05000269/LER-2016-001
| Oconee Nuclear Station, Unit 1 | |
| Event date: | 03-06-2016 |
|---|---|
| Report date: | 05-05-2016 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 51770 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation, 10 CFR 50.72(a)(1)(i), Emergency Class Declaration |
| 2692016001R00 - NRC Website | |
| ML16139A840 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 05/05/2016 |
| From: | Batson S L Duke Energy Carolinas |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| ONS-2016-038 LER 16-001-00 | |
| Download: ML16139A840 (5) | |
Background
This event is reportable per 10 CFR 50.73(a)(2)(iv)(A) as an actuation of the Reactor Protection System (RPS) [JC]. When the reactor trip occurred, Oconee Nuclear Station (ONS) Units 1, 2 and 3, were operating in MODE 1 at approximately 100% power. No significant structures, systems or components were out of service such that they contributed to this event.
The 19KV output of the unit 1 generator is fed to a Main Step-Up (MSU) transformer where voltage is increased to 230KV. The MSU Transformer is located beside the Turbine Building and feeds the generator's power output to the switchyard.
The ONS electrical power system design credits two Keowee Hydroelectric Units (KHUs) [EK] within the owner controlled area of the station as its emergency power sources. Upon a loss of power, emergency power is supplied from both KHU units through two separate and independent routes, an overhead and underground emergency power path.
The ONS Limiting Condition for Operation (LCO) for Technical Specification (TS) 3.8.1 "AC Power Sources" contains the following:
The following AC electrical power sources shall be OPERABLE:
1. Two offsite sources on separate towers connected to the 230 KV switchyard to a unit startup transformer and capable of automatically supplying power to one main feeder bus; and 2. Two Keowee Hydro Units (KHUs) with one capable of automatically providing power through the underground emergency power path to both main feeder buses and the other capable of automatically providing power through the overhead emergency power path to both main feeder buses.
EVENT DESCRIPTION
At approximately 1512 on March 6, 2016, a fault occurred on one of the bushings for the Unit 1 - Main Step-Up (MSU) transformer [EL], which resulted in a Generator [TB] Lockout/Turbine Trip which in turn prompted a reactor trip. At the time of these occurrences a security officer stationed in proximity of the MSU transformer heard a loud noise and observed smoke and flames coming from the MSU transformer and reported it to the Control Room.
This reactor trip is an anticipatory design feature of RPS which trips the reactor based on an "at power turbine trip," prior to Reactor Coolant System (RCS) [RC] parameters (pressure, temperature, level, flow, or reactor power) reaching their reactor trip setpoints.
Reactor trip breaker operation and control rod insertion all occurred as expected and within acceptable time responses. RCS and Secondary plant parameters were maintained such that no safety system actuation occurred.
With regard to nuclear safety systems, operational and plant system response to the Reactor Trip was evaluated as being normal/acceptable.
The reactor trip (RPS actuation) is the reportable attribute of this LER per 10 CFR 50.73(a)(2)(iv)(A).
A "Notification of Unusual Event" (NOUE) was declared at 1520 due to a transformer fire that was started by an electrical fault of one of its bushings. The transformer deluge system activation along with the response of onsite and offsite fire teams promptly contained the fire. The fire eventually caused a transformer overhead conductor to fail and fall to the ground, which triggered a switchyard bus lockout.
Due to the ONS electrical distribution design, the lockout also constitutes a loss of one emergency power path to the site. At 1658, the discovery that the fire had affected a safety system, led the emergency classification to be upgraded to an "Alert" condition. The fire was declared to be completely extinguished at 1708 and the plant exited all Emergency declarations at 2016.
Other than entry into the Technical Specifications associated with the fire's impact on one emergency power path, there was no impact to systems required for safe operation and shutdown of the plant during the NOUE and Alert. The second emergency power path and multiple offsite sources remained operable and had no impact on the operational response to the reactor trip or recovery for the emergency declarations. Units 2 and 3 continued to operate at 100% power throughout the event.
CAUSAL FACTORS
The cause of this Reactor Trip was determined to be an electrical failure of a transformer bushing (the transformer connection point for a conductor going to the 230KV switchyard). The electrical fault initiated a generator lockout and turbine trip which prompted a reactor trip.
CORRECTIVE ACTIONS
1. The 230KV bus was restored.
2. A replacement step-up transformer was tested and installed on Unit 1.
SAFETY ANALYSIS
The Unit 1 Main Step-up transformer's bushing failure caused a generator lockout/turbine trip, that initiated a RPS anticipatory reactor trip. The unit's system response was in line with a loss of electrical load reactor trip. Steam generator pressure control responded as expected and Main Steam Relief Valves (MSRVs) lifted and reseated within specifications. Steam generator level was controlled to appropriate post trip levels without an Emergency Feedwater actuation. RCS pressure, temperature and inventory (i.e., pressurizer level) was also controlled appropriately. All reactor coolant pumps operated throughout the event and no Emergency Core Cooling System (ECCS) or Engineering Safeguards (ES) actuation conditions were met. Thus, the controlled nature of the plant response to the reactor trip did not result in a significant risk to the public.
The transformer fire was contained and controlled without injury, and did not present any radiological safety challenges. The fire affected one out of two emergency power paths during post trip recovery. No other nuclear related plant systems were impacted. The loss of the emergency power path was within the safety allowances provided by Technical Specification actions.
ADDITIONAL INFORMATION
Energy Industry Identification System (EIIS) codes are identified in the text as [XX].
There were no releases of radioactive materials or radiation exposures in excess of limits associated with this event.
SIMILAR EVENTS
A search of the Oconee Corrective Action Program (CAP) database was conducted for the preceding three year period. No other LERs were found related to reactor trips caused by secondary electrical load issues.