ML20216E149

Forwards Two Ltrs Received from NMPC Re Nine Mile Point Unit 1 Core Shroud Related to 10CFR2.206

ML20216E149
Person / Time
Site:	Nine Mile Point
Issue date:	07/26/1999
From:	Hood D NRC (Affiliation Not Assigned)
To:	Judson T AFFILIATION NOT ASSIGNED
References
2.206, NUDOCS 9908020073
Download: ML20216E149 (3)

Text

July 26, 1999 Mr. Tim Judson Syracuse Peace Council 924 Burnet Avenue Syracuse, NY 13203

Dear Mr. Judson:

As requested during our telephone discussion on July 21,1999, regarding your 10 CFR 2.206 petition of May 24,1999, I am forwarding copies of two letters received from Niagara Mohawk Power Corporation (NMPC) regarding the Nine Mile Point Unit 1 core shroud. Enclosure 1 is NMPC's letter dated June 9,1999, that forwards a report summarizing the horizontal and vertical shroud weld inspections performed during the 1999 refueling outage. Enclosure 2 is NMPC's letter dated July 12,1999, that presents a final root cause evaluation of the cap screw that failed in the upper spring assembly of the shroud tie rod.

Copies of both letters are also available in the Reference and Documents Room of the Penfield Library at State University of New York in Oswego.

Sincerely, ORIGINAL SIGNED BY:

9908020073 990726 Darl S. Hood, Sr. Project Manager, Section 1 gDR ADOCK 05 0 Project Directorato l Division of Licensing Project Management Office of Nuclear Reactor Regulation

Enclosures:

1. NMPC letter to NRC dated July 9,1999

2. NMPC letter to NRC dated July 12,1999 cc w/encls: See next page DISTRIBUTION:

_ Docket File (50-220)3

• PUBLIC '

PDI-1 R/F J. Zwolinski/S. Black E. Adensam (ema,li only) 1.n L

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S. Bajwa S. Little D. Hood 3000M , D ,i W. Batemen \.N E. Imbro -

R. Subbaratnam (e-mail RXS2)

OPA M. Oprendek DOCUMENT NAME: G:\PDI-1\NMP1\JUDSON2.WPD *See Previous Concurrence To receive a copy of this document, Indicate in the box: "C" = Copy without attachment / enclosure "E" = Copy with rttachment/ enclosure "N" = No copy l0FFICE PM:PDI-1 , lE LA:PDI-10L) I SC:PDI 1 M l l lN l NAME DHood/rsl M f1 Slitt17 SBajwa Vi l DATE 07/ d /99 " 07/d3/99 07/'7J/99 Official Record Copy

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/ July 26, 1999 l

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Mr. Tim Judson l Syracuse Peace Council 924 Burnet Avenue

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l Syracuse, NY 13203 I o

Dear Mr. Judson:

l As requested during our telephone discussion on July 21,1999, regarding your 10 CFR 2.206 l petition of May 24,1999, I am forwarding copies of two letters received from Niagara Mohawk j Power Corporation (NMPC) regarding the Nine Mile Point Unit 1 core shroud. Enclosure 1 is i NMPC's letter dated June 9,1999, that forwards a report summarizing the horizontal and j vertical shroud weld inspections performed during the 1999 refueling outage. Enclosure 2 is

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NMPC's letter dated July 12,1999, that presents a final root cause evaluation of the cap screw J

that failed in the upper spring assembly of the shroud tie rod.  !

l Copies of both letters are also available in the Reference and Documents Room of the Penfield f Library at State University of New York in Oswego. I Sincerely, 4 l

l DJLih4 l Darl S. Hood, Sr. Project Manager, Section 1 Project Directorate i I l Division of Licensing Project Management i Office of Nuclear Reactor Regulation l

Enclosures:

1. NMPC letter to NRC dated July 9,1999

2. NMPC letter to NRC dated July 12,1999 )

cc w/encls: See next page 1

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I Nine Mile Point Nuclear Station i Unit No.1 Regional Administrator, Region i Mr. John H. Mueller U.S. Nuclear Regulatory Commission Chief Nuclear Officer 475 Allendale Road Niagara Mohawk Power Corp. l King of Prussia, PA 19406 - Nine Mile Point Nuclear Station Operations Building, 2nd Floor Resident inspector Lycoming, NY 13093 U.S. Nuclear Regulatory Commission P.O. Box 126 Lycoming, NY 13093 Charles Donaldson, Esquire Assistant Attorney General New York Department of Law 120 Broadway New York, NY 10271 Mr. Paul D. Eddy State of New York Department of Public Service Power Division, System Operations 3 Empire State Plaza Albany, NY 12223 Mr. F. William Valentino, President New York State Energy, Research, and Development Authority Corporate Plaza West 286 Washington Avenue Extension Albany, NY 12203-6399 Mark J. Wetterhahn, Esquire

. Winston & Strawn 1400 L Street, NW Washington, DC 20005-3502 Gary D. Wilson, Esquire Niagara Mohawk Power Corporation 300 Erie Boulevard West Syracuse, NY 13202 Supervisor Town of Scriba Route 8, Box 382 Oswego, NY 13126

NiagarahMohawk' cw o. % July 9,1999 Pxne 31s 34W63 NMPIL 1449 F.. 3is 349 47s3 vee nesant Nuclear Sakty Assessment and Support U. S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555 RE: Nine Mile Point Unit 1 Docket No. 50-220 DPR-63

Subject:

Generic Letter 94-03, intergmnular Stress Cormsion Cmcking of Core Shmuds in Boiling Water Reactors' Gentlemen:

l During refueling outage number 15 (RFO15), Niagara Mohawk Power Corporation (NMPC) l performed inspections of the Nine Mile Point Unit 1 (NMP1) core shroud. The purpose of l this letter is to provide core shroud inspection results as required by Generic Letter 94-03 and l Boiling Water Reactor Vessel Internals Project Report, titled "BWR Core Shroud Inspection l and Flaw Evaluation Guideline (BWRVIP-01)." These inspection results are provided in an

[ attachment to this letter.

The inspection of the core shroud vertical and horizontal welds satisfied the scope of inspections as approved by the NRC in their letter dated March 24,1999. The results of these inspections demonstrate that the American Society of Mechanical Engineer required structural margins were satisfied for the previous operating cycle and would continue to be satisfied for an additional 2 year operating cycle without any repairs to the core shroud welds.

Inspection results for the V10 weld showed the greatest crack depth change of all the examined welds; however, all examined welds remain bounded by the structural analysis of the core ~

shroud. NMPC decided to pre-cintively repair the V9 and V10 welds using a contingency repair previously approved by the Staff.

