05000353/LER-2016-001

LER-2016-001, Manual Actuation of the Reactor Protection System When Critical Due to Wiring Design Error

Limerick Generating Station, Unit 2
Event date:	06-01-2016
Report date:	07-27-2016
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
Initial Reporting
ENS 51968	10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation
3532016001R00 - NRC Website

LER 16-001-00 for Limerick, Unit 2 Regarding Manual Actuation of the Reactor Protection System When Critical Due to Wiring Design Error

ML16209A265
Person / Time
Site:	Limerick
Issue date:	07/27/2016
From:	Libra R W Exelon Generation Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
LG-16-083 LER 16-001-00
Download: ML16209A265 (4)

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Unit Conditions Prior to the Event Unit 2 was in Operational Condition (OPCON) 1 (Run) at approximately 100 percent power with PPC modification testing in progress. There were no structures, systems or components out of service that contributed to this event.

Description of the Event

On Wednesday, June 1, 2016, Limerick Unit 2 was operating at 100 percent power with PPC (El IS:CPU) modification testing in progress. The modification testing directed closure of a circuit isolation switch which resulted in the trip of both RRPs (El IS:P) due to an error in the modification wiring design. The control room supervisor (CRS) entered the procedure for an unexpected/unexplained change in core flow (OT-112) due to the trip of both RRPs.

The procedure directed a manual actuation of RPS.

All control rods inserted and safety significant systems functioned as expected. Reactor level initially increased then decreased to approximately +0 inches which is less than the +12.5 inch low level setpoint for RPS. Level then stabilized at normal level. Nuclear Steam Supply Shutoff System Groups IIA and IIB isolations actuated on low level at +12.5 inches.

The digital feedwater level control system functioned as designed. Reactor pressure vessel pressure was controlled by the main steam bypass valves.

The investigation of the event identified that a wiring design error caused an unplanned actuation of both RRP trip relays when a circuit isolation switch was closed as directed by the modification acceptance test procedure.

A four-hour ENS notification (#51968) was completed on Wednesday, June 1, 2016, at 1041 hours0.012 days <br />0.289 hours <br />0.00172 weeks <br />3.961005e-4 months <br /> as required by 10CFR50.72(b)(2)(iv)(B) for an actuation of RPS when the reactor is critical. This LER is being submitted pursuant to the requirements of 10CFR50.73(a)(2)(iv)(A) for a manual actuation of RPS.

Analysis of the Event

There was no actual safety consequence associated with this event. The potential safety consequences of this event were minimal. All control rods were verified to be fully inserted following the RPS actuation. The 2A RRP was restarted at 1548 hours0.0179 days <br />0.43 hours <br />0.00256 weeks <br />5.89014e-4 months <br /> on June 1, 2016 and the 2B RRP was restarted at 0326 hours0.00377 days <br />0.0906 hours <br />5.390212e-4 weeks <br />1.24043e-4 months <br /> on June 2, 2016.

The PPC replacement modification was in progress and computer input points were being transferred from the original computer to the new computer. When the second RRP related computer point was placed in service by closure of a circuit isolation switch both 2A and 2B RRP trip relays were energized due to an error in the wiring design.

Cause of the Event

The direct cause of the event was a circuit wiring design error that was implemented in the field and caused energization of the RRP ASD trip coils.

The root cause of the event was a failure of station personnel to appropriately apply THU error prevention techniques to identify the design error and prevent its installation and testing as part of the modification.

Corrective Actions Completed The isolation switch for the mis-wired circuit was opened to enable reset of the ASD trip coils.

The 2A and 2B ASDs were returned to service.

Corrective Actions Planned The wiring design error will be corrected and the Modification Acceptance Test will be revised to ensure the change is correctly tested per the requirements of the Acceptance Test Criteria.

The human performance aspects of the event will be addressed through several management actions that include reinforcement of proper standards and behaviors related to THU error techniques with station personnel.

Previous Similar Occurrences There was no previous RPS actuation in the past five years due to modification testing.

Component data System:

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AD Reactor Recirculation System P Pump 2A-P201-DR Reactor Recirculation Pump G080 General Electric Company 5K46385AA1