05000443/LER-2005-006
| Docket Number Sequential Revmonth Day Year Year Month Day Year N/A 05000Number No. | |
| Event date: | 05-01-2005 |
|---|---|
| Report date: | 06-29-2005 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4432005006R00 - NRC Website | |
I. Description of Event
On May 1, 2005 at 1109, Seabrook Station initiated a manual reactor trip from 17% power.
During initial start-up of the main turbine [TA, TRB] following refueling outage 10, the turbine automatically tripped at approximately 1045 rpm when turbine vibration exceeded the trip setpoint of 12 mils. Following the turbine trip, vibration levels continued to increase to between 22 and 24 mils. As a result, the operators manually tripped the reactor in accordance with station procedures in preparation for breaking condenser vacuum to slow the turbine. The emergency feedwater system (EFW) [BA] actuated on low steam generator levels following the reactor trip.
Condenser vacuum was broken until the high vibration condition cleared and was then subsequently restored to provide a secondary heat sink. The high vibration trip signal reset after approximately 20 minutes with turbine speed below 350 rpm. The main turbine was re-started on May 2, 2005 without incident.
II. Cause of Event
The cause of the turbine high vibration, which resulted in the need for a manual reactor trip, was a turbine rub (contact between the rotor and stationary components in the turbine). The High-Pressure Turbine rotor is a mono-block rotor design. Any rubs on the shaft at or below the rotor's first critical speed will result in rapidly increasing vibration levels. The rub creates a hot spot on the rotor that causes the shaft to bow, which will change the center of mass of the rotor. Once a rub starts it will typically spread to multiple locations as the rotor bows and vibration levels increase.
III. Analysis of Event
This event resulted in valid actuations of the reactor protection [JC] and emergency feedwater systems, meeting the reporting criteria of 10CFR50.72(b)(2)(iv)(B) and 50.72(b)(3)(iv)(A). An immediate report (event #41655) was made to the NRC at 1257 on May 1, 2005. This event is of regulatory significance because it resulted in actuation of systems provided to mitigate the consequences of an accident.
During the event, heat removal via the steam dumps [SB, XCV] to the main condenser [SG, COND] was unavailable for approximately one hour. The unavailability of the condenser following the reactor trip was inconsequential. Because of the low decay heat with the new core, operation of the emergency feedwater system alone prevented any temperature increase sufficient to demand operation of the atmospheric or condenser steam dump valves.
No consequences resulted from this event and, therefore, the event had no adverse impact on the plant or on the health and safety of the public. This condition did not involve a Safety System Functional Failure and no inoperable structures, systems, or components contributed to this event.
The reactor protection and EFW systems functioned as designed in response to this event.
IV. Corrective Action The planned corrective action is to add precautions to operating procedures to minimize thermal distortion and rapid changes in steam flow through the Turbine to reduce the possibility of initiating a rub.
V.Additional Information
The Energy Industry Identification System (EIIS) codes are included in this LER in the following format: [EIIS system identifier, EIIS component identifier].
VI. Similar Events Seabrook Station has had no similar events involving a reactor trip due to high vibrations on the main turbine.