05000315/LER-2013-002
| Donald C. Cook Nuclear Plant Unit 1 | |
| Event date: | 04-24-2013 |
|---|---|
| Report date: | 06-24-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 48964 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation |
| 3152013002R00 - NRC Website | |
Conditions Prior to Event Defueled
Description of Event
On April 24, 2013, the Donald C. Cook Nuclear Plant (CNP) Unit 1 reactor was in a refueling outage and defueled.
At 1411, a failure occurred on power cable [EA][CBL5] 1-1110-1 routed between 1-TR101CD [EA][XFMR], 4kV Buses 1C and 1D Reserve Feed Auxiliary Transformer (RAT) and 4kV circuit breaker 1-1C4 [EB][BKR], Reserve Incoming Feed from RAT 1-TR101CD. Fault protection circuitry caused breaker 12-12CD [EA][BKR] , Reserve Feed Auxiliary Transformers TR101CD and TR201CD Supply Breaker, to open, which de-energized both Unit 1 1-TR101CD and Unit 2 2-TR201CD [EA][XFMR] Reserve Feed Transformers.
1-TR101CD and 2-TR201CD Reserve Feed Transformers are associated with Train A of the offsite power source.
The loss of Train A Reserve Feed caused a valid actuation of the Unit 1 CD Emergency Diesel Generator (EDG) [EK][DG], which automatically started and loaded as designed. The Unit 1 East Component Cooling Water Pump (CCW) [CC][P] and Unit 1 North Non-Essential Service Water Pump [KG][P] auto started and loaded on the EDG as designed.
1-CD EDG auto start. The load shed signal was cleared and 12-PP-31N [DA][P], North Spent Fuel Pit Pump was manually restarted and loaded on to the 1-CD EDG on April 24, 2013 at 1447, which restored the North Spent Fuel Cooling Train. The South Spent Fuel Pit Cooling Train [DA], which is powered by Unit 2, remained in-service. The interruption of the North Spent Fuel Pit Pump operation resulted in a 2 degree Fahrenheit rise in the Spent Fuel Pool temperature. This event did not challenge the key safety function of Spent Fuel Pit Cooling.
During the event, Unit 2 remained stable at 100% power. The Unit 2 2-TR201CD [EA][XFMR] Reserve Feed Transformer was restored to service following isolation of the fault.
Event Notification 48964 documented the actuation and start of the U1 CD EDG in accordance with 10 CFR 50.72(b)(3)(iv)(A), Specified System Actuation of an Emergency Diesel Generator, as an eight hour report on April 24, 2013, at 17:15 EDST.
The valid actuation is reportable as a Licensee Event Report (LER) in accordance with 10CFR 50.73(a)(2)(iv)(A), System Actuation of an Emergency Diesel Generator.
Cause of Event
The event was caused by a failure of a power cable. The cause evaluation determined the power cable failure was a reduction of the insulation dielectric strength due to cable age combined with a stressor of a prolonged pressure point from the cable lay path. The pressure point was a 45 degree corner in the respective cable tray [FA][TY].
Analysis of Event
On April 16, 2013, previous to this event, the supply breaker for the Unit 1 RAT tripped open when performing an equipment clearance restoration of transformer 1-TR101CD. An investigation team was convened for troubleshooting to identify the cause. Electrical "megger" testing was performed and results were in the acceptable range and no grounds were detected. Based on test results, it was determined that the power cables had not failed.
With no failure mode identified, the Unit 1 RAT was energized on April 21, 2013, and the investigation was concluded.
Then, on April 24, 2013, the supply breaker for the Unit 1 and Unit 2 Reserve Feed Transformers opened and reserve feed was lost to Train A for both units. Plant personnel in the area of the affected equipment observed an arc flash and an audible indication behind the Unit 1 RAT. It was noted to have been raining just prior to the event.
During the failure investigation, an industry expert was consulted and provided guidance for medium voltage cable diagnostic "Tan Delta Testing.
Tan Delta diagnostic testing was completed. Test results of one power cable indicated unacceptable test results.
Visual inspections were performed and found the power cable damaged. Replacement of the failed power cable and two additional power cables which shared the cable tray was completed. The three new replacement cables were tested using Tan Delta Acceptance Withstand test guidelines. A Tan Delta Maintenance Withstand test was also performed on 9 cables which were not replaced. Cable testing results were satisfactory.
This event occurred on Unit 1, during a Unit 1 scheduled refueling outage. During the event, Unit 1 was defueled.
All Unit 1 fuel was in the Spent Fuel Pit (SFP) at the time of the event and the unit was performing outage maintenance activities on Train A equipment and electrical supplies. Shutdown risk status remained green during the Unit 1, Train A, TR101CD RAT cable failure as the full complement of Unit 1, Train B 4 kV, power supplies were unaffected and available to Unit 1 Train B equipment. When the Train A RAT de-energized, the Unit 1 CD EDG started and loaded as designed for loss of offsite power conditions, and Unit 1 equipment that was not out of service for outage work at the time, started and operated as designed. Additionally, Operations successfully restarted the North Spent Fuel Pit pump 36 minutes after the pump tripped. The South Spent Fuel Pit pump, powered by Unit 2, remained in service throughout the event. Prior to the event, most heat removal had been on the Unit 1 associated North SFP cooling train. SFP temperature rose 2 degrees Fahrenheit during the time the available Train A electrical equipment and EDG functioned as designed. There was no nuclear safety significance to this event. This event did not challenge the key safety function of Spent Fuel Pit Cooling. This event did not challenge any of the other key safety functions beyond Electrical Power Sources, as defined for shutdown conditions in the Plant Shutdown Safety and Risk Management procedure.
The risk impact to Unit 2 was minimal. Risk posed to Unit 2 was evaluated as an emergent event in the online risk assessment program. The allowed outage/configuration time (AOT/ACT) for the configuration that existed, considering Unit 1 risk significant equipment along with the Unit 2 201CD RAT unavailable, exceeded 1000 hrs.
PRA evaluation risk metrics associated with this event are ICCDP = 3.18E-9 and ICLERP = 1.4E-10, which are below the accepted limits defining low risk (i.e., ICCDP
Corrective Actions
Completed Corrective Actions
Three power cables installed in cable tray 1 EI-P8 were replaced, which included the failed power cable 1110-1. Tan Delta Acceptance Testing was performed and results were satisfactory for the three new cables.
Planned Corrective Actions
Engineering staff will verify electrical testing methodology are in accordance with IEEE recommendations for "withstand testing," and diagnostic tests are in agreement with recommendations in EPRI report 1020805, Plant Support Engineering: Aging Management Program Guidance for Medium-Voltage Cable Systems for Nuclear Power Plants. Procedures will be revised based on the review and recommendations provided.
Previous Similar Events
LERs for CNP Unit 1 and Unit 2 were reviewed for the previous five years and found no similar events.