ML20245D954
| ML20245D954 | |
| Person / Time | |
|---|---|
| Site: | Beaver Valley, Davis Besse, Oconee, Mcguire, Catawba, Summer, Crystal River, McGuire, 05000000, Trojan, Crane |
| Issue date: | 09/06/1988 |
| From: | Taylor J BABCOCK & WILCOX CO. |
| To: | Murley T Office of Nuclear Reactor Regulation |
| References | |
| REF-PT21-88-095-000 JHT-88-171, PT21-88-095-000, NUDOCS 8810070359 | |
| Download: ML20245D954 (6) | |
Text
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Baticock & Wilcox -
nuci, ro.., owon O'
a McDermott cogny JHT/88-171 3315 Old Forest Road September 6, 1988 P.O. Box 10935 '
Lynchburg, VA 24506 0935 (804) 385-2000 a
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Murley, Director j
Dr. Thomas E Office of Nug$eansReactor Regulation
(
U.S. Nuclear Regulatory Commission
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Washington, D.C.
20555
Subject:
Information on B&W Plugs Fabricated from Heat W592-1
Dear Dr. Murley:
The purpose of this letter is to advise you of a condition involving Inconel 600 material used to fabricate a number of steam generator tube plugs and found to possess a
microstructure susceptible to stress corrosion cracking.
This letter describes the condition, identifies the specific plants involved, summarizes our knowledge about its cause, describes B&W work in progress and provides a schedule fc; submittal of further information.
STATEMENT OF PROBLEM Babcock & Wilcox has found that certain Inconel 600 material used to fabricate steam generator tube plugs has a microstructure which may be susceptible to stress corrosion cracking even though this material was specified in a manner believed to ;. ovide resistance to stress corrosion.
Rolled, ribbed, and welded plugs have been fabricated from this material and used in both Once Through Steam Generators (OTSG) and Recirculating Steam Generators (RSG).
Of 25 B&W-supplied rolled plugs that saw reactor plant service and were subsequently removed and examined, B&W found one plug with potential stress corrosion cracks.
The type of tube plug in which the cracks were found (shown on the attached Figure 1) is rolled a short distance inside the tubesheet.
A short " head" is provided outside the tubesheet for later removal, and the closed or " stub" end is inboard of the roll-expanded area.
The cracks were located in the cuter roll trancition of a plug from Heat W592-1.
They were originally thought to have resulted from either the stress relaxation process or the removal process.
Metallurgical examination indicates that the cracks are intergranular.
Intergranular carbide precipitation has been determined by corrosion testing to be a strong contributor to stress corrosion resistance.
This carbide precipitation is obtained by proper mill annealing and heat treatment based on experimental testing.
Heat W592-1 was found to contain intragranular carbides with
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E. Murley/NRC
'l JHT/88-171 September 6, 1988 j
Page 2 little intergranular carbide precipitation.
Its microstructure was examined after the cracks were found.
The plug had been removed using a
TIG relaxation technique.
Metallurgical examinations of archive plug specimens from all other heats of l
alloy 600 material used by B&W have verified that proper carbide i
distribution, and therefore adequate stress corrosion cracking resistance, exists.
l B&W procured this material with a purchase specification that referenced ASME Code material and required a heat treatment of a minimum of 1800F for five minutes.
Heat W592-1 was purchased and received its final heat treatment by the vendor on November 30, 1984.
The Certificate of Conformance which accompanied heat W592-1 showed that it met all specification chemistry requirements.
Our review indicates that heat treatment was performed in accordance with the B&W specification.
B&W is taking steps to evaluate the cause of the undesirable carbide precipitation and will make appropriate changes to procurement specifications if necessary.
Future material will be metallurgically examined to ensure proper carbide distribution prior to use.
POTENTIAL FAILURE MODES AND SAFETY SIGNIFICANCE There are three postulaced failure modes associated with the plug roll transition area cracks; severance of the head could result in a small loose part, severance of the stub end could cause a SG tube stabilizer to move away from its intended location (OTSG cnly), or cracking or severance of the stub end could cause tube leakage.
A safety evaluation of these potential failure modes is in progress.
Our preliminary evaluation of the loose parts concern is that it does not represent a safety hazard.
B&W has recommended to the utilities with installed rolled plugs of heat W592-1 that the plugs remain in service unless subsequent studies n.__ca;;.:
c nacd for corrective act.icn.
PLANTS AFFECTED B&W has notified all plants supplied with tube plugs from heat l
W592-1 of the concerns involving this material and has arranged for return of all unused plugs.
Plants which have installed rolled, ribbed, or welded plugs made of this material are listed below together with the number of plugs known to be installed in each plant.
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vm F
T. E..'Murley/NRC JHT/88-171-September 6, 1988
- Page 3.
i-Plant W592-1 Pluas Beaver Valley I (117)
' Catawba II (6)
Crystal River III (1)
' Davis-Bosse (1)
McGuire I (166)
McGuire II (329)
Oconee I
-(48)
Oconee II (1) l Oconee III (8)
Three Mile Island I (20)
Trojan (2)
V.C.
Summer (244)
B&W ACTIVITIES IN PROGRESE B&W has the. following activities in progress to determine the cause and impact of this potential problem.
Development of Enhanced Non-Destructive ExaminrQon
-(NDE);
Capability We are developing an NDE technique capable of determining whether or_not an installed rolled plug is cracked.
This: technique was tested for the first time in the cold leg channel head of a plant
'in late June 1988.
43 plugs from two different heats were examined, including 26 plugs from heat W592-l'.
Good data was obtained on the inner roll transition.
No defects were P
identified.
Refinements are being made in the inspection b
technique to permit reliable, inspection of. the outer roll transition area.
Deterr.ination cf Inctall:4 I' lug Lccations In addition to utilizing available records of plug serial numbers and locations for each plant, we are developing visual methods for reading plug serial numbers in situ.
Corrosion Test B&W is performing corrosion testing of plugs made from heat WS92-1 material to better quantify its susceptibility to stress i
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corrosion cracking.
This testing will. evaluate the effects of two different installation ' techniques for rolled plugs.
Mechanical loadings will. be applied to the plugs - following the corrosion test.
g Evaluation of Plug Failure Potential and Consequences We are evaluating the potential for cracking or severance of the plug stub ends and. will evaluate the consequences of these failures on primary-to-secondary leakage if needed.
SUBMITTAL OF ADDITIONAL INFORMATION After completion of the accelerated stress corrosion testing described above, B&W will describe the results obtained and advise you of further plans for resolution of the SG plug issue.
We will submit this information in November 1988.
If you have any questions, please call me (804/385-2817) or Mr.
S. P. Hellman (804/847-3936).
Ve truly yours, r g James H.
aylo, Manager Licensing Services JHT/bcc-r cc:
C. E. Rossi/NRC I
E. L. Jordan /NRC C. Robinson /DPC
'P. F. McKee/FPC G. A.. Kammerdeiner/Duquesne Light M. D. Quinton/SCE&G P. C. Hildebrandt/TED T. G.' Campbell /AP&L G. R. Capodanno/GPUN B. L. Curtis/ Allen Nuclear Assoc.
D.
Keuter/SMUD 1
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ROLL TRANSITION
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CRACK ROLL T
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TRANSITION PLUG HEAD r
t FIGURE 1.
ROLLED PLUG 1
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