05000298/LER-2003-005

LER-2003-005, Turbine Trip Failure During Testing Due to Ester Contamination of Turbine Lube Oil

Cooper Nuclear Station
Event date:	09-28-2003
Report date:	11-21-2003
Reporting criterion:	10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident
2982003005R00 - NRC Website

PLANT STATUS

Cooper Nuclear Station (CNS) was in Mode 1 (Run) at approximately 61 percent power. Power had been lowered from 100 percent to support performance of an on-line modification to replace the Control Rod Drive System (EIIS:AA) scram solenoid pilot valves.

BACKGROUND

The main turbine is provided with safeguards against situations that may cause damage to the turbine if it were not taken out of service immediately. These protective devices include a low vacuum tripping device, a low bearing pressure trip, a thrust bearing trip and a remote controlled solenoid actuated trip. They are contained in a protective trip block mounted on the turbine front bearing pedestal. These devices operate independently to actuate the protective trip dump valve. The turbine trip protection features are functionally tested to simulate the trip condition.

One of the electrical signals received by the solenoid trip device is for a reactor vessel high water level. This instrumentation is designed to detect a potential failure of the feedwater level control system (EIIS:JB) that could result in excessive feedwater flow. With excessive feedwater flow, the water level in the reactor vessel rises toward the high water Level 8 reference point causing the trip of the two feedwater pump turbines and the main turbine. The main turbine trip causes a reactor scram which mitigates the reduction in Minimum Critical Power Ratio (MCPR).

The trip block consists of a drain valve, a pivot plate, and the previously described trip actuators. Any one of the trip actuators can cause the trip plate to tip and actuate the drain valve which will depressurize the auto-stop oil header. This action will close the main turbine stop valves and initiate a reactor scram.

EVENT DESCRIPTION

At 2345 hours0.0271 days <br />0.651 hours <br />0.00388 weeks <br />8.922725e-4 months <br /> on September 28, 2003, while performing turbine trip testing, the turbine trip block failed to actuate as demanded while testing the solenoid trip. The low vacuum trip was tested and it did not actuate as required. The surveillance was terminated. Troubleshooting identified initial resistance in the movement of the trip plate in the trip direction. Repeated cycling of the trip plate resulted in no abnormal resistance. The surveillance was again initiated and completed satisfactorily.

When the initial surveillance failed, it was determined that a reactor trip would not occur as required by Technical Specification 3.3.2.2, Feedwater and Main Turbine High Water Level Trip Instrumentation.

Failure to satisfy the operability requirements of Technical Specification 3.3.2.2 resulted in Condition B being entered at 0015 on September 29, 2003 to restore the high water level trip capability within two hours. When the capability was not timely restored, Condition C was entered at 0215 on September 29, 2003 with the required action to reduce power to less than 25% within four hours. At 0530, power was lowered to less than 25% (22.6% power). Trip capability was restored at 0615. Technical Specification 3.3.2.2 Conditions B and C were exited at 0551 on September 30, 2003 based on troubleshooting activities, successful performance of main turbine trip testing, and approval of compensatory measures for the degraded condition.

BASIS FOR REPORT

This event is reportable in accordance with 10CFR50.73(a)(2)(v)(D) as "any event or condition that could have prevented the fulfillment of a safety function of structures or systems that are needed to mitigate the consequences of an accident.

CAUSE

The most probable root cause was determined to be ester contamination of the turbine lubricating oil combined with trace amounts of water, resulting in the formation of hydrolyzed esters that increased the forces required to open the trip valve. The source of the ester contamination is being investigated. In addition, particles collected in clearances around the trip valve, increasing the forces required to open the trip valve.

SAFETY SIGNIFICANCE

The loss of the reactor vessel high level (Level 8) main turbine trip function has a negligible impact on Core Damage Frequency. This condition did not impact the ability of the reactor feed pump turbine trip on Level 8.

Thus, no water carry over would be expected to impact High Pressure Coolant Injection(HPCI) (EIIS:BJ) or Reactor Core Isolation Cooling (RCIC) (EIIS:BN) operation. The main turbine trip block degradation had no impact on initiating event frequency. The trip block inability to trip the main turbine is not an initiator. The only transient which credits the turbine trip block function is the feedwater controller failure. The credit is for the indirect scram that results from the turbine stop valve closure. All other trip functions are designed strictly for turbine protection. The impact on MCPR is bounded in the core reload licensing analyses. Mitigation capabilities for shutting the reactor down to decay heat levels and Emergency Core Cooling System (ECCS) injection were not impacted.

This event constitutes a Safety System Functional Failure in accordance with NEI 99-02, Revision 2 as any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident. Feedwater controller failure is a transient analyzed in the CNS Updated Safety Analysis Report.

CORRECTIVE ACTIONS

Immediate Actions

1. An extra operator was designated to trip the turbine locally if a trip was needed. This contingency action remained in place until trip capability was restored on 09/29/03.

2. An interim action is performing the turbine trip functional test more frequently.

Long-Term Action 1. The particulate contamination in the Turbine Lube Oil System will be removed until the particulate counts are less than one-half of the manufacturer's allowable limits, at a minimum. This action will be completed by 06/01/05. In the interim, the lube oil strainer continues to remove particulates.

PREVIOUS EVENTS

A search of internal operating experience found no past events that were relevant.

ATTACHMENT 3 LIST OF REGULATORY COMMITMENTS© Correspondence Number: NLS2003110 The following table identifies those actions committed to by Nebraska Public Power District (NPPD) in this document. Any other actions discussed in the submittal represent intended or planned actions by NPPD. They are described for information only and are not regulatory commitments. Please notify the Licensing & Regulatory Affairs Manager at Cooper Nuclear Station of any questions regarding this document or any associated regulatory commitments.

OR OUTAGE

Particulate contamination in the Turbine Lube Oil System will be removed until the particulate counts are less than one- half of the manufacturer's allowable limits, at a minimum.

06/01/05 PROCEDURE 0.42 � �