05000335/LER-2016-003

LER-2016-003, Generator Lockout Relay Actuation During Power Ascension Results in Reactor Trip

St. Lucie Unit 1
Event date:	08-21-2016
Report date:	10-20-2016
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
3352016003R00 - NRC Website

LER 16-003-01 for St. Lucie, Unit 1, Regarding Generator Lockout Relay Actuation During Power Ascension Results in Reactor Trip

ML17230A065
Person / Time
Site:	Saint Lucie
Issue date:	08/15/2017
From:	Deboer D Florida Power & Light Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
L-2017-144 LER 16-003-01
Download: ML17230A065 (4)

learned are incorporated into the licensing process and fed back to industry. Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (1-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

Description of the Event

On August 21, 2016 St. Lucie Unit 1 was in in Mode 1 performing normal power ascension evolutions following a maintenance outage. At 1926 EDT when the main generator [EL] reached approximately 8000 amps and at 38% reactor power, unexpected actuation of the Main Generator Inadvertent Energization Lockout Relay [86:EL] caused the main generator to trip, resulting in an automatic reactor trip.

The reactor trip was complicated by the generator lockout preventing the automatic transfer of station auxiliaries to the available startup transformer power, requiring the emergency diesel generators to automatically start and power the safety related buses. Reactor coolant pumps normally powered through the non-safety buses were deenergized, and decay heat removal was via natural circulation and manual control of Auxiliary Feedwater. Due to the blocked automatic transfer to the available startup transformer power, the electric driven main feedwater pumps were not available until power to the non-safety related buses was restored.

Other than the generator lockout, all systems functioned as designed, and the plant stabilized in Mode 3.

Operators declared a Notice of Unusual Event (NUE). The declaration of the NUE was based on the lockout relay preventing the automatic transfer to available startup transformer power. The transfer of power was required to be performed manually by operators following verification of availability. Offsite power remained available at the switchyard throughout the event, and operators restored power to the safety and non-safety related buses from the startup transformers at 2036 and restarted reactor coolant pumps. The unusual event was terminated at 2125 hours0.0246 days <br />0.59 hours <br />0.00351 weeks <br />8.085625e-4 months <br /> following restoration of forced cooling within the reactor coolant system.

Unit 2 was unaffected and remained at 100% power.

Cause

The root cause of this event was a latent error introduced during a 2013 design modification when a wire for the inadvertent energization lockout relay reset circuit was incorrectly removed as part of a modification to the synchronization circuit. This modification allowed the relay to remain energized and undetected following manual synchronization to the grid.

Analysis of the Event

The generator lockout relay was energized due to a missing wire in the non-safety related protection relay circuit. The missing wire had been inadvertently removed during a modification of the automatic synchronization system components implemented in 2013 to restore the automatic synchronization capabilities of the main generator breakers. The wire that had been incorrectly removed was in a section of the circuit that was not intended to be modified, and this wire was not within the scope of the 2013 design modification. The missing wire caused the lockout relay circuit to remain armed after manual synchronization to the grid. The discrepant condition was not immediately detectable to control room operators.

learned are incorporated into the licensing process and fed back to industry. Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

Prior to this event, Operators were performing normal power ascension evolutions following a maintenance outage. The Unit 1 main generator was manually synchronized to the electrical grid.

Previous synchronizations since the 2013 modification had been performed using the automatic synchronization feature, which was not affected by the missing wire. Following the manual synchronization on August 21, 2016, the lockout relay circuit did not reset as designed but remained armed with a trip setpoint of 8000 amps through the main generator.

This licensee event report is being reported in accordance with 10 CFR 50.73(a)(2)(iv)(A) as "Any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph (a)(2)(iv)(B)." This event included automatic actuations of the reactor protection system and the 1A and 1B emergency diesel generators, and manual actuation of the auxiliary feedwater system.

Safety Significance

This condition was mitigated by the automatic system actuations and concurrent Operator actions.

The Conditional Core Damage Probability (CCDP) and Conditional Large Early Release Probability (CLERP) values were evaluated for the stated event and were found to be below the thresholds of 1.0E-6 and 1.0E-7 for CCDP and CLERP, respectively. Therefore, it is concluded that the risk impact of the stated event is not risk-significant.

The health and safety of the public were not affected by this event.

Corrective Actions to Prevent Recurrence

1. The removed wire for the lock out relay circuit was reinstalled.

2. Implemented procedure guidance to verify the inadvertent energization relay is reset prior to exceeding 8000 amps when synchronizing to the grid manually.

Failed Component 86/INAD/1790 Main Generator Inadvertent Energization Lockout Relay Manufacturer General Electric Model: 12HEAG1B237X16 Previous Occurrence No previous similar LERs.