ML16138A447
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MJambor AUG 19 180 EHughes MEMORANDUM FOR:
Richard H. Vollmer, Director, Division of Engineering Denwood F. Ross, Director, Division of Systems Integration Stephen H. Hanauer, Director, Division of Human Factors Safety Roger J. Mattson, Director, Division of Safety Technology Bernard J. Snyder, Program Director, TMI Program Office Brian K. Grimes, Program Director, Emergency Preparedness Program Office FROM:
Darrell G. Eisenhut, Director, Division of Licensing
SUBJECT:
OPERATING REACTOR EVENT MEMORANDUM NO. 80- j1:
STEAM GENERATOR MANWAY STUD CRACKING AT OCONEE UNIT 3 PROBLEM On June 25, 1980, IE resident inspector, IE Region II Office, and B&W were informed by Duke Power Company that during routine cleaning of manway studs, a stud from the Oconee 3 A-OTSG (once-through steam generator) lower primary manway was visually found to be cracked.
Further investigation of all 64 primary manway studs (16 studs on each' of four manways) on both OTSGs, using visual and ultrasonic testing (UT) inspection techniques, revealed a total of eight additional :cratked studs. One of these was found on the B-OTSG upper primary manway, while all of the remaining cracked studs were found to be on the A-OTSG lower primary manway. All of the OTSG primary manway studs were replaced with new studs that had been ultrasomic and magnetic particle tested before Unit 3 was returned to service.
BACKGROUND The two-inch diameter primary manway studs are made of a high strength, low alloy botting material, SA-320 GR-L43 (AISI-4340). Although all of the cracked studs came from two heat lot numbers both of which were supplied as part of the original equipment, the preliminary investigation to date has npt revealed any evidence that the stud cracking is an apparent materials problem.
OFFICE SURNA..
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sURNAME (0.TC DATE NRC FORM 318 (9-76)
NRCM 0240
- u. s. GOVERNMENT PRINTING OFFICEt 1976 -
626-624
Multiple Addressees AUG 1 9 19%
To pinpoint the exact cause of the stud cracking, eight of the cracked studs and two used good studs from the lower manway of both A and B OTSGs (one from each OTSG) were sent to B&W Lynchburg Research Center (LRC) by the licensee for metallurgical examination. Based on the results of the preliminary examination to date, the two studs that have been examined experienced cracking across approximately 98% add 50% of the cross-sectional area and crack propagation appears to have been at least assisted by corrosion. The principal cause of the crack initiation has yet to be determined.
Investigation of manway installation and removal procedures as a possible contributor to the stud cracking has been inconclusive. Records indicate that the lower primary manway on the A-OTSG has only been opened three times. In each instance, the procedures used were found to be in general agreement with the instructions manual supplied by B&W.
The sensitivity of the high gain 11T examination technique (ASME Section V) was also evaluated at the LRC. The preliminary results indicate that a minimum crack depth of 0.1 inch could be detected. However, a more sensitive UT technique used by 8&W is expected to have the capability of detecting crack size of about 0.03 inch which is close to the threshold crack size for stress corrosion cracking.
The primary manway studs on the Unit 1 OTSGs wereealsouultrasonically tested in early July 1980 and found no cracks. Similar testing will be performed on Unit 2 during the next refueling outage. In reviewing the past records, according to B&W, it has been determined that two primary manway studs were found cracked at the Arkansas Nuclear One, Unit 1, in 1978. The subsequent visual, magnetic particle, UT, and metallurgical examinations of the two cracked studs could not conclusively determine the cause of cracking. D&W has also determined that the same configurations and material are used on all of the following plants:
Oconee 1/2/3 North Anna 3/4 TMI 1/2 WNP 1/4 Crystal River 3 Bellafonte 1/2 Arkansas 1 PGE (undesignated site)
Rancho Seco Davis Besse On June 27, 1980 B&W advised its other operating plant utility customers by telephone about the Oconee 3 stud cracking problem. Several utilities have since inspected their OTSG's primary manway studs. On July 28, 1980 IE was notified by Arkansas Nuclear One that the inspection of Unit 1 steam generator manway studs during the current outage had resulted in two stud bolts, located in the A-OTSG lower manway, with defective NDE indications-hedetails.of-the
-redatos rc-st under nv a4 NRC FORM 318 (9-76)
NRCM 0240 U: S. GOVERNMENT PRINTING OFFICEs 1976 - 626*624
Multiple Addressees AUG 19 1980 An LER and a report on this incident were submitted by the licensee and B&W, respectively.
SAFETY SIGNIFICANCE OTSG primary manway stud cracking could result in excessive reactor coolant leakage which could be detected by standard RCPB leakage detection systems. The generic implications of this stud-cracking problem have not been determined by IE. To date, it appears that only two plants have been affected.
SHORT TERM ACTIONS IE has the lead responsibility and considered the repair action taken by the licensee acceptable. IE is also considering issuing an Information Notice to all B&W utilities.
RECOMMENDED LONG TERM ACTIONS Based on the information available to date, we recommend that NRR, in addition to foolow up on the results of metallurgical examination, evaluates and considers imposing the UT technique which is significantly more sensitive than that of ASME Section V for all future examinations of bolts and studs.
We have discussed this problem with the Division of Engineering (V. Noonan) and they have agreed to pursue this'stud:
cracking problem.
Original signed by arrell G. Eisenhat Darrell G. Elsenhut, Director Division of Licensing
Contact:
C.Y. Cheng, X28033 cc:
H. Denton K. Wichman C. Michelson L. Barrett DL ADs C. Berlinger DL BCs J. LaFleur R. Vollmer R. Hartfield, MPA V. Noonan C.Y. Cheng M. Fairtile OFFICE
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