United States Nuclear Regulatory Commission - Protecting People and the Environment

ACCESSION #: 9903110295


PAGE: 1 OF 5

FACILITY NAME: San Onofre Nuclear Generating Station
(SONGS) Unit 2
TITLE: Automatic Start of an Emergency Diesel Generator (EDG)
EVENT DATE: 2/01/1999 LER #: 1999-001-00 REPORT DATE: 3/3/1999
OTHER: Part 21
NAME: R.W. Krieger,
Vice President,
Nuclear Generation
TELEPHONE: (949) 368-6255



On 2/1/1999 (event date), Unit 2 was in Mode 6 (000 percent power) for a planned refueling outage. At 0959 PST, while removing a supply breaker for a 4.16 kV Class 1E bus from its cubicle, a worker discharged the breaker's closing springs, closing the breaker. As a result, power was lost to the bus, Shutdown Cooling was lost, and the Emergency Diesel Generator started. Southern California Edison (SCE) promptly notified the NRC Operations Center (NRC Event Log Number 35336) that an Unusual Event had been declared for the loss of Reactor Coolant System heat removal capability for greater than 10 minutes (10CFR50.72(a)(i)) and an Engineered Safety Feature actuation (10CFR50.72(b)(2)(ii)). This LER is provided in accordance with 10CFR50.73(a)(2)(iv), 10CFR50.73(a)(2)(v), and 10CFR21.

All equipment operated as expected. Operators restored SDC in approximately 26 minutes.

SCE has implemented or planned corrective actions to prevent recurrence.

The incremental increase in core damage probability was "very small."

In the past 3 years, SCE has not reported an event with similar causes and corrective actions.


Plant: San Onofre Nuclear Generating Station Unit 2
Reactor Vendor: Combustion Engineering
Event Date: February 1, 1999
Event Time: 0959 PST
  Unit 2 Unit 3
Mode: 6, Refueling 1, Power operation
Power: 000 percent 99.9 percent
Temperature: 65.3 degrees F 546.5 degrees F
Pressure: Atmospheric 2250 psia


At the time of this event, Unit 2 was in day 30 of a planned 60-day efueling outage. The reactor core was approximately 50 percent reloaded ith no core alterations in progress at the time of the event. The refueling cavity was filled to 23' above the flange. Figure 1 shows the Class 1E 4.16 kV (EB) electrical distribution system for Unit 2.

  • Train A safety components were in-service with bus 2A04 powered by the Unit Auxiliary Transformer (UAT) (FK) through breaker 2A0419. One Saltwater Cooling (SWC) (BS) pump, one Component ooling Water (CCW) (CC) pump, one Containment Spray (CS) (BE) ump, and one Low Pressure Safety Injection (LPSI) (BP) pump to rovide Shutdown Cooling (SDC) and Spent Fuel Pool (SFP) cooling ere being powered from 2A04.

  • Bus 2A04 (EB) was being credited as the alternate source of offsite power to Unit 3 bus 3A04.

  • The Reserve Auxiliary Transformer (RAT) 2XR1 (FK) was disconnected from the switch yard and all three ground disconnect switches on the high side (220 kV) were closed. (See the Cause of the Event section.)

  • Train B safety components were unavailable due to scheduled component work in-progress.

Breaker (BKR) 2A0418 is a 5 kV, 3,000 amp (continuous) ABB model 5HK-350 Class 1E circuit breaker. The breaker is closed by springs. The closing springs are normally released electrically to close the breaker. However, the breaker can be closed manually. The circuit breaker is mounted on a rolling chassis for ease of racking it in and out of the switchgear. (See the Additional Information section.)

Description of the Event:

Scheduled activities were underway to clear the RAT 2XR1 (a primary power supply for the 2A04, Train A 4,16 kV bus, and an alternate power supply to bus 3A04 in Unit 3), to support other maintenance. Problems were encountered while trying to rack out breaker 2A0418, the supply breaker from the RAT. (See Additional Information section.) A multi-discipline team (utility, licensed and non-licensed), assembled to troubleshoot removing the breaker, concluded the likelihood of the breaker inadvertently closing could be reduced by discharging the closing springs using the trip lever on the bottom of the breaker.

On February 1, 1999 (event date), at 0959 PST, when the worker (utility, non-licensed) discharged the closing springs on breaker 2A0418, the breaker closed. When the breaker closed, the high side ground on the RAT 2XR1 created a three phase to ground fault through the ground disconnect switch. The fault current path was from Main/UATs 2XM and 2XU1 through bus 2A04 via breaker 2A0419 to RAT 2XR1 via breaker 2A0418. The fault lasted about 1.5 seconds and the voltage on bus 2A04 dropped to approximately 2.2 kV.


As a result of the undervoltage, breaker 2A0419 opened and bus 2A04 was de-energized. Breaker 2A0418 remained closed, as expected, because its de control power had been removed as part of the operation in progress. Consequently, no alternate source breaker could close because the supply breakers are interlocked such that only one can be closed at a time. EDG 2G002 (EK) started and its breaker did not close. The pumps required for SDC and SFP cooling (one LPSI, one CS, one CCW, and one SWC) also tripped automatically. All systems and equipment operated as expected.

Southern California Edison (SCE) promptly notified the NRC Operations Center (NRC Event Log Number 35336) in accordance with 10CFR50.72(a)(i) that an Unusual Event (UE) had been declared at 1009 PST for the loss of Reactor Coolant System (RCS) heat removal capability for greater than 10 minutes. (The UE was exited approximately 30 minutes later.) Because the start of the Emergency Diesel Generator (EDG) is an Engineered Safety Feature (ESF) actuation, that notification was also provided in accordance with 10CFR50.72(b)(2)(ii).

