05000266/LER-2015-003
| Point Beach Nuclear Plant Unit 1 | |
| Event date: | 03-09-2015 |
|---|---|
| Report date: | 09-23-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2662015003R01 - NRC Website | |
Reported lessons learned are incorporated into the licensing process and fed back to industry.
Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by Internet e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
2. DOCKET
6. LER NUMER
Description of the Event:
At approximately 1730 on March 9, 2015 during the performance of maintenance, it was determined that the as found current limit check of the D-107 Battery Charger did not limit current. Troubleshooting on D-107 Battery Charger sensing and current limit board identified a defective crimp on a wire in a connector which caused an intermittent open circuit that disabled the current limit function.
The crimp on the wire in the connector was repaired, tested and the battery charger was returned to service.
This 60-day licensee event report is being submitted in accordance with the requirements of 10 CFR 50.73(a)(2)(i)(B).
Cause of the Event:
The cause of the plant condition prohibited by Technical Specification LCO 3.8.4. was a defective crimp from original construction of the battery charger.
Analysis of the Event:
The safety-related 125 VDC system consists of four main distribution buses, in addition to two swing buses, each capable of supplying one of the four 125 VDC buses. Each of the four main distribution buses is powered by a battery charger and a station battery. The function of the battery chargers is to supply their respective DC loads while maintaining the batteries at full charge. All of the battery chargers are powered from the 480 VAC Engineered Safety Feature (ESF) system.
The battery chargers are interlocked such that a loss of offsite power will disconnect the battery chargers from their 480 VAC source. A coincident safety injection signal would prevent restoration of the battery chargers unless offsite power is restored to the safeguards buses or safety injection is reset. This limits the loading on the standby emergency power supply during the period immediately following a safety injection signal. During this period, the 125 VDC loads are supplied by their associated station battery until such time as power to the chargers is restored. Two swing battery chargers are available through one of the swing DC distribution buses. The swing battery is capable of being aligned to any one of the four main distribution buses to take the place of the normal battery.
Each battery charger has sufficient capacity to restore the battery from the design minimum charge to its fully charged state within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> while supplying the largest combined demands of the various continuous steady state loads while staying within the capacity of the supply breakers. Additionally, the battery charger is required to be restored arid aligned to the DC Bus within one hour of the onset of an accident with a loss of offsite power to provide adequate voltage and current to all components fed from the system. The charger is normally restored by closure of a contactor from the Control Room. If the battery charger is not restored within one hour, components fed from the system may not have adequate voltage to operate.
The battery charger current limiter is relied upon to limit DC current when the battery charger is restored within one hour after the onset of an accident with a loss of offsite power to carry the DC bus. The DC current must be limited to prevent blowing the fuses in the DC output bus disconnect and tripping the charger 480V AC input transformer supply breaker.
Thus, the current limit function is needed to support design basis requirements.
Evaluation of the data provided that all possible variables could not be bounded to ensure that tripping of the supply breaker would not occur under worst case conditions. Therefore, it was determined that the capability to restore the battery charger to the DC electrical power subsystem to recover the discharged battery while carrying connected loads may not have been able to be performed as designed. This condition may have resulted in an actual plant condition prohibited by Technical Specifications, Technical Specification 3.8.4. DC Sources-Operating.
Corrective Actions:
The crimp on the wire of the battery charger sensing and current limit board connector was repaired. The review for 10 CFR Part 21, Reporting of Defects and Noncompliance is being evaluated in the corrective action program. Additionally, the procedure has been revised to correct the identified deficiencies.
Reported lessons learned are incorporated into the licensing process and fed back to industry.
Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washi n g ton, DC 20555-0001,01 by internet e-mail to Infocollects.Resource©nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503, If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
2. DOCKET
6. LER NUMER
Safety Significance:
During an event in which the affected battery charger supply breaker may have tripped, station operators would either align the swing charger or take actions to locally restart the battery charger with the failed current limiter. Plant procedures that provide guidance to locally restart the battery charger with the failed current limiter had identified deficiencies. The procedure deficiencies may have resulted in an overcurrent condition when the charger with the failed current limiter was repowered. Station operators would diagnose the problem and determine whether further attempts at charger recovery with the failed current limiter were prudent. The charger could have been used to provide the required safety function within the capacity of the AC supply breaker. The swing charger was available except for maintenance and testing activities.
Based on the equipment redundancy for recovering power to the DC Bus and the high percentage of swing charger availability, this issue is of very low safety significance. There was no impact on the health and safety of the public as a result of this event.
Similar Events:
There have not been similar events of operation or conditions prohibited by Technical Specifications.
Component Failure Data:
D-107 Battery Charger Sensing and Current Limit Board Connector - Power Conversion Products