05000374/LER-2003-004
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| 3742003004R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
General Electric - Boiling Water Reactor, 3489 Megawatts Thermal Rated Core Power
A. CONDITION PRIOR TO EVENT
Unit(s): 2 � Event Date: 7/7/03 � Event Time: 2319 Reactor Mode(s): 1 � Power Level(s): 100 Mode(s) Name: Run
B. DESCRIPTION OF EVENT
On 7/7/03, the LaSalle Station Unit 2 main power (MP)(EL] "B' phase disconnect in the switchyard catastrophically failed, resulting in an electrical short to ground. Protective relaying operated to clear the fault and trip the main generator. The reactor scrammed upon loss of the generator.
The failure occurred at the mechanical connection portion (beaver tail and contact finger) of the disconnect switch, and resulted in the disintegration of a significant portion of the "B" phase disconnect switch, rigid bus, and standoff insulator.
The electrical disturbance that caused the reactor scram also tripped the three Unit 2 circulating water (CW)(KE) pumps, which resulted in the loss of the main condenser as a heat sink. While in the expanded level band prescribed by LGA- 001, "RPV Control,' due to loss of the normal heat sink, the crew attempted to stabilize the plant and recover lost systems. While in the expanded level band, the shift crew allowed the Reactor Protection System (RPS)[7C] to automatically actuate four additional times on Reactor Low Level (Level 3, +11 inches) due to the shrink and swell effects from cycling safety relief valves (SRV).
This event was reportable under 10 CFR 50.73(a)(2)(iv)(A) as an event or condition that resulted in the automatic actuation Of the reactor protection system.
C. CAUSE OF EVENT
The root cause of the disconnect failure could not be determined. The severity of the damage left limited physical evidence for analysis. Disconnect misalignment can result in improper seating and subsequent heating of the disconnect; however, this was not a likely cause of this event because there was no misalignment on the AN or "C" phases. Thermography data prior to the event did not indicate an increasing temperature trend on the 'B' phase disconnect. A failure of the standoff insulator could have also caused the disconnect to fail.
The cause of the four additional scram signals received while attempting to stabilize the plant was a lack of a formal operational strategy for operating with a prescribed reactor water level band of -30 to +59.5 inches. LGP-3-2, 'Reactor Scram,' contains cautions that direct the NSO to reset the scram as noon as possible to prevent damage to the CRDs. LGA-001 directs a level band of -30 to 59.5 inches when using SRVE: for reactor pressure control. Because the Level 3 reactor low level scram setpoint is well above the bottom of the band (+11 inches), these procedural directions led the crew to conclude that additional scrams in this condition were expected and acceptable.
D. SAFETY ANALYSIS
The safety significance of this event was minimal. A reactor scram with a loos of the main condenser is an analyzed event. Reactor level and pressure were maintained using Reactor Core Isolation Cooling and the safety relief valves, and the High Pressure Core Spray System was operable throughout the event.
E. CORRECTIVE ACTIONS
1. A condition monitoring program will be developed to inspect and monitor the condition of the insulators, as appropriate. (AT# 166562-21).
2. The "A" phase insulator stack and the remaining insulator stack from the "B" phase will be sent to the vendor (LAPP Insulator) for further analysis. The results will be evaluated to determine whether additional actions are warranted GM 166562-24).
3. The adequacy of procedures MP-4.5.1 "Substation Disconnect Switch Visual Inspection", MP-4.5.2 *Disconnect Refurbishment Procedure", and MP-4.5.3 "Transmission Disconnect Switches Maintenance Requirements" will be reviewed for adequacy and revised as needed (Jaen 166562-22).
4. A formal strategy for operation in the expanded level band will be developed (AT# 166691-17).
F. � PREVIOUS OCCURRENCES LER NUMBER � TITLE 345 KV Line C Phase Insulator On January 31, 2001, LaSalle Unit 1 experienced a turbine generator trip and reactor scram. The cause of the generator trip was a phase-to-ground fault on the "C" phase of the transmission line between the main power transformer and the switchyard. The root cause of the event was build up of bird excrement, which caused tracking across the underslung support insulator. The corrective actions from this event would not have prevented the Unit 2 event.
Insulator Failure and Subsequent Flashover to Ground On March 28, 1990, LaSalle Unit 1 was at 100% power. The "II" phase insulator between the Unit 1 east/west main power transformers and the switchyard failed and flashed over to ground. Ac a result of this flashover, the "B" and "C" phase differential current relays both tripped. Thin caused a Unit 1 main generator lockout trip, which resulted in a main turbine trip and a reactor scram.
The "B" phase insulator was replaced. The remaining insulators were inspected and no significant problems were noted. The physical and material investigation done at that time did not determine a conclusive root cause. No signs of foreign materials or visual cracks could be found among the remains. The failure was deemed an isolated case, and no additional corrective actions were specified.
!VC ['OM 365A (7-2001) G. - COMPONENT FAILURE DATA Pascor Atlantic ITE Electric Type TTR6 disconnect switch rated at 2000 amps