05000424/LER-2009-003
| Vogtle Electric Generating Plant - Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4242009003R00 - NRC Website | |
A. REQUIREMENT FOR REPORT
This report is required per 10CFR50.73(a)(2)(iv)(A) due to a manual actuation of the Reactor Protection System (RPS) and an automatic actuation of the Auxiliary Feedwater (AFW) System.
B. UNIT STATUS AT TIME OF EVENT
Unit 1 was in Mode 1 (Power Operation) operating at approximately 24 percent rated thermal power, preparing to synchronize the main generator to the grid, during power ascension, following completion of an unplanned shutdown.
C. DESCRIPTION OF EVENT
On December 9, 2009 during power ascension from an unplanned shutdown the main turbine was accelerated to 1800 revolutions per minute (rpm) in preparation for synchronization to the grid.
During preparations for synchronization, the main turbine vibration levels increased. Due to the elevated vibration levels on the main turbine, the operators manually tripped the main turbine from the control room in accordance with plant procedures. Once the turbine was tripped and started to coast down, vibration levels momentarily stabilized and then began to increase. Due to the increase in vibration levels on the main turbine, the operators broke condenser vacuum in accordance with plant procedures to slow the turbine to minimize any potential damage to the turbine. Prior to breaking condenser vacuum the reactor was manually tripped because operation at 24 percent power could not be sustained without the main condenser available. Without the main condenser available, the main feedwater pump would trip on low condenser vacuum and the turbine bypass valves would not be available. Therefore, the control room operators manually tripped the reactor at approximately 23:10 hours EST on December 9, 2009. Both motor driven auxiliary feedwater pumps automatically started in accordance with plant design upon trip of the main feedwater pump on low condenser vacuum. All safety systems responded per design and the plant was stabilized in Mode 3.
D. CAUSE OF EVENT
Investigation into the event determined the high vibration experienced by the turbine was as a result of a "rub" between the turbine rotor and the surrounding packing. During the spring 2008 refueling outage on Unit 1, the high pressure turbine rotor, diaphragms, and packing were replaced. The tight clearances associated with new packing introduce the opportunity for rubs and related high vibrations. A rub occurs when a stationary component contacts a rotating component while the turbine is turning. The result of rubbing is localized hot spots on the rotor's surface at the point of contact. The heat of friction developed during a rub causes the rotor material directly under the point of contact to expand with respect to the rest of the rotor.
This uneven material expansion causes thermal distortions resulting in rotor bowing. As a rotor bows, it centerline of mass and center of rotation move relative to each other resulting in changes in rotor vibration. This was found to be a common occurrence at no load or low load conditions as documented in operating experience across the industry. Less severe rubs had occurred on three of the five turbine rolls between the installation of the new equipment and this event.
E. ANALYSIS OF EVENT
Due to high vibration levels on the high pressure turbine, control room operators tripped the turbine in accordance with plant procedures. As the main turbine began to coast down, vibration levels momentarily stabilized and then started to increase. In accordance with plant procedures, the control room operators broke vacuum in the main condenser to minimize damage to the turbine. Also, prior to breaking condenser vacuum and in anticipation of the trip of the main feedwater pump due to low condenser vacuum and the resultant loss of feedwater flow, control room personnel acted appropriately to trip the reactor and prevent a challenge to the automatic reactor trip actuation circuitry. As anticipated, the main feedwater pump tripped due to low condenser vacuum which initiated the automatic start of the motor driven AFW pumps. All control rods fully inserted, and all safety systems responded in accordance with plant design.
Based upon these considerations, there was no adverse affect on plant safety or the health and safety of the public.
This event does not represent a safety system functional failure.
F. CORRECTIVE ACTIONS
1. The turbine was placed on turning gear to remove the bow on the rotor. The unit was subsequently returned to service without problems.
2. Operating procedures were revised to minimize activities performed with the turbine at rated speed prior to connecting the generator to the grid thus minimizing the time at no load or low load conditions.
G. ADDITIONAL INFORMATION
1. Failed Components:
None. Although the main turbine experienced high vibration levels, the cause of the high vibrations was due to a rub developing in the high pressure turbine. Once the turbine coasted down, it was placed on the turning gear. It was subsequently returned to service without problems.
� 2. Previous Similar Event:
A review of Licensee Event Reports for the past three years did not identify a similar occurrence where a manual actuation of the RPS was required as a result of high vibrations associated with the main turbine.
3. Energy Industry Identification System Codes:
Main Turbine System-TA Auxiliary Feedwater System-BA �