05000364/LER-2014-002
| Joseph M. Farley Nuclear Plant, Unit 2 | |
| Event date: | 10-14-2014 |
|---|---|
| Report date: | 12-12-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3642014002R00 - NRC Website | |
Reported lessons teamed are incorporated Into the licensing process and led beck to Industry.
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1. FACILITY NAMI. 6. LER NIJMIIFR 2. 1/011
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NUMI1LR Westinghouse - Pressurized Water Reactor Energy Industry Identification Codes are identified in the text as [XX].
Requirement for Report The reactor trip is reportable per 10 CFR 50.73(a)(2)(iv)(A) for manual actuation of the reactor protection system. Additionally, the reactor trip resulted in a valid actuation of the Auxiliary Feedwater system which is also reportable per 10 CFR 50.73(a)(2)(1v)(A).
Unit Status at Time of Event Prior to the event Unit 2 was operating at 82 percent power and was in a power coast-down prior to a refueling outage.
Description of Event
On 10/14/14 at 0341 CDT, the Unit 2 reactor was manually tripped after a lightning strike in the HVSY led to a phase 3 to ground fault on a 500kV transmission line resulting in a B train Loss of Site Power (LOSP). The fault caused the 2B SAT instantaneous overcurrent relay to actuate and resulted in de-energizing the 2B Startup Auxiliary Transformer (SAT). A missing nut in the Power Circuit Breaker (PCB) protection circuitry caused a high resistance on one side of the current transformer circuit resulting in an imbalance in current flows and an actuation in the differential instantaneous overcurrent relaying.
The B train LOSP in conjunction with the 2B Emergency Diesel Generator (EDG) being out of service for a planned maintenance outage caused a loss of Component Cooling Water (CCW) to the Reactor Coolant Pumps (RCPs). The Unit 2 Abnormal Operating Procedures for loss of CCW and loss of A or B Train Electrical Power were entered and the reactor was manually tripped and the RCPs were secured.
In February and March of 2013, High Voltage Switchyard (HVSY) PCB 944 and protective circuitry were installed as part of a Design Change Package (DCP) for upgrading the HVSY Bank 1 bus. This design change work was performed by Alabama Power Company (APCO) with oversight provided by Southern Nuclear Company (SNC) in accordance with approved oversight procedures. Testing was performed to verify correct installation of the current transformers (CT) for Power Circuit Breaker (PCB) 944 and the associated wiring. These tests included insulation resistance (Megger) testing of CT and cables, continuity checks of CT cables, verification of one ground per circuit, and polarity, ratio and saturation testing for CTs. All of these were satisfactory and the DCP was completed with no issues being identified.
Approximately 18 months after the DCP Installation, a lightning strike on the Farley-Snowdoun 500kV transmission line on 10/14/14, led to a phase 3 to ground fault on the Farley-Snowdoun 500kV line. The Snowdoun line relaying at Farley Transmission Substation operated as a result of the fault and tripped open HVSY PCBs 1112 and 1212. Although these PCBs tripped open, the phase 3 to ground fault did not clear due to a ground fault that was occurring Internal to PCB 1112 on the bus side of the main contacts.
The No.1 500kV bus secondary differential relaying operated as a result of continuation of the fault and tripped HVSY PCBs 1102 and 1132 which cleared the fault.
Prior to the bus fault being cleared, fault current was drawn from the 230kV side of the HVSY through HVSY PCBs 840 and 944. The fault current of over 3000 amps exposed a loose termination In phase 3 of NRc Foam 366A (02-2014) SEQUI.M1AL
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PCB 944's CT circuit when the fault current passed through PCBs 840 and 944. This loose connection created a point of high resistance in PCB 944's CT circuit which caused the 2B SAT instantaneous overcurrent relay to see a difference in the current contributions from PCBs 840 and 944, indicating a fault in the 2B SAT zone of protection and causing the instantaneous overcurrent relay to actuate.
The instantaneous overcurrent relay then tripped HVSY PCBs 840 and 944 resulting in isolation of the 2B SAT which caused a LOSP to the B train power bus. A higher than normal resistance was created by a loose termination in the PCB 944 CT circuit to the instantaneous overcurrent relay that was caused by omitting a 5/16 inch nut when installing PCB 944 protection circuitry.
At the time of the B train LOSP, the 26 EDG was tagged out of service for a planned maintenance outage for replacement of the governor. This caused the diesel to be unavailable to start as designed to pick up the B train loads after the loss of the 2B SAT. One of the loads lost on the B train was cooling water to the RCP thermal barriers and oil coolers. This led to entry into the Abnormal Operating Procedures for loss of CCW and loss of A or B Train Electrical Power, and manually tripping the reactor and securing the RCPs at 0341 CDT.
Unit 2 'B' train power was restored at 0523 CDT.
Cause of Event
The direct cause of this event was the loose termination from a missing nut in the PCB 944 CT circuit connection to the instantaneous overcurrent relay, which in addition to the continued fault on the 500kV bus side of PCB 1112 led to the B train LOSP. The manual reactor trip resulted from the B train alignment of cooling water to the RCP thermal barriers and oil coolers.
The root cause was inadequate verification practices during APCO wiring installations that led to a nut not being installed on its terminal. A contributing cause to this event is not fully understanding the extent of the differences in verification practices performed by the organizations.
Safety Assessment All systems responded as designed to the reactor trip and the B train LOSP. The CCW miscellaneous header was aligned to A-Train, and the B RCP was restarted. The 2C diesel generator was manually started and was aligned to B train loads. All equipment operated as-designed to mitigate the event.
There was no loss of safety function and no radioactive release associated with this event. There was no actual consequence detrimental to the health and safety of the public. During the event all A-Train safe shutdown equipment responded appropriately.
Corrective Action The missing nut was installed and secured for the PCB 944 current transformer. Primary and secondary protective relaying for the 2B SAT was tested satisfactorily. Extent of Condition walkdowns were performed for other circuits in the HVSY and repaired as necessary. To prevent recurrence SNC will work to strengthen standards of the utility performing the maintenance for work performed in the high voltage switchyard, ensuring that for this type of work expectations are communicated, and training and verification practices are strengthened.
MC HAW 366A (0..2014) 1. FAC ILITY NAME 3. PAGE 6. LFR NI MBER 2.110( ICET SLIQIE '11A, IsUMDLR YEAR RI v
Additional Information
Farley Unit 1 and Unit 2 Licensee Event Reports for the previous five years have been reviewed. In 2013 Farley Unit 1 experienced an automatic reactor trip resulting from a loss of the startup auxiliary transformer. LER 2013-001-00 was submitted. That event was caused by a degraded piece of equipment (lightning arrestor) that, over a period of time, had moisture ingress that caused a fault. The failure of the equipment caused the 1E3 SAT to become de-energized resulting in a Unit 1 automatic reactor trip. The details and corrective actions for that event are not similar to this occurrence because it did not result from the human performance tool of Inadequate verification practices.