As acknowledged by the Staff in a letter dated March 24,1999, NMPC will define the scope , )

of the core shroud reinspection at least three months prior to the start of RFOl6.

Very , yours, j Carl D. Terry 1 Vice President Nuclear Safety Assessment & Support  ;

CDT/KWK/ kap i Attachment xc: Mr. H. J. Miller, Regional Administrator, Region I Mr. S. S. Bajwa, Section Chief PD-I, Section 1, NRR Mr. G. K. Hunegs, NRC Senior Resident Inspector Mr. D. S. Hood, Senior Project Manager, NRR Enclosure 1 Records Management _.f4f)-7/pb68/-

Nine Mile Point Nuclear Station PO Bon b3..Lveomno New Yo* 13093 0063

• www nimo com

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RFO-15 Core Shroud Inspection Summary Report I

l June 1999 Inspection performed by: Framatome Technologies ,

Summary report prepared by: Niagara Mohawk Power Corporation i 1

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LIST OF ABBREVIATIONS USED IN THE RFO-015  ;

CORE SHROUD INSPECTION

SUMMARY

REPORT ASME American Society of Mechanical Engineers avg average bot bottom I BWR Boiling Water Reactor

• -BWRVIP Boiling Water Reactor Vessel Internals Project deg degree FTI Framatome Technologies GE General Electric GENE General Electric Nuclear Energy GL Generic Letter HAZ heat affected zone hr hour ID inside diameter IGSCC Intergranular stress corrosion cracking

<in inch ksi in stress intensity L longitudinal max maximum MPM MPM Technologies NE Nuclear Energy NMPC Niagara Mohawk Power Corporation NMP1 Nine Mile Point 1 NRC Nuclear Regulatory Commission OD outside diameter  ;

rev revision RFO refueling outage l Rms root mean square s shear SER Safety Evaluation Report SIN Scan Identification Number T thickness UT- ultrasonic testing 2-

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'NiagarahMohawk' Summa >

This report provides a summary of the NMP1 core shro'ud inspections performed during RFO-15 consistent ,

with the requirements of GL 94-03 and BWRVIP 01. The RFO 15 shroud ultrasonic .UT) inspections were performed by Framatome Technologies BWR Sers ves. GE-Nuclear Energy and Framatome performed the RFO-15 shroud visual (EVT 1) inspections. The inspections performed exceeded the requi ed inspections approved by the NRC in the reference 2 letter and included honiontal weld inspections and base metal

. inspections.

The inspection results remained bounded by the analyses previously submitted and approved by the NRC.

see reference 1. The review of the V9 and V10 indications has concluded that the welds will continue to satisfy the required structural margin for an additional 2 year operating cycle assuming the BWRVIP-14 crack growth rate of 2.2 x 10'in/hr without repair. NMPC has pre-emptively installed the NRC approved NMP1 core shroud vertical weld repair, reference 10. on vertical welds V9 and V10. The pre-emptive shroud repair for the V9 and VIO was installed based on the 10 inspection results which indicated crack growth in the through thickness direction. The UT measurement frorn RFO-14 to RFO-15 showed nominal

- crack depth change greater than the UT uncertainty with the average obsersed crack depth change remaining bounded by the NMPC analysis assumptions and the BWRVIP-14 disposition crack growth.

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I' NiagarahMohawk' Table of Coatents

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1. Introduction

2. Evaluation of Results

3. NMP1 Core Shroud RFO 15 Weld Inspection Summary Table

4. Summary Map of Shroud Inspection Coverage (3 sheets) l

-Nine Mile Point i Shroud RFO-15 Inspection Coserage I Nine Mile Point 1 Shroud RFO-14 and RFO-15 Inspection Results )

-Nir,e Mile Point i Shroud RFO-14 EVT-1 and UT-1 Inspection Coverage

5. V9 Inspection Figure 1. Figure 2

6. . VIOInspection Figure 3 Figure 4 o

References:

1. GE-NE-523-B13-01869-043, rev. 0, April 1997," Assessment of the Vertical Weld Cracking on the NMPI Shroud"

2. NRC Letter, March 24,1999, " Core Shroud Re-inspection Plan for Refueling Outage 15, Nine Mile Point Nuclear Station Unit I"

3. BWRVIP Shroud Vertical Weld Inspection and Evaluation Guidelines, GE-Nuclear Energy report GENE-B13-01980-14, June 1999

4. NRC SER, May 8,1997, " Modifications to the core shroud stabilizer lower wedge retaining clip and evaluation of shroud venical weld crackmg, Nine Mile Point Nuclear Station Unit I"

5. GE-NE-523-B13-01920-39, April 1998, " Structural Margin Assessment to Support current fuel cycle operation of Nine Mile Point Unit I with Shroud vertical weld Indications"

6. MPM report No. MPM-497439 April 1997 " Analysis of Nine Mile Point Unit 1 Shroud Weld V9 and Weld VIO Cracking"

7. 1999 Core Shroud Inspection Examination Report for Nine Mile Point 1 Framatome Technologies, FTI reference No. 1600525, June 1999

8. 1999 H8 IVVI data sheets NMPIRl5-99-8,9

9. 1999 H9 IVVI data sheets NMPIR15-99-8.9 i

10. NRC SER, April 30,1999," Alternative Repair of the Core Shroud Vertical Welds, Nine Mile Point Nuclear Station, Unit No.1 (TAC NO. MA4701)" l Attachments: I I, Detailed RFO-15 UT method description

2. Detailed Shroud weld UT coverage maps RFO-15 l

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N5gere g= Mohawk' INTRODUCTION:

The RFO-15 core shroud inspecnon plans were submitted for NRC review and approval in a letter dated December 30.1998. The NRC approsed the RFO-15 core shroud inspecuan plans in a letter dated March 24.1999 The required core shroud inspections for RFO-15 nduded:

1.

All accessible areas of each vertical weld will be inspected by ultrasome tesung WT) or a combmanon of UT and enhanced visual examinauons (EVT-1 t (V3.V 1. V7.V8.V9.Vlo.V11,V12.V15. VIM

2. UT inspecuons will be performed on all the accessible portions of the rmg segment welds. The ring segment wc!ds are designated as VI and V2 in the upper shroud ring. VS.V6 m the shroud top guide ring, and the Vl3.V14 in the shroud core support plate rmg.

3. Shroud horizontal welds structurally replaced by the shroud repair assemblies do not require inspection.

4. EVT-l will be performed to re-inspect the flaw indications at shroud support weld HS.

5. EVT-1 from the annulus of the shroud support H9 weld in accordance with BWRVIP-38.

O The core shroud inspections were required to be performed in accordance with the requirements in BWRVIP-03, " Reactor Pressure Vessel and Exammation Guidehnes" as discussed in the reference 2 letter.

