05000368/LER-2009-001
| Arkansas Nuclear One - Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3682009001R00 - NRC Website | |
A. Plant Status At the time of the occurrence of this event, Arkansas Nuclear One, Unit 2 (ANO-2) was operating at 84% power following the securing of the "A" Main Feedwater Pump (MFWP) [SJ] due to bearing degradation.
B. Event Description
On March 13, 2009, at 19:18 CDT, the "A" MFWP outboard thrust bearing temperature detector began trending upward. Following a verification of the elevated bearing temperature, the "A" MFWP was removed from service at 19:34 CDT. The Feedwater Control System (FWCS) [JB] responded as designed to the loss of the "A" MFWP by increasing the "B" MFWP speed and modulating open the "B" Main Feedwater Regulating Valve (MFRV). Control room operators commenced a power reduction to 84%. During this period, "B" MFRV began ramping closed.
Upon detecting this abnormal condition the control board operator took the valve control to manual, inserting a full open demand signal. The valve was non-responsive to this action and continued to move in the closed direction resulting in the water level in the "B" Steam Generator reaching approximately 25%. At 21:51 CDT, operators manually tripped the reactor. Although the FWCS entered the Reactor Tripped Override (RTO) mode, it was unable to reposition either the MFRVs or the Main Feedwater Regulating Bypass Valves due to the system being in manual control mode. Therefore, the "B" MFWP was manually tripped at 21:55 CDT. The Emergency Feedwater (EFW) System actuated as designed and restored normal level in the Steam Generators.
The positioner was replaced and Unit 2 returned to 100% power operation, Mode 1, at 17:01 CDT on March 17, 2009.
C. Root Cause
Investigation revealed that a valve positioner for the "B" MFRV had failed causing the valve to be driven in the closed direction.
The MFRVs are controlled by Fisher Controls DVC6000 Series positioners. An integral part of the valve positioner is a current-to-pressure (UP) converter that transforms an electrical signal to a pneumatic signal between the electronic control system and the air operated valve. The I/P converter was found to be sticking. Although a definitive Root Cause has not been found, the vendor believes that the condition was caused by a foreign substance in the clearance area of the armature, internal to the I/P converter.
D. Corrective Actions
Immediate corrective actions included the replacement and diagnostic testing of the positioner, and particulate and moisture testing of the air supply to the valve.
E. Safety Significance
All safety systems were available and performed as designed. There was no radiological release to the public and the event presented no equipment safety or industrial safety issues. The Safety Significance of this event is low.
F. Basis for Reportability A manual reactor trip from power in response to actual plant conditions is reportable pursuant to 10 CFR 50.73(a)(2)(iv)(A).
G. Additional Information
There were no previous similar events reported as Licensee Event Reports by ANO.
Energy Industry Identification System (El IS) codes are identified in the test as [XX].