05000482/LER-2004-005
| Wolf Creek Generating Station | |
| 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 |
| 4822004005R00 - NRC Website | |
Background:
The Turbine Supervisory Instrumentation (TSI) System [EIIS Code: JJ] monitors selected turbine generator parameters and provides corresponding information to Operations personnel as well as input signals to the turbine's Electro Hydraulic Control System (EHC) [EIIS Code: TG] . The Turbine Supervisory Instrumentation maintains a history of turbine data plus assists during operation, maintenance and tests by providing information of specific turbine parameters. The Turbine Vibration Monitoring System is part of the TSI and is designed to sense, record and indicate bearing vibration and phase angle information. The vibration monitoring system provides both local and remote high vibration alarms. In addition, a high-high alarm provides alarm and turbine trip functions.
Plant Conditions Prior to the Event:
MODE —1 Power — 100 percent Normal Operating Temperature and Pressure
Event Description:
On October 7, 2004 Wolf Creek Generating Station (WCGS) was operating at 100 percent steady state power, with the "A" train of the Residual Heat Removal System (RHR) out of service for normal maintenance. At 11:48 A.M., the plant experienced an automatic reactor trip due to indicated high main turbine vibrations. Concurrent with this event, several lightning strikes from an electrical storm were observed in the immediate area, including one recorded in the vicinity of the steel high line tower for the Main Startup Transformer. This lightning strike created an induced electromagnetic field (EMF) that caused the actuation of the Main Turbine High-High Vibration Trip circuitry, which then caused the subsequent automatic reactor trip.
All control rods fully inserted and all safety-related equipment operated as designed. With the exception of the normal maintenance in progress on the "A" RHR pump, no other activities were underway at the time of this event.
Basis for Reportability:
This event is reportable per 10 CFR 50.73(a)(2)(iv)(A), which requires reporting of "Any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph (a)(2)(iv)(B) of this section? Paragraph (B) (1) of 10 CFR 50.73(a)(2)(iv) includes "Reactor protection system (RPS) including: reactor scram or reactor trip?
Root Cause:
The root cause of this event was a lightning induced electromagnetic interference (EMI), triggered by an electrical storm. This EMI caused a false high-high vibration signal from the main turbine vibration monitoring system that actuated the turbine trip logic that tripped the main turbine.
This signal has been verified to be a false input through the verification of normal vibration levels prior to the trip, during the coast down of the turbine following the trip, and during and after the subsequent plant return to full power operation. There were no abnormal vibrations noted in the turbine building, nor were any electrical trips initiated from the generation protection or switchyard protection systems during this event.
Corrective Actions:
Walkdowns of the containment structure, all power block buildings, and switchyard were conducted following this event. No damage to any system, structure, or components due to this event was identified. Main turbine data collected after the turbine trip was reviewed and verified that there was no indication of any damage to the turbine.
An electrical ground on the EHC system was repaired by replacement of an associated relay and power supply.
Safety Significance:
The safety significance of this event is low. All safety related systems, structures, and components performed as required and expected. Nuclear safety was maintained by implementation of station procedures after the automatic shutdown of the reactor. There were no adverse effects on the health and safety of the public.
Operating Experience/Previous Events:
A review of WCNOC License Event Reports submitted over the last 5 years revealed no instances where a reactor trip resulted from EMI generated from a lightning strike in the vicinity of power block structures or the switchyard.