ML19345G179
| ML19345G179 | |
| Person / Time | |
|---|---|
| Site: | Haddam Neck File:Connecticut Yankee Atomic Power Co icon.png |
| Issue date: | 12/07/1973 |
| From: | Hartley F CONNECTICUT YANKEE ATOMIC POWER CO. |
| To: | Anthony Giambusso US ATOMIC ENERGY COMMISSION (AEC) |
| References | |
| AO-73-12, CYH-2532, NUDOCS 8103100220 | |
| Download: ML19345G179 (2) | |
Text
Regulatr/
- D-C3 ONNECTICUT NKEE ATOMIC POWER COMPANY
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BERLIN. CO N N ECTIC U T
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(Jg P.O. BOX 270 H ARTFORD. CONNECTICUT C6101 1ll
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December 7,1973
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Co 50-213 Deputy Director for Reactor Projects kf b{ d
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Mr. A. Giambusso 1
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United States Atomic Energy Commis, ion g y.3 3,,.
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Office of Regulation
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Dear Mr. Giambusso:
In accordance with Section 5.7.2 of the Connecticut Yankee Technical Speci-fications, Abnormal Occurrence 73-8 reported failure of twenty-three (23) steam generator seismic holddown bolts. The fcllowing report of an additional steam generator holddown bolt failure is hereby reported as Abnormal Occurrence 73-12. This report also provides additional information concerning the original twenty-three (23) failed bolts.
Abnormal Occurrence 73-8 concluded that the cause of the failed bolts was overheating at instaliation.
Siace that time several of the original broken bolts were examined by elettron microscopic metallurgical analysis and the mode of fracture was determ;ned to be stress corrosion cracking.
The oxidized appearance of most of the belts tends to indicate that the bolts failed during the early history of the plant, however, the precise time of fracture can not be determined. Appearances 3re, however, that all of these failures occurred at approximately the same time and that the failures are old.
While installing micro switches to monitor each of the 256 bolts, an additional failed bolt was discovered on 11/26. This bolt is located in the #3 steam generator holddown ring at the paint nearest to the center of the containment.
Records indicate that this bolt was ultrasonically tested satisfactorily on 10/18 and was additionally tested by " sounding" with a hamer. At the time of its discovery the bolt head was raised slightly (1/4") from the flange as were twenty-two (22) of the original twenty-three (23) bolts.
This bolt has been examined by electron microscopic metallurgical analysis and the fracture also appears typical of stress corrosion.
Stress corrosion requires an environment, a tensile stress and a material susceptible to stress corrosion cracking. The environment is such that moisture was present in the concrete surrounding the nut during the original installation. Voisture can also seep under the flange of the ring girder to the bolts. This moisture most likely creates the adverse environment for strest, corrosion.
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The twenty-four (24) failed bolts were replaced with bolts constructed of the same material, however, the yield strength has been reduced to lessen the stress corrosion susceptibility. The tensile stress on the replacement bolts caused by preload has been lowered to - 50,000 psi. This results in a reduction of stress below the crack propagation threshold for this material. The replacement bolts have been covered by a protective enclosure which has been sealed to the underside of the ring girder to prevent moisture seepage into the threads of the nut and bolt. All bolts have had an ultrasonic examination to determine any detectable indications. This inspection has been performed on two separate occasions and no significant indications have been detected. The second ultrasonic inspection involved a more refined technique, therefore we have reasonable assurance that all the bolts are sound.
A monitoring system has been applied to each bolt to detect possible bolt failure by detecting any lifting of the bolt from the steam generator support block.
A design alternative that would preclude the need for dependency on the present holddown bolt assemblies insofar as vertical loading is concerned, is presently being engineered. After review and finalization of that design alternative, an evaluation for possible installation during a future plant shutdown will be conducted.
A special. report entitled Steam Generator Holddown Bolt Failure will be submi',ted within sixty days.
Very truly yours, l4 F. W. Hartley Plant Superintend nt FWH/smb
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