05000316/LER-2012-002

LER-2012-002, Unit 2 Reactor Trip from Generator Trip Due to Incorrect Relay Setting

Docket Number
Event date:	3-0-2012
Report date:	06-26-2012
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
3162012002R00 - NRC Website

Conditions Prior to Event 91 percent reactor power.

Description of Event

On April 30, 2012, at 2328 hours0.0269 days <br />0.647 hours <br />0.00385 weeks <br />8.85804e-4 months <br />, during power ascension following a recent refueling outage, Donald C. Cook Nuclear Plant (CNP) Unit 2 Reactor tripped automatically from a trip of the main turbine [TRB]. All control rods [AA] fully inserted and the auxiliary feedwater system (AFW) [BA] started and performed as designed.

Immediately following the reactor trip, the Reactor Coolant System (RCS) [AB] temperature responded as expected and lowered to approximately 544 degree Fahrenheit (F), then rose to a maximum of 548 degree F approximately 8 minutes following the reactor trip. RCS temperature then slowly lowered over the next 35 minutes reaching a minimum RCS temperature of 539 degree F on May 1, 2012, at 0013 hours1.50463e-4 days <br />0.00361 hours <br />2.149471e-5 weeks <br />4.9465e-6 months <br />.

The main steam isolation valves (MSIVs) [ISV] were subsequently closed in accordance with the reactor trip response procedure, which was effective in controlling the cooldown. Decay heat was being removed through the steam generator atmospheric relief valves (PORV) [RV]. The cause of the RCS cooldown was due to leakage past a steam dump [JI] valve [V] to the condenser [SG].

All major plant components functioned as designed; as such, there were no safety system functional failures. The reactor trip was reported in accordance with 10 CFR 50.72(b)(2)(iv)(B) and the AFW actuation was reported in accordance with 10 CFR 50.72(b)(3)(iv)(A).

The main turbine tripped due to the actuation of the main generator unit differential and overall differential lockout relays [86]. A multifunction digital overall differential relay [87] operated the lockout relays and had a phase 2 and phase 3 differential indication observed. A walkdown did not identify any indication of an actual electrical fault.

The reactor trip and AFW system actuation are reportable as a Licensee Event Report (LER) in accordance with 10 CFR 50.73(a)(2)(iv)(A).

Cause of Event

A multifunction digital overall differential relay was installed during a recent refueling outage modification which replaced three overall differential relays and the "V/Hz Over-Excitation Relay" with the single device.

The new relay actuated due to an incorrect current transformer winding compensation setting within the new relay. The incorrect setting caused an invalid measurement of current imbalance that was significant enough at 91% power to trip the differential relay element. The relay setting was not verified following installation as prescribed by the modification. The modification process did not require formal verification of tasks implementing the modification.

Analysis of Event

The event is not considered to be risk significant as there were no risk significant equipment failures to pose elevated risk. It is recognized that there was an actuation of a main generator protective circuit and all systems responded as designed.

Based on review of the control room logs and the post-trip review, all plant systems performed as designed to shut down the unit and remove decay heat. No risk-significant equipment functions were affected or failed and no significant operator actions outside those required for normal trips were required.

Corrective Actions

Completed Corrective Actions

The overall differential relay setting changes were implemented and monitored during the restart evolution as load was increased to 100%.

Planned Corrective Actions

Planning and modification procedures will be revised to address task verification of modification implementation.

Previous Similar Events

LERs for CNP Unit 1 and Unit 2 for the past three years were reviewed for similar events. The following was identified:

05000315-2011-001-00, Unit 1 Reactor Trip Due To Main Turbine Trip On September 07, 2011, at 0854 hours0.00988 days <br />0.237 hours <br />0.00141 weeks <br />3.24947e-4 months <br />, Donald C. Cook Nuclear Plant (CNP) Unit 1 Reactor tripped automatically due to a trip of the main turbine. All control rods fully inserted and the auxiliary feedwater system (AFW) started and performed as designed.

The cause of the event above, Unit 1 Main Turbine trip, was an incorrectly installed thrust probe, which was due to an over reliance on field engineering guidance for maintenance activities in lieu of detailed planning instruction. Corrective actions included revising project guidance to improve the quality of work order instructions.

The cause of the event, for this LER, was that the modification process did not require a formal verification of activities required to implement the modification.

The causes are different in each event and the corrective actions taken in the prior event would not have precluded this event.