05000456/LER-2007-001

LER-2007-001, Unit 1 Reactor Trip Following a 345 Kv Transmission Line Lightning Strike

Docket Number
Event date:	06-27-2007
Report date:	08-27-2007
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
Initial Reporting
4562007001R00 - NRC Website

A. Plant Operating Conditions Before The Event:

Event Date: June 27, 2007 Event Time: 0921 Unit: 1 MODE: 1 Reactor Power: 100 percent Unit 1 Reactor Coolant System (RCS) [AB] Temperature: 587 degrees F. Pressure: 2236 psig

B. Description of Event:

Prior to the event, an in-series impedance relay was in a tripped state due to a failed fuse in a potential monitoring circuit of the 1D reactor coolant pump (RCP). There were no additional structures, systems or components inoperable at the beginning of the event that contributed to the severity of the event.

On the morning of June 27, 2007, Braidwood Unit 1 was operating at full power.

At 0921 hours0.0107 days <br />0.256 hours <br />0.00152 weeks <br />3.504405e-4 months <br />, Braidwood Unit 1 experienced an automatic reactor trip from full power. A lightning strike in close proximity to the site caused a grid disturbance that resulted in actuation of protective relays for the 1D RCP. The shutdown of the 1D RCP resulted in a rapid reduction of coolant flow in the 1D reactor coolant loop and subsequent Reactor Protection System (RPS) [JG] actuation to trip the Unit 1 reactor.

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 event was initiated by a lighting strike 4.3 miles from the plant that created a single phase to ground fault on a 345 kV transmission line. The ground fault suppressed the B-phase voltage to 48% of nominal until 345 kV protective breakers at Braidwood and the East Frankfort Transmission Substation isolated the line about 3 cycles (0.051 seconds) after fault initiation. The suppressed B-phase voltage created a momentary phase imbalance on the Braidwood RCPs resulting in elevated current on the unaffected A and C-phase. The C-phase current on the 1D RCP exceeded the trip setpoint of an instantaneous overcurrent protective relay. Normally, the RCP trip on momentary overcurrent would have been blocked by an in-series impedance relay; however, at the time of the event, the impedance relay was in a tripped state due to a failed fuse in a potential monitoring circuit.

The Root Cause of the Unit 1 trip was determined to be a latent design weakness in the RCP protective scheme that permitted the existence of an undetected component failure creating a grid disturbance-intolerant vulnerability.

Contributing to the this event, the Bus 159 potential transformer secondary side C-phase fuse open-circuited preventing proper voltage application to the 1D RCP impedance protective relay.

3. 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 actuation of the Reactor Protection System was valid for this plant condition and occurred without incident. The reactor trip is designed to prevent Departure from Nucleate Boiling caused by the reduction in total core flow. This event is explicitly described under Updated Final Safety Analysis Report section 15.3.1, Partial Loss of Reactor Coolant Flow.

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.

For a short period of time following the reactor shutdown, Unit 1 entered Limiting Condition of Operation Action Requirement (LCOAR) 3.7.5, Condition A for the 1B Auxiliary Feedwater Pump when the diesel oil Day Tank level dropped to 74%. Operators were dispatched to add fuel to the Day Tank to exit the LCOAR. Although the 1B AFW pump was declared inoperable during this time, the pump remained in operation until it was no longer needed to maintain steam generator levels. The 1B LCOAR was entered at 10:07 am and exited at 10:45 am when the Day Tank low-level alarm cleared. The 1B AFW pump day tank level was above the Technical Specification limit prior to the event so the 1B AFW pump was capable of performing its design function.

This event did not result in a safety system functional failure.

E. Corrective Actions:

Corrective actions in response to this issue include:

• Revising identified procedures to perform monitoring of the integrity of critical fuses within trip-sensi ve protective relaying circuits, and

• Implementing design changes to eliminate the identified vulnerability from the RCP protective relaying system, or if design changes are not appropriate, developing alternate actions to address the vulnerability.

. Previous Occurrences:

There have been no similar Licensee Event Report events at Braidwood Station in the last three years.

G. Component Failure Data:

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