05000412/LER-2006-001
| Beaver Valley Power Station | |
| Event date: | 04-02-2006 |
|---|---|
| Report date: | 05-24-2006 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4122006001R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
Westinghouse-Pressurized Water Reactor {PWR} Main Generator Excitation System {TL}
CONDITIONS PRIOR TO OCCURRENCE
Unit 2: Mode 1 at 100 percent power.
There were no systems, structures, or components that were inoperable at the start of the event that contributed to the event other than as described below.
DESCRIPTION OF EVENT
On April 2, 2006, the Beaver Valley Power Station (BVPS) Unit No. 2 reactor tripped at 1402 hours0.0162 days <br />0.389 hours <br />0.00232 weeks <br />5.33461e-4 months <br /> due to a Main Turbine trip from a Main Generator Loss of Field Protection Trip. The first out computer point was Main Generator Loss of Field along with Generator Protection Generator Trip. Following the reactor trip, safety systems functioned as designed. Although both safety related emergency busses correctly switched automatically to their respective offsite power feed when the Main Generator tripped, both Emergency Diesel Generators (EDGs) automatically started due to a momentary under voltage condition caused by the initial Main Generator trip. Both EDGs ran unloaded. All three Auxiliary Feedwater System pumps started as expected on low steam generator levels. The plant was stabilized in Mode 3 with primary and secondary parameters returned to normal hot standby values.
Subsequent investigation discovered that an open circuit existed on a series connected field pole of the Exciter on the BVPS Unit No. 2 Main Generator. The open circuit was a tear in the No. 2 series field pole terminal which connected the No. 1 coil to the No. 2 coil. The tear was across the entire terminal. The open circuit can best be described as a fracture which propagated until the connector had insufficient current carrying capability. With the open circuit resulting from the failed pole connection, the Main Generator tripped on loss of field.
This No. 2 series field coil was the only coil in which a support block was not installed between the top and bottom terminals.
The No. 2 EDG experienced a control power alarm when it was shutdown from its unloaded condition approximately one and one half hours after the reactor trip. This was due to a failure of a diode in the EDG's voltage regulator motor operator controller bridge following the normal small voltage surge experienced whenever the EDG is shutdown. This failure was unrelated to the reactor-generator trip event. The EDG would have functioned satisfactorily prior to its manual shutdown.
CAUSE OF EVENT
The direct cause of the reactor trip was a turbine trip, caused by a main generator trip. The main generator tripped the main turbine, which will then cause a reactor trip if power is above 49 percent.
The root cause of the generator trip has been determined to be less than adequate workmanship, manufacturing and/or disassembly inspections. Initial inspection by the vendor (post-trip) identified that the block which supports the field pole #1 and field pole #2 terminals was not present. With this block missing, the field pole connection is subject to increased flexing during normal operation and also during maintenance activities. This increased flexing led to fatigue failure of the Exciter Field. The missing support block on pole #2 should have been installed prior to shipment to Beaver Valley Power Station. It was indeterminate if the support block was removed during one of the Exciter inspections/disassemblies (performed by the vendor at a ten yeafirequency), or never installed during the manufacturing process.
REPORTABILITY
On April 2, 2006, Unit 2 experienced a reactor trip at 1402 hours0.0162 days <br />0.389 hours <br />0.00232 weeks <br />5.33461e-4 months <br />. This is reportable pursuant to 10 CFR 50.72(b)(2)(iv)(B) as an actuation of the Reactor Protection System when the reactor is critical and pursuant to 10 CFR 50.72(b)(3)(iv)(A) as an event that resulted in the valid automatic actuation of the Reactor Protection System, Auxiliary Feedwater System and Emergency Diesel Generators. BVPS notified the Nuclear Regulatory Commission per 10 CFR 50.72 via EN 42467 at 1546 EST on April 2, 2006.
This reactor trip is reportable pursuant to 10 CFR 50.73(a)(2)(iv)(A) as an event that was not part of a pre-planned sequence which resulted in the valid automatic actuation of: the Reactor Protection System per 10 CFR 50.73(a)(2)(iv)(B)(1), and of the Auxiliary Feedwater System per 10 CFR 50.73(a)(2)(iv)(B)(6), and of the onsite emergency AC electrical power systems - Emergency Diesel Generators per 10 CFR 50.73(a)(2)(iv)(B)(8).
SAFETY IMPLICATIONS
At the time of the reactor trip, no components credited in the current PRA model were unavailable. All safety systems functioned as designed. This event is bounded by the Design Basis "Loss of External Electrical Load and/or Turbine Trip" described in Section 14.1.7 of the BVPS Unit 1 Final Safety Analysis Report (UFSAR).
The plant risk associated with the BVPS Unit 2 reactor trip that occurred on April 2, 2006, due to the main generator trip is considered to be low. This is based on the conditional SAFETY IMPLICATIONS (Continued) core damage probability for the event when considering the actual component unavailability that was present at the time of the reactor trip.
Based on the above, the reactor trip safety significance on April 2, 2006, was very low.
CORRECTIVE ACTIONS
1. The exciter field coil was replaced and a support block installed.
2. The existing main exciter preventative maintenance task is being revised to ensure that the proper details exist on disassembly and reassembly of the exciter field pole connectors.
3. A generator/exciter preventative maintenance template is being developed to include a mechanical inspection of the exciter which includes the exciter stator series lead connections and a verification of the presence of all the stator field lead support blocks.
Completion of the above and other corrective actions are being tracked through the BVPS corrective action program.
PREVIOUS SIMILAR EVENTS
A review found one prior BVPS Unit 1 and no prior BVPS Unit 2 Licensee Event Reports within the last ten years involving a main generator problem.
- LER 1-99-010, "BVPS Unit 1 Manual Reactor Trip Due to Main Unit Generator Voltage Regulator Malfunction". This manual trip was initiated following cyclic field current variations observed in the control room, subsequently determined to be from a degraded voltage regulator logic drawer. The cause of this event was not similar to the cause of the Main Generator event on April 2, 2006.
COMMITMENTS
There are no new commitments made by FirstEnergy Nuclear Operating Company (FENOC) for BVPS Unit No. 2 in this document.