05000373/LER-2011-001
| Lasalle County Station Unit 1 | |
| Event date: | 02-01-2011 |
|---|---|
| Report date: | 03-25-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| 3732011001R00 - NRC Website | |
LaSalle County Station (LSCS) Unit 1 is a General Electric Boling Water Reactor with 3546 Megawatts Thermal Rated Core Power.
A. CONDITION PRIOR TO EVENT:
Unit(s): 1 Reactor Mode(s): 1
B. DESCRIPTION OF EVENT:
Event Date: February 1, 2011 Event Time: 1918 CST Mode(s) Name: Power Operations Power Level: 100 percent On February 1, 2011, at 1918 hours0.0222 days <br />0.533 hours <br />0.00317 weeks <br />7.29799e-4 months <br /> CST, LaSalle Unit 1 automatically scrammed from 100% power.
The scram was due to a main generator load reject caused by a fault on the C-phase of the 1W Main Power Transformer (1W MPT)(MP)[EL]. All control rods fully inserted, all systems responded as expected to the scram, and Emergency Core Cooling Systems were not challenged.
Following repair and restoration of the 1W MPT, the unit was restarted on February 10, 2011, and subsequently synchronized to the grid on February 11, 2011. Unit 1 returned to full power at 1815 CST on February 12, 2011.
This event is reportable under 10 CFR 50.72(b)(2)(iv)(B) and 10 CFR 50.73(a)(2)(iv)(A) as an event or condition that resulted in a valid, automatic actuation of the Reactor Protection System. The NRC was notified of this occurrence via ENS 46582, at 2154 hours0.0249 days <br />0.598 hours <br />0.00356 weeks <br />8.19597e-4 months <br /> CST on February 1, 2011.
C. CAUSE OF EVENT:
The root cause analysis concluded that the event was caused by external bushing flashover due to ice and snow buildup on the C-phase bushing, which diminished its creepage length rating (defined as the contoured surface distance of an insulator between conducting surfaces). The condition was further aggravated by the presence of sodium chloride, imbedded with small amounts of limestone and soil silicates that reduced the insulative capability of the bushing in localized areas and further reduced the creep distance.
Adverse weather conditions were present at the time of the scram. These included blizzard conditions, wind gusts of up to 54 mph and sustained winds of 40 mph just prior to the MPT transient from a direction of 48 degrees (i.e., winds from the NE blowing SW), with an air temperature of 22 degrees F.
D. SAFETY ANALYSIS:
The safety significance of this event was minimal. All control rods fully inserted and all systems responded as expected to the scram. The Emergency Core Cooling Systems were not challenged.
This event did not meet the NEI 99-02 definition of an unplanned scram with complications. There were no safety system functional failures.
E. CORRECTIVE ACTIONS:
Corrective Actions:
® The C-phase bushing, the C corona ring and the MPT mechanical relief valve were replaced ® A comprehensive engineering strategy plan to significantly reduce the potential for high voltage bushing flashover will be developed ® A subject matter expert review will be conducted of the cleaning frequency and methodology guideline to determine best practices for maintaining bushing cleanliness across high risk seasonal salt and snow accumulations Corrective Action to prevent Recurrence:
® MPT bushings will be replaced with a more robust anti-flashover design that exceeds minimum Basic Insulation Level (BIL) rating, adding more margin to the creep length
F. PREVIOUS OCCURRENCES:
On May 21, 2009, at 1635 CDT, LaSalle Unit 1 automatically scrammed from 100% power. The scram was due to a generator lockout signal caused by a failure of the surge arrestor on the A phase of the 1W MPT. The cause of surge arrestor failure was determined to have been due to a manufacturer's defect.
G. COMPONENT FAILURE DATA:
Manufacturer: Westinghouse (W120) Model: Type 0