ML20077A660
| ML20077A660 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 05/07/1991 |
| From: | WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP. |
| To: | |
| Shared Package | |
| ML20077A658 | List: |
| References | |
| SG-91-04-021, SG-91-4-21, NUDOCS 9105100301 | |
| Download: ML20077A660 (8) | |
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ATTACHMENT
SUMMARY
REPORT ON THE EXAMINATION OF THE STEAM GENERATOR TUBE R11-C14 1
NORTH ANNA POWER STATION UNIT 1 i 1
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'l PREPARED BY:
WESTINGHOUSE ELECTRIC CORPOR ATION FOR
. VIRGINIA ELECTRIC AND POWER COMPANY i
l 9105100301 910507 PDR ADOCK 05000338:
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SG 91 04 021
SUMMARY
ON THE EXAMINATION OF NORTH ANNA UNIT 1 STEAM GENERATOR TVBE Rll-Cl4 Tube Rll Cl4 from Steam Generator B of North Anna Unit I was cut below the
' second hot leg support plate and pulled for examination to characterize corrosion cracking suspected to exist at the first support plate crevice region. This summary provides preliminary examination data that is considered significant. A more complete description of the data obtained and its analysis will be presented in the final report. ,
l Nondestructive Examination Visual examination of the pulled tube showed circumferential cracking at and just crack belowexisted networks the location of the first support-plate top edge. The and the other centered near (Zerodegree divider plate and 90g20gt two locations, one centered near when looking in the primary floworupwarddirection.) Dimensional characterization showed a maximum ovality of 0.028 inches at this location with the apex of the ovality occurring near the centers of the crack networks. Dimensional characterization also showed that the dented support plate (denting was observed by field eddy current data prior to the tube pull) acted as a die during the tube pull, reducing the tube diameter above the support plate i approximately 0.008 inches. At the tubesheet top, the explosive expansion ;
transition was approximately 0.75 inches long with a diametrical expansion 1 of 0.018 inches. The center of the expansion transition was located approximately 0.1 inches above the estimated location of the tubesheet top.
X Ray radiography clearly showed a complex network of circumferential cracking between 0.0 and 0.2 inches below the support plate top edge. The cracking appeared to be almost continuous around the circumference. In addition, faint radiographic indications of circumferential cracking were sporadically observed _around the circumference at the support plate bottom edge location. The cracking at the bottom edge location. appeared to be restricted to a more narrow band than the cracking at-the top edge-location. No crack indications were observed at~the tubesheet top location.
l.aboratory eddy current testing was performed using-bobbin and RPC probes. Bobbin probe data-showed a 99% deep indication at the first support plate crevice region. RPC examinations found-two major circumferential cracks at the support plate top edge location, the same as in the field examinations. The cracks were-1600 and 1400 long, The cracking at the top edge location had RPC signals that were approximately 30 volts in size. -In-contrast, 5 to 6 volt readings were obtained in the field, indicating that the cracks were significantly opened by the tube pull At the support plate bottom edge location, laboratory RPC examinations found 3-to 5 regions around the circumference with circumferential crack indications. -The largest of these indications was 800 long and had a signal strength of 3.7 volts.= A review of the field' j ,RPC; data showed thatuat least one, possibly two, of these regions were b_ ~~
observed at the support plate bottom location. No field or laboratory eddy current indications were called at the tubesheet top location.
Ultrasonic testing (VT) in the laboratory showed circumferential indications near the support plate top edge and smaller circumferential indications near the bottom edge in both the radial and axial aim scans.
No axial indications were detected in the circumferential aim scan.
Similar to the RPC data, two major circumferential indications were found at the top edge location where they were almost continuous around the circumference. At the bottom edge, four to five locations, more or less uniformly locatea around the circumference, had circumferential indications; the largest was 80 to 900 long. In addition, two small circumferential indications were observed at other locations. One was located within the center of the support plate crevice region near 400 and the other 0.7 inches above the support plate top edge near 1400.
