ML19329B083
| ML19329B083 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 10/19/1976 |
| From: | Roe L TOLEDO EDISON CO. |
| To: | |
| Shared Package | |
| ML19329B081 | List: |
| References | |
| 131, NUDOCS 8001300674 | |
| Download: ML19329B083 (2) | |
Text
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. EDISON
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October 19, 1976 '- .
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Serial No.131 '.~' '. #
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, Docket No. 50-346 . .
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.' Mr. James G. Keppler li - .
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- y. Regional Director, Region III .
.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-
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. 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
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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
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power supply circuit. Diode failure could occur when simultanecusly the~ '
' breaker load current was less than 15-20% frame size current rating and -
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- 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-
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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 - -
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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,
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. be required to pass pre-operational testing under the Applicant's test
. program before being considered acceptable for use.
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Safety Implications '
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- If the above conditions had gone uncorrected, the zener diode coulci have .
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- 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
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associated essential electrical equipment during an electrical fault.
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With the diode opened, excessive control voltage would be available in
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e the power supply and power sensor circuits. In this latter c.ase, no *
- immediate safety consequence would have necessarily occurred, however
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electrical fault protection of associated essential electrical equipment
- could have been adversely affected should the high voltage have caused
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the power sensor to fail. .
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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 - - -
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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
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Office of Inspection and Enforcement
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U.S. Nuclear Regulatory Commission
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