a-non. doc-051398
1 2-3 i
electron microprobe analysis to examine if the weld dilution and/or weld metal composition !
played a role in the cracking. Bulk wet chemistry analyses of the type 403 base metal, type 309 i weld buttering and type 308 weld deposits were performed. The spot and the bulk chemistry analyses were conducted to examine if the weld metals met the specification requirements. The Edax analysis results of the crack deposits are illustrated in Figures 2-79 through 2-84. The spot l chemistry analysis msults of the weld and base materials by microprobe are summarized in l Table 2-3. The bulk chemistry analysis results of the base and weld materials are summarized in Table 2-4. The auger spectroscopy analysis results of the fracture surface are illustrated in Figure 2-85. The ESCA analysis msults of the oxide layer on the fracture face are illustrated in !
Figures 2-86 and 2-87. The microprobe elemental chemistry profiles across the weld e.re illustrated in Figure 2-88.
2.6 MECHANICAL PROPERTY AND RESIDUAL STRESS MEASUREMENTS l Vickers hardness traverse measurements were conducted across the weld interface on polished
- sections of the G-9A weld to assess the approximate strength levels of the base metal and weld metal regions. Weld residual stress measumments were conducted on G-5A tube by the hole l drilling technique with strain gage rosette at three different locations at the weld interface as I per ASTM E837-9 procedure. The measumrrants were made at 3.11 in, from the internal lip which would place them at the 403/309 interface. These measurements were taken at 120 deg.
spacing. The results of the hardness survey across the weld interface are illustrated in Figure 2-89. The msults of the residual stress measurements are summarized in Table 2-5.
2.7 REVIEW OF DESIGN AND FABRICATION RECORDS l
As part of the root cause assessment effort of the motor tube cracking, the vendor supplied l design and fabrication records were myiewed to examine the fabrication and inspection I sequence employed for the CRDM housing tubes and in particular the G-9A weld. These results are included in the discussion of the root cause. A mproduction of the original design drawing of the motor tube is included in Figure 2-90.
2.8 MECHANISTIC AND ROOT CAUSE ASSESSMENTS Based on the msults of the various tests and evaluations discussed above, an assessment of the mechanism and cause of cracking in the Prairie Island Unit 2 G-9 motor tube was developed.
These assessments also included consideration of the results of the UT examinations conducted at site on similar motor tube welds in some of the units that are currently in outage and the ,
msults of this review of fabrication records. Based on the results of the mechanistic assessments, chemistry evaluations and fabrication history myiew, an assessment of the root cause was conducted in an effort to reflect on the potential of cracking in similar welds curmntly in service. The msults of the evaluations and assessments are summarized in i Section 3.
Examinations and Tests May F98 c:\4177a-non. doc-051398 I
2-4 Table 2-1 Summary of Dimensional Measurements of the MotorTbbe Section Clock Iscation Diameter Following Transverse de Longitudinal Cut As Received Following Transverse Cut !
OD ID OD ID (inch) (inches) (inches) (inch) .
0-180 4.230 3.902 4.235 3.921 l 45-225 4.231 3.917 4.233 3.924 l 90-270 4.230 4.231 3.910 l l
l i
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t Examinations and Tests May 1998 0:\4177a-non doc-051398 t_____________________________-_______________-__________ _ _ _ . _. ._ _ _ _ . _ . _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
')
2-5 1 j
Table 2-2 l G-9A Weld Metallography Sample Identification Summary i I
Clock Location Met. Mount No. Sample No.
300 M2268 G9D5A l 260 M2267 G9D3A 220 M2266 G9DIA 220 (Auger Spectroscopy) G9CIB 150 M2265 G9B2 l 90 M2270 C9BIC1 60 M2269 G9B1A1 l
I l
1 l
I l
l l
Exanunations and Tests May 1998 i a:\4177a-non. doc-051398
2-6 Table 2-3 Summary of Electmn Micmprobe Quantitative Chemistry l Chemistry Analysis Results of the Base and Weld Materials in G-9 Weld Cr Mn Fe Ni Si-1 304 21.671 1.627 72.61 9.898 0.415 AISI Spec. 18-20 2.0 max Bal. 8-10.5 1.0 max 304 2 308 18.992 1.187 77.624 7.210 0.355 AWS Spec. 18-21 2.5 max Bal. 9-11 0.9 max.'
308 l
3 309 23.79 1.862 67.408 12.196 0.399 l
l AWS Spec. 22-25 2.5 max Bal. 12-14 0.9 max 309 4 403 12.699 0.423 87.905 0.404 0.245 AISI Spec. 11.5-13 1.0 max -
0.5 max 403 i
l 1
I l
l i -
Examinations and Tests May 1998 i o:\417/a-non. doc-051398
- _ _ _ _ _ _ _ _ - _ _ _ _ _ _ _ _ - _ _ _ _ _ _ _ _ _ _ _ _________________- _ _ a
2-7 j
\
l l
l 1
Table 2-4 Bulk Chemistry Analysis Results of the Base and Weld Materials of G-9A Weld j Sample Identification Analytical 9803-0294W E7 9803-0295W E7 9803-0296W E7 Parameter Method Units PCI 403 (3/30/98) PCI 309 (3/30/98) PCI 308 (3/30/98) i M etals Sihmr (Total) A-296 ppm <2.0 <2.0 <2.0 Aluminum (Total) A-296 ppm <20 <20 <20 Arsenic (Total) A-296 ppm 33 25 23 l
Boron (Tctal) A-296 ppm <40 <40 <40 !
