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) | |
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. um ese.sa<a October 19, 1976
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.e Serial No.131
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, Docket No. 50-346
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.' Mr. James G. Keppler li -
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Regional Director, Region III
.Dir'ectorate of Regulatory Operations C
U.S. Nuclear Regulatory Commission
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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 General Electric identified a failure mode of the zener diode in the power supply circuit.
Diode failure could occur when simultanecusly the~
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' breaker load current was less than 15-20% frame size current rating and 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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.. 7 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
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- If the diode had shorted, insufficient trip voltage would have
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The consequence would have been failure to protect associated essential electrical equipment during an electrical fault.
With the diode opened, excessive control voltage would be available in t
the power supply and power sensor circuits.
In this latter c.ase, no e
- 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.
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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.
.Yo.urs very truly.
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,Dr. Ernst Volgenan, Director Office of Inspection and Enforcement U.S. Nuclear Regulatory Commission
.i Washington, D. C.
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