ML082200065
| ML082200065 | |
| Person / Time | |
|---|---|
| Site: | Palisades  |
| Issue date: | 08/06/2008 |
| From: | Schwarz C J Entergy Nuclear Operations |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| TAC MD8649 | |
| Download: ML082200065 (9) | |
Text
Entergy Nuclear Operations, Inc. Palisades Nuclear Plant 27780 Blue Star Memorial Highway Covert, MI 49043 Tel269 764 2000 August 06,2008 Technical Specification 5.6.8 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Palisades Nuclear Plant Docket 50-255 License No.
DPR-20 Response to Request for Additional Information on 2007 Steam Generator Tube Inspection Report (TAC No. MD 8649) Dear Sir or Madam: By letter dated April 7, 2008 (ML080980422), Entergy Nuclear Operations, Inc. (ENO) submitted information pertaining to the 2007 steam generator tube inspections at the Palisades Nuclear Plant.
By electronic mail dated June 17, 2008, the Nuclear Regulatory Commission requested additional information on the 2007 steam generator tube inspection report. Enclosure 1 provides the EN0 response for the requested information.
Summary of Commi"rents
- This letter contains no new commitments and no revision to existing commitments.
J. Schwa Site Vice President Palisades Nuclear Plant Enclosure CC Administrator, Region Ill, USNRC Project Manager, Palisades, USNRC Resident Inspector, Palisades, USNRC ENCLOSURE 1 RESPONSE TO REQUEST FOR ADDIITIIONAL INFORMATlON ON 2007 STEAM GENERATOR TUBE BNSPECTBON REPORT By letter dated April 7, 2008, Entergy Nuclear Operations, Inc. (ENO) submitted information perlaining to the 2007 steam generator tube inspections at the Palisades Nuclear Plant (PNP). By electronic mail dated June 17, 2008, the Nuclear Regulatory Commission (NRC) requested additional information on the 2007 Steam Generator Tube inspection Report. The requested information and EM0 responses follow. NRC Request I. One tube was identified that had an indication attributed to tube-to-tube wear.
Regarding this indication and this degradation mechanism, please address the following:
-. a. Please discuss the root cause for this degradation mechanism (i.e., why have the tubes moved in close proximity such that tube-to-tube wear is occurring).
mase discuss I-ke exl-ent to which this mechanism been obsewed in similarly designed and operated steam generators (include in this discussion historic growth rates, a general discussion on the severily of the indications defected, and the region of the the bundle aNected).
EN0 Response a. The tubes have not moved into this condition; the condition has been present since steam generator manufacture.
No formal root cause analysis has been pedormed.
The likely cause is due to manufacturing tolerances associated with tube bending for the square bend region. If the horizontal run distance getween square bend centers is on the high side of "re tolerance range and the next higher row tube has a horizontal run distance between square bend centers on the low side of the tolerance range, the tube-to-tube gap just below the square bend would be reduced. Another possible cause is a square bend with bend angle not equal to 90 degrees. While not common, this condition has been observed in other similar designed steam generators.
Indications at other units have experienced depth of penetration similar to that obsewed at PNP. An observation in 2005 from a plant with similar steam generators shows the proximity condition occurred at the square bend to horizontal run section of tubing. As this condition is related to steam generator manufacture and there have been no industry reports of high depth wear at these locations, the implied growth rates are considered extremely low.
Page 1 of 8 NRC Request b. Please discuss whether the integriv of the diagonal bars adjacent to this tube was verified.
For example, for the tubes surrounding the tube in row 136, column 77, were the diagonal bars verified to be at the correct axial elevation? (Refer to Information Notice 2005-29, 'Steam Generator Tube and Supporl Configura'alion'y.
In addition, were any visual inspections of the bawing pedormed (i.e., on the exterior of the tube bundle)? EN0 Response b. Integrity of the diagonal bars was verified by
+PointTM (+Pt) rotating pancake coil (RPC) testing which shows both bars are located at the same elevation. Bobbin data from the last two refueling outages were reviewed for the characteristic bahing shift eddy current signature; no such signatures were reported.
NBC Request c. Please discuss the results of any r/isual inspections in this area.
EN0 Response 6. A "bottom up" visual inspection was perFormed of the center of tubesheet region from the secondary side of the tubesheet. No failed diagonal bars were identified.
The diagonal bars positioning observed at the last refueling outage (1 R1 9) inspection was consistent with prior visual inspections.
- d. Please discuss the results of any rotating probe inspections pedormed in row 238, column 77, prior to the plugging of this tube in 1998. EM0 Response d. No +Pt RPC examination was pedormed on row 138 (R138), column 77 (C77), prior to 1998. Page 2 of 8
- e. Please &cuss whether there is any evidence that the tubes adjacent to the tube in row 136, column 77 are in close proximi@ either through visual examination or from a review of the eddy current data (historic or present).
