05000250/LER-2010-003
| Docket Number Sequential Revmonth Day Year Year Month Day Yearnumber No. | |
| Event date: | 09-23-2010 |
|---|---|
| Report date: | 11-19-2010 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 46274 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation, 10 CFR 50.72(b)(3)(iv)(A), System Actuation |
| 2502010003R00 - NRC Website | |
DESCRIPTION OF THE EVENT
On September 23, 2010, with Unit 3 operating at 100% power, an unplanned automatic reactor trip occurred at approximately 17:14:40 when an electrical flashover on the high side of the Unit 3 Generator Step Up Transformer [EL, XFMR] occurred. All systems responded as designed At 17:52, a notification (EN 46274) was made to the NRC Operations Center in accordance with 10 CFR 50.72(b)(2)(iv)(B) due to actuation of the Reactor Protection System with the reactor critical and 10 CFR 50.72(b)(3)(iv)(A) due to Auxiliary Feedwater System actuation. Unit 3 reactor [AB,RCT] and turbine [TA,TRB] tripped due to a generator differential protection relay trip. This event was entered into the Corrective Action Program as AR 582206582206 All systems functioned as normal with the exception of Control Rod G5 in Control Bank A which indicated 18 steps. The unit entered and exited E-0 "Reactor Trip" and ES-0.1 "Reactor Trip Response." All 4kV buses had power from the Unit 3 Start Up Transformer. Heavy weather (rain and wind) conditions existed at the time of the reactor trip.
CAUSE OF THE EVENT
The event was evaluated to determine the root cause and contributing causal factors. The root cause was an external flashover to ground of the "C" phase high voltage (HV) bushing. The root cause did not originate with the GSU transformer or connected 230kV transmission system. The one cause that could not be refuted was the potential for rapid contamination of the GSU transformer bushing due to steam/canal water carryover effluent from the circulating water system condenser water box priming system. If this was the cause, the priming system deficiencies that could have contributed to the rapid contamination event include a clogged moisture separator drain line loop seal, failed condenser water box check valves, and a corroded silencer of the circulating water condenser water box primary system ejector that no longer performed moisture separation and did not prevent saline laden steam and water droplets from exiting the system. Nonetheless, evidence supporting this potential cause was not found..
ANALYSIS OF THE EVENT
At the time of the event, it was raining. However, no lightning was recorded at the plant at the time of the event based on the National Lightning Detection Network and eyewitness accounts. Wind speed was approximately 9 miles per hour with wind gusts. Interviews revealed that a heavy rain continued after the event. Consequently, any contamination evidence was likely removed.
REPORTABILITY
The event is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A) due to automatic actuation of the Reactor Protection System when an unplanned reactor trip occurred and 10 CFR 50.73(a)(2)(iv)(B) due to automatic actuation of the Auxiliary Feedwater System.
ANALYSIS OF SAFETY SIGNIFICANCE
The plant responded as expected to the automatic reactor trip. Although one control rod indication remained at 18 steps, the control rod was subsequently verified to be fully inserted and the rod position indicator was appropriately adjusted.
CORRECTIVE ACTIONS
Immediate Corrective Actions:
- Replace GSU Transformer Surge Arresters (all phases)
- Replace 230kV Stand Off Insulators (all phases)
- Internal inspection of the GSU transformer
- Generator Radial Lead Seals were replaced
Corrective Actions to Prevent Recurrence:
1. Implement long term action plan for the Unit 3 and 4 Circulating Water System to mitigate degradation issues that lead to canal water carry over. Action plan to improve the following:
o Silencer condition and performance o Moisture separator (314T34) drain line operation o Water box vacuum tank check valves operation o Vacuum air leaks.
2. Replaced Unit 3 GSU Transformer High Voltage Bushings (all phases) with longer bushings.
(Complete) 3. Replace. Unit 4 GSU Transformer High Voltage Bushings (all phases) with longer bushings.
Contributing Cause Corrective Actions:
1. Unclogged moisture separator drain loop seal. (Complete) 2. Replaced the silencer of the Circulating Water Condenser Water Box Priming System Ejector.
(Complete) 3. Replaced all four condenser water box vacuum tank check valves (Complete)
ADDITIONAL INFORMATION
component function identifier (if appropriate)].
FAILED COMPONENTS IDENTIFIED: Unit 3 Step-Up transformer PREVIOUS SIMILAR EVENTS: None