05000348/LER-2013-001
| Joseph M. Farley Nuclear Plant, Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3482013001R00 - NRC Website | |
Westinghouse - Pressurized Water Reactor Energy Industry Identification Codes are identified in the text as [XX].
Description of Event
At 2105 on June 11, 2013, while Unit 1 was operating in Mode 1 at 100% reactor power, the Unit 1 B-Train Startup Transformer [XFMR] was automatically de-energized by protective relay actuation. This resulted in the loss of power to the 1B and 1C Reactor Coolant Pump (RCP) buses [BU] and an automatic reactor trip on an RCP undervoltage signal. The de-energized transformer also resulted in a B-Train Engineered Safety Feature (ESF) Bus [BU] Loss of Off- Site Power (LOSP). The 1B Emergency Diesel Generator[DG] automatically started and re- energized the B-Train ESF Buses and the B-Train LOSP sequencer [2] automatically started LOSP loads. All control rods completely inserted during the reactor trip. Forced flow of reactor coolant was maintained by the 1 A RCP [P] which remained running throughout the event. All three Auxiliary Feedwater (AFW) Pumps [P] automatically started and provided makeup water to the steam generators. The secondary steam dumps remained in service to remove decay heat to the main condenser. No secondary or primary reliefs lifted during the event. All plant systems responded as designed to the reactor trip and B-Train LOSP. The operating crew responded to the event by entry into the appropriate emergency response procedures (FNP-1-EEP-0.0, "Reactor Trip or Safety Injection", and FNP-1-ESP-0.1, "Reactor Trip Response") and stabilized the plant in a hot-standby condition. The A-Train ESF buses remained energized from an operable off-site power source via the 1 A Startup Transformer throughout the event with an operable emergency diesel generator available as a backup power source. There were no major components or systems that were out of service at the time of the event that contributed to the severity of the event. The reactor trip, diesel generator auto-start, and AFW auto-start are reportable as system actuations per 10 CFR 50.73(a)(2)(iv)(A).
Immediate investigation of the event identified a failed lightning arrester [LAR] at the phase 2 230kV terminal of the 1B Startup Transformer. All three lightning arresters on the 1B Startup Transformer were replaced and the transformer was returned to service, supplying the B-Train ESF Buses, on June 13, 2013, at 0429. The Unit 1 reactor returned to critical operation on June 13, 2013 at 1632.
Cause of Event
Investigation of the event determined that the lightning arrester on the phase 2 230kV terminal of the 1B Startup Transformer had shorted to ground resulting in protective relay actuation which opened the switchyard supply breakers to the startup transformer.
The failed lightning arrester is a type 'VN' arrester, model number 215920, manufactured by Ohio Brass Company. The arrester has been in service since 1992. A failure analysis conducted by the manufacturer determined the root cause of the failure was an undetected compromise of the arrester sealing plate during factory assembly resulting in extremely slow moisture ingress into the arrester during its service life. Farley performs industry-accepted monitoring and testing activities on lightning arrestors. This particular failure mode was not detected by these monitoring and testing methods.
Safety Assessment All systems responded as designed to the reactor trip and the B-Train LOSP. The B-Train ESF buses were automatically re-energized from the B-Train emergency diesel generator.
The A-Train ESF Buses remained powered from off-site power with an operable emergency diesel generator available to supply backup power if needed. No major components or systems were out of service during the event. 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.
Corrective Action A root cause analysis of this event identified that all lightning arresters of this vintage may have a higher susceptibility to moisture intrusion due to the manufacturing and acceptance test procedures that were in place at the time. Corrective actions include the implementation of an accelerated schedule for replacement of lightning arresters of the same vintage.
Additional Information
Similar previous events Unit 1 — Unplanned B Train LOSP During Switchyard Breaker Testing