ML18040A176
| ML18040A176 | |
| Person / Time | |
|---|---|
| Site: | Nine Mile Point  |
| Issue date: | 04/30/1996 |
| From: | Terry C NIAGARA MOHAWK POWER CORP. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| Shared Package | |
| ML17059B123 | List: |
| References | |
| GL-94-03, GL-94-3, NMP1L-1067, TAC-M90102, NUDOCS 9605070004 | |
| Download: ML18040A176 (7) | |
Text
CATEGORY j.
REGUI ATII INFORMATION DISTRIBUTIONFSTEM (RIDE)
ACCESSION NBR:9605070004 DOC.DATE: 96/04/30 NOTARIZED: YES DOCKET FACIL:50-220 Nine Mile Point Nuclear Station,- Unit 1, Niagara Powe 05000220 AUTH.NAME AUTHOR AFFILIATION TERRY,C.D.
Niagara Mohawk Power Corp.
RECIP.NAME RECIPIENT AFFILIATION Document Control Branch (Document Control esk)
SUBJECT:
Forwards proprietary
& nonproprietary versions of "Core Shroud Repair 270 Degree Tie Rod Assembly Assessment,"
C requested by NRC during 960325
& 26 telcons.Proprietary assessment withheld from public disclosure per 10CFR2.790.
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, (g V RIIIASA A U MODMAWK NIAGARAMOHAWKPOWER CORPORATION/NINE MILEPOINT NUCLEARSTATION. P.O. BOX 63, LYCOMING,N.Y.13093/TEL. (315) 349-7263 FAX (315) 349-4753 CARL D. TERRY Vice President Nuciear Engineering April30, 1996 NMP1L 1067 U. S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555 RE:
Nine MilePoint Unit 1 Docket No. 50-220
Subject:
Generic Letter 94-03, "Intergranular Stress Corrosion Cracking ofCore Shrouds in Boiling Wafer Reactors" (TAC1Vo. M90102)
Gentlemen:
Niagara Mohawk Power Corporation (NMPC) has performed a review ofthe final inspection of the core shroud repair installed during the spring 1995 refueling outage.
The results ofthe review were discussed with the NRC in telephone conferences on March 25 and 26, 1996. Atthat time, we indicated that one ofthe deviations identified in our March 23, 1995 submittal was incorrectly described in that submittal. The deviation concerns the lower spring wedge which leans against a recirculation nozzle weld at the 270'ocation.
Our original inspection indicated that the contact area between the lower wedge and the reactor pressure vessel wall appeared to be approximately 2/3 ofthe wedge area.
Our most recent review has determined that the actual contact area is approximately 9% ofthe wedge area.
This discrepancy was reported under 10CFR50.72 (b)(1)(ii)(B)on March 22, 1996 and 10CFR50.73(a)(2)(ii)(B) on April22, 1996.
NMPC committed to supplement our original shroud repair submittal with an analysis ofthe as installed 270'ie rod assembly by April30, 1996. The supplemental submittal is being submitted to the NRC stafffor review and approval as an alternative repair, pursuant to 10CFR50.55a (a)(3)(i). The supplemental submittal provides a technical evaluation ofthe adequacy ofthe as installed condition ofthe 270'ie rod assembly to confirm the effectiveness ofthe core shroud repair. The supplemental submittal is included with this letter as Enclosure 1.
'Enclosure 1 provides information and analyses requested by the NRC staff during the above referenced telephone conferences.
Achronology ofthe events that eventually led to the discovery ofthe 270'ower wedge deviation is discussed along with an uncertainty analysis ofthe installed condition based on the videotape and computer aided drafting (CAD) review. Structural and seismic evaluations concluded that the as installed condition ofthe 270'ie rod assembly is acceptable and that all safety functions provided by the shroud repair are maintained.
The evaluations performed have bounded the installation uncertainty associated with the lower wedge location on the nozzle blend radius.
The first evaluation concluded that adequate friction exists to maintain contact stability ofthe lower wedge on the nozzle blend radius under all original design loading conditions.
The second evaluation assumed the wedge slips to the bounding angle of 9605070004 960430 PDR ADQCK 05000220
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Page 2 uncertainty and the third evaluation assumed that contact stability is lost such that the lower wedge is assumed to slip offthe nozzle rendering the lower spring ineffective. The third evaluation provides added confidence that the horizontal and vertical displacements ofthe core shroud willbe maintained below allowable limits under seismic loading conditions assuming no friction. Aflowinduced vibration analysis was also performed assuming the absence ofcontact during normal operation.
Although the evaluations have concluded that the as installed condition ofthe core shroud repair is acceptable, NMPC is considering the implementation ofa modification to the 270'ie rod assembly during the NMP1 spring 1997 refueling outage.
However, specific design details, costs and durations are not well defined at this time. Modification options that would move the lower wedge offthe nozzle blend radius such that full contact with the vessel wall is achieved, are currently under evaluation. Barring any unforeseen hardships associated with a modification, NMPC willsubmit its modification plans to the NRC by May 30, 1996. In the event that a modification is determined to be impractical compared to the modification costs and risks and compared to the safety significance ofthe as installed condition, NMPC willapprise the NRC of our alternate plans by May 30, 1996.
Enclosure 1 is considered by its preparer, General Electric, to contain proprietary information exempt from disclosure pursuant to 10CFR2.790.
Therefore, on behalf ofGeneral Electric, Niagara Mohawk hereby makes application to withhold this document from public disclosure in accordance with 10CFR2.790(b)(1).
An affidavit executed by General Electric detailing the reasons for the request to withhold the proprietary information has been included as. Anon-proprietary version ofthis document has been included with this letter as.
Very truly yours, C. D. Terry Vice President - Nuclear Engineering CDT/TWR/1mc Attachment xc:
Regional Administrator Region I Mr. D. S. Hood, Senior Project Manager, NRR Mr. B. S. Norris, Senior Resident Inspector Records Management
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