The Framatome Technologies (FTI) methods have been quali0ed in accordance with BWRVIP-03 and the qualinearion is documented in BWRVIP-03. rev i UT demonstration 21 and 22. A detailed discussion of i

the UT inspection methods used during RFO 15 by FTl is included as attachment 1. I This inspection report provides the results of the core shroud inspection results consistent with BWRVIP- j 01 and GL 94-03 reporting requirements. I 1

EVALUATION OF RESULTS: '

The RFO-!5 shroud inspection satis 0ed the five inspection commitments of the reference 2 NRC letter.

The shroud inspection included additional inspection of the core shroud base metal adjacent to the V9 and VIO vertical welds and horizontal weld inspections at the intersection of selected shroud vertica! weius.

The achieved shroud LTT inspection coverage on a per weld basis is provided in attachment 2. The RFO-15 shroud re-inspection results are provided in the summary table. A summary level discussion is provided for the indications identified in the summary table.

The overall conclusion based on the RFO-15 core shroud inspctions is that the core shroud vertical welds will continue to satisfy the ASME required structural margins. The results also demonstrate that the required structural margins would continue to be satis 0ed for an additional 2 year operating cycle assuming no vertical weld repair on the V9 or V10 welds.

RING SEGMENT WELDS (VI.V2. VS. V6. VI3. V14i All the ring segment welds were located and inspected using UT methods. The inspection coverage achieved included all accessible regions of the weld using the primary UT technique. No indications were identified. The UT inspections performed exceeded the requirements of BWRVIP-07 for these welds (i.e.

BWRVIP requires only single sided EVT-1 inspections). De conclusion reached based on these inspections is that the ring segment welds have no cracking and the tie rod repair assumptions are considered validated. In addition, the inspection results of the Hl,H2 and H6b horizontal welds demonstrate that the tie rod repair assumptions associated with these ring welds are conservative (i.e. they assumed the horizontal welds to be 100% through-wall cracked).

Shroud Verrical Weld V3 The RFO-14 inspection of the V3 weld has identined 2 indications on the right side of the V3 weld. The indications were: 1) ID 1.5" long,23%T and 2) OD .8" long ,8%T. The RFO-15 inspection coverage was

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L -N:agara@ Mohawk' expanded and the locations were more pr'ecne than inat used i sr RFO.11 The RFO.14 identined i F long ID indication corresponds with an RFO-15 lack of fusion UT indication th.it shows no surface connecuen p The RFO-14 OD mdication could not be identified and the res new of the RFO-14 data shows that the OD

.. E"long indication was only idenufied using the OD creeping wave with a depth that was not measurable l This indication is not considered real. The RFO 15 identified ID indications located seseral inches from the weld centerline (4.9" 4.5". and 5 m have been characterized as associated with the locanon of top guide ID attachment welds. This cracking has no impact on the shroud structural integrity or the top guide inmerity per BWRVIP-26. Durmg RFO-15 the required top guide hold down assemblies were inspected j ' with no indications identified.

Shroud Verrucal Weld V4 The RFO-14 mspection of the V4 weld had identified 2 indications: 1) 20" long ID connected. 40GT

, g indication on the left of V4 and 2) 1.5" long,53%T indication on the right side of V4 The RFO-15 inspection of this weld coverage was expanded and more precise than that used for RFO-14.

No indications were identified on the right side of the V4 weld. The inspection identified a 26.8" long

' indication on the left side of V4. The indication detected in this weld is not characteristic ofIGSCC. It's characteristics are more typical of fusiv.i line inclusions or lack of fusion. The fusion line indication is in the lower 1/3 of the weld on the left side and is coincident with an indication at the ID toe of the weld. An indication is also detected at the weld root intermittently along the length of the weld. Both of these l; indications are manufacturing induced and do not appear to be service induced. The signal at the toe of the l weld appears to be geometric but may also be caused by very shallow cracking. The fusion line indication does not appear to be connected to the root indication or the indication at the weld toe. However, a l-- conservative analysis would consider the fusion line indication as the upper tip of the flaw and the ID l indication at the weld toe as the base of the flaw. The difference between these signals would be l considered a conservative estimate of the total flaw depth.

A supplernental EVT-1 from the ID surface over approximately a 6" region was performed to determine if the UT indication was ID connected. The inspection showed no indications. The conclusion reached based on this data is that the V4 weld loes not have an IGSCC related crack that requires additional disposition.

The existing reference 1 analysis bounds the RFO-15 data and establishes the integrity of this weld for I cycle of operation assuming IGSCC conditions. Since the indication is not ID connected and not IGSCC, crack growth assumptions for disposition are not applicable.

Shroud Venicai Weld V7 and V8 l This weld was inspected using UT methods during RFO-14 with no indications identifi:d. The RFO-15 l inspections were also UT with no V7 or V8 weld indications identified. The RFO-15 UT coverage included l an additional several inches on either side of the vertical weld. This expanded coverage identified two axial indications associated with the H4 weld as noted in the summary table. These indications are H4 axial indications typical of core shroud IGSCC behavior and are not indicative of vertical weld V7 or V8 cracking. These indications satisfy the venical weld acceptance criteria if the criteria is conservatively applied.

htgud Venical Weld V9 and VIO Additional detailed vertical weld flaw evaluations are not required since the vertical weld V9 and VIO vertical weld repair has been installed. The indications associated with the secondary cracking, associated with the vertical weld V9 and V10 were reviewed relative to the integrity of the core shroud and the impact on the repair The evaltntion concluded that these indications do not impact the vertical weld repair function.

The RFO-15 UT coverage was expanded compared to the RFO-14 coverage. The previously identified visual and UT indications were re-inspected in RFO-15 using UT methods. The inspection of the secondary l

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s NbgarahMohawk cracking has not indicated any sigmficant change in Jepth or length compared to that pres tously identified in RFO-14 This cracking wa> investigated dunng RFO-11 uung pnmarily sisual methods (EVT-1). These inspections identified significant areas of cold work and grinding operations which corresponded to the secondary cracking. Visual inspection of the shroud surface during RFO-15 also shows significant evidence of grinding and local depressions typical of the use of spreader bars and ID/OD attachment weld used for jacking purposes. Res new of construction photos also show significant evidence of these manufacturing processes These practices are also known to have been typicai and are discussed in BWRVIP-01. The boat sample analysis from the V9 and V10 welds showed that the crack initiation corresponded with regions of sigmficant cold working. The measured crack length and depth of the RFO-14 indications remain unchanged; which implies that no significant stresses are acting on these cracks consistent with the cold work condition as the source of the local susceptibihty and stress conditions imtiating the cracking. In conclusion, the presence of the secondary cracking outside of the heat affected zone is considered consistent with the extent of condition previously esaluated for NMPI in the reference 4 NRC SER.