(Destructive examination showed that these latter indications were not caused by corrosion.) A number of axial and circumferential indications were observed above, below and near the tubesheet top location where the expansion transition was located. Only one of these was judged likely to be a corrosion related indication. It was a 0.2 inch lon indication located 1 inch above the tubesheet top near F0g axial . (Subsequent destructive examination showed that this indication location did not have corrosion degradation.) The remaining UT indications at the tubesheet top region were judged to represent tube pulling marks and/or surface deposits.
Destructive Examination The first support plate crevice region was found to have two regions with circumferential cracking of OD origin. One was located at the support plate top edge and the other at the support plate bottom edge, No cracks were found within the support crevice region between the cracking at the support plate edges. The more extensive corrosion occurred at the support plate top edge where the cracking was located from the support plate top edge to 0.2 inches below. The microcrack network at the support plate top edge formed a continuous 3600 macrocrack with two regions with deep (greater than 40% deep) penetrating microcracks. One of the two regions with deeper cracks was 1240 long and was located from 3480 to 1120 .
The other was 1130 long and was located from 1680 to 2810 . Both regions had locations were the cracking was throughwall. The maximum length of throughwall cracking was 900 . The two regions were more or less diametrically opposite and were centered riear the apex of the tube ovality. Table 1 and Figure 1 present a summary of the crack depths at selected locations by SEM fractography.
The circumferential cracking at the support plate bottom edge occurred in two separate microcrack networks which formed two continuous macrocracks separated by regions without any cracking. The cracking was located from the support plate bottom edge to 0.15 inches above the bottom. One region with cracking was 1350 long and was located from 3150 through360f0 0 to 900 . The second region was 1120 long and was located from 135 to 2470 . Again, the two regions were centered near the apex of the local ovality. Table 1 and Figure 1 present a summary of crack depths found at the support plate bottom edge. The crack depths for one of the two cracking regions was determined by SEM fractography. At the other 1
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regicn, where the cracking was judged to be less deep based on visual
- examination of deformed tube sections, the crack depths were determined by metallography. Consequently, the depth of cracking is not as extensively characterized in comparison to SEM fractographic observations. The maximum depth of cracking at the support plate bottom edge was 98%
throughwall.
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' The morphology of the individual circumferential microcracks within the support plate crevice region varied from pure IGSCC to IGSCC with significant IGA components. Where the individual microcracks joined together, only intergranular features were usually observed. Only in a few cases did dimple rupture features predominate on the ledges located between the numerous individual microcracks.
At the tubesheet top location, within the center of the expansion transition, a semi-continuous network of circumferential and axial, OD origin, microcracks was found. The maximum length of axial cracking was 0.04 inches. The maximum 0 length of continuous circumferential cracking was approximately 80 . The circumferential cracking was deeper than the axial cracking and the maximum depth of circumferential cracking was 21%
throughwall. Table I presents a summary of crack depths found at the tubesheet top region.- The_ depths at the tubesheet top region were determined by both SEM fractography (on the half of the tube circumference judged to have the deepest cracks based on visual examination of deformed tube sections) and by metallography; The cracking at the tubesheet top was intergranular; but since the metallographic sections went through locations where the cracks were never deeper than 3 to 4% throughwall, it was difficult to determine the extent that the morphology was either SCC or IGA. As a consequence, the more generic term of intergranular corrosion (IGC) is chosen to dt. scribe the morphology.
Chemical Analyses Energy dispersive spectrometry (EDS) was performed on the tube 0D surfaces adjacent to cracks and on crack fracture faces. Of the elements detected, only K, Cu, Cl, and Pb are judged as candidate elements which might have been involved in corrosion degradation. While-all four of these elements were present in at least 50% of the areas examined by EDS at the tubesheet top, only Cu was observed in at least 50% of the areas examined at the first support plate crevice region. However, Pb was found in 3 of 7 areas examined by EDS at the support plate crevice region. Electron-probe microanalyses (EPMA) of cross sections of the tube wall showed that Pb was concentrated in a 0.0001 inch thick layer that was present on the tube.
