05000499/LER-2001-002

LER-2001-002,

Event date:
Report date:
4992001002R00 - NRC Website

DESCRIPTION OF EVENT:

On March 1, 2001, preparations were underway to align the main 345kV switchyard breakers for North Bus maintenance. At approximately 0700 CST switchyard breakers Y600 and Y610 were opened. The manual disconnect for the "Skyline" offsite transmission feed was then opened to take the "Skyline" circuit out of service while the North Bus was out of service. After the "Skyline" manual disconnect was opened, breakers Y600 and Y610 were closed.

Unknown to the operating and switchyard crews, the C phase pole of breaker Y600 remained opened (indicated fully closed). At 0821 switchyard breaker Y590 was opened. With the C phase pole of breaker Y600 still opened, the Control Room received electrical panel alarms and Circulating Water Pump 21 tripped offline on a phase balance current relay (46 relay). The open C phase pole created a phase imbalance condition that was sensed by generator and Circulating Water motor protection relays once breaker Y590 was opened. Standby Bus 2F and Auxiliary Bus 2H high voltage alarms and a generator negative phase sequence current alarm were received. This was followed closely by the trip of the remaining operating Circulating Water Pumps 22 and 24, requiring the operating crew to initiate a manual reactor trip in accordance with plant procedures. All control rods fully inserted. The Auxiliary Feedwater System actuated on low-low steam generator water level as expected. All safety equipment operated as required. The control room crew closed the main steam isolation valves due to the condenser not being available.

CAUSE OF EVENT

The linkage mechanism that operates the breaker Y600 C phase pole failed due to a linkage connection pin falling out of the linkage. Inspection revealed that a bushing that is required between the linkage pin and the operating linkage was not installed. The bushing was most likely left out during fabrication. The lack of a bushing creates increased friction and tolerances leading to accelerated wear of the components.

CORRECTIVE ACTIONS

1. Switchyard breaker Y600 was satisfactorily repaired and tested.

2. The operating mechanism for switchyard breaker Y640 was inspected for generic implications. The operating mechanism and linkage pins were found to be satisfactory. Switchyard breaker Y640 is the only breaker installed in the switchyard with an operating mechanism the same as breaker Y600.

3. Switchyard inspection procedures will be reviewed to determine if improvements are needed. The review will specifically cover receipt inspection of new breakers installed in the switchyard to ensure breakers are not installed with similar operating mechanism manufacturing defects. This action will be completed by July 18, 2001.

ANALYSIS OF EVENT:

A notification was made to the Nuclear Regulatory Commission on March 1, 2001 at 1255 pursuant to 10CFR50.72(b)(2)(iv)(B) for an actuation of the Reactor Protective System and 10CFR50.72(b)(3)(iv)(A) for the actuation of specified systems.

The conditional core damage probability (CCDP) for a reactor trip is approximately 2x10-07.

ADDITIONAL INFORMATION:

In the past three years there have been no similar events regarding failure of the operating mechanism of a switchyard breaker. However, there were two events regarding switchyard breaker failures in March 1999 and June 2000 that were documented on Condition Reports 99-3690 and 00-9916, respectively.

The Y600 breaker is a 362 kV Mitsubishi 300 SFMT 50E breaker, serial number 970240101.

345 kV Switchyard Normal Operation 'BLESSING I �

SKYLINE

I DOW VELASCO 27 'LON HILL

HOLMAN

IW A PARRISH DOW VELASCO 18 345 KV NORTH BUS Y620 Y590 z Y500 Y510 � 1(14o Y570 --/630 Y600

MN

Y640 Y610 Y586 � Y550 345 KV SOUTH BUS SBT 2

BKR OPEN CI

CDI03026 (04/23/01) GEN 2 UAT 2

SBT I

UAT 1