The sertical weld V9 and VIO RFO-15 inspection results are compared to the RFO-14 results in the following table. The results show tFat the V9 indications remain essentially unchanged. The VIO results show evidence of a change in crack depth. The change in depth is converted direr v to an assumed crack growth rate based on =14,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> of operation. A detailed error analysis or statistical treatment of the data has not been performed at this time. An estimate of the uncertainty associated with the change in ciack depth and corresponding crack growth can be derived based on the BWRVIP qualification quoted rms uncertainty. The combined uncertainty between RFO 14 data to RFO-15 data is .16" (V(.1082 +

.124-)=.164")(RFO-14 data .108", RFO-15 data .124"). This corresponds to an uncertainty in the UT based crack growth rate of 1.17E-5 in/hr assuming 14.000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> of operation.

V9 LEFT SIDE MAIN CRACK DATA Weld RFO-15 RFO-14 Change Crack Growth location FTl UT methods FT1 LTT (inches) 1.17E-5 in/hr Distance from Sizing (based on 14.000 H4 (inches) hrs) 5 to 22 0.647 0.658 -0.011 22 to 34 1.020 0.838 0.181 1.30E 05 34 to 44 0.770 0.685 0.085 6.10E-06 44 to 54 0.616 0.502 0.113 8.09E-06 54 to 72 0.440 0.437 0.003 2.38E-07 l

72 to 84 0.365 0.477 -0.111 l avera9e 0.64646 0.588794 0.058 4.12E-06 l max 1.24 1.16 l VIO RIGHT SIDE MAIN CRACK DATA

' Weld RFO-15 RFO-14 Change Crack Orowth location FTl UT methods FTl LTT (inches) i 1.17E-5 in/hr Distance Sizing (based on from H4 14,000 hrs)

(inches) 7 to 19 0.700 0.518 0.183 1.31 E-05 19 to 23 0.990 0.678 0.311 2.22E-05 23 to 29 0.913 0 8M 2 0.301 2.15E-05 29 to 41 0.861 0.518 0.343 2.45E-05 41 to 53 0.717 0.548 0.170 1.21 E-05 53 to 65 0.812 0.508 0.304 2.17E-05 65 to 77 0.627 0.474 0.153 1.09E-05 77 to 83 0.393 0.424 -0.032 average 0.751 0.535 0.215 1.54 E-05 max 1.06 0.87 1

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Niagara hMohawk' The change in crack depth for the V9 weld is close to the measurement tolerance. The change in crack depth for the VIO weld shows crack growth close to the BWRVIP-14 disposition crack Frowth rate at severallocations. Supplementalinspections of the VIO weld at selected locations were performed using the GE suction cup scanner used during RFO 14. These inspections indicated a change in crack depth on the order of.20 to .25 inches in the locations predicted to be .3 inches using the FTl methods ofinspection and sizing. The crack depth change of.25" corresponds to a crack growth of 1.8x 10 ' in/hr.

The measured change in crack depth is consistent with the potential crack growth predicted by the BWRVIP-14 crack growth correlation and the GE PLEDGE correlation assuming stress intensity in the range of 20 to 25 ksi Vin. The NMPI evaluation of the vertical weld through wall stress intensity included analyses which predicted the potential for the stress intensity to increase from approximately 10 to 15 ksi Vin for cracking in the .2" to .4" depth range to a stress intensity of between 20 and 25 ksi Vin for cracking in the .4 to .8" range, see reference 6. The observed change in crack depth is greater for the indications in the .5" to .8" range when compared to the .2" to .4" range consistent with this stress intensity solution.

The structural analysis described in references I and approved in NRC SER, reference 4, remain bounding for the V9 and VIO welds assuming an additional 2 year operating cycle and the BWRVIP-14 disposition crack growth rate. In addition, both the V9 and V10 vertical welds satisfy the applicaHe BWRVIP generic acceptance criteria assuming an end of operating interval based on a 2 year operating cycle and a crack growth of 2.2E-5 in/hr.

Vertical Weld VII This weld was inspected using ID/OD EVT-1 methods during RFO-14 and no indications were identified.

The RFO-15 inspections were UT and no indications were identified, Verrical Weld V12

. His weld was inspected using ID/OD EVT-1 methods during RFO-14. The RFO-14 inspecta documented an approximate 6" long indication in the right HAZ at the H5 intersection. The RFO-15 inspection UT techniques are capable of more precise sizing and location identification. Ris inspection showed 2 axial indications initiating from the H5 weld propagating parallel to the weld,3.6" and 4.7" to the l

right of the vertical weld centerline. Dese indications are bounded by the reference I structural analysis and do not require additional structural evaluation. This type of cracking pattern is typical of NMP1 and general industry experience with core shroud IGSCC in that local surface cold work conditions created during manufacturing create conditions which lead to this type of cracking. Dese localized conditions I cannot lead to structurally significant venical weld cracking since the core shroud has extremely high flaw tolerance as is demonstrated in the reference I analysis.

Yerrical Weld V15 and V16 he RFO-14 inspection of the V15 and V16 venical welds identified ID initiated cracking at the H6B weld intersection. The review of the RFO-14 UT data hwi identified that these indications were actually ID attachment welds. Review of the visual identification from the RFO-14 inspection tapes revealed that the weld crown was machined flush which made venical weld identification difficult.

The RFO-15 inspection method was designed to confirm the venical weld based on the UT signature. The RFO-15 inspections showed no indications.

Horizontal Weld Insoections Hl.H2.H4.HS. H6b he horizontal welds were re-inspected at the venical weld intersections of welds V3,V4,V9,V10, V15 and V16. The horizontal weld results show no significant change in crack depth compared to RFO-14 data. The 1

Niag;amhMohawk" horizontal weld inspections demonstrate that the vertical weld structural analysis and the vertical weld repair assumptions remain conservative. The integrity of the urtical weld intersecuon provides redundancy to the vertical welds, and demonstrates redundancy for the tic rod repairs.

ShroudSupport %'eldj]

The RFO 15 required inspection of the H8 weld included the previously identified indications using EVT 1 methods from the annulus. The re-inspection results show no measurable change based on EVT-1 sizing methods.

ShroudSuevort n' eld H9 The H9 weld is the core shroud ' support vessel attachment weld. This weld was inspected using EVT-1

- methods from the anaulus region. The inspections were performed after cleaning procedures were used.

This inspection was performed consistent with the BWRVIP-38 guidelines currently under NRC review.

This inspection achieved was 100% of the weld circumference with no indications identified.

