ML20009F627
| ML20009F627 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 03/23/1981 |
| From: | Parker W DUKE POWER CO. |
| To: | James O'Reilly NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II) |
| References | |
| NUDOCS 8107310364 | |
| Download: ML20009F627 (2) | |
Text
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DUKE POWER COhiPANY Powza Duizmixo
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422 Sourn Cnuncu STnzer, CruatoTTE N. C. asa4a
s[c75 E.*o'o c'Uo.
March 23, 1981
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iS Mr. James P. O'Reilly, Director U.S. Nuclear Regulatory Commission h
Region II UQ 101 Marietta Street, Suite 3100 9
gO O 1981 A Atlanta, Georgia 30303 p-C>
Re:
Oconee Unit 3 s
/
Docket No. 50-287 T}
G D ar Mr. O'Reilly:
Please find enclosed a copy of Babcock & Wilcox Report LRC-9092, "Oconee 3 Cracked OTSG Primary Manway studs -- Interim Report", dated August 1980.
This report presents the results of the detailed metallurgical examination per-formed on the manway studs at the Babcock & Wilcox Lynchburg Research Center.
Altnough this report is titled as an interhn report, it does in fact represent the final report on the Babcock & Wilcox examination of the studs.
A total of ten studs were sent to Babcock & Wilcox for examination. Of these ten, eight studs were cracked and two were not cracked. While all ten studs were examined in some detail, the studs identified by Babcock & Wilcox as V-1 and UT-4 were subjected to the most comprehensive examination.
Babcock & Wilcox has concluded that the crack initiation and propagation mechanism was hydrogen embrittlement / stress corrosion cracking. Each of the three potential contributing elements classically associated with stress cor-rosion cracking (environment, stress, and material properties) was evaluated Babcock & Wilcox has and no single element was identified as the rcot cause.
identified potential actions in the above referenced areas that could be taken l
to address the stud cracking problem.
To address the stud cracking, ultrasonic testing (UT) was performed on all OTSG manway studs at Oconee.
In addition, magnetic particle testing (MT) was per-formed on all manway studs on both OTSG's on Oconee 3 and on the "A" OTSG on Oconee 1.
UT was also performed on similar bolting on )ther equipment with These tests found no other problems with bolting at Oconee.
similar environments.
l In addition, Duke Power Company is implementing the following long-term actions:
l 1.
Revised assembly / disassembly procedures have been developed to insure a lower stud operating stress level. Revised cleaning and handling procedures l
have been developed to insure that the stud and closure seal system is un-damaged, clean, and dry; thus improving the overall environment.
I 8107310364 810323 g
PDR ADOCK 05000287 P
/p
r ydI Mr. James P. O'Reilly, Director March 23, 1981 Page 2 2.
The use of stud tensioners will be evaluated, incorporating the revised assembly / disassembly procedures. The use cf stud tensioners would reduce the preload stress by approximately 50 percent.
In order to utilize stud tensioners, replace studs will be required. Any new stud will have a specified minimum and maximum yield strength. A UNR type thread is rec-commended rather than UN type.
The UNR type thread results in a 3arger thread root radius, thus reducing the stress concentration factor.
3.
Hardness testing will be performed to survey the studs in stock and in service. This data will be used to locate studs with relatively high yield strengths.
Consideration will be given to discarding studs with extremely high yield strengths.
4.
A regular in-service inspection program is being performed on steam gene-rator manway bolting and on similar applications until the cause of the above failure has been definitely identified and alleviated.
Duke is not convinced that Molykote G played any significant role in the cracking. Molykote G decomposition products were found only on one of the three fracture surfaces examined. Boron and lithium were found on each of the three surfaces examined. Duke is presently evaluating a plan to perform additional corrosion tests to determine if Molykote G accelerates stress corrosion com-pared to primary coolant.
Duke believes that the above actions will. reduce the susceptibility of the af-fected bolting to stress corrosion cracking so that the margin to stress cor-rosion cracking threshold is increased, thus allowing continued safe operation.
The enclosed report indicates that a total of eleven cracked studs were identified at Oconee 3.
In Reportable Occurrence Report R0-287/80-11, dated July 16, 1980, Duke reported the discovery of only nine cracked studs. These studs were identi-fied using visual and UT inspection techniques. A subsequent inspection using the MT inspection technique identified +wo additional cracked studs. The operational location of these additional studs is not known. Of these two, one was apparently cracked only in the thread form and was undetectable by high-gain The other stud was reinspected with a high-gain UT technique and the crack UT.
was detected.
It is not clear whether the original UT inspection was performed in accordance with the ASME Code or utilized the high-gain technique.
In addition to the eight cracked studs sent to Babcock & Wilcox for examination, two studs were provided to Battelle Laboratories, Columbus, Ohio in August 1980 at the request of the NRC.
Duke Power requests that a copy of the examination results be provided to it when these results become available.
Ver truly yours,/
s~
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'N William O. Parker, Jr
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