Also present in this layer were the base metal elements, as well as 0, Si, A1, Mg, Ca and 2n. At the one location where a thicker surface deposit was present w top this base-layer, Na and S were also detected.
X ray diff', action of OD deposits collected from the first support plate crevice region of Tube Rll-Cl4 indicated the presence of magnetite, copper metal (not oxide), and two forms of magnesium silicate minerals. Since magnesium silicate minerals can not exist in acidic environments, the minerals must have formed in a neutral or basic environment. Since no hematite or copper oxide was found, a reducing environment is indicated.
Electron Spectrometry for Chemical Analysis (ESCA) was performed on the OD
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~ deposits adjacent to a support plate crevice region crack at both the surface of the deposits and at deeper locations reached by sputtering.
Elements detected were Mg, Cr, Fe, Ni, 2n, C, 0, Cu, Si, Al, Na and Pb.
Sodium was incorporated into the oxide in the 0.1 to 0.8 weight percent range. Lead ranged between 0.2 and 2.0%. Little, if any, carbonate was present. Silicon was present as s Dicates. Most oxygen showed silicate or hydroxide bonding rather than oxide bonding. The iron and nickel photoelectron signals also indicated surface hydroxides. The copper was metallic even at the surface. The presence of hydroxides further supports the presence of either a caustic or a aeutral environment.
ESCA was also performed on the crack face. Elements detected were Mg, Cr, Fe, 2n, C, 0, Si, Al, Na, Pb, S, C1 and B. Metal hydroxides predominated. Lead was oxidized rather than metallic. Sulfur was present in both oxidized and reduced forms. Boron was bonded to oxygen. Chlorine could have been present as a transition metal chloride or an organic.
Auger Electron Spectrometry (AES) was also performed on the crack face.
Boron distribution was spotty. Sulfur and chlorine were found on both the crack face and on the lab fracture, indicating that they were laboratory contaminants. Depth profiles of Ni, Fe and Cr showed Cr depletion at both the crack tip and the crack center. Chromium depletion occurs in caustic environments, but the extent of Cr depletion was small and the observations could be used to support either a neutral or a caustic environment.
Summary and Conclusions Intergranular, OD origin, circumferential stress corrosion cracking was found at the top and bottom edges of the first support plate crevice region of Tube Rll-Cl4 The morphology of the cracking ranged from SCC to SCC with significant IGA components. The cracking at the top edge was the deeper and the more extensively located. The cracking was continuously located around the circumference. Two regions, of 1240and 1130 lengths, had cracking that was greater than 40% deep. The maximum length of throughwall cracking was 900 . The cracking at the bottom support plate edge was also significant and had a similar distribution. The regions with the deepest cracks were centered near the apex of the tube ovality. The tube within the support plate region was dented (0.028 inches).
At the tubesheet top region, minor circumferential and axial, OD origin, intergranular corrosion was sporadically present near the center of the expansion transition which was 0.75 inches long with a diametrical expansion of 0.018 inches. The circumferential cracking was the deeper and more extensively distributed. it was up to 21% throughwall.
Eddy current data provided an accurate description of the degradation found at the support plate top edge. At the support plate bottom edge, eddy current data, especially ^.he field eddy current data, providad a less complete description of the degradation even though the cracking was deep. Ultrasonic testing in the laboratory provided slightly more complete description of the cracking, but it also gcnerated indications not caused by corrosion degradation.
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The c'hemistry data obtained was consistent with either a caustic or a neutral environment. It is suspected that local alkaline environments were responsible for the cracking observed; however, it is also possible that Pb played a role in the cracking.
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. crevice region of: Tube Rll-Cl4. . Crack front positions are' based on SEM fractography except for crack front on-bottom sketch near-1800 which is based solely on-metallography.
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