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p Niagara hMohawk' N.\lPI Core Shroud RFO 15 Weld Inspection Summar3 Table Weld / Location Weld Inspection Shroud Exam Results Length Coverage surface Type (in) Nr:e 1 ID/OD VI- ring weld 6 27.5 % 1009 UT No Indicanons 68' weld OD volume V2 - ring weld 6 259 weld 1004 UT No indicanons 248' volume OD V3 31.25 88.6% ODSD UT No V3 weld indicauons.

O' ID right side, length 2.3" max depth 25%T (4.9~ from weld)

ID right side, length 3". max depth 19%T(5.6" from weld >

ID left side, length 1.7" max depth 23%T (4.5" from weldi 0 V4 31.2f 86% UT ODSD UT UT indications typical oflack of fusion 180' -6" evt-1 ID EVT-1 EVT-1 ofID shows no indicauons.

V5 - ring weld 2 30% ODSD UT No Indications 124' V6 - ring weld 2 17.5 % ODSD UT No Indications 304*

V7 18.5 33.51 % ODSD UT No V7 weld Indications 45' 2" axial from H4 located 3.4" right of V7 (17%T) 3.2" axial from H4 located 4.9" right of V7 (25%T)

V8 18.5 34.95 % ODAD UT No Indications 225' V9 90.12 95.48 % ODSD UT main crack left side, main crack length = 77.2", see figure 1.2 23' Average depth = 42%T. max depth 83%T 30 secondary cracks left and right side: typical length = 2",

tvpical depth .2" to .5" VIO 90.12 96 37 % ODSD .UT main crack right side, main crack length = 75" , see figure 3.4 203' Average depth = 49%T, Max depth = 70%T 3 I secondary cracks left and right side: typical length = 2",

typical depth .2" to .5" Vil 63.5 93.86 % OD/ID UT No indications 110" V12 63.5 93.54 % ODSD UT No V12 weld indications 290*.

2.5" axial from H5 located 3.6" right of V12 (15%T)

=3" axial from H5 located 4.7" right of V12 (10%T)

V13 - ring weld 4.5 46.67 % ODSD UT No indications 123' V14 - ring weld 4.5 42.22 % ODSD IJr No indications 303' Vl5 22.13 85.18 % ODSD UT No lndications 33' Vl6 22.13 78.63 % ODSD UT No indications 213'

- HI @ V3 .(+ . 24") 42.6" ODSD- UT No indications Odeg - Planned H1 @ V4 =(+ . 24") 66.9" ODSD UT No Indications 180 deg Planned H2 @ V3 - .(+ . 24") 37.83" ODSD IJr No Indications Odeg Planned H2 @ V4 -(+ . 24") 44.17" ODSD IJr 2 indications top H2 180 deg planned (l84.5 deg - 182.5.17%T). (l91 deg- 189 deg. 27%T)

NiagarahMohawk' l l l r Weld / Location Weld inspection Shroud Exam Results Length Coserage surface T,s pe (in) ID/OD H4 @ V9 =( + - 24" ) 94.49' ODSD UT OD-bot (8.25 to 6 3 deg), length = 3 ", mas depth = 189T) 23 deg planned OD-hot <8.7 to i I 97 deg). length = 5.1", max depth = 22.GT) ,

OD bot (18.23 to 19.5 deg), length = 2", max depth = 299T)

ID-bot (27.33 to 25.0 deg ), length = 3.6", max depth = 259Ti l OD-bot (39.80 to 36.21 deg) length =5.6", max depth =339T) ,

H4 W V10 -( + - 24") 95.55" ODSD UT 203 deg planned OD-top (196.25 to 199.34 deg). length = 4.8". max depth =NM !

OD-bot (l89.98 to 203.17 deg), length =10", max depth =40GT) 1D-bot (197.92 to 203.69 deg), length =9", max depth =33%T)

OD-bot (207.32 to 208.73), length =2.2", max depth =27%T)

OD-bot (212.32 to 213.22). length =1.4". max depth =19%T) l H5 @ V9 -(+.24") 94.27" ODSD UT ID-top (14.55 to 16.09 deg), length = 2.4", max depth =5%

23 deg planned ID-top (11.4 to 12.6 deg), length =1.9", max depth =17%

OD-top (9.55 to 10.96 deg), length =2.2", max depth =16%T OD-top (21.24 to 23.03 deg), length =2.8", max depth =20%T OD top (16.62 to 19.44 deg), length =4.4", max depth =21%T OD-bot (15.53 to 17.97 deg), length =3.8". max depth =25%T H5 @ VIO =(+ - 24") 88.53" ODSD UT OD-top (186.9 to 188.57 deg). length =2.6", max depth =169T 203 deg planned H6b @ V15 -(+.24") 96" ODSD UT No Wications 33 deg planned H6b @ V16 -(+ . 24") 94.2" ODSD UT No Indications 213 deg planned H8 RFO-13 & 5deg OD EVT-1 No change in recorded length based on EVT-1 sizing methods RFO-14 80 deg

, Identified 85 deg ,.

Indications 132 deg )

270 deg 352 deg H9 100 % 100 % Top EVT-1 No indications Shroud Support side weld Note 1: UT coverage based on coverage by 3 UT transducers,45deg s. 60deg L,80deg L NM: Depth below the measurement threshold T: 1.5" Shroud thickness 1

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i Crack depth parameters for M x. depth =1.21" the main crack running B2O Avg. depth =0.79" along the left side of V9.

t Values are listed for each scan that detected the main crack. The subscan designation appears to the Max. depth =1.24" far left.

B3O Avg. depth =0.96" i B4a ""* d*Pth =' o'" d Avg. depth =0.74" h

Max. depth =0.82" B5O Avg. depth =0.56" i l Max. depth =0.59" l B6O Avg. depth =0.43" l I

f, Max. depth =0.65" B7a Avo. depth =0.4s-B8a Figure 1

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• INDICATIONS 19 & 23 . = 19 i

ARE PART Or THE l

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- i V9 H5 23' V9 BASE METAL INDICATIONS Figure 2

V10 H4 Max. depth =0.41 Avg. depth =0.36 "

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4' Avg. depth =0.85 A2a ,

Values are listed for each l

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far right. Avg. depth =0.91" a f l 1

1 Max. depth =0.99" A4a Avg. depth =0.81" 1

I l Max. depth =0.98" Avg. depth =0.76" A5a 1 i

-l Max. depth =1.03" Avg. depth =0.75" A6a l

i Max. depth =0.90" Avg. depth =0.57 A7a i

4l' Max. depth =0 49 "

A8a Avg. depth =0.35-mm - .

