05000395/LER-2009-004

LER-2009-004, Manual reactor trip due to main steam dump system fault 05000

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
3952009004R00 - NRC Website

PLANT IDENTIFICATION

Westinghouse - Pressurized Water Reactor

EQUIPMENT IDENTIFICATION

ITY00408E, Delta T-Tavg Control Summing Amplifier

IDENTIFICATION OF EVENT

On December 16, 2009, VCSNS was performing a downpower to repair a faulty main turbine control valve when high vibration was observed on the #2 turbine bearing. The main turbine was manually tripped at 1414 hours0.0164 days <br />0.393 hours <br />0.00234 weeks <br />5.38027e-4 months <br /> with reactor power at approximately 46%. Following trip of the main turbine, the main steam dump system failed to operate. With increasing RCS and main steam pressures, operators initiated a manual reactor trip at approximately 1415 hours0.0164 days <br />0.393 hours <br />0.00234 weeks <br />5.384075e-4 months <br />.

EVENT DATE

December 16, 2009 Condition Report CR-09-05566 was written to address event.

REPORT DATE

February 11, 2010

CONDITIONS PRIOR TO EVENT

Mode 1, 46% Power

DESCRIPTION OF EVENT

On December 16, 2009, while VCSNS was performing a downpower to repair faulty main turbine control valve #4, main turbine vibration was observed increasing on the #2 bearing. Some vibration was expected due to faulty main turbine control valve #4 being open when it is normally closed at less than 90% power. At the prejob brief, the shift supervisor had determined that if vibrations reached 9 mils, the turbine would be tripped. At approximately 1414 hours0.0164 days <br />0.393 hours <br />0.00234 weeks <br />5.38027e-4 months <br /> with the reactor at 46% power and turbine vibration approaching 9 mils on bearing #2, the main turbine was manually tripped. Following the trip of the turbine, the main steam (MS) dump system failed to operate. MS pressures increased resulting in actuation of MS power operated relief valves (PORVs) and MS safety valves. At 1415 hours0.0164 days <br />0.393 hours <br />0.00234 weeks <br />5.384075e-4 months <br />, operators manually tripped the reactor. Main turbine vibrations increased to greater than 15 mils on the #1 and #2 turbine bearings necessitating breaking of main condenser vacuum per station operating procedure. Both motor driven emergency feedwater pumps were manually started at 40% steam generator level. During the event, pressurizer pressure exceeded 2300 psig resulting in opening of a pressurizer PORV for a short period of time. Pressurizer level decreased below the low level setpoint due to the plant cooldown which resulted in a letdown isolation. Operators promptly restored letdown per plant procedures. Operators entered the reactor trip recovery procedure, stabilized the plant in Mode 3, and remained in Mode 3 until repairs of the MS dump system circuit card and main turbine control valve #4 were made.

CAUSE OF EVENT

A malfunction of MS dump system circuit card ITY00408E, Delta T - Tavg Control Summing Amplifier, was determined to be the cause of the event. Troubleshooting by maintenance and engineering personnel revealed the output of the circuit card had failed low.

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ANALYSIS OF EVENT

The main steam (MS) dump system is designed to dump main steam to the main condenser, or atmosphere, to remove decay heat and facilitate pressure control of the RCS during plant trips, transients, and turbine load rejections. Failure of the MS dump system to actuate could result in unplanned reactor trips. The MS power operated relief valves (PORVs), MS safety valves and pressurizer PORVs responded to system pressures as designed. The MS PORVs and safety valves dumped steam to the atmosphere to control plant cooldown when the MS dump system failed. The reactor protection system (RPS) responded as designed when reactor trip breakers opened reducing reactor power. There were no automatic engineered safeguard feature (ESF) system actuations. Both motor driven emergency feedwater pumps were manually started at 40% steam generator level prior to reaching their automatic setpoint. All safety systems responded as required to protect the plant and public and there were no safety consequences resulting from this event.

Actual vibration levels on main turbine bearings #1 and #2 during the coastdown were reviewed and were found to be consistent with expected vibration levels for a coastdown with a rub. Vibration levels were normal throughout startup and loading confirming no deleterious effect on turbine components.

From a probabilistic risk assessment (PRA) perspective, this event was not risk significant. The MS dump system is modeled for failure to close on an ATWS (Anticipated Transient Without Scram) event, but is not modeled for failure to open. The reactor trip initiating event frequency is based on industry experience as well as plant specific events and therefore includes the potential for steam dump system failure to cause a reactor trip. Since all PRA credited functions were successful, there is no calculated increase in Core Damage Frequency (CDF) or Large Early Release Frequency (LERF).

CORRECTIVE ACTIONS

The cause of the event was determined to be the failed circuit card ITY00408E, Delta T-Tavg Control Summing Amplifier. The card's output had failed low. The card was replaced. Post maintenance testing was performed satisfactory and the MS dump system was returned to service. The card has been placed in the station's preventive maintenance program and is scheduled for refurbishment at an 8 year frequency.

PRIOR OCCURRENCES

None. There were no prior failures of circuit card ITY00408E identified for the MS dump system during the past three years.

�NRC FORM 366A (9-2007) PRINTED ON RECYCLED PAPER