ML19305A195
| ML19305A195 | |
| Person / Time | |
|---|---|
| Site: | Yellow Creek  |
| Issue date: | 01/16/1979 |
| From: | James O'Reilly NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II) |
| To: | Aydelott W, Gilleland J, Niav Hughes TENNESSEE VALLEY AUTHORITY |
| References | |
| NUDOCS 7901220233 | |
| Download: ML19305A195 (9) | |
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NUCLEAR REGULATORY COMMISSION c, ( [,g REGION il
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ATLANTA. GEORGIA 303J3 N 161979
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In Reply Refer To:
RII:JP0 50-438, 50-439 50-259, 50-260 50-296, 50-518 50-519, 50-520 50-521, 50-553 50-554, 50-327 50-328, 50-390 50-391, 50-566 50-567 Tennessee Valley Authority Attn:
}k. N. B. Hughes Manager of Power 830 Power Building Chattanooga, Tennessee 37401 Gentle::2en:
The enclosed Circular 79-02 is forwarded to you for inf ormation.
If there are any questions related to your understanding of the suggested actions, please contact this office.
Sincerely, wmes P. O'Reilly D rector
Enclosures:
1.
List of IE Curculars Issued in the last 12 months 7801220233
JAN 1 g 1979 t
Tennessee Valley Authority cc w/ enc 1:
J. E. Gilleland Assistant Manager of Power 830 Power Building Chattanooga, Tennessee 37401 W. W. Aydelott, Project Manager Bellefonte Nuclear Plant P. O. Box 2000 Hollywood, Alaba=a 35752 J. F. Cox 400 Co==erce Street W9D199 Knoxville, Tennessee 37902 J. G. Dewease, Plant Superintendent Box 2000 Decatur, Alaba=a 35602 R. T. Hathcote, Project Manager Hartsville Nuclear Plant P. O. Box 2000 Hartsville, Tennessee 37074 G. G. Stack, Project Manager Sequoyah Nuclear Plant P. O. Box 2000 Daisy, Tennessee 37319 J. M. Ballentine Plant Superintendent Sequoyah Nuclear Plant P. O. Box 2000 Daisy, Tennessee 37319 T. B. Northern, Jr.
Project Manager Watts Bar Nuclear Plant P. O. Box 2000 Spring City, Tennessee 37381 W. P. Kelleghan, Project Manager Phipps Bend Nuclear Plant P. O. Box 2000 Surgoinsville, Tennessee 37873 M. M. Prir.a. Project Manager Yellow Creak Nuclear Plant P. O. Box 2000 4
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i UNITED STATES NUCLEAR REGULATORY COMMISSICN OFFICE OF INSPECTION AND ENFORCE}ENT WASHINGTON, D.C.
20555 January 16, 1979 IE Circular No. 79-02 FAILURE OF 120 VOLT VITAL AC POWER SUPPLIES Description of Circumstances:
On September 16, 1978, Arkansas Nuclear One - Unit 2, while in hot functional testing preceding initial criticality, suffered a degrada-tion of both of the independent off-site power sources. This produced an undervoltage condition on the Engineering Safety Features (ESF) Buses and caused an inadvertent ESF actuation.
The licensee determined that the ESF actuation occurred on a loss of at least two of the uninter-ruptable 120 volt vital AC power sources.
Immediate investigation revealed that all four of the Solidstate Controls, Inc., (SCI) inverter static switches had automatically transferred to the alternate power supply (the 120 volt AC being supplied by transformers directly of f the ESF buses rather than from the inverter output.)
Figure 1 (attached) shows the functional interconnection of the 120 volt AC power supply system.
A single conclusive cause of the undesired SCI inverter static switch transfer could not be identified. However, the following problems were discovered during subsequent investigation.
(a)
The SCI inverters have circuitry to monitor the incoming DC voltage level which, on sensing a low voltage (nominally 104 DC volts for these inverters) will trip both input breakers to the inverter component after an adjustable time delay.
The setting of these time delay relays were not verified during either preoperational testing er subsequent maintenance. The time delay in necessary in order to accommodate transient loading conditions which may be encountered.
(b) On one SCI inverter, a DC fuse within the inverter component was found blown.
The vendor indicated that this fuse will blow due to an excessive DC voltage to the inverter component caused by a transient on the 480 AC input.
The licensee has subsequently reset the taps on the 480V to 120V three-phase AC input transformer to limit the DC voltage to the inverter to less than the nameplate maximum rating in the event of a high AC input voltage transient.
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