Barium (Total) A-296 ppm <4.0 <4.0 <4.0 Beryllium (Total) A-2% ppm 2.4 <0.40 <0.40 Calcium (Total) A-296 ppm <200 <200 <200 Cadmium (Total) A-296 ppm 200 130 160 Cobalt (Total) A-2% ppm 200 810 660 Chromium (Total) A-296 ppm 110000 190000 200000 Copper (Total) A-296 ppm 820 1400 1400 Iron (Total) A-296 ppm 670000 430000 530000 I
Potassium (Total) A-296 ppm <100 <100 <100 l 1
Magnesium (Total) A-296 ppm <100 <100 110 Manganese (Total) A-296 ppm 3400 15000 17000 Molybdenum (Total) A-296 ppm 2300 1700 1200 l Sodium (Total) A-296 ppm <200 <200 <200 Nickel (Total) A-2% ppm 3300 96000 100000 Phosphorous (Total) A-296 ppm 140 250 240 Lead (Total) A-296 ppm 71 25 37 Sulfur (Total) A-2% ppm 72 84 140
]
Antimony (Total) A-296 ppm 65 <20 50 Selenium (Total) A-296 ppm 6800 11000 12000 1
Tin (Total) A-2% ppm 1800 1700 2000 l Examinations and Tests May 1998 o:\4177a-non doc-051398 1
_ _ _ _ _ _ _ _ _ _ _ _ . _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ .)
2-8 Table 2.4 Bulk Chemistry Analysis Results of the Base and Weld Materials of G-9B Weld (Cont.)
Sample Identification Analytical 9803 0294W E7 . 9803-0295W E7 ^ 9803-02%W E7 Parameter Method Units PCI 403 (3/30/98) PC1309 (3/30/98) PC1308 (3/30/98) .
Metals i Titanium (Total) A-296 ppm <1.0 4.5 <1.0 Thallium (Total) A-2% ppm 650 790 920 Vanadium (Total) A-296 ppm 500 390 440 Zine (Total) A-296 ppm 100 290. 320 1
1 l
Examinations and Tests May 1998 '
o:\4177a-nortdoc-051398
)
i
2-9 i
Table 2-5 Summary of Residual Stress Measurement Results Tangential Axial Principal Stress, ksi Hole # Stress *, kai Stress, ksi Max. Min.
1 -17.8 -17.6 -18.2 -17.9 2 +3.5 +5.8 +93 0.0 L
'l 3 -10.8 -21.4 -10.3 -21.9
- Hoop Stress Examinations and Tests May 1998 o:\4177a-non. doc-051398
I 2-10 l
Intentionally Left Blank l
I Examinations and Tests May 1998 o \4177a-non. doc-051398 i
2-11 Prairie 7 'and Spec G9 Area Magnification Comment Fig 2-1 2/25/98 0.3x 3 -
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Figure 2-1 Appearance of G-9A Weld in the As-Received Condition Examinations and Tests May 1998 o:\4177-2-non. doc-051398
2-12 PrairieIsland Spec G9 Ama Magnification Comment Fig 2-2 2/25/98 0.5x 304 403 '
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i Motor Section Tube Figure 2-2 OD Surface Appearance of G-9A Motor Tube Lower Weld in the As-Received Condition Examinations a-i Tests May 1998 o:\4177-3-non. doc-051398
2-13_
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? l l l Figure 2-3 Close Up View of the Canopy Weld and G-9A Motor Tube Transition Weld Showing the Location of Leak (18(F)
Examinations and Tests May 1998 o:\4177-3-non. doc-051398 l
l
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2-14 l
1 Prairie Island Specimen G9 Area Magnification Comment Fig 2-4 2/25/98 i
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Figure 2-1 OD Surface Appearance of G-9 Tube Upper Weld Tube Sample Following Diametric Sectioning Examinations and Tests May 1998 o:\4177-4-non. doc-051398
i 2-15 Prairie Island Specimen G9 Area Magnification Comment Fig 2-5 2/25/98
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l Figure 2-5 ID Surface Appearance of G-9 Tube Upper Weld 'B' Following Diametric Sectioning I
Examinations and Tests May 1998 o:\4177-4-non. doc-051398 I
I
{ 2-16 Prairie Island Specimen E7 Area Magnification Comment l'
Fig 2-6 2/25/98 p,77wwmmmmmmm -
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I Figure 2-6 As-Received Appearance of the E-7 Motor Tube Lower Weld Sarnple (E-7A)
Exarninations and Tests May 1998 o:\41774-non doc-051398
2-17 Prairie Island Specimen E7 Area Magnification Comment Fig 2-7 2/25/98 w
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Examinations and Tests May 1998 o;\4177-4-non. doc-051398
(
2-18 Prairie Island Specimen G5 Area Magnification Comment Fig 2-8 2/15/98
. , - - ~.,. ,_:.,.
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Examinations and Tests May 1998 o:\4171-5-non. doc-051398
l 2-19 Prairie Island Specimen G5 Area Magnification Comment Fig 2-9 2/25/98 l mn, n n,,g,w;c:. . , n un,mrn. ,,
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Examinations and Tests May 1998 o:\4177-5-non. doc-051398 i
2-20 Prairie Island Specimen CRDM Area Magnification Comment Fig 2-10 2/25/98 0.7x
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Figure 2-10 ID Surface Appearance of G-9A Tube Weld Sample After Sectioning and Removalof120 Segment Examinations and Tests May 1998 0:\4177-5-non. doc-051398
2-21 Prairie lsland Specimen CRDM Area Magnification Comment Fig 2-11 2/25/98 sj 403 SST Tube
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Examinations and Tests May 1998
]
o:\4177-6 non. doc-051398 i A
A 1
l i
2-22 Prairie Island Specimen CRDM Area Magnification Comment Fig 2-12 2/13/98 l
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l 2-23 1
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1 Prairie Island Specimen G9 Atea Magnification Comment Fig 2-13 2/15/98 ID 180 Deg.
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Examinations and Tests May 1998 o:\4177-6-nmt. doc 051398
2-24 Prairie Island Specimen G9 Area Magnification Comment l Fig 2-14 2/25/98 ID 180 Deg.
