ML20137Z401
| ML20137Z401 | |
| Person / Time | |
|---|---|
| Site: | Nine Mile Point  |
| Issue date: | 04/18/1997 |
| From: | Hood D NRC (Affiliation Not Assigned) |
| To: | NRC (Affiliation Not Assigned) |
| References | |
| TAC-M98170, NUDOCS 9704240144 | |
| Download: ML20137Z401 (10) | |
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A April 18, 1997 i
LICENSEE:
Niagara Mohawk Power Corporation i
FACILITY: Nine Mile Point Nuclear Station Unit No. 1 i
SUBJECT:
SUMMARY
OF TELEPHONE CONVERSATIONS OF APRIL 10 and 11, 1997, ON CORE l
SHROUD CRACKS and REPAIR (TAC N0..M98170) 1 On April 10 and lip 1997, the NRC staff participated in telephone conference i
calls with Niagara Mohawk Power Corporation (NMPC and licensee) regarding the j
licensee's letter of April 8, 1997, on core ~ shroud welds and tie rod assemblies (also called core shroud-stabilizers) for Nine Mile Point Nuclear Station, Unit 1.
Participants are listed in Enclosure 1.
i The purpose of the call was to identify the'NRC ' staff's requests for i
additional information (see Enclosures 2 and 3) and determine if the use of proprietary information is necessary for an adequate response.
Otherwise, the NRC staff intends to discuss these requests in a public session.
Enclosures 2 i
and 3 were faxed to the licensee by the NRC prior to the telephone calls, f
The licensee confirmed that the questions do not reveal proprietary information and can be asked in public sessions. The NRC staff also requested that the licensee provide written responses to these enclosures folle s ' the 4
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pending public meeting.
i Sincerely,
/S/
l Darl S. Hood, Senior Project Manager Project Directorate I-1 Division of Reactor Projects - I/II i
Office of Nuclear Reactor Regulation j
Docket No. 50-220 j
Enclosures:
1.
Participants i
2.
Requests regarding shroud cracks
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3.
Requests regarding lower wedge cc:
See next page l
DOCUMENT NAME: - G:\\NMPl\\NMP198170.RAI To receive a copy of this document, indicate in the box:
"C" - Copy without t
-attachment / enclosure "E" - Copy with attachment / enclosure "N" - No copy
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DATE 04/g/97 04/lQ197 04/f9/97 04/ /97 04/ /97 Offici al Record Copy 7
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April 18, 1997 LICENSEE: Niagara Mohawk Power Corporation i
FACILITY: Nine Mile Point Nuclear Station Unit No. 1
SUBJECT:
SUMMARY
OF TELEPHONE CONVERSATIONS OF APRIL 10 and 11, 1997, ON CORE t
SHROUD CRACKS and REPAIR (TAC NO. M98170)
.l I
On April 10 and 11, 1997, the NRC staff participated in telephone conference calls with Niagara Mohawk Power Corporation licensee's letter of April 8, 1997, on core s(NMPC and licensee) regarding the hroud welds and tie rod assemblies (also called core shroud stabilizers) for Nine Mile Point Nuclear Station, Unit 1.
Participants are listed in Enclosure 1.
The purpose of the call was to identify the NRC staff's requests for additional information (see Enclosures 2 and 3) and determine if the use of proprietary information is necessary for an adequate response. Otherwise, the NRC staff intends to discuss these requests in a public session.
Enclosures 2 and 3 were faxed to the licensee by the NRC prior to the telephone calls.
The licensee confirmed that the questions do not reveal proprietary information and can be asked in public sessions. The NRC staff also requested that the licensee provide written responses to these enclosures following the pending public meeting.
Sincerely, Darl S. Hood, Senior Project Manager Project Directorate I-l Division of Reactor Projects - I/II Office of Nuclear Reactor Regulation Docket No. 50-220
Enclosures:
1.
Participants 2.
Requests regarding shroud cracks 3.
Requests regarding lower wedge cc: See next page
a Niagara Mohawk Power Corporation Nine Mile Point Nuclear Station Unit No. I cc:
Mr. B. Ralph Sylvia Resident Inspector Executive Vice President U.S. Nuclear Regulatory Commission Generation Business Group P.O. Box 126 and Chief Nuclear Officer Lycoming, NY 13093 Niagara Mohawk Power Corporation Nuclear Learning Center Charles Donaldson, Esquire 450 Lake Road Assistant Attorney General Oswego, NY 13126 New York Department of Law 120 Broadway Mr. Richard B. Abbott New York, NY 10271 Vice President and General Manager -
Nuclear Mr. Paul D. Eddy Niagara Mohawk Power Corporation State of New York Nine Mile Point Nuclear Station Department of Public Service i
P.O. Box 63 Power Division, System Operations Lycoming, NY 13093 3 Empire State Plaza Albany, NY 12223 Mr. Martin J. McCormick, Jr.
