05000312/LER-1978-016, Forwards LER 78-016/01T-0

Forwards LER 78-016/01T-0

ML20197B995
Person / Time
Site:	Rancho Seco
Issue date:	11/06/1978
From:	Mattimoe J SACRAMENTO MUNICIPAL UTILITY DISTRICT
To:	Englken R NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION V)
Shared Package
ML20197B999	List: 05000312/LER-1978-016, Forwards LER 78-016/01T-0
References
NUDOCS 7811140230
Download: ML20197B995 (2)

LER-2078-016, Forwards LER 78-016/01T-0

Event date:
Report date:
3122078016R00 - NRC Website

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^w SACRAMENTO MUNICIPAL UTILITY DISTRICT L1 1708 59th Street, Box 15830. Sacramento. California 95813;(916)452 3211

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November 6, 1978 j

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Director of. Regulatory Operations .

attic Hr. R. H. Engelken flRC Operations Office >-

Region V

'990 North California Boulevard Walnut Creek Plaza, Suite 202 I W#ingt Neck California 94596 Re: Operating License DPR-54  !*

Docket No. 50-312 j' Reportable Occurrence 78-16 tear Fr. Engelken: [i In accordance with Technical Specifications for Rancho Seco Nuclear Generating Station, Section 6.9.4.lb and Regulatory Guide 1.16, Revision 4 Section C.2.a(2), the Sacramento Municipal Utility District is hereby submitting a fourteen-day followup report to Reportable Occurrence 78-16 which was initially reported to your office October 25, 1978. ,:

On October 24, 1978, the "A" diesel generator was paralleled to the "A" Nuclear Service Bus running at partial load, while the "B" diesel generator was out of service due to maintenance work associated with modifications specified in the Fire Hazards Analysis Report. This config-4 uration met the requirements of Technical Specifications Section 3.7.2c  ;

for continued reactor operation.  ;

At approximately 11: 50 a.m., Breaker 4A01, Nuclear Service Transformer Supply to 4A/3A bus, tripped due to overcurrent on one of its three phases.

This lef t the "A" diesel generator carrying the load on the 4A/3A bus. At that time it was noticed that the bus voltage was excessive (>5KV). An attempt was made to lower the bus voltage, but was unsuccessful. The bus voltage limitation was determined to be 4.8KV and, due to being in excess of that value, the "A" diesel generator was taken off lin.e. Breaker 4A01 was reset and closed, thereby re-energizing the 4A/3A bus from its normal supply.

l Initial trip of 4A01 was determined to be caused by the loss of voltage ~

regulation on the diesel generator, thereby causing an overload condition.

Since the "B" diesel generator was already out of service, the shutdown of the "A" diesel generator constituted "inoperability" of both diesel generators simultaneously. A reactor shutdown was initiated as specified in Section 3.7.3 of the Technical Specifications.

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Ah [L E C T Rit SYSTEM StRVING MORE THAN 600.000 IN THF HFARI Of cal 910R4i4

.R. H. Engelken Novemt>er 6,1978 Technicians, troubleshooting the voltage regulator circuitry, discovered the primary fuses on the control power circuit for the voltage regulator had failed. Thl3 in essence caused loss of D.C. excitation to the saturable reactor (control type transformer), allowed the A.C. to run the regulator to its maximum and rendered the vol tage regulatcr ci rcui t inoperable.

Procurement of replacement fuses was expedited. Technicians continued

• their investigation as to reason for the fuse failure, but found no other problems. Upon receipt of the replacement fuses, they were installed and at 1745 the "A" diesel generator was started. After satisfactory _ completion of its surveillance procedure, the unit was declared operable and left paralleled to the "A" Nuclear Service Bus. The: requirements of Technical -

Specifications Section 3.7 2c were satisfied, the reactor shutdown was terminated and the reactor returned to full power.

The fuses which failed are Bussmann type HVU-l rated at SKV and I amp.

Presently District engineers are performing tests to determine if heat may have been a factor in the fuse failure.

The plant transient associated with this event was a reduction in reactor power from 100% to approximately 35% in preparation for shutdown prior to regaining operability of the "A" diesel generator.

Respectfully submitted, y .V ?kL l

J J. Mattimoe

' Assistant General Manager and Chief Engineer JJM:RJR:HH: sal Enclosure cs: Director, MIPC (3) '

Director, IE (30)

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