0.5000 in 304 SST -
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Figure 2-14 Light Optical Macrograph Illustrating the Location of Crack at the Weld Interface on the ID Surface of G-9 Tube (180 location)
Examinations and Tests May 1998 o:\4177-7-non doc-051398
! )
2-25 i
I l PrairieIsland Spedmen G5 Area Magnification Comment Fig 2-15 2/25/98 ID 180 Deg. ,
i G-9A TUBE SECTIONING LAYOUT l
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l I
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l G9C1B Leak-184 180*
LEAK j Figure 2-15 Sectioning Procedure Employed for G-9A Motor Tube Weld Sample Illustrating the Identification and Location of Samples Taken for Various Investigations and Tests i
Examinations and Tests May 1998 o:\4177-7-non. doc-051398 a__-_________---___-__________-_ _ _ _ _ _ _ __ _ _ _. __ _ ._>
l l
, 2-26 1
Prairie Island Spec. M2265-G9B2B Area Magnification Comment Fig 2-16 2/25/98 ID 149 Deg. 2x 304 SS 300 ,
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l Figure 2-16 Metallography Results of the G-9A Weld Showing the Location of Cracking in the Buttesg, and the Weld Overlay and Weld Repair Regions at 149 Location Exandnations and Tests May 1998 0:\4177-7-non. doc-051398
l l
2-27 I Prairic Island Spec. M2265-G9B2B Area Magnification Comment Fig 2-17 2/25/98 ID 149 Deg. 25x T
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Figure 2-17 Metallography Results of G-9A Weld Illustrating the Cracking Morphology at 149 Location Examinations and Tests May 1998 c:\4177-7-non. doc-051398
1 2-28 i
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Prairie Island Spec. M2265-G992B Area Magnification Comment )
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2-30 l PrairieIsland Spec. M2265 - G9B2B Area Magnification Comment Fig 2-20 2/25/98 ID 149 Deg. 200x
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2-31 1 l Prairie Island Spec. M2265 - G9B2B Area Magnification Comment Fig 2-21 525/98 ID 149 Deg. 2J0x l l l
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2-39 PrairieIsland Spec M2267-G9D3A Area Magnification Comment ! Fig 2-29 2/16/98 1A 100X 7
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Examinations and Tests May 1998 o \4177-18-non. doc-051398
2-73 Prairie Island Specimen G9D2A Area Magnification Comment Fig. 2-63 2/15/98
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Examinations and Tests May 1998 o \4177-18-non. doc-051398
! 2-74 l Prairie Island Specimen G9D2A Area Magnification Comment l Fig. 2-64 2/15/98 _
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2-75 Prairie Island Specimen CRD Area Magnification Comment Fig. 2-65 2/21/98
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2-76 Prairie Island Specimen CRD Area Magnification Comment Fig. 2-66 2/21/98 Pre-Service Fracture l Laboratory Fracture
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2-78 Prairie Island Specimen CRD Area Magn.ification ll Comment Fig. 2-68 2/21/98 i Figure 2-68 Higher Magnification SEM Fractograph Illustrating the Laboratory Fractured Dimpled Morphology at 300 Location Examinations and Tests May 1998 o.\ 4177-18-non doc-051398
2-79 Prairie Island Specimen G9C2 Area Magnification Comment Fig. 2-69 2/15/98 Fracture Face l O.D., Surface
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2-80 PrairieIsland Specimen G9C2 Area Magnification Comment Fig. 2-70 2/15/98 Oxide covered Pre-Service Fracture Face Machine Marks on the O.D., Surface
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Examinations and Tests May 1998 o \417718-non. doc-051398
2-81 Prairie Island Specimen G9C2 Area Magnification Comment Fig. 2-71 2/15/98 a
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l l l 2-84 1 1 Prairie Island Specimen G9 Area Magnification Comment l Fig. 2-74 2/23/98
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2-85 PrairieIsland j Specimen G9 Area Magnification Comment . 1 Fig. 2-75 2/23/98
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2-87 Prairie Island Specimen G9 Area Magnification Comment Fig. 2-77 2/23/98 7,m.,/Mj3,?"M54T,G.:.7,f pw E
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2-88 PrairieIsland Spec G9D2A Area Magnification Comment Fig. 2-78 2/15/98
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2-89 PrairieIsland Spec G9D2A Area Magnification Comment Fig. 2-79 2/15/98 l X-RRY: 0 - 20 kev Lives 36s Preset 77s Remai ni ng: 41s Real: 50s 28'4 Dead ' C u F t C i r i 1 0
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2-90 Spectrum file : PRCRD2 P r . I s 1, G9C2. p h29, area.1 LIVETIME(spec.): 36 ENERGY RES AREA 8.2 72.35 21530 TOTAL AREA: 172086 Pebk at .00 kev omitted? Peak at .92 kev omitted? FIT INDEX:44.57
.ELMT APP. CONC ERROR (WT%)
OK : 1 44.730 .806 SiK : 1 .349 .077 CrK : 1 33.110 .476 FeK : 1 84.613 .871 NiK : 1 7.393 .492 CuK : 1 .52.644 1.074 ZAF CALCULATIONS
...[ 3 iterations]
20.00 kV TILT: .00 ELEV:35.OO AZIM: .00 COSINE:1.OOO Spectrum: P r . I sl , G9C2, p h29, area 1 All elmts analysed.NORMALISED ELMT ZAF %ELMT +- Error ATOM.t I CK : 1 1.469 14.154 +- .255 37.117 SiK.: 1 .540 .300 +- .067 448 CrK : 1 1.069 14.399 +- .207 11.619 FeK : 1 .985 39.933 +- .411 30.001 NiK : 1- .922 3.726 +- .248 2.663 i CuK : 1 .890 27.489 +- .561 18.153 .l TOTAL '100.000 100.000
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i 1 Figum 2-79 (Cont.) Energy Dispersive X-ray Chemistry Analysis Results of the Crack Deposits on the Fmshly Opened Fractum Face in the Tube Segment G-9C2 Between 150 and 220' Clock Locations Examinations and Tests May 1998 i
- o:\4177-19-non. doc-051398
2-91 PrairieIsland Spec G9D2A Area Magnification Comment Fig. 2-80 2/15/98 X-RAY: 0 - 20 kev Live: 36s Preset: 36s Remai ni ng: Os Reat: 95s 20% Dead F e C r l 0 l g b S,4 i e .' N i & , e] : i:' ; ,-= , ki ' h . --
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i 2-92 PrairieIsland Spec G9D2A Area Magnification Comment Fig. 2-81 2/15/98 A X-RRY: 0 - 20 kev Live: 77s Presets- 63s Rema!ni ng: 49s Reat: 100s 23*4 Dead F t C u C
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2-93 Prair eIsland Spec G9D2A Area Magnification Comment i Fig. 2-82 2/15/98 l l l ! X-RAY: 0 - 20 kev l Lives 43s Preset 43s Remai ni ng Os i Real: 63s 32% Dead i l F i i l t l l l l C i I r 1 0 l l
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2-94
) Prairie Island Spec G9D2A Area Magnification Comment Fig. 2-83 2/15/98 l
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i 2-95' jPrairie Island Spec G9D2A Area Magnification Comment l Fig. 2-84 2/15/98 X-RRY: 0 - 20 kev Lives 36s Preset 36s Remaining: Os Reat: 48s 25% Dead F t l i C 0 l i 6 4 L Z i H C [ Ir l h N I ll l H i Ml1
