ML19329B083

Deficiency Rept Re GE AK-3-25 & AK-3-50 Volt Ac Circuit Breakers,Power Sensors & Supplies,Supplementing 760806 Interim Deficiency Rept.Power Supply Circuit Redesigned to Eliminate Failed Zener Diode Mode

ML19329B083
Person / Time
Site:	Davis Besse
Issue date:	10/19/1976
From:	Roe L TOLEDO EDISON CO.
To:
Shared Package
ML19329B081	List: ML19329B083 ML19329B089 ML19329B094 ML19329B095 ML19329B096 ML19329B097
References
131, NUDOCS 8001300674
Download: ML19329B083 (2)

Text

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TOLEDO

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. EDISON

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. um ese.sa<a October 19, 1976 '- .

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Serial No.131 '.~' '. #

, Docket No. 50-346 . .

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.' Mr. James G. Keppler li - .

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.Dir'ectorate of Regulatory Operations -

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el 799 Roosevelt Road J.. . "-

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Dear Mr. Keppler:

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' Th'is letter is submitted in accordance with 10 CFR 50.55(e) as a con-

.. .* smnmation of the Interim Report dated August 6,1976 regarding defi-

. ciencies in the Davis-Besse Nuclear Power Station Unit No. 1, General

. Electric, type AK-3-25 and AK-3-50 480 volt AC circuit breakers, power sensors and supplies. ., . ,

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Description of the Deficiency

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General Electric identified a failure mode of the zener diode in the power supply circuit. Diode failure could occur when simultanecusly the~ '

' breaker load current was less than 15-20% frame size current rating and 5 '

the gate current of the shunting SCR's (silicon controlled rectifiers) was in the medium to upper range of the specification limits. With .

these two conditions existing, the thermal rating of the diode is ex-ceeded, which may lead to failure of the zener.

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. Corrective Aceion .

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General Electric redesigned the power supply circuit t:o eliminate the - -

stated failure mode. The Applicant has arranged to replace all subject

• ** power supplies and power sensors at Davis-Besse Unit I and is presently -- --

" . having General Electric rework the breakers. Each reworked breaker will,

. be required to pass pre-operational testing under the Applicant's test

. program before being considered acceptable for use.

Safety Implications '

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• If the above conditions had gone uncorrected, the zener diode coulci have .

• " failed.

• If the diode had shorted, insufficient trip voltage would have ~

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.been available. The consequence would have been failure to protect t

associated essential electrical equipment during an electrical fault.

With the diode opened, excessive control voltage would be available in e the power supply and power sensor circuits. In this latter c.ase, no *

- immediate safety consequence would have necessarily occurred, however electrical fault protection of associated essential electrical equipment

• could have been adversely affected should the high voltage have caused the power sensor to fail. .

, ,,.7 A The scheduled repair of the affected breakers and acceptable pre-oper-

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'ational testing of the devices prior to station operation assures that - - -

the health and safety of the public are not jeopardized. . . .

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.Yo.urs very truly.

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,Dr. Ernst Volgenan, Director Office of Inspection and Enforcement U.S. Nuclear Regulatory Commission Washington, D. C. 20555 .

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