PNO-V-88-048, on 880717,unit Manually Tripped & Natural Cooldown Commenced as Result of Ground Fault in 12 Kv non- Vital Buses.Event Investigation Team Formed to Determine Location & Cause of Ground Fault

PNO-V-88-048:on 880717,unit Manually Tripped & Natural Cooldown Commenced as Result of Ground Fault in 12 Kv non- Vital Buses.Event Investigation Team Formed to Determine Location & Cause of Ground Fault

ML20151H052
Person / Time
Site:	Diablo Canyon
Issue date:	07/18/1988
From:	Mendonca M, Narbut P NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION V)
To:
References
PNO-V-88-048, PNO-V-88-48, NUDOCS 8808010120
Download: ML20151H052 (2)

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l .. A K W FME!MINARYNOT*FICATIONOFEVENTORUNUSUALOCCUPRENCE--PNO-V-88-48 Date July 18, 1988 I

This preliminary notification constitutes EARLY r..* ice of events of POSSIBLE safety or public interest significance. The information presented is as initially received tvithout verification or evaluation and is basically all that is known by Region V staff on this date.

FACILITY: PACIFIC GAS & ELECTRIC COMPANY s Emergency Classification X Notification of Unusual Event DIABLO CANYON UNIT 2 DOCKET NO. 323 SAN LUIS OBISP0 COUNTY, CA /

3}/ -

Alert Site Area Emergency l,3 General Emergency Not Applicable i

SUBJECT:

SHUTDOWN ON NATURAL CIRCULATION l

f l On July 17, 1988, Unit 2 was manually tripped and a natural circulation cooldown was  !

cocrnenced as a result of a ground fault in 12KV non-vital buses.

Starting at 6:02 a.m. with the Unit at 100% power, numerous ground alams were received l l on Unit 2 12 KV bus D. This bus providet power to non-vital loads, i.e., one Circulating  !

Water Pump and two Reactor Coolant Pumps (RCPs). The Control Room was notified at t 6:10 a.m. that Oa ground bank for auxiliary transfomer 21, which feeds non-vital 12 KV buses D&E from the main generator, was smoking. Bus D was switched to startup

transformer 21 which is supplied by 230 KV power at 6

30 a.m. Reactor power was reduced '

' to 50% and the Circulating Water Pump on bus D was secured at 7:15 a.m. Ground alams continued to corne in, and at 7:15 a.m. the ground bank for startup transfertner 21 was reported to be en fire. The reactor was manually tripped at 7:17 a.m. and reactor coolant pumps on bus D were secured by 7:19 a.m. Ground fault alams were new indicated on bus E and one reactor coolant pump on that bus was secured at 7:20 a.m.

Upon the trip, all loads were automatically switched to startup transformers. As ground l alams continued to come in, the remaining RCP was manually tripped. At 7:46 a.m.,

a startup transfomer 21 phase differential current trip was received which isolated 230 i KV power from the startup transfomer for both unit startup transfomers. At this point, all diesel generators star *.ed and vital Unit 2 loads were sequenced on. All related components responded as designed. ,

At 7:57 a.m. an UE was declared when a Safety injection was received due to high steem '

line differential pressure between Steam Generators. The unbalanced temperatures and steaming rates on the Steam Generators, due to the staggered tripping of RCPs, ,

caused the steam line differential pressure condition. At 8:07 a.m., the Safety Injection was secured.

I At 8:24 a.m., the 4.16 KY auxiliary transforrers were fed through the main transformer, '

following separation from the main generator, allowing all 4.16 KV buses, including {

vital loads, to be powered by 500 KV off-site power.  !

Following the safety injection, a number of wat?r hatner events were reported in the  !

secondary plant. This did not affect the shutdown, since the main steam and feedwater '

isolation valves were closed.

The licensee's operators initiated natural circulation cooldown at a rate less than or equal to 25'F/ hour, and initiated RHR cooldown at 8:06 p.m. The plant is currently at about 220*F on RHR. The liceasee is currently evaluating teminction of the UE.

g o g 2o eso7 e -

t PNO-V-se-o40 PNU 0 J

o PG5E's President and the Vice President for Nuclear Power Generation made a site visit to assure appropriate response to the event. A licensee's Event Investigation Team was forred to determine location and cause of the ground fault and effects of the water hanner event. The Resident Inspectors responded to the event and observed plant e.ooldown operations until RHR was initiated.

The licensee notified the media and moderate attention was received.  ;

This information is current as of 10:30 a.m., July 18, 1988.

CONTACT: H. Mendonca, RV P. Narbut ,

FTS 463-3720 805-595-2354  :

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. DISTRIBUI!ON BY 5520: FAX TO: i Da te 1/ / J ' Time _ // ! d Ass Date Time 1

Chairman Zech EDO RI INPO l

! Comm. Roberts AE00 RI! NSAC

, Comm. Carr PA0 RI!! LICENSEE 1 Comm. Rogers SLITP RIY RESIDENT INSPECTOR

{ SECY NRR HQS. OPERATIONS OFFICER ,

CA hMSS 1 PDR ARM MAIL TO:

i OGC O! DC5 (Original)

! ACRS OIA DOT:(Trans. Only. Applicable State)

GPA RES RV FORM 211 Revised: 8/19/87  :

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