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l 2-97 1 i I l l Prairie Island Spec G9D2A Area Magnification Comment l t l Fig. 2-86 2/15/98 X eutvey C:\PRAIRETCRDWeC18\1\ SURVEY.DTS AM 30-06-2001 R CAE = 100eV STEP = 600.227 meV SCANS =1 TIME = tem 20.00s l LABEL M -n i I I I I 240- - l 220- - 200 n - 1 Cu ! 180. 0 l i 100- - 1 l 140- - l to 120- Te - o 1 100- Cr ~ go_ _ e- _ C 40~ Cu _ 20- - l o _ l i l I i ~~ 1000 soo soo 400 tw 0 Birus6ng Energy (eV) Figure 2-86 Electron spectroscopy Chemical Analysis (ESCA) Results Illustrating the Pmsence of Zinc and , Copper in the Oxide Layer on the Fracture Face Oaminations and Tests May 1998 o:\4177-19-non. doc-051398
2-98 lPrairieIsland Spec G9D2A l Area Magnification Comment i Fig. 2-87 2/15/98 ' aurvey C:\PRAIRE\CRDWDC18\1\ SURVEY.DTS AM 30-06-2001 CAE = 100eV BTEP = 500.227 meV SCANS =1 TME = tem 20.00s M mn i I l l l 4e- _ 44- - C 42- - 40- . 36- - l 30- _ 34- _ 32- - t 30 - 28 - a - g 2.- - 24 - gg- - 20- - 18- Zn - l 10-Zn ~ i 14- Si En cu - 12 - 10 - Fe - r 8- - e - 4- I l i I I _ 260 200 1so 100 So Bindme Energy (eV) f Figure 2-87 Electron Spectmscopy Chemical Analysis (ESCA) Results of the Oxide Layer on the Fractum Face Showing the Presence of Zinc, Boron and Copper Examinations and Tests May 1998 l o:\417719-non. doc-051398
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3-1 l 3 RESULTS AND DISCUSSION Examination Results The results of the surface examinations are illustrated in Figures 2-1 through 2-14. The c as-received OD sr . m condition of the affected G-9B motor tube lower weld sample is illustrated in Figuies 2-1 through 2-3. The OD surface appeared smooth with no evidence of mechanical distress or corrosion depmits. A circumferential, shallow machining mark on the lower section tube is seen in Figures 1-1 and 1-2 in the as-received condition. The machiMng mark was introduced at the site in an attempt to separate the lower section tube by a transverse cut prior to shipping to hot cell facilities, but was apparently not carried through. Also apparent in Figures 2-1 through 2-3 is the weld overlay on the canopy seal joint which was originally considered as the leak source at the site. Figure 2-3 is a close-up view clearly revealing the OD surface crack through which the leak had occurred. The OD surface crack associated with the through wall leak was oriented circumferentially measuring approximately 0.5" long and is positioned in the tapered transition region of the motor tube just above the canopy weld. The OD surface appearance did not reveal the presence of a weld at this location. The OD and ID low magnification surface examination results of five additional weld samples from the three part length CRDM motor tubes from Prairie Island Unit 2 am also illustrated in Figures 2-4 through 2-9. The OD and the ID surface appearance of the G-9 tube upper weld (G-9B) am illustrated in Figures 2-4 and 2-5 mspectively. The ID surface appearance clearly revealed a darker oxide layer for the 403 stainless steel motc,r tube portion compared to the 304 stainless steel extension tube which appeared bright and shiny due to its higher msistance to oxidation at service temperatures. The typical OD surface appearance of E-7 and G5 motor tube lower and upper weld samples is illustrated in Figures 2-6 through 2-9. The lower section tube was separated by sectioning along the machined groove and a 120 axial strip was sectioned out from the remaining motor tube containing the transition weld in order to facilitate closer ID surface examinations and plan for additional sectioning of the weld sample. The ID surface condition is illustrated at low magnification in Figure 2-10. As was the case for the upper weld samples, the lower weld sample also revealed darker oxide color on the ID surface of the Type 403 stainless steel center section of the motor tube. The overall results of the low-magnification OD surface examinations of the six motor tube weld samples (G-9A, G-9B, E-7A, E-7B and G-5A and G-5B) showed no evidence of mechanical distress, surface attack or cracking with the exception of the 0.5-inch long OD surface crack at the G-9A weld where the leak occurred. The results of the higher magnification surface exarmnations by borescopy and light optical microscopy are illustrated in Figures 2-11 through 2-14. The results of the borescopic ID surface examination results of the G-9B weld are illustrated typically in Figures 2-11 and 2-12. In general, the borescopic examination results were not adequate to diffemntiate mechanical scratch marks from cracks although the dissimilar metal weld interface could be easily detected. The higher magnification light optical microscopy examination results of the ID surface condition of the G-9A weld sample are shown in Figures 2-13 and 2-14. Clear evidence of a circumferentially oriented crack running along the weld interface can be seen here. Some Results and Discussion May 1998 o:\4177a-non. doc-051398
! l l l 3-2 evidence of minor scratches and tooling marks can also be seen. No evidence of any other cracks could be found. Based on the asults of the surface examinations, a sectioning plan was developed for the G-9A weld sample to provide specimens for various tests and evaluations conducted under the current investigation. The sectioning layout including the location and identification of samples is illustrated in Figure 2-15. The results of the dimensional measurements are summarized in Table 2.1. The results confirmed no appreciable change in dimensions due to sectioning, suggesting the absence of significant residual stresses in the weld. The results of the metallographic examinations are illustrated in Figures 2-16 through 2-43. Light optical metallographic examinations were conducted on axial sections taken from six circumferential locations around the G-9A weld and from three circumferential locations (at 0 , 120 and 240 clock locations) in each of the remaining five welds; namely, E-7A&B, G-5A&B and G-9B welds. A summary of sample and met. mount identification numbers corresponding to each of the clock locations where sections were taken from the G-9A welds is provided in Table 2-2 for traceability. The purpose of the metallographic examinations was to establish the microstructure in the weld and base materials, the presence of cracking, the crack initiation sites and propagation directions, the cracking morphology and its relation to the local microstructure. Figure 2-16 illustrates the metallography results of G-9A weld on a section taken at 149 clock location. The macrograph indicates the layout of the 309 buttering