05000219/LER-2017-005

LER-2017-005, 1 OF 3

Oyster Creek Nuclear Generating Station
Event date:	10-09-2017
Report date:	01-03-2018
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
2192017005R00 - NRC Website

LER 17-005-00 for Oyster Creek Nuclear Generating Station Regarding Failure of the Emergency Diesel Generator #2 During Surveillance Testing Due to a Broken Electrical Connector

ML18009A436
Person / Time
Site:	Oyster Creek
Issue date:	01/03/2018
From:	Gillin M F Exelon Generation Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
RA-17-083 LER 17-005-00
Download: ML18009A436 (4)

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 e-mail to 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.

Oyster Creek Nuclear Generating Station

3. LER NUMBER

2017 - 00 005

Plant Conditions Prior To Event

Event Date:

October 09, 2017 Event Time: 03:12 hours ET Unit 1 Mode: Run Power Level: 100%

Description of Event

On 10/09/17, during the bi-weekly EDG #2 Load Test (station procedure 636.4.013), a Generator Lockout (86G) signal was received which tripped the EDG output breaker. The diesel had run for 4 minutes loaded in the procedurally prescribed band of 2600-2800 KW prior to receiving the 86G lockout signal. The EDG #2 trip is documented under Issue Report (IR) 04060815. This failure resulted in EDG #2 being declared inoperable, and entry into an unplanned 7-day LCO (TS Section 3.7.D) at 0312.

Troubleshooting identified a broken electrical ring lug connector on one current transformer (CT5) that provides an input to a differential current relay (87G). When the ring lug connector became detached from the terminal point, the relay sensed a current imbalance and the protective relay logic actuated. The differential current relay (87G) provided an input to the Generator Lockout circuit, causing the lockout relay (86G) to trip the EDG#2 output breaker. Repairs were completed on 10/10/17 at 0557. EDG #2 was satisfactorily tested and declared operable at 0805, and the 7-day LCO was exited.

Cause of Event

The investigation determined the electrical ring lug connector failure was due to fatigue cracking that was initiated by stresses caused by bending and twisting of the electrical lug beyond limits specified in industry guidelines.

Analysis of the Event

The ring lug that failed was sent to a metallurgical lab for failure analysis. The metallurgical analysis determined the ring lug had failed due to fatigue cracking. The investigation performed for the EDG#2 trip concluded the fatigue cracking was initiated by historical stresses caused by bending and twisting of the electrical lug beyond limits specified in industry guidelines. Based on a review of recent maintenance records for EDG#2 and a review of plant procedures and work practices, the investigation concluded the ring lug was most likely distressed during initial installation in the 1990's.

Assessment of Safety Consequences

Two EDG units serve as the Standby Power Supply for Oyster Creek Nuclear Generating Station (OCNGS) by providing an emergency source of power to the 4.16 kV buses 1C and 1D in the event of a loss of normal power.

The EDG units are designed to start and load automatically, if required. Non-essential loads are automatically shed by under-voltage sensing devices on loss of offsite power to ensure that the EDG units are not overloaded. The capacity of the EDG units is sufficient to sequentially energize for starting all safety-related pumps and auxiliaries required for a safe shutdown of the reactor in the event of a Design Basis Accident. The EDG units are independent of each other, except for a common bulk fuel storage supply, and are provided with auxiliary systems to ensure reliable starting and continuous operation with no Operator attention. Power to start the EDG units is self-contained and is not dependent on the availability of any other source of normal plant power at the moment of initiation.

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 e-mail to 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.

Oyster Creek Nuclear Generating Station

3. LER NUMBER

2017 - 00 005 The Generator Lockout relay (86G) is an auxiliary relay that is operated upon the occurrence of abnormal conditions to maintain associated equipment out of service until it is reset. The lockout (86G) relay is sealed-in when the Differential Current Relay (87G) is initiated. The failure of the wire lug would not have prevented EDG #2 from being able to start. However, in the as found condition EDG#2 was unable to start and load to supply power to the safety buses for the required mission time of 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />. Because EDG #1 was operable and available to supply power to its associated loads in the event of a loss of offsite power, there was no loss of safety function.

Corrective Actions

1. The broken ring lug connector forthe current transformer (CT5) was replaced. EDG#2 was tested and restored to operable status on 10/10/17 at 0805 hours0.00932 days <br />0.224 hours <br />0.00133 weeks <br />3.063025e-4 months <br />.

2. All ring lug connectors were inspected in the current transformer and potential transformer cabinet for EDG#2.

Four other bent and/or twisted ring lug connectors were identified and replaced on 10/10/17.

3. Extent of condition inspections were performed on all ring lug connectors in the current transformer and potential transformer cabinet for EDG#1.

4. Two bent and/or twisted ring lug connectors were replaced on EDG#1 on 12/28/17.

5. Revise electrical ring lug installation and EDG inspection procedures to include inspection for bent and/or twisted lugs.

Previous Occurrences

There have been no similar, previous events resulting from a failure of a wire lug on an EDG at Oyster Creek.

Component Data Component IEEE 805 System ID IEEE 803A Component Emergency Diesel Generator EK DG