ML19246A766
| ML19246A766 | |
| Person / Time | |
|---|---|
| Site: | Cooper  |
| Issue date: | 05/22/1979 |
| From: | Seyfrit K NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV) |
| To: | Pilant J NEBRASKA PUBLIC POWER DISTRICT |
| References | |
| NUDOCS 7907060327 | |
| Download: ML19246A766 (1) | |
Text
CENTRAL FILES
- " '%,g UNITED STATES PDR:HQ 8,
NUCLEAR REGULATORY COMMISSION QR
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611 RYAN PLAZA DRIVE, SUITE 1000 gg((
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AR LINGT ON, T E XAS 76012 May 22, 1979
' Docket No.
50-298 Nebraska Public Power District ATIN:
J. M. Pilant, Director Licensing & Quality Assurance Post Office Box 499 Columbus, Nebraska 68601 Gentlemen:
Enclosed is IE Bulletin No. 79-11 which requires action by you with regard to your power reactor facility with an operating license or a construction permit.
Should you have questions regardinb this Bulletin or the actions required of you, please contact this office.
Sincerely,
~
Ja,
.b' Karl V. Seyfrit 1
Director
Enclosures:
1.
List of IE Bulletins Isseed in Last Twelve Months Cooper Nuclear Station 300
,23rd cc:
L. C. Lessor, Superintendent Post Office Box 98 Brownville, Nebraska 68321 7 907060 'sf 7
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT IE Bulletin No. 79-11 Date: May 22, 1979 Page 1 of 2 FAULTY OVERCURRENT TRIP DEVICE IN CIRCUIT BREAKEES FOR ENGINEERED SAFETY SYSTEMS Discussion:
We have received information from Westinghouse and an NRC licensee relating to the potential failure of a circuit breaker in an engineered safety system of a nuclear power plant. This circuit breaker had a defect in one of its three time delay dashpots which resulted in a reduced time delay for over-current protection.
The defect was a small hairline crack in the end cap of the dashpot.
Further investigation by this licensee disclosed that 7 out of 17 spare dashpot end caps and 2 non-engineered safety feature breakers also had similar defects.
The circuit breaker is a Westinghouse t,pe DB-75.
Westinghouse type DB-50 breakers also use the same type of dashpot and end cap.
DB-50 and -75 breakers are used extensively in PWR's and some BWR's may also have the same breakers.
Similar make and model circuit breakers, when used for scram purposes, do not require the overcurrent trip feature and thus are not of concern. The end cap crack tefect, if severe enough, could result in premature tripping of the circuit breaker because of insufficient time delay in overcurrent protection, i.e., the notor starting (inrush) current could cause the breaker to trip inadvertently and thus prevent the motor start.
The defects reported by the licensee in April 1979, occurred in the replacement end caps which were provided to solve the problem described in IE Bulletin 73-1.
The subject of Bulletin 73-1 was end caps made of a black phenolic material.
As a result of that Bulletin, the black end caps were replaced with a new type made of fiber-filled polyester.,aterial called " navy gray."
Prior to the April 1979 report, there have been no reports of suspect " navy gray" end caps either from scheduled testing or unusual behavior in service. The manufacturer of the " navy gray" end caps believes the crack defects may be linked to a raw material batch problem. That is, the molding ingredient materials used may have neared the end of their shelf life 'efore use.
It is not believed the end caps, af ter fabrication, have a significant shelf life limit, due to the low residual stress and low crack propagation probabilities.
Description of Event:
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The following information was obtained April 12, 1979, and a subsequent meeti DUPLICATE DOCUMENT the licensee.
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