on 403 base metal, the 308 weld morphology and the location of cracking at the weld interface. A significant observation is the evidence of what appears to be weld repair on the ID surface extending into close proximity of the buttering layer. No evidence of cracking was seen at any location other than the buttering interface with the base metal. The white numbering along the weld interface indicates where higher magnification micrographs illustrating microstructure at the weld interface are taken. The black rectangles are the areas where Edax analyses of the weld chemistry were done. Figures 2-17 through 2-19 are the higher magnification micrographs of the weld clearly illustrating the location of cracking in the buttering as well as the cracking morphology. The cracking followed the weld-interface but was contained totally within the buttering layer. The cracking initiated on the ID surface where it appeared wider and progressed radially outward. The cracking is discontinuous and is interconnected with intercellular and interdendritic cracks. Evidence of microfissuring and lack of fuon can also be seen. The cracking morphology seen in Figures 2-17 through 2-19 resembles that of solidification cracking (Refs. 2,3) induced during the weld freezing process. Figure 2-20 illustrates the 308 weld overlay microstructure. Figure 2-21 illustrates weld microstructure in the buttering away from the interface. Figure 2-22 illustrates grain structure and inclusions in the 304 basemetal (in the as-polished condition). Figures 2-23 through 2-31 illustrates metallography results and additional cracking behavior from other sections of the weld. These results suggest a significantly different cracking behavior in the weld. The major cracking near the interface here is transcrystalline and appears to have followed hard regions where a martensite phase is formed in the mixing zone. The cracking also followed intercrystalline morphology around equiaxed grains formed at the Results and Discussion May 1998 o:\4177a-non. doc-051398
3-3 interface near the mixing zone prior to the dendritic weld structure. Typical microstructure of the weld interface showing the martensite layer and the equiaxed grains in the mixing zone are illustrated in Figures 2-32 through 2-33. The type of cracking behavior seen here is consistent with the hydrogen cracking reported (Ref. 4) during post weld heat treatment in the cladding layer. The cracking behavior and the mechanism of their occurrence will be further discussed later in the section. Typical metallography results of the G-9 upper weld (G-98) and the upper and lower welds in the G-5 and E-7 motor tubes are illustrated in Figums 2-34 through 2-43. The metallography here included axial sections at 0 ,120 and 240 clock locations in each of the 5 welds. In each case close examinations at higher magnification were conducted in the weld regions including the weld interface. The results of the metallographic examinations showed no evidence of cracking in any of the five welds examined. The results of the fractographic examination of the freshly opened cracks in the G-9A weld are illustrated iri Figures 2-44 through 2-78. The examinations were conducted by light optical and scanning electron microscopy techniques and included cracks in the circumferential weld samples identified in Figures 2-15 and 2-44. The cracks in all the samples with the exception of two samples designated for mechanical tests (260 -300 ) and NDE tests (0 -60 ), were opened and the fracture faces were examined. Figure 2-45 illustrates a low magnification light optical fractograph of 150 to 220 sample (Sample No. G9C2) containing the leak location (at 184 ). Slightly higher magnification fractographs of this sample are illustrated in Figures 2-45 through 2-48. Several significant observations can be made from examining these fractographs. The cracking was initiated on the ID surface and progressed radially outward close to the OD surface where a small ligament of (laboratory induced) bright overload fracture is seen. Through-wall crack penetration is seen at the leak location corresponding to the 180 to 184 clock location. The fracture was covered with a dark oxide layer which tumed lignter as the crack progressed towards the OD surface. The presence of dark oxide suggests that the fracture face was subjected to higher than service temperatures during its occurrence or during the life of the crack. The f acture appeared highly irregular and seems to have followed the weld bead morphology at the interface. No evidence of bench marks (or crack arrest lines) was seen, suggesting that the crack growth was not intermittent and that cyclic loads did not play a role in the cracking process. Evidence of secondary cracking was present. Similar observations can be made from the low magnification fractographs of other samples taken from locations around the cireteference as illustrated in Figures 2-49 through 2-52. The results of the higher magnification scanning electron fractography of the freshly opened fracture faces discussed above are illustrated in Figures 2-53 through 2-77. Figure 2-53 is low magnification SEM fractography illustrating the identification of locations where higher magnification SEM fractography was conducted. Figures 2-54 and 2-55 illustrate lower magnification fractographs showing the freshly opened surfaces at the leak location and evidence of secondary cracking. Higher magnification SEM fractographs of freshly opened crack at the 150 and 185 clock locations are illustrated in Figures 2-56 through 2-61. Figure 2-56 illustrates an overload fractured ligament at 185* adjacent to the leak location, close to the OD surface. The SEM Results and Discussion May 1998 o:\4177a-non. doc-051398