Vice President Mr. F. William Valentino, President Nuclear Safety Assessment New York State Energy, Research, and Support and Development Authority Niagara Mohawk Power Corporation Corporate Plaza West Nine Mile Point Nuclear Station 286 Washington Avenue Extension P.O. Box 63 Albany, NY 12203-6399 Lycoming, NY 13093 Mark J. Wetterhahn, Esquire Mr. Kim A. Dahlberg Winston & Strawn General Manager - Projects 1400 L Street, NW Niagara Mohawk Power Corporation Washington, DC 20005-3502 Nine Mile Point Nuclear Station P.O. Box 63 Supervisor Lycoming, NY 13093 Town of Scriba Route 8. Box 382 Mr. Norman L. Rademacher Oswego, NY 13126 Plant Manager, Unit 1 Nine Mile Point Nuclear Station Gary D. Wilson, Esquire P.O. Box 63 Niagara Mohawk Power Corporation Lycoming, NY 13093 300 Erie Boulevard West Syracuse, NY 13202 Ms. Denise J. Wolniak Manager Licensing Niagara Moi..wk Power Corporation Nine Mile Point Nuclear Station P.O. Box 63 Lycoming, NY 13093 Regional Administrator, Region I U.S. Nuclear Regulatory Commission 475 Allendale Road King of Prussia, PA 19406
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PARTICIPANTS i
April 10, 1997
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NMPC' NRC M. McCormick R. Wessman S. Leonard K. Manoly G. Inch R. Hermann i
J. Rodabaugh (GE)
J. Rajan R. Corieri D. Hood D. Baker
- E. Gray (RI)
T. Gleason (GE)
T. Moslak (RI) et al.
PARTICIPANTS l
April 11, 1997 NMPC NRC i
M. McCormick R. Hermann D. Baker M. Banic L. Carrell S. Bajwa i
T. Oldfield E. Gray (RI) 1 G. Inch T. Moslak (RI) l S. Leonard et al.
t R. Corieri I
S. Ranganath (GE)
G. Deaver (GE) et al.
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REQUEST FOR ADDITIONAL INFORMATION REGARDING 1-CORE SHROUD WELD CRACKING AND TIE ROD REPAIR NINE MILE POINT NUCLEAR STATION, UNIT ONE The following enclosures refer to Niagara Mohawk Power Corporation's letter (NNP1L 1200) to the NRC, dated April 8, 1997:
j Enclosures 1 and 8:
" Assessment of the Vertical Weld Crackina on the NMP1
)
Shroud" i
1.
Please discuss how the uncertainty factors used in crack growth calculations (UT and VT measurements of flaw length and depth) for the i
vertical welds were determined and also address how the BWRVIP f
guidelines on inspection uncertaintias were met.
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2.
It appears that the indication maps for vertical welds V10 and V9 are j
not consistent with the corresponding flaw data plots.
In the flaw data plots, the uncracked areas are much larger than that shown on the corresponding indication maps.
Please explain the discrepancy and discuss its impact on the determination of reinspection interval.
1 3.
In Figure 5-6, only the crack growth in the wall thickness (depth) l direction was considered.
Please provide justification for not considering the crack growth in the axial direction (parallel to l
i vertical weld) and the potential crack initiation and growth in the uncracked areas.
If we assume that cracks will initiate and grow in the uncracked areas, what would be the acceptable reinspection interval for i
vertical weld V97 The bounding case would be assuming the uncracked j
areas to be cracked with zero depth.
j 4.
Limited inspection was performed on horizontei welds H8 (30% of j
circumference) and H9 (26 inches in circumference in one area). Some minor cracking was found in weld H8.
Please provide your basis that the extent of the weld inspection was adequate and that weld will retain its i
integrity at the end of the proposed period of operation assuring the l
core will remain adequately supported.
1 5.
In Appendix C of Inspection Summary, the description of cracking at i
vertical weld V9 is not consistent for the UT and VT examinations. The l
results of VT examinations reported that the majority of cracking is at the left hand side of heat affected zone (HAZ) on the out side diameter l
(00) surface while the results of UT examinations indicate it was on the right hand side HAZ.
Please clarify the inconsistency in the cracking description.
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3 6.
Leakage Estimate It appears that potential leakage from the uninspected areas assumed to be throughwall cracked were not included in the leakage estimate.
If this additional leakage was included, would it affect the conclusion presented.
- Please provide bases for assuming a crack opening of 0.001 inch in the I
circumferential welds.
2
- In vertical welds, the leakage flow area was assumed to be 3 in.
Please provide the bases for the assumed flow area.
- Was NRC approved methodology used in the leakage estimate?
Please ide.tify any assumptions made in leakage estimate that were not 7
previously approved by NRC.
The LEFM analysis to determine the minimum required ligament for a vertical weld with the circumferential welds. assumed failed appears to 7.
have been done using the loads from the pressure differential across the shroud and the loads from faulted conditions such as LOCA and MSLB.
Much discussion is pre %ed regarding the stress and related stress intensities from fit-up and welding.
Explain why these were not included in the analysis and address their significance.
What is the basis far the stress intensity of 20 that is used?
8.
" Shroud Reoair Anomalies--Nine Mile Point Unit 1. RF014" Enclosures 2 and 8:
When will the metallography be available for the latch that is suspected In our view the conclusion that the failure is based 9.
of failing by SCC 7 on SCC is speculative without the supporting information.