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NiagarahMohawk' Attachment 1 Framatome Technologies L'T Description (Total 12 pages) l 1

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l

Nine Mile Unit-1 (hr0-15) Core Shroua Ultrasonic and Visual Examination Report i

l

1.0 INTRODUCTION

This report addresses the Nine Mile Unit-1, core shroud ultrasonic and visual examinations performed by Framatome Technologies (FTI) during the penod April 24 to May 14,1999. The work scope included ultrasonic examination of the sixteen vertical welds (V1 - V16) and a twenty-four inch segment on eitner side of the vertical / horizontal weld intersection for welds V3, V4, V9, V10, V15, and V16. The intersecting welds are identified as H1@V3, H1@V2 for example. The l visual examinations included the ID of V4 and the OD boat sample excavation l region for weld V10.

Ultrasonic Examinations I l Vertical Welds 24" Segment on either side of the l Horizontal / Vertical Weld Intersection l V1 i V9 l

_H1@V3 V2 V10 i H1@V4 l l V3 i V11 l H2@V3 i V4 l V12 i H2@V4 l l V5 V13 H4@V9 l V6 V14 H4@V10 l V7 V15 H5@V9 l V8 I V16 I H5@V10 l l l H6b@V15 l l I I H6b@V16 l

1 Visual Examinations Wold Description of examination l

V4 ID surface of V4 V10 OD boat sample excavation '

This report contains a description of the examination technique, examination results and coverage, calibration records, certifications, and reference documentation. The report has been prepared for Niagara Mohawk's Nine Mile Point Unit-1under FTl contract number 1600525. The information contained herein is fumished to Nine Mile solely for their use in conjunction with the work scope performed under this contract.

I

Nine Mile Unit-1 (hc 0-15) Core Snrouc Ultrasonic and Visual Examination Report

2.0 DESCRIPTION

of TECHNIQUES Ultrasonic examinations:

The Nine Mile Unit-1 core shroud welds were ultrasonically examined using three Framatome designed tools, namely the Flange tool, the Upper tool. and the Lower tool. The flange tool was designed for scanning from the top surface of the flange (top of the upper ring). The upper tool was designed for scanning from the OD sunace of the shroud between the H1 and H2 elevations, and the

, , lower tool was designed for scanning also from the OD surface but between the H1 and H6B elevations.

The flange tool was used to examine the horizontal / vertical weld intersections H1@V3 and H1@V4 using a straight beam technique, in addition, ring segment welds V1 and V2 were also examined using angle beam techniques. In both l cases, scanning was performed from the top surface of the flange. The upper tool was used to examine V4 and also the underside of H1@V4. The lower tool was used to examined the remainder of the welds. Each weld and the tool (s) used to perform the examination are sumrnarized in the table below.

Wold l Flange j Upper i Lower l Wald i Flange l Upper I Lower ,

V1 X l l j H1@V3 X j l X V2 i X l l H1@V4 X l X l X V3 l 1 X l H2@V3 I I X V4 X l j l H2@V4 l l 1 X V5 I I X 1 i H4@V9 l l l X-V6 l l X l H4@V10 l l I X V7 l X l H5@V9 l l X V8 i X H5@V10 l l X V9 X ,

H6b@V15 l X V10 X l H6b@V16 I X V11 X V12 X l V13 X

! V14 X i V15 i X i V16 l X l 2

L'

s --

Nine Mile Unit-1 (RFO-1. ; ore Snrouc Ultrasonic and Visual Examination Report Each of the three tools were positioned circumrerentially arcunc :ns L cc... s periphery using the pusher-cart assembly. The pusher-cart was attacneo to the core cover via a ring assernbly which permitted circumferential movement of the cart. After the core cover was inserted into the shroud, the pusher-cart was positioned circumferentially and the appropriate tool inserted into the cart for scanning.

Flange Tool:

The flange tool was designed for scanning only from the top surface of the i upper ring (the flange). Movement in the circumferential direction (y) was i accomplished by moving the pusher-cart. Movement in the index I direction (x) was performed hydraulically in the radial direction toward the vessel centerline. Thus, the scan pattem resembled a pie like section as shown below. The transducer package used with the flange tool consisted of four side by side separate transducers, also illustrated below.

1 y

m 1

/

m Top view of a circumferential scan with a radialindex.

Directions are positive as shown l

0* 60' 45' 80' 1

p K P T Side view of flange tool transducer package.

Positive scan direction coincides with 45* and 60' beam directions as shown c

l 0.-,.....- n Nine Mile Unit-1 (RFO t., Core Shrouc Ultrasonic and Visual Examination Report l

Upper Tool:

The upper tool was designed for scanning from the OD surface of the core shroud in either the circumferential or vertical directions. The pusher-cart again was used for circumferential tool movements while a lead screw provided the vertical up and down tool mcvements. The circumferential (y) movement was defined as clockwise positive (looking down onto the vessel) and the vertical (x) movement as positive upward. The synergistic positions of the alpha (a) and beta (p) joints permitted the transducers to be positioned either horizontally or vertically pointing left/ l right or up/ down respectively.- Two transducer packages were used with l the upper tool, one side by side package and one tandem package. Each package consisted of three transducers with nominal angles of 45*,60*,

and 80*. The tool coordinates and transducer package arrangements are illustrated below.

l l

X 1

+

Y l

j 80* 45' 60*

r ,

) C 4-  % 4 Q

80*h 45' y 60* y lliustration of the Upper tool coordinates and transducer packages at the a = right and p= out positions 4

i

2 0 fie.cn l Nine Mile Unit-1 (RFO-1w Core Shroua Ultrasonic and Visual Examination Report

)

I 1

A a = up o

a = left a = right i

i i

l a = down Y

60' a 45' 4 80' a 80' 45" 60' l 4-- --* 5 l 60' 45' 80* 45' 60*

80* I i

+ 4--  % v v v S = in p = Out lilustration of the Upper Tool's a and p positions .

5 k.

1

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'Nine Mile Unit-1 (RFO-15, Jore Shrouc Ultrasonic and Visual Examination Report Lower Tool:

The lower tool was designed for scanning from the shrouJ OD surface in

~ both the horizontal and vertical directions. The pusner cart again was used with this tool, however only for positioning circumferentially, Unlike the flange tool and upper tool, the lower tool did not rely upon the pusher-cart for circumferential movements. Instead, the lower tool had its own local"x/y" movements for performing horizontal and vertical scanning.