3-4 fractographs in Figures 2-57 through 2-61 illustrate the typical fracture morphologies of the pre-existent fracture and the laboratory fracture regions separated by the crack tip boundary at the 150 location. The fractum morphologies show that the pre-existent fractum is covered with oxide all the way up to the crack-tip region. Evidence of secondary cracking is seen occasionally. The laboratory fracture followed classical dimpled morphology suggesting the materialis sound and ductile with no evidence of embrittlement. Similar observations can be made from the fractographs taken from the 240 and 300 locations as presented in Figures 2-f 2 through 2-64 and Figures 2-65 through 2-68, respectively. Higher magnification SEM fractographs of the leak region (180 to 184*) are illustrated in Figures 2-69 through 2-71. The fractographs illustrate the presence of oxide all the way up to the OD surface at the leak location. No evidence of an overload ligament that could have triggered the on-set ofleak in the part length penetration is apparent (Figures 2-70 and 2-71) although there is some subtle evidence oflight oxide covered dimples seen in Figure 2-70. Higher magnification fractograph in Figure 2-71 suggests some smoothening of the fracture striace at the leak suggesting potential impact loading of mating faces of the crack prior to leak. Figures 2-72 through 2-78 illustrate the crack depth mapping procedure by employing SEM fractography conducted at every 10 interval around the circumference. Precise crack depth measumments were made by detecting the onset of dimpled overload fracture boundary at each location. The results of the crack depth measurement results are summarized in Figure 2-78. While conducting these measurements, it became apparent that them is a region before the crack-tip is reached where ductile fractured islands are prevalently distributed in the pre-existing (oxidized) fracture face. This is characteristic of hot cracking where islands of sound materialligaments are retained around liquid metal during freezing process. A close examination of the fracture region prior to the crack tip confirmed the presence of a " mixed zone" consisting of a region where freshly fractured bright ductile ligaments are present in the pre-existing fracture face. This is a significant observation since the ligaments not only impose a torturous path for the leakage but they also contribute to appreciable stiffness and strength to the structure. 'Ihe width of the mixed zone was measured at several locations around the circumference as illustrated in Figures 2-72 through 2-77 and the results are included in the crack depth profile plot in Figure 2-78. The results of the chemistry assessments are summarized in Figure- 2-79 through 2-88 and in Table 2-4. Energy dispersive X-ray analysis results of randomly selected locations on the fracture face are illustrated in Figures 2-79 through 2-84. The results show evic'ence of widely spread copper on the fracture face. Edax analysis was also conducted on the ID surface scrapings to establish if coolant contaminants had any role in the cracking process. The results showed no evidence of contaminants in the ID surface residue. The results of the Auger Electron Spectroscopy (AES) of the dark oxide layer on the fracture face are illustrated in Figure 2-85. The results clearly showed evidence of sulfur, boron and copper tied-up in the oxide layer suggesting a possible role of contaminants in the cracking. The results of the Electron Spectroscopy Chemical Analysis (ESCA) of the oxide layer am illustrated in Figures 2-86 and 2-87. The results of the AES and ESCA chemistry analyses illustrated above showed that the dark oxide layer on the fracture face was rich in chromium. The elements S, B, and Cu were detected at all intergranular fractum locations. The boron layer was very thin (10 m). The boron most likely originated from the primary coolant. Copper and sulfur were Results and Discussion May 1998 o:\4177a-non. doc-051398
3-5 found deeper in the oxide (40 pm). The sulfur was present as sulfide. The copper was oxidized at the surface. The ESCA results confirmed the pmsence of copper, sulfur and boron in the oxide and further suggested the presence of zinc occasionally on the fracture face. These observations are important since the presence of low melting species has significant implications on the mechanistic aspects of cracking. Figure 2-88 illustrates the electron microprobe elemental concentration profiles taken across the base and weld metal regions. The results of the total chemistry spot analyses conducted in the base metal and weld metal regions as indicated in Figure 2-88 are summarized in Table 2-3. The microprobe chemical analysis results illustrated in Figure 2-88 and Table 2-3 suggests no evidence of any detrimental elements such as copper, boron, sulfur and zinc in the weld metal. These results show that the source of contaminants is extraneous to the weld rod chemistry and that the low melting species are most likely introduced from contamination from external sources during welding. The results of the microhardness traverse across the base and weld metal regions at the crack location are illustrated in Figure 2-89. Although the hardness values confirmed the presence of a hard zone in the mixing region at the interface, the absolute hardness values suggest that the weld has gone through post weld heat treatment (PWHT) and that significant tempering of the martensite layer occurred during the PWHT. This is further confirmed by thelack of (high) tensile stmss at the weld interface as measumd by the hole drilling technique. Summary of Results The cracking is located in the Type 309 stainless steel buttering at the Type 403 stainless steel (SST) base metal interface of the G-9 tube lower weld. The cracking initiated on the ID surface and progressed radially outward. A circumferential, through-wall crack of ~0.5 inches length was observed at 180 -184 clock location on the OD surface where the leak occurred. The cracking is circumferential and covered 360* around the inside diameter. The cracking depth varied from 65% to 100% depending on the circumferential locations. Two types of cracking behavior were seen in the G-9A weld sam}.le. The first type of cracking is comprised of discontinuous segments connected with intergranular/ intercellular ligaments suggesting that the cracking may have occurmd during solidification. The second type of cracking was intergranular and transgranular and followed the hard martensitic layer formed at the interface. Evidence of a 0.03 in. to 0.07 in. wide zone consisting of descrete and freshly fractured ligaments connecting the two mating crack faces was seen extending around the cimumference positioned prior to the crack-tip. This most likely contributed to resistance to leakage and additional strength for the cracked penetration. Results and Discussion May 1998 c:\4177a-non. doc-051398
3-6 The fracture face was covered with dark high temperature oxide all the way up to the OD surface where the leak occurmd, suggesting that the cracking most likely occurred during welding or post-weld heat treatment. No evidence of " beach marks" or fatigue striations was pmsent on the fracture face.
- The analysis of ID surface deposits showed no evidence of contaminants such as halogens suggesting that coolant chemistry did not play any role in the cracking process. This was also confirmed with the crack deposits.
