05000374/LER-2003-001
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv), System Actuation |
| 3742003001R00 - 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: 1/10/03 � Event Time: 1156 Reactor Mode(s): 1 � Power Level(s): 090 Mode(s) Name: Run
B. DESCRIPTION OF EVENT
On January 10, 2003, at 11:56, Unit 2 was manually scrammed in anticipation of an automatic scram due to a decreasing reactor water level.
The event was initiated by a trip of the 2B Condensate/Condensate Booster Pump (CD/CB)[SD) on instantaneous overcurrent. The 2D CD/CB pump was out of service for planned maintenance and was not available to auto-start to compensate for the loss of the 2B pump, which resulted in the condensate system running at two- thirds capacity, The overcurrent condition on the 2B CD/CB pump caused a momentary (55msec) low voltage condition on 6.9 kV Bus 252 and the downstream sub-buses. The low voltage condition along with a latent failed level switch on the heater drain tank caused the 2A and 2C Heater Drain pumps to trip. This produced a significant reduction in condensate flow to the feedwater system, and resulted in an automatic trip of both Turbine Driven Reactor Feed Pumps (TDRFP)(FW)[SJ] on low suction pressure. When the Control Room Operators could not maintain the reactor in the band of 20 to 50 inches and in control as specified by the Unit Supervisor, the Unit was manually scrammed.
The Unit 2 Station Air (SA)[LF] Compressor tripped due to a loss of control power as a result of the voltage reduction on Bus 252. The Unit 2 'A' and 'C' Circulating Water (CW)(KE] pumps tripped due to a loss of synchronization that occurred during the fast bus transfer of 4.16 kV Bus 241X from the Unit Aux Transformer to the Station Aux Transformer.
Other plant equipment responded normally to the scram. Operators restored normal reactor water level using the Motor Driven Reactor Feed Pump.
An ENS call was made at 1505 hours0.0174 days <br />0.418 hours <br />0.00249 weeks <br />5.726525e-4 months <br /> on January 10, 2003, in accordance with 10 CFR 50.72(b)(2)(iv)(B). This event is reportable under 10 CFR 50.73 (a) (2) (iv) (A) as an actuation of the reactor protection system (RPS) when the reactor is critical.
C. CAUSE OF EVENT
The trip of the 2B CD/CB pump was due to a broken lug on the 6.9 kV cable connection to the 'B' phase of the motor. The lug broke away, resulting in a cloud of ionized gas. This caused a three phase fault, which resulted in the instantaneous overcurrent trip of the motor. The `B' phase lead was found disconnected and out of position inside the lead box. The lead box cover was found on the floor, some four feet north of the pump, along with the majority of the associated box cover fasteners. The 2B CD/CB Pump motor was tested, and passed all electrical tests, which indicated that the 'B' phase lead failed in service.
The root cause of this event was an inadequate original installation, in that the field fit up of the lead box resulted in a vibration-induced fracture and failure of the 'B' phase motor cable lug on the 2B CD/CB pump. The routing of the cable into the motor termination box placed additional weight and stresses on the cable lug (i.e., the weight of the cable conductors and tension due to cable bends).
The cause of the trip of the 2A and 2C HD pumps was a failure in the two out of two logic circuit, in which a failed heater drain tank level switch caused one relay in the logic to be in the tripped condition. This relay failure, when combined with the reduced voltage experienced during the 2B CD/CB motor overcurrent, resulted in the completion of the second part of the logic, which tripped the 2A and 2C HD pumps. The failed level switch was corrected as part of the troubleshooting and recovery efforts to return the unit to service.
D. SAFETY ANALYSIS
The safety significance of this event inserted and all systems responded as Cooling Systems were not challenged.
water flow is an analyzed event. The increase as a result of this event.
E. CORRECTIVE ACTIONS
Corrective Actions:
was minimal. All control rods fully expected to the scram. The Emergency Core The loss of a CD/CB pump or reactor feed likelihood of a severe accident does not 1. Large cable routings will be reviewed and terminations in junction boxes attached to large motors will be inspected (AT# 139037-25, 26).
2. The lug failure will be reviewed to determine applicability to other Exelon stations (AT# 139037-28).
3. Engineering will evaluate if a typical motor fault will cause a loss of loads on station busses, and recommend possible changes to prevent loss of loads (AT# 139037-23).
4. A review will be performed to identify and evaluate plant systems for latent failures that could potentially affect plant production (AT# 139037-21).
Corrective Action to Prevent Recurrence:
5. The 2B CD/CB motor terminations will be repaired to ensure that stresses on the lugs are minimized (AT# 139037-19).
F. PREVIOUS OCCURRENCES
On July 24, 1996, the CD/CB pump 2CDO1PB auto tripped. Investigation found that overcurrent relay 2551-AP115 was tripped at Bus 252. Inspection of the 2B CD/CB pump found that the 'A' phase motor pigtail had burned off at the lug. A new 3/0 3/8" hole lug was attached and the lugs for the 'B' and 'C' phase motor leads were also replaced. The repair was performed under WO 9607053501. The cause of the problem was identified in the work order as an improperly installed and sized lug. This was a different cause than the scram on January 10, 2003.
G. COMPONENT FAILURE DATA
Penn-Union, Connector, Lug, 4/0 Cable, Part No. BBLU-4/0D