ML18065A216
| ML18065A216 | |
| Person / Time | |
|---|---|
| Site: | Palisades  |
| Issue date: | 10/25/1995 |
| From: | Smedley R CONSUMERS ENERGY CO. (FORMERLY CONSUMERS POWER CO.) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| GL-95-03, GL-95-3, NUDOCS 9511010036 | |
| Download: ML18065A216 (7) | |
Text
consumers Power POW ERi Nii MICHlliAN'S PROliRESS Palisades Nuclear Plant: 27780 Blue Star Memorial Highway, Covert, Ml 49043 October 25, 1995 U S Nuclear.Regulatory Commission Document Control Desk Washington, DC 20555 DOCKET 50-255 - LICENSE DPR PALISADES PLANT
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RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION - GENERIC LETTER 95 CIRCUMFERENTIAL CRACKING OF STEAM GENERATOR TUBES In a letter dated September 25, 1995, the NRC transmitted a request for additional information for Generic Letter 95-03, "Circumferential Cracking of Steam Generator Tubes": Attachment 1 provides the requested information. is a notarized statement affirming the original submittal dated June 27, 1995.
As requested in the September, 1995 NRC letter, this corrects the original notarized statement which was incorrectly dated (1994 instead of 1995).
SUMMARY
OF.COMMITMENTS This letter contains no new commitments and no revisions to existing commitments.
Richard W. Smedley Manager, Licensing CC Administrator, Region III, USNRC P*roj ect Manager, NRR, USN RC NRC Resident Inspector - Palisades Attachm~nts (2) r-9511010036. 95,1025 1
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A OHS' ENERGY COMPANY
ATTACHMENT 1 CONSUMERS POWER COMPANY PALISADES PLANT DOCKET 50-255 RESPONSE TO REQUEST FOR AODITIONAL INFORMATION GENERIC LETTER 95-03 CIRCUMFERENTIAL CRACKING OF STEAM GENERATOR TUBES 3 Pages
RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION FOR GENERIC LETTER 95-03 CIRCUMFERENTIAL CRACKING OF STEAM GENERATOR TUBES NRC REQUESTED INFORMATION (1)
It was indicated that circumferential cracking has not been identified at tube support plate intersections or in low row U-bends for inservice Combustion Engineering (CE) steam generators and that these areas are not considered areas of susceptibfl.ity applicable to Palisades.
Provide the basis for this statement given that these areas have exhibited cracking in a CE steam generator that was replaced and given that these areas have exhibited c.ircumf.erential cracking in steam generators manufactured by another vendor (i.e., at dented regions and in low radius U-bend tubes).
CPCo RESPONSE (1)
Circumferential cracking at tube support plate intersectio~s has been identified in the retired CE steam generators from Millstone and Palisades.
The retired Millstone~2 steam generators were CE Series 67 units that contained seven full eggcrate tube support structures, two partial eggcrates, two partial drilled tube s~pport plates, two diagonal (batwing) supports and three vertical supports. All of these were fabricated from carbon steel.
In 1991 Millstone-2 shut down twice due to a through wall circumferenti~l crack at the intersection of an
~ggcrate support~ The postulated failure mechanism was stress corrosion cracking initiated at the outer diameter (OD).
Denting related expansion and shifting of the eggcrat.es, and locking of the tubes at the batwing and vertical supports, produced the undesired stresses. It is believed that the denting and expansion were* caused by the buildup of magnetite on the carbon steel eggcrate supports.
The retired Pali sades steam generators were *unique with respect to the design Of the tube supports which consisted of solid tube support plates with flow holes, plates without flow holes but with flow slots, full eggcrates, and partial drilled plates. At the time of retirement, the retired Palisades steam generators had not experienced circumferential cracking at the top of the tubesheet, but had ~xperienced circumferential cracking at locatio.ns above the tubesheet.
The postulated failure mechanism for the Palisades circumferential cracks was OD initiated stress corrosion cracking.
The tubes were considered to be locked into the tube support plates as a result of denting.
During operation, the plates and tubes moved relative to one another, thereby imposing bending stresses on the tubes.
The bending stresses then initiated circumferential cracks.
It is believed that the current Palisades steam generator design is not susceptible to this degradation mechanism.