EN0 Response e. Due to the steam generator design, the only tubes that can experience a proximity condition for this area of the steam generator are located in the same column; the diagonal bars provide spacing in the row direction.
Visual examination of internal bundle locations in triangular pitch steam generators is difficult and likely will not produce data sufficient to determine this proximiv condition.
Eddy current data was reviewed with no evidence of tube proximiv on R134 C77. f. It was indicated that the indication attributed to tube-to-tube wear could not be identified in the bobbin coil data. Nonetheless, the bobbin coil data for all tubes in rows 130 througk 738 was re-reviewed with no abnormal signals being deieected.
In addition, the ro tatingr probe data from a N tubes with wear scars at I-he diagonal bar (holleg and cold-leg) for the tubes in rows 134 througk 138 was reviewed and all wear observed in these tubes was associated with the diagonal bar.
Please discuss, fie extent of condition (i.e., which tubes could potentially come in contact).
Why was the rotating probe data for tubes in rows less than 134 not re-reviewed to confirm that all of the wear was associated with a supporl? ln addition, discuss the exl.ent to which rotating probe examinatrons were performed in this region to confirm the absence of additional indications of tube-to-tube wear (given that the indication could not be detected with the bobbin coil data).
EN0 Response::
- f. Plus Point WPG data was obtained only for those locations with bobbin reporled wear scars. A11 other locations in rows 134 through 138 with wear scars were found to have experienced wear with the diagonal bar, not tube-to-tube wear. The bobbin data for large row tubes was reviewed for observation of diagonal bar-cold leg (DBC) or diagonal bar-hot leg (DBH) reports in the same column and successive rows; no such locations were observed.
If a tube-to-tube contact condition were present in any of the other locations it is expected that such a condition would have been reporled either in the production or resolution process. The potential for Page 3 of 8 tube proximi@ is reduced with decreasing vertical span length (i.e., decreasing row count) from the square bend to the closest eggcrate as the eggcrate forms another tube positioning control point. Additional RPC testing was not performed as the lack of bobbin coil detection is associated with the shallow indication depth. Mad more significant depths associated with tube-to-tube proximity been present, a bobbin coil signal would have been produced.
This is why a review of all bobbin data for rows 130 and higher was pedormed.
No alypical signals were noted.
Additional RPC testing was not performed as the bobbin coil can detect a tube-to-tube proximity condition. The review of bobbin data for rows 130 through 138 included a thorough review of the appropriate channel for observation of potential tube-to-tube contact; no such conditions were reporled.
- g. How would tube-to-tube wear be distinguished from wear attributed to a transient loose parl? Have any wear indications been observed in the free span that were attributed to loose pans (and in an area where tube-to-me contact could occur)? Were any tubes returned to service with wear attributed to loose parts? EN0 Response g. Wear due to tube-to-tube proximi@ can be distinguished from loose parl wear using several parameters. Tube-to-tube wear involves extended longitudinal lengths with very gradual depth tapers. Tube loose part wear typically involves a very short affected length with rapid depth tapers. Eddy current data (both bobbin and +Pt RPC) can be used to identify the presence of a foreign object.
No wear was identified with eddy current data, and no foreign obj&cts were identified in an area where tube-to-tube contact could occur. For the three tubes plugged due to foreign object tube wear the locations were at vertical straps (with objects assumed to be wedged between the tube and vertical strap assembly) or just above an eggcrate suppoTI. These structures control tube positioning and spacing, thus, there is no potential for tube-to-"rube wear at these locations. The two tubes, one in each steam generator, with wear attributed to loose parts were removed from sewice by tube plugging.
MRC Request 2. Your repoH indicates that the possible loose parts in both steam generators were reviewed during the foreign object search and relrieval and lhat no loose parts required removal in steam generator "A" since only small sludge rocks Page 4 of 8 were identified Hovvever, the repot? also indicates that three tubes were plugged in steam genera'alor
'%" as a result of interaction with a possible loose pad. Please clariiy this discussion. Were only possible loose parl indications near the top of irhe tubesheet inspected visually (during the foreiw object search and retrieval)?
Was the part near the tube in row 129, column 108, at the 8th cold-leg tihe support verified to be present? In response to I-he possible loose parl and the associated volumetric indication in the tube located in row 129, column 108, this tube was plugged and stabilized and a nearby tube (row 128, column 109, with just a possible loose part indication) was plugged. If the possible loose part was not removed from this location, please discuss I-he likelihood that this tube could wear as a result of the possible loose pat? and lead to tube severance which could then impact other adjacent non-pluggred tubes). Is monitoring of this non-stabilized tube planned for future outages?