Chemistry analysis msults confirmed the presence of low melting species such as copper, sulfur, zinc, and boron in the oxide layer on the fractum face. The chemistry analysis of weld and base materials showed that they meet the specification mquirements. No evidence of contammants such as copper, boron, sulfur and zinc is found in the weld chemistry. Metallographic examination of axial sections through the five similar welds in the three motor tubes (G-9, G-5, and E-7) at Unit 2 confirmed no evidence of cracks. UT examination of 60 other part length CRDM motor tube welds in the industry confirmed no indications. Fabrication Sequence The type 403 stainless steel tube motor tube is joined to 304 stainless steel end forgings via a full penetration weld. The weld materialis Type 308L. The welds am subjected to both radiographic, penetrant test, and visual examination in accordance with the mquirements of the ASME code in effect at the time of fabrication. The specified (Ref.1) sequence of welding, heat treatment and non-destructive examination is listed below for the G-9 motor tube.
- Machine butter pmparation on 403 tube e Irr machined surface befom buttering e Butter with 309 weld wim e Machine butter on both ends
- PT examine butter surface
- RTbutter weld
. Heat tmat after buttering per ASME Code Case 1337, Part 3
. The mcords indicated a rework on the weld approximately 10 months after the date of original fabrication.
Results and Discussion May 1998 o:\4177a-non doc-051398
3-7 A myiew of records confirmed that the vendor (Royal Industries) changed the weld deposit material to Alloy 82 applied to the 403 approximately one year after the original fabrication of the Prairie Island tubes. It is also interesting to note that the G-9A weld contained a repair on the ID in close proximity to the crack in the region between the 149 and 220 clock locations where the deepest cracking (and leak) occurred. It is not clear if the repair was an attempt to fix the indication that was missed. Experience with making a repair with 308 adjacent to 403 indicates that it could be very difficult, especially with limited access ID locations such as the case hem. Although normally not expected, it is conceivable as an isolated occurrence, that while making an unsuccessful weld repair the crack in the buttering could be driven radially outward further inside the weld. In addition, the FT and RT results could potentially be masked by the dissimilar metal interface considering the fact that hot cracks are generally very tight. Industry Weld Records Review Under a separate effort supported by WOG funding, Westinghouse conducted a review of motor tube fabrication records for all PLCRDMs in Westinghouse units currently in service. History books and material identification reports prepared by Royal Industries were retrieved and reviewed (Ref. 5) for all plants with the exception of Ginna and Beaver Valley Unit 1. A listing of all twenty-nine (29) domestic plants that have PLCRDMs fabricated by Royal Industries installed is provided in Table 3-1. In the case of Ginna, the only information that could be located at this point, was for a spam PLCRDM supplied by Royal Indur+ ries, this was not installed on the reactor vessel closure head. The weld wire heat number used to butter the failed PLCRDM at core location (G-9) (Ser. No. RA70-90) (X1419 U309) at Prairie Island Unit 2 was also used for the same application on twenty (20) other PLCRDMs. The following is a list with the application plant and serial number. Plant Serial Number D. C. Cook Unit 2 (Total 6) RA70-079, RA70-084, RA70485, RA70-087, RA70-089, RA70-092 Diablo Ctayon Unit 2 (Total 8) RA70-083, RA70-101, RA70-102, RA70-103, RA70-110, RA70-112, RA70-114, RA70-122 Indian Point Unit 2 (Total 1) SB-42 North Anna Unit 1 (Total 5) RA70-097, RA70-098, RA70-106, RA70-107, RA70-113 I Results and Discussion May 1998 o \4177a-non. doc-051398 j a___-___-____-_---_____ _ _ _ _ _ _ _ - _ _ - _ _ _ _ _ _ _ - _ _ _ _ - _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ - _ _ _ _ _ - - _ - _ _ _. . _ - - - _ _ _ _ _ - _ _ - _ _ _ _ _ ,
3-8 The following conclusions were reached from the initial review process (Ref. 5). 1 1. The weld wire heat number used to butter the failed PLCRDM at Prairie Island Unit 2 was also used for the same application on several other PLCRDMs installed.
- 2. Eight (8) plants have part lengths CRDMs that utilize Alloy 82 buttering and weld instead of 309/308L stainless steel.
3. The results of the chemical composition reported on the certified material test reports for weld wim heat number X1419 U309 were compared with ASME Section II, Part C requirements. In all instances, the value mported on the certified material test report was within ASME Code limits.
- 4. This initial fabrication record review could not identify any unique characteristic for the (G-9) weld and no direct conclusion could be drawn.
Mechanistic and Root Cause Considerations Based on the examination results and the evidence presented above, consideration of three types of cracking mechanisms seem mlevant to the curmnt part length CRDM cracking issue at Prairie Island Unit 2. Hot cracking during solidification e Cold cracking during PWHT Reheat cracking during PWHT Hot cracking or solidification cracking occurs during welding at J hnilar metalinterface when the resulting strains or (displacements) can not be accommodated by the remaining liquid metal during the critical freezing range leaving interdendritic or intercellular cracks and microfisums upon solidification. Factors that contribute to hot cracking include: 1. The magnitude of strains developed at the interface during welding.
- 2. Weld metal ductility and delta ferrite content.
- 3. Freezing range 4.
