05000251/LER-2015-002
| Turkey Point | |
| Event date: | 5-12-2015 |
|---|---|
| Report date: | 7-13-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2512015002R00 - NRC Website | |
(02-2014") Estimated burden per response to comply with this mandatory collection request: 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />.
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 LICENSEE EVENT REPORT (LER) intemet e-mail to lnfocollecto.Resource@nrc.gov, snd to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget,CONTINUATION SHEET 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.
DESCRIPTION OF THE EVENT
On May 12, 2015 at approximately 0430 hours0.00498 days <br />0.119 hours <br />7.109788e-4 weeks <br />1.63615e-4 months <br /> with the Unit 4 reactor [AC, RCT] at approximately 80% rated thermal power, an automatic reactor trip occurred in response to a turbine [TA, TRB] trip. The turbine trip was caused by a generator [TB, GEN] differential lockout that opened the generator output breaker [TB, GEN, BKR]. During the reactor trip response, the Auxiliary Feedwater (AFW) System [BA] automatically initiated as expected. The unit was subsequently stabilized in Mode 3. All systems responded correctly to the trip.
Troubleshooting identified an open circuit across the terminal block points associated with the secondary of the differential protection neutral side phase 'A' current transformer (CT) [TB, GEN, XCT]. Wiring was found burned and a stud in the secondary terminal was found loose. Subsequent inspection found that a lug connecting the field wiring to the CT leads had failed. The failed lug caused an open circuit resulting in the generator lockout.
Event Notification No. 51065 was made to the NRC Operations Center in accordance with 10 CFR 50.72(b)(2)(iv)(B) for a valid actuation of the reactor protection system (RPS) while critical and 10 CFR 50.72(b)(3)(iv)(A) for a valid Engineered Safety Feature actuation of AFW. This report is in accordance with 10 CFR 50.73(a)(2)(iv)(A) due to valid actuations of the RPS and AFW.
CAUSES OF THE EVENT
The direct cause of the lug failure was inadequate tightness of the CT connection.
The root cause was that the vendor recommended torque value for a stud lugged connection, that connects field wiring to the CT, was not used during the engineering change (EC) and work order planning process. The tightening requirement for this type of connection is considered to be skill of the craft; therefore, no torque specification was listed in the EC or work instructions.
Contributing causes:
1. A periodic maintenance activity to check the wiring connections for the CTs had not been established.
2. Operating experience relating to the frequency of main generator CT leads failing was not considered.
ANALYSIS OF THE EVENT
The protection scheme. of the main generators at Turkey Point contains a high speed differential relay that monitors the current flow entering and leaving each generator winding to guard against phase to phase shorts or grounds. When a current difference occurs, the relays actuate the lockout relay which causes the generator breaker to open.
(02-2014) Estimated burden per response to comply with this mandatory collection request: 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />.
4,1 '. Reported lessons learned are incorporated into the licensing process and fed back to industry.
- .W j# Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch(r-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001,or byLICENSEE EVENT REPO RT (LER) intemet e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office of Informationand Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget,CO NTINUATIO N SHEET Washington, DC 20503. If a means used to impose an information collection does not display a currently valid 0MB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
The generator differential lockout was caused by a loose connection on the 4G03A CT. This CT is installed on the neutral side of the generator and provides a secondary current signal to the generator differential relay. The relay compares the generator neutral current to the generator output current, which should be equal. When the loose connection caused an open circuit (secondary current went to zero), the differential relay operated and initiated the generator lockout.
The 4G03A CT was installed in 2013 during an extended power uprate outage. The connections were made hand tight using skill of the craft, which is typical for secondary connections. The connection was inadequate and eventually caused the open circuit. The connections on the eleven other CTs installed as part of the generator upgrade were checked and two other loose connections were found and corrected.
The new CTs employ a mechanical compression stud for the electrical connection. During installation, the CT wires were lugged and connected at the top of the mechanical compression stud (the wires were not stripped back and inserted into the slot and compressed). The type of stud on the new generator CTs differs from all other CTs used at Turkey Point.
AFW actuation on steam generator lo-lo level was as expected based on the review of data from previous reactor trips from 100% and simulator trips from 100%.
ANALYSIS OF SAFETY SIGNIFICANCE
At the time of the event, Unit 4 was at approximately 80% power level. The generator differential lockout caused a turbine trip which resulted in the automatic reactor trip. Plant response was as expected. All control rods fully inserted. All systems responded as designed. The unit transitioned to Mode 3. As a result, the safety significance of the event is considered very low.
CORRECTIVE ACTIONS
Corrective action is in accordance with condition report AR 2047137 and includes:
1. A temporary modification was made to disconnect/disable the input of the CT with the failed lug from the differential protection scheme.
2. Other similar CT connections on the Unit 4 main generator were inspected and tested to ensure adequate tightness.
3. Preventive maintenance procedure 0-PME-090.03 and electrical specification SPEC-E-012 will be revised to include connection torque requirements per the vendor work instruction manual for the type of terminal used in the CTs.
4. A preventive maintenance task will be implemented to periodically inspect and test the Unit 3 and 4 main generator CT connections.
5. Similar CT connections in the Unit 3 generator will be inspected during the next refueling outage.
(02-2014) Estimated burden per response to comply with this mandatory collection request: 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />.
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 LICENSEE EVENT REPO RT (LER) intemet e-mail to lnfocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget,CO NTINUATIO N SHEET 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.
ADDITIONAL INFORMATION
component function identifier (if appropriate)].
FAILED COMPONENTS IDENTIFIED: None.
PREVIOUS SIMILAR EVENTS: None in the last five years.