05000457/LER-2009-001

LER-2009-001, Reactor Trip on Over Temperature Delta Temperature due to a Signal Spike on One Channel With Another Channel Placed in the Tripped Condition for Surveillance Testing

Docket Number
Event date:	04-24-2009
Report date:	06-23-2009
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation
4572009001R00 - NRC Website

A. Plant Operating Conditions Before The Event:

Event Date: April 24, 2009 Event Time: 1141 Unit: 2 MODE: 1 Reactor Power: 100 percent Unit 2 Reactor Coolant System (RC) [AB]: Normal operating temperature and pressure

Description of Event:

There were no structures, systems or components inoperable at the beginning of the event that contributed to the severity of the event.

On April 24, 2009, a Technical Specification (TS) surveillance calibration of the 2B pressurizer [AB] pressure loop was being performed. This surveillance requires that the channel under test be placed in the tripped condition. In support of this surveillance, Operations entered TS 3.3.1 "Reactor Trip System (RTS) Instrumentation," Conditions A, E, and K for one channel being inoperable, and the 2B over temperature delta temperature (OTDT) logic coincidence is made up.

At 1141 hours0.0132 days <br />0.317 hours <br />0.00189 weeks <br />4.341505e-4 months <br />, while the 2B pressurizer pressure channel calibration was in progress, a momentary/spurious signal spike occurred for indeterminate reasons on the 2D OTDT channel (narrow range cold leg temperature spiked low). With the 2B OTDT bistable already placed in the tripped condition, the spike on the 2D OTDT Operator response to the trip was proper and all safety related systems, structures, and components operated normally during this event. Following the reactor trip, all four Unit 2 steam generators [S]] reached their low-2 reactor trip setpoints and the Unit 2 pressurizer reached its low pressure reactor trip setpoint, all of which are expected responses on a trip from full reactor power. The auxiliary feedwater system [BA] actuated on low-2 steam generator levels, as expected, to maintain steam generator levels.

This event is reportable under 10 CFR 50.73(a)(2)(iv)(A), any event or condition that resulted in manual or automatic actuation of any of the systems listed in 10 CFR 50.73(a)(2)(iv)(B) including any event or condition that results in actuation of the reactor protection system (RPS) when the reactor is critical, and actuation of the PWR auxiliary feedwater system.

C. Cause of Event

The Unit 2 reactor tripped due to a momentary/spurious signal spike on the 2D OTDT channel while the 2B OTDT logic. The investigation of this event found no issues with human performance, equipment failure, or plant activities that could have caused the signal spike.

the Reactor Trip System and Engineered Safety Features Actuation System (RTS/ESFAS) [JE] protects against unplanned or stray trip signals on a single channel, which would otherwise result in a reactor trip, while still capturing legitimate trip signals seen by multiple channels. However, this design is not fault tolerant. During � error, spurious transient, or channel failure in a coincident channel initiates an inadvertent reactor trip or safeguards actuation. This design has resulted in several events within the industry including unit trips while at power.

Therefore, the root cause of the Unit 2 reactor trip is determined to be the design of the RTS/ESFAS, which places a loop in a trip condition for testing, increases vulnerability during testing conditions.

D. Safety Consequences:

There were no safety consequences impacting plant or public safety as a result of this event.

The OTDT function is a RTS function. The OTDT function is provided to ensure that the design limit departure from nucleate boiling ratio is met. This trip function also limits the range over which the overpressure delta­ temperature trip function must provide protection.

The reactor trip system responded automatically due to the to the OTDT trip signal received. 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.

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

E. Corrective Actions:

The corrective action to prevent recurrence is to pursue a modification which bypasses a tested channel rather than placing it in the tripped state during surveillances.

F. 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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