Has an evaluation been performed that demonstrates that a sufficient ligament exists in the as found vertical welds to assure that the rings 10.
postulated in designing the shrouds would remain as rings rather than ring segments?
Has an analysis of the cracked shroud been performed assuming the tie rods were not functional (not there for analysis purposes) considering 11.
Based on such an the as found condition of the shroud weld cracking?have maintained the required A analysis would the shroudfactors and therefore a en considered operable for without consideration of the condition of the tie rods?
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"Nine Mile Point Unit 1 Core Shroud Crackino Evaluation" 12.
Much discussion is provided regarding the fabrication factors that could have effected the residual stress distribution in the shroud that resulted in cracking taking place at the OD rather than the ID.
Analytic work was reported to have been done to demonstrate this.
4 Please provide the analysis.
13.
Information in the submittal states that hardness of the stainless shroud was limited to Rockwell "B" value of 90. Are records available of the areas that were ground? Is a reference or any other information available relating the hardness value to martensite content or to the possible reduction in the IGSCC cracking threshold?
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REQUEST FOR ADDITIONAL INFORMATION REGARDING THE REDESIGN
'0F THE LOWER WEDGE SUPPORT OF THE CORE SHROUD TIE ROD ASSEMBLY NINE MILE POINT NUCLEAR STATION, UNIT ONE The following requests are based upon Niagara Mohawk Power Corporation's i
letter (NMPil 1200) to the NRC, dated April 8, 1997:
I i
1.
The deflection of the C-spring in the tie rod assembly is likely to stress the latch attached to the lower wedge if it is not sliding 4
i freely. What is the estimated contribution from the C-spring deflection j
towards the latch stresses during heat up?
i 2.
Provide'the structural details of the nut' locking device on the upper part of the tie rod assembly.
j 3.
Provide a clear photograph of the failure surface of the failed latch on the lower wedge to assess the failure mechanism.
(The Xeroxed copy provided as figure 2 (page 26) is not sufficiently legible).
4.
You state that metallurgical examinations related to the failed latch assembly on the lower wedge will be performed to confirm that stress corrosion is the failure mechanism. When will these examinations be completed?
5.
Explain why two latches (alloy X 750) are used in the upper support assembly while only one is used in lower wedges.
Provide details as to how theyiare attached to the lower springs and the lower wedges.
6.
The maximum tie rod looseness that could have been caused by the oversized holes in the shroud support cones has been calculated.
Is this sufficient to cause loss of both initial and thermal preload?
7.
Identify any plant operating, transient, or test conditions during which the toggle bolts on the lower tie rod anchors could slide down the oversized holes in the shroud support cones and remain there due to high frictional forces likely to exist at the hole surfaces.
8.
It is not clear how the angle on the lower wedge is designed to increase the possibility of sliding in both directions.
9.
Hive you considered the possibility of lowering the radial contact force at the lower wedges to assure sliding at the vessel / lower wedge and lower wedge / lower spring interfaces? What are the potential problems if the radial contact force is decreased at the lower wedge?
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What is the difference between condition 2 (All horizontal welds have through-wall cracks) and condition 3 (All horizontal weld cracks are 360 degrees, with no ligament remaining) identified in Section 4.0,
" Consequences to Previous Plant Operation," Shroud Repair Anomalies, Nine Mile Point Unit 1, RF014, GE Report GENE B13-01733-407 11.
Provide an evaluation of the safe operation of the shroud and the tie rod assembly with the existing ligaments in the horizontal and vertical welds.
12.
Justify the statement in the shroud repair anomalies report that the design of the latch will accommodate all potential vertical displacements without exceeding the ASME Code limits. Justify any apparent discrepancy between this and the new latch design report.
13.
The maximum possible differential vertical displacement of the lower wedges and the probable wedge movement have been determined. Since the potential exists that the wedges may not always slide at the spring interfaces due to unanticipated forces on the wedge during various operational transients, the design of the latches should be based upon the maximum estimated vertical displacement.
14.
Provide justification of structural integrity of the latches after a loss of feedwater heating event.
15.
During certain plant operating and test conditions (e.g., during a hydrotest), the radial contact force on the wedges is likely to be minimal.
Under such conditions, the wedges could potentially slide circumferentially along the vessel surface and remain stuck in that position. This could impose an additional torsional moment on the latch during subsequent plant operation, and appears not to have been factored into the present design.
Please discuss this potential and any considerations given to it in the design.
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. DISTRIBUTION:
L Hard Copy Docket File PUBLIC PDI-1 Reading l
OGC ACRS E-Mail S. Collins /F. Miraglia R. Wessman G. Carpenter R. Zimmerman R. Hermann B. Sheron S. Varga.
J. Strosnider K. Manaly J. Zwolinski K. Wickman C. Cowgi1T, RI S. Bajwa R. Frahm, Sr.
L. Doerflein, RI l
D. Hood T. Green-W. De.o, ED0 l
S. Little J. Rajan G. Holahan l
D. Ross M. Bani W. Koo G. Lainas W. Gleaves E. Gray, RI 1
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