Presence of harmful low melting elements such as sulfur, phosphorous, copper, etc. Due to the dissimilar thermal expansion properties of the austenitic 309 stainless steel buttering and the martensitic 403 material, the strains that develop at the interface during freezing are expected to be very significant. Good weld metal ductility and delta ferrite contents in excess of 4% are required to accommodate the high residual stresses that are likely to develop at the interface. Furthermore, weld metal chemistry is important since it controls the freezing range during which it is susceptible to cracking. Since elements such as sulfur, boron and copper Results and Discussion May 1998 o:\4177a-non. doc-051398
3-9 have a lower solubility in austenite, they are rejected into the last remaining liquid and thus concentrate at the dendritic boundaries. This segregation of impurities that lower the effective solidus temperature of the last material to solidify pmmotes hot cracking. The observed interdendritic cracking and microfissuring in some sections of the weld, the dissimilar nature of the weld interface and the presence of harmful elements such as sulfur, boron, copper and zine detected in the fractum face all suggest that hot cracking is a likely mechanism effecting the G-9A weld. The second type of cracking behavior seen in the G-9A weld resembles the character of cracking that has been observed in stainless steel claddings (Ref. 4). It has been a recognized fact that at the interface between ferritic steel and stainless steel carbon migtation occurs from the base metal into the fused zone during welding PWHT. This leads to the development of a hard zone (Ref. 4) at the interface rendering the weld susceptible to hydrogen embrittlement. A narrow martensite band is known to form in the as-welded condition within the zone of partial mixing. PWHT results in carbon migration and intense carbide precipitation within the weld metal together with formation of a decarburized zone in the base metal. Although decomposition (tempering) of the original interfacial martensite is expected to occur during PWHT, higher hardness is expected to be maintained over a 100 m zone adjacent to the interface. The hard zone is higher in hardness and lower in ductility rendering it susceptible to hydrogen cracking during thermal cycles and this is even more likely in the presence of stmss concentration such ) as notch or in the current case, a hot crack. Thus the pre-existing hot crack can induce additional crack growth in the susceptible hard zone during post weld heat treatment. The suggested mechanism assumes a pre-cracked condition by hot cracking which in tum was caused by the introduction of contaminants during weld fabrication. Stress relief cracking is caused by creep strains associated with stress relaxation of residual welding stresses during PWHT and occurs generally in course grained regions of the weld heat affected zones. The extent of creep cavitation that occurs along grain boundaries is strongly sensitive to the presence of contaminants such as phosphorous, arsenic copper and sulfur. Since the observed cracking is primarily in the partial mixing zone of the weld metal at the interface and not in the HAZ, the stress relief cracking is not considered as a likely contributor mechanism for the G-9A weld cracking. The mechanistic assessments considered above suggest that the Prairie Island Unit 2 part length CRDM G-9A motor tube weld cracking is caused by solidification cracking during weld fabrication followed by crack extension and /or additional cracking in the hard zone of the partial mixing zone of the weld metal during post weld heat treatment. Since examination of 60 similar welds in the Westinghouse units have confirmed no cracking it appears that the initial solidification cracking (which was most likely caused by the introduction of contaminants during weld fabrication) may have served as a precurser for rendering the weld susceptible to further PWHT cracking. The results of the review of the available fabrication mcords suggested nothing specific to the weld wire, welder or welding procedure that isolates the G-9A weld from other welds. The presence of copper, sulfur, boron and zine contaminants tied up in the high temperature oxide layer on the fracture face is an important contributing factor to the occurrence of the observed solidification cracking. Microchemistry assessments of the weld metals confirmed that the contaminants are extraneous to the weld wire chemistry and are most Results and Discussion May 1998 oM177a-non. doc-051398
3-10 likely introduced from external sources during weld fabrication. It follows from the above that contamination of the weld during weld fabrication is the " root cause" of cracking in the Prairie Island Unit 2 PLCRDMs G-9A weld. 1 l l l i i i Results and Discussion May 1998 0:\4177a-non. doc-0513%
3-11 Table 3-1 Domestic Plants Where Royal Industries PLCRDMs are Currently Installed Plant Number of PLCRDMs Ginna 4 Prairie Island Unit 1 4 Point Beach Units 1 and 2 4 Kewaunee 4 Farley Units 1 and 2 5 (Alloy 82) Robinson Unit 2 8 Beaver Valley Units 1 and 2 5 in Unit 1,1 in Unit 2 (Alloy 82) Turkey Point Units 3 and 4 8 Surry Units 1 and 2 5 North Anna Units 1 and 2 5 D. C. Cook Units 1 and 2 8 McGuire Units 1 and 2 8(Alloy 82) Indian Point Units 2 and 3 8 Diablo Canyon Units 1 and 2 8 Salem Units 1 and 2 8 Sequoyah Units 1 and 2 8 (Alloy 82) Watts Bar Unit 1 8 (Alloy 82) l l Results and Discussion May 1998 o:\4177a-non. doc-051398 l
]'
I' 4-1 l. p M 4 CONCLUSIONS Based on the overall results of the investigation it is concluded that: The' observed cracking in the G-9 part-length CRDM housing motor tube lower weld at Prairie Island Unit 2 station originated from weld fabrication.
.- ' No evidence of additional service growth of the defect could be identified.
. Metallographic examination of five other welds in the motor tubes at Unit 2, fabricated during the same time period confirmed no evidence of any defects. UT examination of
, some sixty part length motor tube welds in Westinghouse units confirmed no cracking.
. Root cause considerations suggested that introduction of contaminants such as sulfur, boron, copper and zine from an unknown external source during the weld fabrication most likely contributed to the cracking of Prairie Island G-9 motor tube.
. The occurmnce of part length motor tube cracking is an isolated event limited to the Prairie Island G-9A motor tube weld.
1 Conclusions May 1998 o:\4177a. doc-051396 ~ _ _ _ = - - - _ _ _ _ _ _ - _ _ _ - - _ _ _ _ _ _ _ - - - - _ _ - - - - - _ _ _ _ - - - - - - _ - - - - - - - - _ _ - - - - - - - - - - - - - - - - - - - _ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - . -
5-1 5 LIST OF REFERENCES
- 1. Royal Industries, " Motor Tube Center Section Drawing No.121E005-1," August 1968.
- 2. C. D. Lundin and C. P. D. Chou, " Hot Cracking Susceptibility of Austenitic Stainless Steel Weldments," Welding Research Council Bulletin No. 289, November 1983.
l
- 3. M. J. Cieslak, A. M. Ritter and W. F. Savage, " Solidification Cracking and Analytical Electron Microscopy of Austenitic Stainless Steel Weld Metals," Welding Research l I Council, Supplement to the Welding Journal, January 1982, p.1-5.
l 4. M. F. Gittos and T. G. Gooch, "The Interface Below Stainless Steel and Nickel-Alloy Claddings," Welding Research Supplement No. 461-5, December 1992. i
- 5. D. E. Boyle and A. Paterson, "Part Length CRDM Housing Issue Initial Fabrication Records Review," Westinghouse Internal Report No. NSD-E-MSI-98-070, March 1998.
l List of References May 1998 o:\4177a-non. doc-051398
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