05000374/LER-2002-002
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 3742002002R00 - 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: 05/30/02 � Event Time: 1551 Hours Reactor Mode(s): 1 � Power Level(s): 100 Mode(s) Name: Run
B. DESCRIPTION OF EVENT
On 05/30/02, at 1551, the 2B Diesel Generator (DG)[EK] was started and brought to full load for post-maintenance testing. The DG operated normally for approximately 15-20 minutes, then reactive load (VAR) began to vary erratically.
The control room operator was unable to control VAR manually using the voltage regulator motor-operated potentiometer (MOP).
The 2B DG was unloaded and the MOP was wiped (exercised) in accordance with LOS- DG-M3 "1B(2B) Diesel Generator Operability Test." The DG was returned to full load, and VAR continued to vary erratically. It was then unloaded and shutdown.
Troubleshooting determined the cause of the event to be a failure of the voltage regulator range potentiometer R3. Because the other four LaSalle DGs use the same R3 potentiometer, operability tests were conducted in accordance with TS 3.8.1., RA B.3.1., on the 0, 1A, 1B, and 2A DGs, in order to determine that they were not inoperable due to a common mode failure. All were tested successfully.
The R3 potentiometer and the MOP were replaced, and the 2B DG was returned to service at 1350 on 5/31/02.
The 2B DG provides emergency power to the single-train High Pressure Core Spray (HP)[BG] system; therefore, this failure was determined to be reportable under 10 CFR 50.72(b)(3)(v)(D) as a failure that could have prevented fulfillment of a safety feature needed to mitigate the consequences of an accident. The NRC was notified via an ENS call at 0008 on 5/31/02.
This event is considered a safety system functional failure.
C. CAUSE OF EVENT
The root cause was a poor design choice by the vendor, Basler Electric Co., because the potentiometer used in the R3 application is a low-cost component that is frequently noisy even when new.
D. SAFETY ANALYSIS
The safety significance of the event was minimal. The 2B DG was out-of-service for maintenance when the failure occurred, and was restored to operable status within the Technical Specification allowed outage time. Since the 2B DG provides emergency power to High Pressure Core Spray, this event is reportable under 10 CFR 50.73(a)(2)(v)(D) as a condition that could have prevented fulfillment of a safety function. Normal AC power was available to HP at all times.
Had the R3 potentiometer failed with the 2B diesel generator operating in an emergency mode (isochronous), the failure would have resulted in actual changes in bus voltage, which could have been outside Technical Specification tolerances.
Had this resulted in the loss of HP, all other ECCS systems and Reactor Core Isolation Cooling (RI) [BN] were operable and available to mitigate the consequences of an accident.
E. C CORRECTIVE ACTIONS
Corrective Actions:
1. The R3 potentiometer and the MOP were replaced, and the 2B DG was tested and returned to service (Complete).
2. Operability tests were conducted in accordance with TS 3.8.1., RA B.3.1. on the 0, 1A, 1B, and 2A DGs, in order to determine that the operable DGs were not inoperable due to a common mode failure (Complete).
Corrective Action to Prevent Recurrence:
3. New periodic maintenance items were created to wipe the R3 potentiometers and check them for proper operation every 24 months (Complete).
4. System Engineering will investigate an alternative to the Basler R3 potentiometer through Engine Systems, Inc. (ESI), the LaSalle EMD diesel vendor (AT# 110032-17).
F. C PREVIOUS OCCURRENCES There was a previous failure of the R3 potentiometer in 1995, on the 2B DG. The failure occurred at approximately 15 years of service life. The corrective actions included replacement of the R3 potentiometers in all five DGs.
In 1999, the entire voltage regulator was changed out on the 2B DG, and shortly thereafter the R3 potentiometer failed. This was considered an infant component failure, and the potentiometer was replaced. In 2001, a faulty R3 potentiometer was identified on the 2A DG during replacement of a MOP.
The information available prior to the 2002 R3 failure indicated that the maintenance being performed was adequate to ensure the reliability of the voltage regulator.
G. C COMPONENT FAILURE DATA Manufacturer: Basler Electric Co.
Potentiometer: 150 ohms, 4 watt, 270° range of operation Part#: 04768