05000457/LER-2007-001
| Braidwood Station | |
| Event date: | 08-23-2007 |
|---|---|
| Report date: | 10-22-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| 4572007001R00 - NRC Website | |
A. Plant Operating Conditions Before The Event:
Event Date:�August 23, 2007 � Event Time: 1530 Unit: 2� MODE: 1� Reactor Power: 100 percent Unit 2 Reactor Coolant System (RCS) [AB] Temperature: 581 degrees F, Pressure: 2238 psig
B. Description of Event:
There were no additional structures, systems or components inoperable at the beginning of the event that contributed to the severity of the event.
On August 23, 2007; Braidwood Unit 2 was operating at full power.
At 15:24 hours, circulating water (CW) [KE] pump 2A tripped due to high differential level across the inlet traveling screens. Operations entered the appropriate abnormal operating procedure.
At 15:29 hours, while Operations was in the process of taking the proceduralized required actions to stabilize the event, CW pump 2B tripped due to a high differential level across the inlet traveling screens. In response, operator actions included isolating the 2B and 2D waterboxes, and initiating a 580 MWe ramp down at 20 MWe/minute.
At 15:30 hours, due to condenser backpressure exceeding allowable limits, Operations manually tripped Unit 2.
Operator response to the trip was proper and all safety related systems, structures and components operated normally during this event. The auxiliary feedwater system [BA] actuated, as expected, in this event to maintain steam generator levels.
C. Cause of Event
The root cause of the Unit 2 trip was that the traveling screen instrumentation experienced a sudden and false high differential level indication brought on by a momentary plugging of the upstream bubbler tube from agitated debris during a period of high winds. An evaluation determined that a pluggage of less than 0.2 seconds would be adequate to cause a spike sufficient in size to reach the pump trip point.
On August 21, 2007, the station experienced a fish kill in the cooling lake. At that time, Braidwood began monitoring of CW pump inlet traveling screens for cleanliness and.proper operation. The traveling screens and associated level monitoring components performed as expected.
On August 23, 2007, prior to the trip, severe weather conditions, including high winds, moved into the area.
During the high winds, Unit 2 experienced the automatic trips of the 2A and 2B CW pumps due to sudden spiking of the inlet traveling screen water level differential. The loss of substantial CW flow resulted in condenser backpressure to increase beyond allowable limits and therefore Operations manually tripped Unit 2.
Following the Unit 2 trip, the operators continued with response activities associated with the conditions existing with a manual Unit trip. Those responses brought the Unit to an orderly, safe and expected condition.
Troubleshooting activities included the calibration of the traveling screen instrumentation for the 2A and 2B CW pumps. All instruments were found to be within tolerance. During these checks, one failed component was � observed. The 2A level differential switch was found stuck in the closed position and replaced. This component failure did not contribute to this event. All other components and systems were found to be functioning normally and as expected.
D. Safety Consequences:
There were no safety consequences impacting plant or public safety as a result of this event. All safety related systems, structures and components operated normally during this event. The loss of multiple CW pumps and the resulting high condenser backpressure are events well within the ability of the operator to control, which include the removal of Unit 2, as necessary.
During the reactor shutdown, all required safety systems responded appropriately. There was no loss of any function that would have prevented fulfillment of actions necessary to 1) Shutdown the reactor and maintain it in a safe shutdown condition, 2) Remove residual heat, 3) Control the release of radioactive material, or 4) Mitigate the consequences of an accident.
The 2A and 2B auxiliary feedwater pumps started on Low Low steam generator levels as expected. The 2C condensate / condensate booster pump automatically started on low net positive suction head, as expected.
This event did not result in a safety system functional failure.
E. Corrective Actions:
The corrective action to prevent recurrence will be to install a time delay to preclude trips of the circulating water pumps due to sudden and false high differential level indication.
F. Previous Occurrences:
There have been no similar Licensee Event Report events at Braidwood Station in the last three years.
G. Component Failure Data:
Manufacturer Nomenclature Model Mfg. Part Number N/A N/A N/A N/A