EN0 Response No metallic foreign objects were identified at the top of tubesheet region. For those tubes plugged due to foreign objects, the objects were located in upper bundle regions.
The object located above the 8th cold leg support was repofled by eddy current examination. There is essentially no potential for this object to result in a "eub severance. The location is near the periphery, and high in the bundle.
At this location, flow directions are vertically oriented and any tube wear is likely due to tube vibration against the object. Such wear will have low growlh rates and can be self-limiting. Without sufficient cross-flow velocities there is essentially no potential for a tube severance, either of the tube repofled with wear, or the adjacent tube without wear.
The industry position regarding stabilization of such indications in vertically oriented flow locations is considered an extreme consewatism. Monitoring of the non-stabilized tube is not planned for future refueling outages. However, as all tubes are inspected ukng the bobbin coil at each outage, those currently active tubes surrounding these locations will be inspected each refueling outage. NRC Request 3. On page 6 of your April 7,2008, leMer, you irnply that the tube in row 136, column 77, was plugged due to interaction with a possible loose parl; however, previously you indicated that this tube was plugged as a result of tube-ilo-me wear. Please clarify.
Page 5 of 8 EN0 Response 3. R136 677 was plugged due to tube-to-tube wear. This is discussed on pages 4 and 5 of the Steam Generator Tube inspection Report. The inclusion of W136 C77 with the tubes on page 6 as attributed to loose pat? wear is in error. NRC Request 4. Regarding Table 4A in your April 7, 2008 letter, please clarify the information contained within the column entitled "+Pt [+PointTM] Depth Voltage." ls this column providing the de th of the indications based on the voltage of the $ signal from the +PointT coil? EN0 Response 4. This column provides the indication depth using a regression of +Pt signal amplitude and indication maximum depth, developed from pulled tube data. NRC Request 5. Tables 4B and 46 in your April 7, 2008 le~er, contain all indications due to tube wear for tubes that remain in service. In Table 4B, the tube in row 129, column 108, is listed. Please clarily whether this tube was plugged.
If so, please confirm tht this I-able only contains indications that remain in service. Similarly, Table 46 contains the tube in row 70, column 13. Please clarify whether this We was plugged. If so, please confirm that this table only contains indications that remain in service. EN0 Response P 5. The tube in row 129, column 108, in Table 4B, was stabilized and removed from service by plugging.
A comparison of Table 43, steam generator E-508 indications for tubes that remained in sewice, and Table 6, condition monitoring results and identification of tubes plugged, identified that there were eight line items in Table 4B that contained tubes with indications that were removed from sewice in the 2007 refueling outage. The following line items in Table 4B contain those tubes that did not remain in service: Page 6 of 8 The tube in row 70, column 13, in Table 4C, was stabilized and removed from service by plugging.
A comparison of Table 4C, steam generator E-50B -. indications for tubes that remained in service, and Table 6, condition monitoring results and identification of tubes plugged, identified one line item (the tube in row 70, column 13) in Table 46 contained an indication and was removed from service in the 2007 refueling outage. The following line item in Table 4C contains that tube.
The remaining tubes in Tables 4B and 46 remained in sewice. No. 834 Z NRC Request 6. Please confirm hat your examinations of dents/dings included dents/dings on both the hot-leg and cold-leg side of the steam generator.
If not, please proviale the technical basis for not inspecting the dentsldings on the cold leg in light of operating experience that crackingr at dentsldings can occur on the cold leg (prior to being obsewed on the hot-leg) and the finding of axial crack in a greater than 5 volt ding at the Pourlh veflical strap (row 23, column 102, in steam generator B). SG B EN0 Response 6. All dents (i.e., >2V as reported by bobbin coil analysis) at eggcrates, both hot and cold legs, all freespan dings >5V (as reported by bobbin coil analysis) from hot leg top of tubesheet to cold leg top of tubesheet, and all dents 23.5V Row 70 Page 7 of 8 Column 13 Depth in Percent 13 Location VS4 Elevation
-0.79 Status cTS (as repofled by bobbin coil analysis) at verlkal straps and diagonal bars were inspected with the +Pt coil. MRC Request During your 2007 steam generator tube inspections, the indication at the hot-leg diagonal bar in the tube located in row 99, column 138, was sized at 7-percent through-wall. This indication had previously been sized at 17-percent through wall and is located near a tube that had previously been plugged due to wear with the wrap around bar (row 99, column 140). Please discuss any insights on why the size of the indication appeared to have decreased between inspecl"ions.
Are these results consistent with the uncertainly associated with sizing war indications?
EN0 Response 7. These results are consistent with bobbin coil non-destructive examination sizing uncerlainty, parlicularly for such low amplitude signals.
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