ML20137H596

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PNO-IIT-85-002A:on 851121,unit Tripped.Control Room Operators Manually Scrammed Reactor Per Procedures for Loss of Vital bus.In-plant Power Restored from Switchyard. Relationship of Events Yet to Be Determined
ML20137H596
Person / Time
Site: San Onofre Southern California Edison icon.png
Issue date: 11/24/1985
From: Lanning W, Martin T
NRC
To:
References
PNO-IIT-85-002A, PNO-IIT-85-2A, NUDOCS 8512020429
Download: ML20137H596 (2)


Text

f DATE: 11/24/85 PRELIMINARY NOTIFICATION OF EVENT OR UNUSUAL OCCURRENCE-PNO-IIT-85-2A This preliminary notification constitutes EARLY notice of events of POSSIBLE safety or public interest significance. The information presented is preliminary, requires further evaluation and is basically all that is known by the IIT on this date.

FACILITY: Southern California Edison Company Emergency Classification Unit 1 X Notification of Unusual Event Docket No. 50-206 Alert Site Area Emergency General Emergency Not Applicable

SUBJECT:

Status Report from NRC Incident Investigation Team The Incident Investigation Team (IIT) arrived onsite on November 23, 1985 to begin its investigation of the events surrounding the November 21, 1985 plant trip at San Onofre Unit 1. The sequence of events was initiated by an electrical fault on the "C" auxiliary transfcrmer that was subsequently cleared through relay protection. Control room operators responded by manually scranning the reactor pursuant to procedures for loss of a vital bus. The main generator was also tripped, which resulted in total loss of in-plant power.

The switchyard remained energized from offsite power. As designed, the diesels started but did not automatically energize the emergency busses. Operators restored in-plant power from the switchyard in about 4 minutes. Other significant highlights of the incident include a water hammer associated with the "B" steam generator, a ruptured feedwater heater, a feedvater leak from a check valve in the "B" steam generator feedwater regulating valve bypass piping, and loss of indicated level in all three steam generators. The relationship of these events is yet to be determined.

As of 9 p.m. PST, November 24, 1985, the team has interviewed all operating staff on duty at the time of the initiating event and has viewed the involved equipment. The team has negotiated with the licensee a list of quarantined equipment and a procedure for disassembly, examination and maintenance. A preliminary sequence of events will be available on the morning of November 27, 1985.

A press conference was held on November 23. Major wire services and regional media have reported the Unusual Event and the formation and charter of the IIT.

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