05000424/LER-2006-001
| Vogtle Electric Generating Plant - Unit 1 | |
| Event date: | 04-17-2006 |
|---|---|
| Report date: | 06-09-2006 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4242006001R00 - NRC Website | |
A) REQUIREMENT FOR REPORT 10 CFR 50.73 (a)(2)(iv) requires this report 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 in Mode 1 (Power Operation) at 33% rated thermal power. Other than that described herein, there was no inoperable equipment that contributed to the occurrence of this event.
C) DESCRIPTION OF EVENT On April 15, 2006, while at 100% power the Unit 1 Control Room crew was challenged by an erratic response of the Loop 3 Main Feed Regulating Valve (MFRV), 1FV0530, while operating in automatic.
The crew was able to stabilize the valve in manual mode, however, over the next 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> the control of the valve continued to degrade. On April 16, 2006, it was decided that Unit 1 would be taken to Mode 3 (Hot Standby) for investigation of the issues with the Loop 3 MFRV. On April 17, 2006, at 0026 EDT, Unit 1 was manually tripped at 33% power when Steam Generator (SG) number 3 water level was observed to be slowly increasing with the Loop 3 MFRV, 1FV0530, unable to control level in either automatic or manual. The Main Feedwater system isolated and the Auxiliary Feedwater (AFW) system actuated. Control room operators responded to throttle the AFW valves to control SG water level, and the unit transitioned to stable operation in Mode 3. The NRC Operations Center was notified of this event on April 17, 2006, at 0120 EDT.
The failed transducer was procured from the vendor as a replacement transducer for the obsolete model I/P transducer. The obsolete model was originally installed in the plant because of its reliability and tight shutoff feature not available from other manufacturers. Plant Vogtle utilized the tight shutoff feature to ensure VP or positioner calibration drift did not cause valve leakage. Prior to installation at Plant Vogtle, the replacement model I/P transducer was evaluated in 1999 during AOV diagnostic testing and in the Steam Generator Blowdown (SGBD) System. Through testing and use in the SGBD system, the replacement model I/P transducer proved to be reliable.
The vendor has made manufacturing changes since the initial evaluation of the replacement model I/P transducers. When these changes were made to the non-safety related component, the vendor did not change the model number, outline drawing, installation and operating manual, or otherwise denote the changes. These changes degraded the reliability of the replacement model I/P transducers, which caused unanticipated equipment failures.
D) CAUSE OF EVENT On April 17, 2006, Unit 1 was manually tripped at 33% power because of the inability to control Steam Generator water level using the Loop 3 Main Feed Regulating Valve (MFRV). A review of the event found that the Loop 3 MFRV, 1FV0530, operated erratically due to a failed I/P transducer. Failure analysis results showed two failures of the I/P transducer. One failure was the air supply input connector and the resultant electrical to pneumatic conversion process for a valve internal to the controller, and the second failure was the electronic circuit board.
E) ANALYSIS OF EVENT A Unit 1 shutdown was in progress at the time of the reactor trip due to concerns with the Loop 3 Main Feed Regulating Valve (MFRV). The unit shutdown had commenced from 100% power on April 16, 2006 at 1603 EDT. The unit was manually tripped at 33% power on April 17, 2006 at 0120 EDT due to the inability to control Steam Generator (SG) number 3 water level using MFRV 1FV0530. Following the reactor trip, the Main Feedwater system isolated and the Auxiliary Feedwater (AFW) system actuated.
Control room operators responded to throttle the AFW valves to control SG water level, and the unit transitioned to stable operation in Mode 3. 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 ACTION MFRV I/P transducers were replaced prior to the unit restart.
2) A schedule for the replacement of all I/P transducers of the same make and model as the failed transducer has been developed. There are approximately 33 I/P transducers of the same make and model remaining in various plant applications.
3) Digital positioners were installed in the Unit 2 MFRVs during the Fall 2005 refueling outage, MFRVs during the Fall 2006 refueling outage.
G. ADDITIONAL INFORMATION
1) Failed Components:
VP Transducer 2) Previous Similar Events:
There have been no previous events with a similar cause in the last three years.
3) Energy Industry Identification System Codes:
Main Feedwater System — SJ Auxiliary Feedwater System — BA P RC FORM 366A (1-2001)