05000328/LER-2003-002
| Sequoyah Nuclear Plant (Sqn) Unit 2 | |
| Event date: | 01-24-2003 |
|---|---|
| Report date: | 03-25-2003 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 3282003002R00 - NRC Website | |
i. PLANT CONDITION(S) Unit 2 was in power operation at approximately 100 percent power.
II. D DESCRIPTION OF EVENT
A. Event:
On January 24, 2003, at 0835 Eastern Standard Time, Operations personnel observed that one of the Unit 2 refueling water storage tank (RWST) (EIIS Code BQ) level transmitters had failed high and another transmitter was observed to be reading higher than normal. Inspections by instrument maintenance personnel revealed that the transmitters were malfunctioning because of freezing at the sense lines to the transmitters inside the junction boxes. The failures occurred during cold weather, approximately 6 degrees Fahrenheit. Temporary heat was placed around the area of the junction boxes. After adding temporary heat, the two transmitters returned to scale.
One transmitter was calibrated and returned to operable status that same day. The other transmitter had to be replaced because it could not be calibrated. The new transmitter was installed, calibrated, and returned to operable status the following day.
B. Inoperable Structures, Components, or Systems that Contributed to the Event:
None.
C. Dates and Approximate Times of Major Occurrences:
January 24, 2003, at During performance of a surveillance instruction it was found 0835 EST that a Unit 2 RWST level transmitter had failed high and another was reading higher than normal.
January 24, 2003, at Operations entered Limiting Condition for Operation (LCO) 0845 EST 3.0.3 based on two inoperable RWST channels required for RWST/Containment Sump swapover.
January 24, 2003, at TVA Applied for and received enforcement discretion from 1252 EST NRC for the failure of the two RWST level channels.
January 24, 2003, at One of the two RWST level channels was calibrated and 1345 EST declared operable. Operations exited LCO 3.0.3.
D. Other Systems or Secondary Functions Affected:
Although other sense lines were affected by the cold weather, none were determined to be associated with a loss of freeze protection because of damaged cables,
E. Method of Discovery:
The failed RWST level instruments were discovered during routine performance of a shiftily channel check surveillance.
F. Operator Actions:
Control room operators evaluated the condition and took action to maintain the plant in a safe condition. The appropriate TS LCO was entered, plant management was notified, and work documents were initiated to troubleshoot and restore the failed transmitters.
G. Safety System Responses:
No safety system responses were required.
CAUSE OF THE EVENT
A. Immediate Cause:
The immediate cause of the event was the failures of the RWST level transmitters because of the cold weather.
B. Root Cause:
The two RWST level transmitters malfunctioned because the heater circuits designed to prevent freezing in the junction boxes in which they are located did not operate properly.
The malfunctions of the heater circuits were the result of damaged cables supplying power to the heater strips. Evaluation of the cables determined that the root cause of the damage to both cables was excessive pull tension, during installation in December 1982. This was verified by a microscopic examination of the ends of the conductor strands at the damaged spots. The evidence suggests that the excess tension resulted from an attempt to pull in kinked cables.
C. Contributing Factor:
There were no contributing factors to the event.
IV. ANALYSIS OF THE EVENT
The RWST provides a source of borated water to the emergency core cooling system (ECCS) for a loss of reactor coolant accident event. The supply of water is used during the injection mode of the event. The injection mode continues until manual actuation is taken or the defining logic automatically switches the RWST water supply to the containment sump at which time the recirculation mode is entered. The RWST level transmitters provide level indication and automatic transfer of ECCS suction from the RWST to the Containment sump upon low RWST water level and sufficiently high containment sump water level. The RWST level system is comprised of four level channels with a logic combination of two out of four channels and a safety injection signal to perform the safety function.
At the time of the condition, two of the four RWST level transmitters were operable and able to meet the required safety function. The reduction in the minimum number of operable channels did not cause a malfunction of the operable channels. Operators are trained to perform procedurally required manual switchover; the procedure requires the operators to perform a manual switchover to the containment sump should the automatic switchover fail. Additionally, compensatory actions were established during the time when only two RWST level transmitters were operable, As part of the compensatory actions taken; a dedicated operator was used to monitor RWST level and perform the switchover to the sump should the automatic function fail.
Also, monitoring of the RWST water level was increased from every 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> to every two hours.
V. ASSESSMENT OF SAFETY CONSEQUENCES
Based on the above Analysis of The Event and the compensatory actions taken, this event did not adversely affect the health and safety of plant personnel or the general public.
VI. � CORRECTIVE ACTIONS
A. Immediate Corrective Actions:
Temporary heat was placed around the area of the junction boxes. After adding temporary heat, the two transmitters returned to scale. One transmitter was calibrated and returned to operable status that same day. The other transmitter had to be replaced because it could not be calibrated. The new transmitter was installed, calibrated, and determined operable the following day. The damaged cables were replaced.
B. Corrective Actions to Prevent Recurrence:
The other RWST junction box heater cables are being replaced, to ensure that a similar problem does not exist.
VII. � ADDITIONAL INFORMATION
A. Failed Components:
The malfunctions of the heater circuits were the result of cables damaged during installation, supplying power to the heater strips. The cables are: 2/c, 12 AWG, seven strands, manufactured by American Insulated Wire (AIW), TVA type PXMJ, mark letter WGB, no separator tape over the conductor, paper separator tape over the non-twisted conductors (the cable was assembled in a "twin-flat" arrangement).
x ham.
B. Previous LERs on Similar Events:
A review of previous reportable events for the past three years did not identify any 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).
VIII. COMMITMENTS
None.