The eggcrate supports in the
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- cur-rent Pali sades steam generators have been fabricated from 409 stainless steel. The stainless steel eggcrate supports, along with our rigid secondary side chemistry environment wi.11 work to inhibit the formation of magnetite, which is believed to have caused the denting problems in the retired Millstone and Palis~des steam generators.
Bobbin inspections performed thus far have not identified this problem at support plate intersections. Furthermore, in July, 1995, a visual inspection of the upper tube bundle region on the secondary side of one active Palisades steam generator showed that no deposit build-up or other degradation mechanisms were occurring in this area.
2 With respect to the retired CE steam generators, there were no occurrences of circumferential cracking in small radius U-bends.
As a preventive measure early in the life of the Palisades retired steam generators, the eleven inner rows of tubes wefe plugged due to wastage problems.
However, only a small number of inner row tubes on the retired Millstone steam generators were plugged prior to retirement.
There were several occurrences in the Millstone steam generators of circumferential cracking in 90 degree bends (10 inch radius bends) at or adjacent to diagonal supports, and in horizontal runs at or adjacent to vertical supports; these have been described as cracks in the bend region. All circumferential cracks identified to date in active CE steam generate.rs have been at or very near the top of the tubesheet on the hotleg side.
There have been no reported circumferential indications in the upper part of the tube bundles, including tight radius U-bends.
Several plants have conducted RPC tests of the tight radius U-bends.
These included Maine Yankee, and Palo Verde-I, 2, and
- 3.
NRC REQUESTED INFORMATION (2)
Please clar:ify the statement that "any eddy current indication '
identified as a crack will be structura11y evaluated for proper repair."
CPCo RESPONSE (2)
It is currently our position to plug or sleeve any tube showing a cracking indication, regardless of size. The wording used in our letter of September 25, 1995 was intended to include tube stabilization that may be required in addition to plugging or sleeving, depending on the speciflc location of the flaw.
NRC REQUESTED INFORMATION (3)
During the Maine Yankee outage in July/August 1994, several weaknesses were identified in its eddy current program as detailed in NRC Information Notice 94-88, "Inservice Inspection Deficiencies Result in Severely Degraded Steam Generator Tubes," In Information Notice 94-88, the staff observed that several circumferential indications could be
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traced back to earlier inspections when the data was reanalyzed using terrain plots. These terrain plots had not been generated as part of the original field analysis for these tubes.
For the rotating pancake coil (RPC) examinations performed at your plant at locations susceptible to circumferential cracking during the previous inspection (i.e.,
previous inspection per your Generic Letter 95-03 response), discuss the extent to whic.h terrain plots were used to analyze the eddy current data. If terrain plots were not routinely used at locations susceptible to circumferential cracking, discuss whether or not the RPC eddy current data has been reanalyzed using terrain mapping of the data.
If terrain plots*were not routinely used during the outage and your data has not been reanalyzed with terrain mapping of the data, discuss your basis for not reanalyzing your previous RPC data in light of the findings at Maine Yankee..
Discuss whether terrain plots will be used to analyze the RPC eddy current data at locations susceptible to circumferential cracking during your next steam generator tube inspection (i.e., the next* inspection per your Generic Letter 95-03 response).
CPCo RESPONSE (3)
For purposes of clarifying terms used in our GL 95-03 response, "previous inspection" refers to the inspection conducted in June 1993, "next inspection" refers to the inspection conducted in July, 1995.
Terrai.n plots were generated and analyzed for all RPC inspection performed in June, 1993 and July, 1995.
It has been, and will remain, our policy to generate and analyze terrain plots for all RPC inspections. June, 1993 was the first inspection in which RPC examinations were performed on the Palisades replacement steam generators..
ATTACHMENT 2 CONSUMERS POWER COMPANY PALISADES PLANT DOCKET 50-255 OATH & AFFIRMATION FOR JUNE 27, 1995 SUBMITTAL 1 Page
CONSUMERS POWER COMPANY To the best of my knowledge, the contents of the submittal dated June 27, 1995 entitled "Response to Generic Letter 95 Circumferential Cracking of Steam Gene*rator Tubes,". are truthful and complete.
By\\? y_)\\ t:'.S-~~--:t\\
Richard W Smedley
~
Manager, Licerising Sworn and subscribed to before life this r:25 ~ day *of g)~
J Alora M. Davis, Notary Public
- Berrien County, Michigan (Acting in Van Bureh County, Michig~n)
My comm.ission expires August 26, 1999 1995.
[SEAL]
- .. ~.