05000397/LER-2003-006
| Docket Number | |
| Event date: | |
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| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(i) |
| 3972003006R00 - NRC Website | |
Event Description
On February 13, 2003 with Columbia Generating Station (Columbia) operating in Mode 1 at 100% power, the Division 1 Emergency Diesel Generator (EDG-1) was declared inoperable at 2100 hours0.0243 days <br />0.583 hours <br />0.00347 weeks <br />7.9905e-4 months <br /> due to a lack of reasonable assurance of operability based on a degrading trend in the generator north bearing vibration levels. Technical Specification (TS) 3.8.1, Condition 13, was entered. The amount of time required to replace the bearing exceeded the 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> completion time requirements of TS 3.8.1, Condition B.4. No other inoperable equipment contributed to this event.
Energy Northwest requested and received enforcement discretion from compliance with the completion time requirements of TS 3.8.1, Condition 13.4, for an additional 11-day period. The purpose of this enforcement discretion was to allow sufficient time to replace the generator bearings and restore EDG-1 to operable status.
During the activities to replace the generator north bearing it was determined that both EDG-1 generator bearings were damaged. After replacing both generator bearings and reassembling EDG-1, vibration testing and analysis indicated the newly installed bearings were degrading. Analysis of the bearing vibration data led to uncertainty regarding the ability of the new bearings to fulfill their design basis function of operating for at least 30 days after a design basis accident. Energy Northwest decided to disassemble the generator to determine the extent and cause of the bearing damage. Therefore, EDG-1 was not restored to operable within the time limit specified by the NOED and a plant shutdown commenced at 1945 hours0.0225 days <br />0.54 hours <br />0.00322 weeks <br />7.400725e-4 months <br /> on February 27, 2003. On February 28, 2003 the plant entered Mode 3 at 0449 hours0.0052 days <br />0.125 hours <br />7.423942e-4 weeks <br />1.708445e-4 months <br /> and Mode 4 at 1721 hours0.0199 days <br />0.478 hours <br />0.00285 weeks <br />6.548405e-4 months <br />. No other inoperable equipment contributed to this event.
A four hour event notification telephone call was made to the NRC Operations Center at 2337 EST on February 27, 2003 pursuant to 10 CFR 50.72(b)(2)(i) after the plant shutdown was initiated (Event Number 39625).
There were no failures of significant plant equipment during the shutdown. The shutdown was uneventful with the exception of receiving Oscillation Power Range Monitor alarms when in the area of increased awareness and a preheater steam supply valve failed to close. Alternate means to detect and suppress thermal hydraulic instability oscillations confirmed no instability was present. The preheater steam supply valve was manually closed by an operator. There were no adverse consequences to other plant equipment or to the health and safety of the public as a result of this required shutdown.
Following the plant shutdown, the generator bearings were replaced a second time and followed by post-maintenance testing.
Cause of the Event
A formal root cause analysis concluded the initial cause of the degraded bearings was electrical arcing damage due to a breach located somewhere in the south end bearing insulation system. However, the source of the initial insulation breach could not be determined.
A bearing failure analysis determined fluting existed in both the north and the south bearings of the generator. Fluting is a type of metal pitting damage that resembles a repeating pattern of long axial grooves in a stripe-like appearance. The fluting was caused by electrical arcing damage. Electrical arcing damage occurs in a bearing when a path exists for shaft circulating currents to pass through the bearings due to a breach in the bearing's electrical insulation system.
The bearing failure analysis also concluded that there was spallation on the outer race of the south bearing. This spallation was attributable to a combination of electrical arcing damage, bearing race material (thru-hardened martensitic steel vs. case hardened carburized steel) and misalignment wear.
The cause of the misalignment wear is suspected to be due to a minor misalignment of the bearing housing to the bearing pedestal that was introduced during generator reassembly in 1991.
The cause of the bearing failures following replacement was determined to be an incorrectly sized insulated dowel pin used during reassembly which resulted in a breach of the bearing electrical insulation system. The organizational behaviors that contributed to the dowel pin modification will be reviewed further for possible improvements and additional corrective actions will be performed if needed. The primary barrier that failed to prevent damaging the new bearings was the lack of detection of a rotor shaft circulating current which was inadvertently introduced by the dowel pin. If the current flowing through the new bearings had been detected, then it is possible the integrity of the electrical insulation could have been restored before bearing damage occurred.
Contributing causes include weaknesses in the Vibration Monitoring Program and the Oil Analysis Program. These programs were not robust enough to detect the actual cause of the north bearing degradation, or the fact the south bearing was also degraded.
Safety Significance
EDG-1 is a 4400 KW generator mounted between tandem V-20 cylinder diesels. It supplies the Division I safety related bus in the event that normal and backup power supplies are lost. With one EDG inoperable, the remaining operable EDG's and offsite circuits are adequate to supply electrical power to the onsite Class 1E Distribution System. Thus, on a component bases, single failure protection of the required feature's function may have been lost; however, the safety support function was not lost. An identical diesel generator system is installed to support the Division 2 safety related bus. Verification was made that the same failure mechanism did not exist on the Division 2 and Division 3 (HPCS) EDGs. The basis for requesting the NOED included establishing compensatory measures sufficient to assure the plant risk remained low. There were no safety consequences associated with this event and the risk significance of the event was minimized by preplanned compensatory measures.
26158 R2 I DOCKET (2) � LER NUMBER (6) 05000397 NUMBER � NUMBER
FACILITY NAME
Columbia Generating Station
Corrective Actions
Corrective actions were taken to replace the degraded bearings, assure the integrity of the electrical insulation system, and verify bearing to shaft alignment.
Corrective Actions to Prevent Recurrence
Additional corrective actions are planned to enhance Columbia's ability to detect and diagnose electrical arcing damage at the onset prior to bearing degradation. This includes improving the Vibration Monitoring Program, Oil Analysis Program, and insulation integrity testing frequency and methodology.
Previous Similar Events
The two previous events involving EDG bearing failures that occurred at Columbia are listed below.
Past Similar Event: LER 90-012 Event Date: � 5/27/1990 Failed Component: Division 1 EDG generator bearing failure due to improper lubrication Manufacturer: � 5407 Model: � L-1101I Past Similar Event: LER 91-006 Event Date: � 4/12/1991 Failed Component: Division 1 EDG generator bearing degradation due to contaminated oil Manufacturer:
� 5407 Model: � L-11011 The previous similar events listed above involved bearing degradation but neither event was caused by electrical arcing damage. Therefore, due to the significant difference in the causes of these events, the actions to prevent recurrence for these past events would not have been effective in precluding the bearing degradation that was experienced due to a breach in the bearing electrical insulation system.
26155 R2