05000328/LER-2008-002
| Sequoyah Nuclear Plant Unit 2 | |
| Event date: | 11-9-2008 |
|---|---|
| Report date: | 12-19-2008 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 3282008002R00 - NRC Website | |
I. PLANT CONDITION(S)
Unit 2 was in Mode 3.
II. DESCRIPTION OF EVENT
A. Event:
On November 9, 2008, at 1821 Eastern standard time (EST) with Unit 2 in Mode 3, the Unit 2 reactor trip breakers were opened from the main control room (MCR) because of indications of Shutdown Bank "A" Rod E-11 (El IS code AA) dropping into the reactor core.
At the time the reactor trip breakers were opened, the MCR operators were in the process of withdrawing shutdown banks in preparations for entry into Mode 2. Rod E-11 rod position indication dropped to zero and the rod bottom light was lit. The MCR operators entered the appropriate abnormal operating procedure and opened the reactor trip breakers based on indications that were available. All other shutdown banks and control banks were inserted at the time the reactor trip breakers were opened. All safety-related equipment operated as designed. Subsequent to the manual reactor trip, investigation revealed that the plant computer trace for Rod E-11 showed that the failure was in the rod position indication (RPI) system and was not a dropped rod. Further investigation showed that the problem was a broken wire on the E-11 coil stack on top of the reactor head. This report is being submitted in accordance with 10 CFR 50.73 (a) (2) (iv) (A), as an event that resulted in a valid actuation of the reactor protection system.
B. Inoperable Structures, Components, or Systems that Contributed to the Event:
None.
C. Dates and Approximate Times of Major Occurrences:
November 9, 2008 Shutdown Bank "A" control rods were pulled to 222 steps in at 1804 EST� preparation for going to Mode 2.
November 9, 2008 RPI on Rod E-11 rapidly decreased to zero.
at 1809 EST November 9, 2008 Manual actuation of the reactor protection system was at 1821 EST initiated because of indications of a dropped rod (E-11) when withdrawing Shutdown Bank "A.
D. Other Systems or Secondary Functions Affected:
No other systems or secondary functions were affected by this event.
E. Method of Discovery:
Operators noticed RPI E-11 dropping to zero when withdrawing Shutdown Bank "A" in preparations for entry into Mode 2.
F. Operator Actions:
Manual actuation of the reactor protection system was initiated because of indications of a dropped rod (E-11) when withdrawing Shutdown Bank "A." Control room personnel maintained the unit stable in hot standby, Mode 3.
G. Safety System Responses:
The plant responded to the reactor trip as designed.
III.�CAUSE OF THE EVENT A. Immediate Cause:
The cause was a broken RPI secondary coil stack wire.
B. Root Cause:
The most probable cause is that the coil stack wire was flexed and weakened during normal refueling outage activities on top of the reactor head. Then the vibration from the control rod being withdrawn could have caused the wire to fail.
C. Contributing Factor:
The RPI coil stack wire insulation has degraded over time because of heat.
IV. ANALYSIS OF THE EVENT
The plant systems responded to the reactor trip as designed. All of the rods were fully inserted as required. The unit was cooled down to Mode 5 to repair the broken rod position indication coil stack wire for Rod E-11.
V.�ASSESSMENT OF SAFETY CONSEQUENCES Based on the above "Analysis of The Event," this event did not adversely affect the health and safety of plant personnel or the general public.
VI.�CORRECTIVE ACTIONS A. Immediate Corrective Actions:
The broken coil stack wire was repaired.
B. Corrective Actions to Prevent Recurrence:
The RPI reactor head cable replacement modification (currently scheduled for the next refueling outage) will provide a new coil stack top plate, connector, and new cable to the vertical panel. It will prevent future flexing of the coil stack wire.
VII. ADDITIONAL INFORMATION
A. Failed Components:
The RPI coil stack wire broke.
B. Previous LERs on Similar Events:
A review of previous reportable events did not identify any previous similar events.
C. Additional Information:
None.
D. Safety System Functional Failure:
This event did not result in a safety system functional failure in accordance with 10 CFR 50.73(a)(2)(v).
E. Unplanned Scram with Complications:
This condition did not result in an unplanned scram with complications.
VIII. COMMITMENTS
None.