This LER is provided in accordance with:

1. 10CFR50.73(a)(2)(iv). The start of the EDG was an ESF actuation.
2. 10CFR50.73(a)(2)(v). Because of the plant's electrical configuration, specifically the ground on the RAT 2XR1 and the dc control power removed from breaker 2A0418, the EDG was unable to fulfill its safety function of powering bus 2A04.
3. 10CFR21. As described in the Additional Information section, a contributing cause of this event was a mis-positioned roller on breaker 2A0418.

Cause of the Condition:

This event was caused by workers closing breaker 2A0418 by discharging the closing springs. Based on previously observing that discharging the closing springs using the lever on the bottom of the breaker when the breaker was removed from its cubicle did not close the breaker, the workers assumed, but did not confirm, similar operation when in the racked-in position (cognitive personnel error).

Two contributing conditions were identified as necessary for the direct cause to have resulted in the event,

1. The switching order for the RAT directed installing the high side grounds before racking out the low side breakers. Without the grounds installed, closing breaker 2A0418 would not have resulted in the loss of bus 2A04.
2. Breaker 2A0418 did not rack out properly. The cause of the difficulty in racking out the breaker is discussed in the Additional Information section.

Corrective Actions:

Operators restored SDC and SFP cooling approximately 26 minutes after the event by clearing the fault, reenergizing bus 2A04 by closing breaker 2A0419, and restarting CCW, SWC, LPSI and a CS pump.

SCE conducted a station Work Stand Down to brief employees on the event, and to reinforce management's expectation regarding following programs, recognizing error likely situations, response to deviations from normal expected outcomes, and the significant consequences of this event including loss of SDC, declaration of an UE, and local containment evacuation.


Management required approval by the Outage Managers and Operations Manager or Vice President for emergent work and use of safety related blanket Maintenance Orders on Defense In Depth credited components and systems.

Inspections, tests and evaluations of affected electrical equipment and the EDG showed no apparent or predicted damage.

Breaker 2A0418 was removed from its cubicle for inspection. (See the Additional Information section.)

Corrective actions planned or taken to address the causes include:

  • Developing program controls that set Management's expectations for work control of high consequence activities.

  • Sharing lessons learned from this event, including the need to validate assumptions, with appropriate Operations, Maintenance and Technical personnel.

  • Revising the switching order procedure to include racking out low side breakers prior to closing ground disconnects.

Safety Significance:

The risk impact assessment of the loss of 2A04 considered the following:

1. Bus 2A04 recovery, SDC, and SFP cooling restoration scenarios.
2. The risk impact of the loss of SDC. Without a concurrent loss of inventory event, the times to bulk boiling and core uncovery were conservatively estimated to be 28 hours and 170 hours (approximately 7 days), respectively.
3. Loss of offsite power events.
4. Loss of inventory events.
5. Seismic events.

The incremental increase in core damage probability was less than 1E-8, and, based on Regulatory Guide 1.174, is characterized as "very small,"

Additional Information:

  • The difficulty in racking out breaker 2A0418 was caused by an incorrectly placed shutter roller pin which is mounted on the chassis of the breaker. The breaker had been recently overhauled by the vendor (ABB Service Incorporated of The Woodlands, Texas) who incorrectly reinstalled the shutter roller pin into a spare hole on the side of chassis. With the shutter roller pin in the wrong hole, the shutter roller pin became trapped behind the lever arm which actuates the cubicle stab shutter. The lever arm mechanically interfered with the movement of the roller when racking out of the breaker. SCE notified the vendor, and the vendor is evaluating this condition in accordance with 10CFR21.

  • SCE will revise its breaker overhaul procedure to show proper location of the shutter roller pin.

  • In the past 3 years, SCE reported (LER 2-1998-021) an emergency chiller unit was made inoperable when workers failed to implement programmatic requirements for lifting and landing electrical leads on the low temperature cutout switch and the retest did not reveal this error. The event reported herein had similarities to the chiller event (misunderstanding of how equipment operated and failure to verify assumptions). However, the team involved in deciding to discharge the closing springs were working within the bounds of an approved work process and had the authorization of the Shift Manager. Retest was not a factor because discharging the springs directly caused the event.

Figure 1 - "Class 1E 4.16 kV Electrical Distribution System"

ATTACHMENT TO 9903110295 PAGE 1 OF 1
An EDISON INTERNATIONAL Company Vice President
Nuclear Generation

March 3, 1999

U. S. Nuclear Regulatory Commission
Document Control Desk
Washington, D.C. 20555

Subject: Docket No. 50-361
30-Day Report
Licensee Event Report No. 1999-001
San Onofre Nuclear Generating Station, Unit 2


This letter provides Licensee Event Report (LER) 1999-001 in accordance with 10CFR50.73(a)(2)(iv), 10CFR50.73(a)(2)(v), and 10CFR21 for an occurrence involving an Engineered Safety Feature actuation (automatic start of an emergency diesel generator).

The health and safety of neither the public nor plant personnel were affected by this occurrence.

Any actions listed are intended to ensure continued compliance with existing commitments as discussed in applicable licensing documents; this LER contains no new commitments. If you require any additional information, please so advise.


LER No. 1999-001

cc: E. W. Merschoff, Regional Administrator, NRC Region IV
J. A. Sloan, NRC Senior Resident Inspector, Units 2 and 3
Institute of Nuclear Power Operations (INPO)

P. O. Box 128
San Clemente, CA 92674-0128
Fax: 714-368-6183


Page Last Reviewed/Updated Thursday, March 29, 2012