05000400/LER-2008-003
| Harris Nuclear Plant - | |
| Event date: | 08-19-2008 |
|---|---|
| Report date: | 10-17-2008 |
| 4002008003R00 - NRC Website | |
Energy Industry Identification System (EIIS) codes are identified in the text within brackets [ ].
I. DESCRIPTION OF EVENT
At the start of this event the plant was in Mode 3 to support main condenser and Control Rod Drive Mechanism (CRDM) [AA] canopy seal weld repairs. Plant temperature was 557 degrees and plant pressure was 2215 psig. There were no structures, systems, or components that were inoperable at the start of the event which could have contributed to the event, other than the two Digital Rod Position Indication (DRPI) coil stacks [AA] that were being retested to restore Operability following maintenance on a CRDM canopy seal.
On 8/19/08 at 0859 Surveillance Test OST-1112, Rod Position Indication Test 18 Month Interval Modes 3 - 5, was being performed for DRPI testing following the post-maintenance disconnection/testing of two DRPI coil stacks during the forced outage. This test was being performed in accordance with Limiting Condition for Operation (LCO) 3.0.5 to restore Operability to the DRPI system following repairs on the CRDM canopy seal. The Main Control Room (MCR) received a Rod Control Urgent alarm upon initial withdrawal of Control Bank C. Rod motion was stopped and Control Bank C-Group One indicated one step and Group Two indicated three steps withdrawn. All other control rods remained bottomed. The operators entered Technical Specification (TS) 3.1.3.3 which requires action to restore DRPI or open reactor trip breakers within eight hours. Local inspection revealed a phase failure on a Movable Gripper in Power Cabinet 1AC, and reactor trip breakers were opened by operators at 0905. Adequate shutdown margin was maintained and all rods withdrawn as part of this testing were capable of being tripped into the core at all times.
Additional troubleshooting revealed a failed Movable Gripper fuse for phase C located in the bus duct above Power Cabinet 1AC (Shawmut Amptrap, model number A60X30). In addition to the failed fuse, three other fuses with cracked end caps were replaced in the bus ducts. A termination at the fuse block for Power Cabinet 1AC stationary coil was found loose and tightened. The cause of the fuse failure was determined to be age related degradation, as it had most likely been in use for more than 22 years. This fuse failure subsequently caused the Rod Control Urgent alarm. The Reactor Protection System responded as designed when the reactor trip breakers were opened by operators, and there were no other safety system actuations. The control rods were successfully retested at 1010 on 8/20/08, per OST-1112 for rods in Control Banks A, B, and C, and Shut Down Bank B following the maintenance activities, and the system was declared Operable at 1300 on 8/20/08.
II. CAUSE OF EVENT
The root cause of this failure was that the existing preventative maintenance (PM) program did not prevent age related failures of fuses. The current PM requirement is to perform Westinghouse Rod Control System Enhanced Maintenance every third refueling outage. As part of this maintenance, the vendor inspects and cleans the bus duct disconnect switches. Additionally, bus duct fuses are removed and inspected for proper part number markings, loose end caps, cracks and overall integrity. The fuse is replaced if faults are identified during this inspection. The stationary and movable gripper coil fuses are replaced on a periodic interval per OEM recommendation. Radiography and scanning electron microscope evaluation of the failed fuse determined that the degradation most likely occurred at the fuse element/solder interface over time.
This failure is not considered a human performance event as current industry practice is to inspect and replace bus duct fuses as needed. The failed fuse was operated in accordance with design specification and showed no evidence of a strong voltage surge.
III. SAFETY SIGNIFICANCE
The Rod Control malfunction during OST-1112 was an Urgent Failure originating in the Rod Control Power Cabinet 1AC. At the time or the failure, the plant was in Mode 3. The malfunction precluded motion of all Control Bank rods and Shutdown Banks A and B. Shutdown Bank C could have been moved using the bank select mode since its motion is not controlled through the standard Rod Control circuits. The availability of systems or components that are needed to maintain safe shutdown conditions, remove residual heat, control the release of radioactive material, or mitigate the consequences of an accident were not impacted by this event. The operating staff performed the required actions for the trip and there were no adverse safety consequences.
Potential Safety Consequences:
Reactor coolant boron concentration was more than adequate to ensure shutdown margin was maintained during the performance of OST-1112. Had this event occurred at power, all Contrbl and Shutdown Bank rods were capable of being tripped into the core to ensure adequate reactor protection.
IV. CORRECTIVE ACTIONS
The failed fuse and three degraded fuses in the Rod Control system were replaced and the control rods were successfully retested at 1010 on 8/20/08, per OST-1112 for rods in Control Banks A, B, and C, and Shut Down Bank B following the maintenance activities. The system was declared Operable at 1300 on 8/20/08.
Planned corrective actions to prevent reoccurrence includes replacing, during the next refueling outage, Rod Control bus duct and DC Hold Cabinet fuses and establishing a PM to replace the Rod Control bus duct and DC Hold cabinet fuses on a periodic basis.
V. PREVIOUS SIMILAR EVENTS
failure. Diagnostic tests and visual examinations were completed for the installed Power Cabinet Cards.
Cycler Logic Card in the Westinghouse Rod Control System. The most likely mechanism for the Slave Cycler Logic Card failure was aging of a Z4 chip.
Cycler Counter Card in the Westinghouse Rod Control System. The most likely mechanism for the Master Cycler Counter Card failure was aging of the Z13 and Z15 logic chips.
The events detailed in the above LERs relate to failures of active components such as circuit cards. The current structured PM program of card inspection and testing will not prevent a failure of passive components such as fuses. As a result of this event, the existing PM program will be modified to include periodic replacement of bus duct and DC Hold Cabinet fuses to prevent recurrence.