05000373/LER-2001-001

LER-2001-001,

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation
3732001001R00 - NRC Website

PLANT AND SYSTEM IDENTIFICATIONq General Electric - Boiling Water Reactor, 3489 Megawatts Thermal Rated Core Power

A. CONDITION PRIOR TO EVENT

Unit(s): 1/2 � Event Date: 01/31/01 � Event Time: 2147 Hours Reactor Mode(s): 1/1 � Power Level(s): 100/100 Mode(s) Name: Run/Run

B. DESCRIPTION OF EVENT

At 2147 hours0.0248 days <br />0.596 hours <br />0.00355 weeks <br />8.169335e-4 months <br /> on January 31, 2001, LaSalle County Station, Unit 1, experienced a turbine generator trip and subsequent reactor scram. Initial indications were of an electrical fault at or near the Unit 1 main power transformer (MPT)[EL].

Witnesses heard a loud boom, and an associated bright flash was seen several miles away. The operators responded to the scram in accordance with management expectations, and the plant responded as expected, with the following exceptions:

• The lA Circulating Water pump tripped during the power supply fast transfer. The slip guard relay was found tripped. The relay was reset and the pump was restarted without additional corrective action.

• The Division 1 Alternate Rod Insertion failed to reset on scram recovery. A work request was written, and Electrical Maintenance reset the power converters and the system reset successfully.

• A low oil level alarm was received on the 1B Reactor Recirculation pump when the attempt was made to restart it. A containment entry was made to inspect the pump motor, and the oil reservoir level was found about five pints low. There was no sign of a large leak. The reservoir was refilled and the pump started.

• There was an electrical perturbation on Unit 2 that resulted in a loss of the 2A heater drain pump and the 24B and 25B feedwater heaters. Power was stablized at 930 MWe. The heaters were restored and Unit 2 was returned to full power at 0625 hours0.00723 days <br />0.174 hours <br />0.00103 weeks <br />2.378125e-4 months <br /> on February 1, 2001.

The cause of the main generator trip was determined to be a phase-to-ground fault on the "C" phase of the transmission line between the MPT and the switchyard.

The fault activated the differential relay scheme for the MPT, which led to the unit trip and activated the deluge system for the transformers. System digital fault recorders and relaying targets showed that the system responded properly to the fault.

The MPT consists of two Westinghouse 7,000,000 series 625 MVA three phase transformers connected in parallel. Aluminum cable connects the transformers to a common aluminum hard bus, which extends to an A-frame tower, where it is supported by an underslung insulator. Aluminum cable from the 345 KV switchyard is then bolted to the aluminum bus via a pad connection. A strain bell insulator is used to provide lateral support for the cable between the A-frame tower and the next transmission tower.

A track of bird excrement approximately one half inch wide was found on the top of the "C" phase underslung support insulator. Arcing was evident across the underslung insulator, as was flashover across the strain-bell insulator. The flash ended at the corona ring at the end of the strain-bell insulator, which was heavily damaged. The transmission line was also damaged. An inspection of the surrounding area found no other secondary damage from the event.

This event is reportable pursuant to 10 CFR 50.73(a)(2)(iv) as an event that resulted in an automatic actuation of the reactor protection system.

C. CAUSE OF EVENT

The root cause of the event was a build up of bird excrement, which resulted in tracking across the underslung support insulator. The most probable initiator of the tracking was the weather conditions just prior to the event. The air temperature had increased and a light, misty rain was falling, which is postulated to have washed the excrement down the side of the underslung insulator.

D. SAFETY ANALYSIS

The safety consequences of this event were minimal. No equipment failed to operate during this event. The potential for a turbine generator trip is an analyzed condition of moderate frequency (Updated Final Safety Analysis Report, Section 15.2.3, "Turbine Trip".) The reactor was safely shut down and recovery performed without incident.

Corrective Actions:

1) The damaged insulators and transmission line were repaired.

2) The other Unit 1 insulators and the MPT were inspected and cleaned.

Corrective Action to Prevent Recurrence:

3) Substation Engineering will investigate and design appropriate measures to prevent birds from roosting on "A" frame structures.

4) LaSalle Station, with the assistance of Transmission and Distribution Engineering, will determine long term plans to address the bird issue.

TITLE

Insulator Failure And Subsequent Flashover To Ground On March 28, 1990, the "B" phase support insulator on the same Unit 1 tower failed. This was a catastrophic failure, typical of internal breakdown as approved to the surface damage caused by the tracking and phase-to-ground arcing in this most recent event. There is no indication that this most recent trip was caused by a failure of the insulator.

G. � COMPONENT FAILURE DATA Since no component failure occurred, this section is not applicable.