05000335/LER-2001-007

LER-2001-007,

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv), System Actuation
3352001007R00 - NRC Website

Description of the Event

On June 5, 2001, St. Lucie Unit 1 was in Mode 1 at 100 percent reactor power. At 1048 hours0.0121 days <br />0.291 hours <br />0.00173 weeks <br />3.98764e-4 months <br />, St. Lucie Unit 1 experienced an unexpected trip of the 1A2 reactor coolant pump [EIIS:AB:P] (RCP). This was immediately followed by an automatic reactor trip from the reactor protection system (RPS) on low reactor coolant flow.

The Unit 1 turbine generator tripped following the reactor trip. Plant electrical distribution systems automatically transferred from the unit auxiliary transformers to the unit start up transformers as required for onsite power. Approximately one minute later, the 1B main feedwater pump [EIIS:SJ:P] (MFP) tripped and the auxiliary feedwater system actuated to provide cooling flow to the steam generators. All safety-related equipment operated per design.

Cause of the Event

Although no definitive root cause was found for the RCP trip, FPL determined that the most probable cause was a spurious actuation of the Westinghouse SA-1 motor protective relay device [EIIS:AB:P:87]. This determination was based on vendor notifications, known industry history, and the symptoms of the trip.

The 6900-volt switchgear uses a motor protective relay, SA-1, that provides motor protection for faults less than approximately 50% of the motor locked rotor current.

This relay senses the current flow through all three phases of the motor power cables. Any differential in the current flow that exceeds a factory setpoint results in a trip of the motor circuit breaker. This relay responds to motor conditions different than the motor overcurrent relays. It was noted during the review of similar industry events and vendor notifications that the silicon controlled rectified (SCR) in the SA-1 relay was susceptible to random firing. The SCR initiates a trip signal based on a differential current in the motor windings in the three motor phases. Spurious trip signals from the SCR were dependent on "history, ambient temperature, voltage, and time." As a conservative measure, FPL replaced the SA-1 relay. The SA-1 relay that was removed was extensively tested and no problems were found with the removed relay.

The MFP trip was caused when recirculation valve FCV-09-1B2 [EIIS:SJ:FCV] failed to open which resulted in a MFP low flow trip. The faulty valve positioner was replaced with a new positioner.

Analysis of the Event

This event is reportable under 10 CFR 50.73(a)(2)(iv) for two conditions. The first condition is as any event or condition that caused a manual or automatic actuation of the reactor protection system, including reactor scram or trip. The second condition is as any event or condition that caused a manual or automatic actuation of the auxiliary feedwater system.

Analysis of Safety Significance Reactor trips are analyzed events, and pose no significant safety issues. The trip was uncomplicated, and minor secondary equipment issues were dispositioned. FPL confirmed that Westinghouse SA-1 differential current relays are not used in any safety-related circuits at St. Lucie. Therefore, this event had no adverse impact on the health and safety of the public.

FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3)

Corrective Actions

1. The 1A2 RCP SA-1 relay was replaced and the retest was completed satisfactorily.

2. The valve positioner for FCV-09-1B2 was replaced and the retest was completed satisfactorily.

3. FPL is considering the feasibility of several design alternatives to the SA-1 relays.

Additional Information

Failed Components Identified Component: � Motor Protective Relay Device Manufacturer: Westinghouse Model: � SA-1 Component: � Valve Positioner for FCV-09-1B2 Manufacturer: Moore Industries Model: � 74H/FR Similar Events Power to the 1A2 Reactor Coolant Pump," documents a similar trip of the 1A2 RCP.