05000388/LER-2005-005

LER-2005-005, Reactor Automatic Scram due to a Main Generator Lockout

Docket Number
Event date:	6-6-2005
Report date:	9-16-2005
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
Initial Reporting
ENS 41746	10 CFR 50.72(b)(2)(xi), Notification to Government Agency or News Release, 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation, 10 CFR 50.72(b)(3)(iv)(A), System Actuation
3882005005R01 - NRC Website

PLANT CONDITIONS AT TIME OF EVENT

Unit 1, Mode 1, 100% Unit 2, Mode 1, 100%

EVENT DESCRIPTION

At 1233 hours0.0143 days <br />0.343 hours <br />0.00204 weeks <br />4.691565e-4 months <br /> on 6/6/2005 with Unit 2 in Mode 1 at 100% power, the reactor automatically scrammed as result of a turbine trip from a main generator (EIIS Code: TB) lockout. The main generator exciter (EIIS Code: TL) voltage regulator circuit had failed to positively respond to changes to the offsite grid which resulted in a loss of exciter field, causing the lockout. The most probable cause was an increased resistance in the generator exciter field circuitry. This led to an inability of the system to properly respond to the grid changes.

All control rods inserted and all safety systems responded as designed in response to the automatic scram.

Immediately following the scram, two Safety Relief Valves (EIIS Code: SB) opened and then properly reseated in response to the transient. The Main Steam Bypass Valves were subsequently able to control reactor pressure. Reactor water level dropped to -6 inches and was restored to +35 inches by normal feedwater level control. Several balance of plant systems were impacted by the generator voltage perturbation but there were no unusual electrical transients or challenges to the station's safety-related electrical systems. There also were no challenges to containment or evidence of fuel failure.

The event was determined to be reportable under 10 CFR 50.72, reference ENS Notification EN #41746.

The automatic scram was an unplanned actuation of a system designed to mitigate the consequences of a significant event and is reportable per 10 CFR 50.73(a)(2)(iv)(A).

This event resulted in no actual adverse consequences to the health and safety of the public.

CAUSE OF THE EVENT

On June 6, 2005 when Unit 2 experienced changes in the offsite grid, the generator output voltage decreased in response to this change. The generator excitation system output voltage that controls generator output, also decreased as designed. The controlling circuitry for the excitation system has a minimum voltage requirement for it to perform properly, thereby maintaining stable operation of the system. Because of an increased resistance in the circuitry, the actual minimum voltage required for stable operation was higher than an established value. When voltage dropped below the actual minimum voltage requirement, the excitation system was no longer capable of properly functioning, resulting in a loss of exciter field.

ANALYSIS / SAFETY SIGNIFICANCE

Actual Consequences The actuation of the generator lockout caused a turbine trip, resulting in a reactor scram. All control rods inserted and safety systems functioned as designed. The health and safety of the public was not affected.

Potential Consequences Challenges to nuclear safety can result from equipment failure or human errors during recovery of the reactor to normal shutdown conditions. The potential consequence results in a negligible increase in the probabilistic risk to the health and safety of the public.

CORRECTIVE ACTIONS

Completed Actions

1) Inspected generator voltage regulator circuit for potential faults/failures.

2) Replaced possible degraded potentiometer in voltage regulator circuitry.

3) Changed the existing minimum exciter voltage limit setting to increase the range of generator voltages to which the regulator circuitry can respond.

Planned Actions

1) On Unit 1 and Unit 2, identify high resistance paths in the voltage regulator circuits and correct.

ADDITIONAL INFORMATION

None