05000315/LER-2015-001
| Donald C. Cook Nuclear Plant | |
| Event date: | 06-01-2015 |
|---|---|
| Report date: | 07-29-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown |
| Initial Reporting | |
| ENS 51106 | 10 CFR 50.72(b)(2)(i), Tech Spec Required Shutdown |
| 3152015001R00 - NRC Website | |
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Donald C. Cook Nuclear Plant Unit 1 05000315 NUMBER NO.OF
INTRODUCTION
On June 1, 2015, at 0231 hours0.00267 days <br />0.0642 hours <br />3.819444e-4 weeks <br />8.78955e-5 months <br />, Donald C. Cook Nuclear Plant Unit 1 Reactor [RCT] was manually tripped from Mode 1 at approximately 17 percent rated thermal power. Unit 1 was removed from service due to the inability to restore an inoperable emergency diesel generator (EDG) [EK] [DG] to operable status within 14 days in accordance with Technical Specification (TS) 3.8.1, AC Sources - Operating.
The Unit 1 AB EDG was declared inoperable on May 18, 2015, for scheduled maintenance. During a post maintenance test run, the Unit I AB EDG tripped after approximately 16 minutes on HI-HI bearing temperature due to crankshaft bearing No. 4 failure.
Both Unit I and Unit 2 reactors were operating at 100 percent power prior to the event.
EVENT DESCRIPTION
On May 18, 2015, at 0010 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br />, the Unit 1 AB EDG was removed from service for scheduled maintenance which included draining of the lube oil system [LA] to replace a strainer transfer valve and cooler thermostatic bypass valve. On May 21, 2015, at 1049 hours0.0121 days <br />0.291 hours <br />0.00173 weeks <br />3.991445e-4 months <br />, the EDG was started for post maintenance testing. At approximately 16 minutes following the start, the EDG automatically tripped on HI-HI bearing temperature. A failure investigation team found, through identification of melted babbitt material within the crankcase, that the No. 4 main bearing had been damaged.
Further evaluation of the damage confirmed that the time required for repairs would exceed the TS completion time. This required a planned Unit 1 shutdown which took place on June 1, 2015, at 0231 hours0.00267 days <br />0.0642 hours <br />3.819444e-4 weeks <br />8.78955e-5 months <br />, in accordance with TS 3.8.1, AC Sources - Operating.
EVENT ANALYSIS
shutdown. All systems responded as expected with no complications.
COMPONENT
Worthington, Emergency Diesel Generator
ASSESSMENT OF SAFETY CONSEQUENCES
NUCLEAR SAFETY
Failure of the Unit I AB EDG during its post maintenance test run resulted in no actual nuclear safety impacts. The failure of this EDG occurred at a time when the EDG had already been declared inoperable in support of routine maintenance. All normal and reserve offsite power sources, the opposite Train Unit 1 CD EDG, and the Supplemental Diesel Generators were Operable and administratively guarded.
The risk assessment determined this event created a low risk significant event assuming the No. 4 main bearing resulted in a total loss of the Unit 1 AB EDG to perform its safety function.
INDUSTRIAL SAFETY
There was no actual or potential industrial safety hazard resulting from the Unit 1 AB EDG No. 4 main bearing failure.
RADIOLOGICAL SAFETY
There was no actual or potential radiological safety hazard resulting from the Unit I AB EDG No. 4 main bearing failure. The EDGs are located outside the radiological controlled area and were not being relied on to supply emergency power to safety related or any radiological equipment at the time of the event. This condition did not result in any unplanned radiological exposure, release, or contamination.
PROBABILISTIC RISK ASSESSMENT (PRA) The risk significance of the Unit 1 AB EDG No. 4 main bearing failure and the extended outage was calculated at 5E-6 Incremental Conditional Core Damage Probability and 4E-7 Incremental Conditional Large Early Release Probability. The risk assessment was performed assuming the Unit 1 AB EDG was non-functional, and the full power Probabilistic Risk Assessment models which are bounding for shutdown modes. This is considered to be low risk significance per Regulatory Guide 1.174, An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Bases, acceptance guidelines, but above the threshold requiring risk management actions per NUMARC-93-01, Industry Guideline for Monitoring the Effectiveness of Maintenance at Nuclear Power Plants.
ROOT CAUSE
The station accepted leaving air within the lube oil system which degraded the hydrodynamic film wedge and resistive capacity of the No. 4 main bearing leaving it susceptible to electrical pitting and eventual wiping of the bearing babbitt material.
CONTRIBUTING CAUSES
The station developed a culture to accept air in the EDG lube oil system due to prior experience that did not result in damage to the equipment.
The station failed to consider the exciter [EXC] air gap measurement indications and take the necessary actions to investigate or correct the condition.
CORRECTIVE ACTIONS
Immediate Corrective Actions Taken Completed repair and replacement activities for the 1 AB EDG, including but not limited to replacement of the connecting rod bearing and air gap correction for the generator and exciter.
Corrective Actions to Preclude Repetition Completed installation of high point vent valves in the 1 AB EDG lube oil system.
Revised applicable procedures to include venting of the lube oil system and removed a note that allowed air to remain in some portions of the lube oil system.
Planned Corrective Actions
Revise the EDG system monitoring plan to include trending of the EDG shaft voltage.
Revise applicable procedureto require that generator and exciter air gap measurements be corrected if found to be out of tolerance, and add measurement of shaft voltage as a preventive maintenance task.
PREVIOUS SIMILAR EVENTS
A review of the past three years Licensee Event Reports identified no similar events.