05000424/LER-2001-001
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| Report date: | |
| 4242001001R01 - NRC Website | |
A. REQUIREMENT FOR REPORT
This report is required per 10 CFR 50.73 (a)(2)(iv) because an unplanned actuation of the reactor protection system occurred.
B. UNIT STATUS AT TIME OF EVENT
At the time of this event, Unit 1 was operating in Mode 1 (power operation) at 100% of rated thermal power. The generator was operating with Rectifier Bridge 1 out of service and Rectifier Bridges 2, 3 and 4 in service.
C. DESCRIPTION OF EVENT
Following a lightning strike on the unit's 230 kV line on August 18, 2001, anomalies were found on phase A silicon controlled rectifiers (SCRs) of the main generator Rectifier Bridge 1 in the form of failed LEDs and resistors. Upon removing Rectifier Bridge 1 from service for repairs, an anomaly (flickering LEDs) began to occur on phase C of Rectifier Bridge 2 due to a voltage imbalance.
Following consultation with the generator vendor, it was decided to repair Bridge 1, return it to service, then remove Bridge 2 from service for repair.
On August 24, 2001, after repairing Rectifier Bridge 1, personnel were placing the bridge back in service in accordance with procedure 13830-1, "Main Generator Operation." Upon closure of the disconnect switch for Rectifier Bridge 1, a trip of the main generator occurred on loss of field excitation, causing an automatic turbine/reactor trip at 2307 EDT. The first-out annuciator seen by operators in the control room was "Turbine Trip/P-9 Reactor Trip." All control rods were observed to fully insert, and a main feedwater system isolation (FWI) and an auxiliary feedwater system (AFW) actuation occurred as expected. Control room operators acted properly to control steam generator water levels and stabilize the unit in mode 3 (hot standby).
D. CAUSE OF EVENT
An investigation found that, upon returning Rectifier Bridge 1 to service, both phase A SCRs failed in Bridge 4 due to a short circuit. This led directly to the loss of generator excitation field and the generator/turbine/reactor trip. Several possible reasons for the short circuit were proposed, and the failed SCRs were sent to a laboratory for failure analysis. The analysis determined that overheating of the SCRs was the cause of the failures. Although possible causes for the overheating have been identified, the root cause of the overheating, and of this event, cannot be determined with certainty.
E. ANALYSIS OF EVENT
The reactor protection system, the main feedwater isolation function, and the auxiliary feedwater actuation function performed as designed. Control room operators acted properly to control steam generator water levels and stabilize the unit in mode 3 (hot standby). Based on these considerations, there was no adverse effect on plant safety or on the health and safety of the public as a result of this event.
This event does not represent a safety system functional failure.
F. CORRECTIVE ACTIONS
1) Rectifier bridges were repaired and the unit was returned to service.
2) Based on new information received from the generator vendor, operating and maintenance procedures have been revised and generator training for appropriate engineering and maintenance personnel is scheduled for later this year.
3) The failed SCRs were sent to a testing laboratory for failure analysis. The analysis determined that overheating of the SCRs was the cause of the the failures. Although possible causes for the overheating have been identified, the root cause of the overheating, and of this event, cannot be determined with certainty.
G. ADDITIONAL INFORMATION
1) Failed Components:
Silicon Controlled Rectifiers manufactured by General Electric Corporation Part # 44C338754G01 2) Previous Similar Events:
trip caused by problems with the generator control circuits.
3) Energy Industry Identification System Code:
Main Generator System - TB Main Generator Excitation System — TL Reactor Control System - JD Main Feedwater System — SJ Auxiliary Feedwater System - BA