After the tool was inserted into the pusher cart and positioned circum _ferentially, three primary axes of motion manipulated each transducer package, namely "a", "x", and "y" . Alpha permitted rotational movement of the transducer packages (about a vertical line) for positioning at either the +90 or -90 position. (The +90/-90 line is equivalent to a horizontal axis). After positioning alpha, the vertical welds were scanned using a raster type pattern with "x" as the horizontal scanning axis and "y" the index axis. Likewise, for scanning horizontal welds, "y" was the scan axis and "x" the index axis. For both vertical and horizontal scanning, the *x =0" position was defined as the centerline of the tool with positive "x" movement away from the mast in both the +90 and -90 alpha directions. The tool's ay =0" position was defined to be the bottomed-out position of the "y" drive with positive *y" movement in the upward direction.

- Two transducer packages were used with the lower tool. Each package contained a 45',60', and 80* transducer. One package was configured in a side by side arrangement and the other in a tandem arrangement.

Each package was mounted on a pneumatically actuated, independent arm, referred to as "p1" and "p2", for keeping the transducers in contact ,

with the shroud's surface. Generally, scanning was limited to one side of l the weld, per scan, due to the raised weld crown preventing movement  ;

across the weld. Thus, scans were first performed from the right-hand  ;

side by starting from the top and working downward. Subsequently, the i left-hand side was examined in a similar manner. When scanning from i one side of the weid, the transducer package was first butted against the weld crown then pulled back the required scan distance before indexing.  !

Shown below is a sketch of the lower tool depicting the transducer arrangements and the tool's local "x/y" coordinate system.

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Nine Mile Unit-1 (RFO-15) Core Shroud Ultrasonic and Visual Examination Report )

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l I

O 60' 45* 80 \ 80 I I

m O .} E 45'E 60*dl 60'a 45' 80' u V 80* 45' 60' a = -90 f a = +90 l 1

l A16" l y+ '

l l

l t

x+ x+

l c  ;

24" 0" 24" lllustration of the Lower Tool (top) transducer arrangements at the +90 and -90 alpha positions. (bottom)"x/y" directions and scan distances 7

, '/15 Nine Mile Unit-1 (RFO-15) Ccre Shroua Ultrasonic and Visual Examination Report Data Tracking:

All ultrasonic data was acquired using the Framatome designed ACCUSONEX software interfaced with either the Framatome VXI system or RD-Tech Tomo system. The data was subsequently analyzed using the ACCUSONEX analysis system in accordance with Framatome in-service inspection procedure 54-1S1-107-3 " Remote Ultrasonic Examination of Boiling Water Reactor (BWR) Core Shroud Assembly Weld Seams". This data was subsequently stored on optical media to serve as a permanent record of the examination results. As a means of data tracking, each scan was referenced by a " Scan Identification Number" (SIN) and a "Subscan" number. The SIN identifies the particular weld examined, and the subscan (an alphanumeric identifier) denotes the part of the weld examined.

1

! Vertical welds:

The illustration below depicts the subscan arrangement for examining vertical welds. Here only the A1 and B1 scans will be referenced, but in l reality the scans can range from A1 to A8 and B1 to B8. The "A" scans l indicate the 45'/ 60' beam angles are pointing CW or to the left, and the i "B" scans indicate the angles are pointing CCW or to the right. In addition, the "A1/ A1a" scans or "B1/ B1a" scans indicate the 45'/60' angles are pointing toward the weld. Likewise, the "A1b" or " Bib" scans indicate the 45'/60* angles are pointing away from the weld. Hence, the presence of the lower case "b" indicates the primary angles 45*/ 60* are pointing away from the weld with the 80' pointing toward the weld.

l l 60' 45' 80* 60* 45' 80' 4- 4-- --> 4- 4-- ->

Alb A1 or A1a 80* 45' 60' 80' 45" 60*

4-- -+ --+ 4- ->  %

B1 or B1a B1b i l

l I Vertical weld subscan identification S

e.

1 Nine Mile Unit-1 (RFC-15) Core Shroud Ultrasonic and Visual Examination Report i

)

Horizontal welds:

The horizontal welds requirea four subscans, "A", "B" "C", ano v as '

shewn below, if a subscan required multiple scans to cove-its region, then each scan was appended numerically (A1., A2, .). If a sinole scan i overlapped subscan regions, then the scan was identified by botn subscans, (B-C) for example. Shown below is the subscan arrangement '

used for examination of the horizontal welds.

80' ll 45'l I 80" yll 45"I l V V 60'V V' 60'V D A l

C B 60*y 459h 80 f 60*f 45th 80*y I

l Horizontal weld subscan identification 9

Nine Mile Unit-1 (RFu-15) Core Shroud Ultrasonic and Visual Examination Report When scanning honzontal weld H1 or vertical welds V1 cr.o a, ese scans were performed from the top surface of the flange (top or the upper ring) and denoted as positive and negative as shown below. Notice the subscans performed below the weld are still referenced as "B" and "C Positive Negative

,' =

D' 60' 45' 80' C B 45th 60*f459h 80 f 60'f 80'f l

Subscans for the H1 horizontal weld And vertical welds V1 and V2

[ in

E i r

Nine Mile Unit-1 (RFO-15, Jore Shrouo

{

Ultrasonic and Visual Examination Report

)

Visual Examinations: '

Visual examinations were performed on the ID surface of V4 snd the OD boat sample region adjacent to V10. The V4 examination was performed to verify the previously called (RFO-14) ID connected indication was indeed lack of fusion as

, reported by Framatome during RFO-15. The V10 boat sample region was visually examined for comparison with the RFO-14 results. Both examinations were performed using a camera tethered by a rope for manipulation. Results of a

each examination were recorded to a VCR cassette for future reference.

The visual examination calibration and data sneets are located in the examination results section (3) under welds V4 and V10. Personnel certifications are included in the certification section (6).

1 i

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.s.,

Nine Mile Unit-1 (RFO-15) Core Shrouc Ultrasonic and Visual Examination Report 3.0 EXAMINATION RESULTS / COVERAGE Ultrasonic Examinations:

This section contains the ultrasonic examination results for the Nine Mile Unit-1 inspection. Each of the welds examined contain the following information-Examination summary sheet Examination coverage table and coverage map Illustration depicting how the weld was scanned Julian date to calendar date conversion table Examination worksheet l On the following page is a legend depicting the horizontal weld coverage. One l pcJr.t te note is that on each horizontal coverage map is shown 48 inches of horizontal veld length. This is used to illustrate the 24 inches of coverage

, needed on *ither side of the weld.

l The length of the vertica! weld coverage spans fcom the upper to the iower l

attaching horizontal welds.

i l Visual Examinations:

The ID surface of V4 and the OD boat sample region adjacent to V10 were visually examined. V4 was examined to verify the previously called ID connected indication during RFO-14 was indeed lack of fusion as reported by Framatome during RFO-15. Thus this visual examination confirmed there was no cracking on the lD surface of V4. The V10 boat sample region was examined for comparison with the RFO-14 results.

