05000301/LER-2010-003
| Point Beach Nuclear Plant - Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown |
| 3012010003R00 - NRC Website | |
Event Description:
On December 13, 2010, at 1205 CST, preparations for a Technical Specification (TS) required shutdown were initiated on Unit 2. TS Limiting Condition for Operation (LCO) 3.1.4, Rod Group Alignment Limits, was not met as a result of Surveillance Requirement (SR) 3.1.4.2 not being met. The shutdown was subsequently completed at 1703 CST, December 13, 2010.
This 60-day licensee event report is being submitted in accordance with the requirements of 10 CFR 50.73(a)(2)(i)(A), the completion of any nuclear plant shutdown required by the plant's Technical Specifications.
Event Analysis:
On December 10, 2010, at 1350 CST, during performance of TS surveillance procedure TS-06, Rod Exercise Test, rod control urgent and non-urgent failure alarms occurred while stepping Control Bank B rods [AA] out.
Power cabinet 2AC had an urgent alarm locked in, and power cabinets 2AC and 1BD had the non-urgent alarm locked in. These alarms were verified locally in the Unit 2 rod drive room. Unit 2 remained at 100% power.
Initial troubleshooting and repair reset the power supply crowbars and replaced the blown fuse, FU 21. The test was recommenced on December 10, 2010, at 2212 CST and Control Bank B was stepped out one step as procedurally required with no alarms. When Control Bank B was subsequently stepped inward one step, both rod control urgent and non-urgent failure alarms were received. Both of the times that Control Bank B rods failed to move, the movable gripper coil power supply fuse (FU 21) blew for control rod F-6. When the F-6 control rod fuse failed, it resulted in outward rod motion of one additional step. Troubleshooting continued through the weekend of December 11 and 12, 2010.
Discussions with NSSS support engineers suggested a short between cables or a short to ground affecting the F-6 control rod movable gripper coil circuit which caused the FU 21 fuse to blow. Initial megger checks from cable to station ground determined that the resistance was as low as 5 MO. Subsequent megger readings varied but were as low as 18 O.
At 2143 CST on December 12, 2010, the surveillance procedure for Control Banks A, C and D and Shutdown Banks A and B was completed. However, the failure of Control Bank B rods to step in correctly resulted in the inability to satisfy SR 3.1.4.2. Unit 2 was shut down on December 13, 2010, when LCO 3.1.4 was not met because of SR 3.1.4.2 not being met. Troubleshooting and repairs had been unsuccessfully attempted between receipt of the alarms on December 10, 2010, and the shutdown on December 13, 2010.
Safety Significance:
When the .F-6 control rod fuse failed, it resulted in outward rod motion of one additional step. The outward step occurred outside of the active fuel region and had no impact on core reactivity. Additionally, control rods tripped as required during the shutdown on December 13, 2010. Thus, the safety significance of the event was low. There was no impact on the health and safety of the public as a result of this event. This is not a safety system functional failure.
Cause:
A degraded field connection for the F-6 control rod caused control rod fuse 21 to blow, which ultimately resulted in the urgent and non-urgent alarms being received during performance of control rod exercises and the inability to complete the surveillance test. Following the shutdown, when a connection on a patch panel in containment was opened, checked and re-seated, the fault condition cleared as validated by numerous megger readings indicating the field connection had become degraded.
Corrective Actions:
The following corrective actions were taken:
® All system cards were tested, repaired, and returned to the system.
® All rod control cables were tested, and cable issues were addressed.
® All +24 and -24 V DC power supplies in all power cabinets were replaced.
® Post-maintenance testing demonstrated that the system now performs as designed.
The following corrective actions will be taken and are being tracked in the site's corrective action program:
® Unit 2 control rod drive mechanism (CRDM) head and patch panel connections will be inspected during the Unit 2 spring 2011 refueling outage.
® Unit 1 CRDM head and patch panel connections will be inspected during the fall 2011 refueling outage.
® Meggar/ECAD testing will be added to the CRDM PM schedule.
® The Unit 1 24 V DC power supplies will be replaced during the fall 2011 refueling outage.
Previous Occurrences:
None Failed Components Identified:
None