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vertical weld centerline horizontal weld centerline A1a .

15.94 1.21 to 9.01 A2a 11.82 7.01 to 23.01 i l A3a & A3a.1 11.0 & 17.0 17.01 to 33.01 & 17.01 to 33.01 i i A4a & A4a.1 1 11.9 & 16.5 29.01 to 45.01 & 29.01 to 45.01 l J A5a & A5a.1 i 11.8 & 17.3 41.01 to 57.01 & 41.01 to 57.01 i l A6a , 14.84 53.01 to 69.01  :

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A8a i 11.93 77.01 to 88.41 a

Scan Scan distance back from , index Distance downward from vertical weld centerline horizontal weld centerline i B1b 11.25 5.97 to 12.37 I B2b 10.65 11.37 to 27.37 .

I B3b 9.75 1 20.37 to 36.37 l l B4b i 9.75 4 32.37 to 48.37  !

I B5b I 9.75 44.37 to 60.37 1 ,

I B6b l 9.75 i 56.37 to 72.37 I j

B7b i 9.75 i 68.37 to 84.37 i 88b J 9.75 I 80.37 to 88.37 l Scan l Scan distance back from j Index Distance downward froni )

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B1a i .18.9 i 0.97 to 10.37 I j i

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l 10.37 to 26.37 l B3a  : 10.9 < 22.37 to 38.37 i B4a i 10.5 i 34.37 to 50.37 B5a i 11.5 i 44.37 to 60.37 B6a 11.7 l 56.37 to 72.37 B7a 11.95 1 68.37 to 84.37 B8a 11.7 1 78.37 to 88.17 Scan Scan distance back from Index Distance downward from vertical weld centerline horizontal weld centerline Alb i 8.05 1.01 to 9.01 A2b  ! 9.25 7.01 to 23.01

_ A3b l 8.95 17.01 to 33.01

A4b i 9.25 l 29.01 to 45.01 A5b i 9.25 1 41.01 to 57.01

( A6b i 9.55 i 53.01 to 69.01 A7b i 9.05 I 65.01 to 81.01 A8b i 9.25 -

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! I vertical weld centerline horizontal weld centerline A1a i 15.6 1.41 to 9.01  ! ,

A2a 16.2 7.01 to 23.01  !

A3a , 15.75 19.01 to 33.01  !

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44.37 to 60.37 1 B6b i 9.25 1 56.37 to 72.37 B7b i 9.05 l 68.37 to 84.37 B8b i 9.55 I 80.37 to 88.17 Scan Scan distance back from j index Distance downward from vertical weld centerline -

'orizontal weld centerline B1a i 11.9  ! 1.17 to 12.37 B2a I 9.92 1 10.37 to 26.37 I B3a  ; 10.17 22.37 to 36.37 i B4a i 10.25 i 32.37 to 48.37 85a i 10.13 1 44.37 to 60.37 I B6a i 9.8 I 56.37 to 72.37 )

B7a I 9.8 l 68.37 to 84.37 j B8a I 10.0 l 80.37 to 87.97  ;

i Scan Scan distance back from Index Distance downward from vertical weld centerline horizontal weld centerline A1b 8.47 l 1.01 to 9.01  ;

A2b & A2b.1 8.95 & 14.45 i 7.01 to 23.01 & 7.01 to 23.01 j A3b & A3b.1 8.75 & 13.55 l 19.01 to 33.01 & 21.01 to 35.01 j A4b & A4b.1 8.65 & 12.05 1 29.01 to 45.01 & 31.01 to 47.01 A5b & A5b.1 1 8.25 & 13.55 l 41.01 to 57.01 & 41.01 to 57.01 f A6b & A6b.1 i 7.45 & 12.85 i 53.01 to 69.01 & 57.01 to 73.01 I A7b -l 7.65 i 65.01 to 81.01

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vertical weld centerline horizontal weld centerline A1 9.67 1.89 to 10.89 A2 9.56 6.89 to 22.89 A3 1 9.2 18.89 to 34.89 A4  ! 8.6 30.89 to 46.89 i A5 t 9.5 42.89 to 58.89 i A6 i 8.7 54.89 to 62.69 Scan i Scan distance back from index Distance downward from l 1 vertical weld centerline '

horizontal weld centerline l B1 1 9.7 3.85 to 14.25 i 82 I 8.35 10.25 to 26.25 i 83 8.6 22.50 to 38.25 l B4 , 8.9 34.25 to 50.25 B5 I 9.6 46.25 to 62.25 B6  ! 10.1 58.25 to 62.25 i

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Niagara ([zS Mohawk' Richard 8. Abbon Phone 31S 3491812 Vice President Fan 315 349 4417 NuclwEngineenng

• July 12, 1999 NMPIL 1451 U. S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555 RE: Nine Mile Point Unit 1 Docket No. 50-220 DPR-63

Subject:

Generic Letter 94-03, 'Intergmnular Stress Cormsion Cmcking of Core Shmuds in Boiling Water Reactors, ' Final Root Cause Evaluation of a Cap Screw Failurefor a Stabilizer Assembly (Tie Rod)for Nine Afile Point Unit 1 Gentlemen: .

By letter dated May 21,1999, Niagara Mohawk Power Corporation (NMPC) requested Staff i approval of a proposed modification to each of the four tie rods pursuant to l 10CFR50.55a(a)(3)(i). As stated in that letter, the modification replaces the design function of the failed cap screw and other cap screws that have the potential for future failure in the upper spring. NMPC also stated that the root cause evaluation was preliminary and that it would i provide a final root cause evaluation within 30 days following restart of the unit. The purpose of this letter is to provide the final root cause evaluation.

The cause, as stated in our letter dated May 21,1999, was confirmed to be intergranular stress corrosion cracking in the alloy X-750 cap screw material due to large sustained stresses.

Specifically, these stresses were from differential thermal expansion of dissimilar materials fastened by the cap screw.

As also stated in that letter, a potential contributing cause was the sustained stresses that were attributed to the torquing of the cap screw associated with the original assembly of the tie rods.

NMPC has determined that the torquing process was properly controlled during initial l assembly of the tie rods to provide reasonable assurance that the cap screws were not over

) torqued. Therefore, this potential contributing cause has been eliminated.

Furthermore, as stated in the same letter, a second potential contributing cause was the stresses associated with friction between the reactor pressure vessel wall and the upper spring contact points. Based on a re-examination of the wear marks on the reactor pressure vessel wall and destructive testing of two cap screws from the 166 degree tie rod upper spring upper contact, NMPC has eliminated friction as a potential contributing cause, l

l L.

Enclosure 2