05000249/LER-2004-006
| Dresden Nuclear Power Station | |
| Event date: | 10-31-2004 |
|---|---|
| Report date: | 01-19-2005 |
| 2492004006R00 - NRC Website | |
Dresden Nuclear Power Station Units 2 and 3 are General Electric Company Boiling Water Reactors with a licensed maximum power level of 2957 megawatts thermal. The Energy Industry Identification System codes used in the text are identified as [XX].
A. Plant Conditions Prior to Event:
Unit: 03� Event Date: 10-31-2004 � Reactor Mode: 5� Mode Name: Refueling Power Level: 0 percent Reactor Coolant System Pressure: 0 psig
B. Description of Event:
Dresden Nuclear Power Station, Units 2 and 3, had been experiencing increasing trends iri vibration levels on both Main Turbine Generators [TA][TB], bearings 9 and 10 since May 2004. The Main Turbine Generator bearing 9 is located between the electric generator and the low-pressure turbine.
Bearing 10 is located between the electric generator and the exciter [TL]. Numerous efforts and reviews during the summer and fall of 2004 were not successful in resolving the vibration.
Dresden Unit 3 entered refueling outage D3R18 on October 26, 2004. As part of the outage scope, the Main Turbine Generator was internally inspected. On October 31, 2004, the inspection identified that the Unit 3 Main Turbine Generator Rotor had a significant crack in the Main Turbine Generator Rotor shaft near the turbine end coupling. The crack was approximately 13 inches in length. This finding resulted in the decision to remove Unit 2 from service and conduct an inspection of its rotor shaft. On November 1, 2004, a crack was identified on the Unit 2 rotor shaft. The Unit 2 crack was approximately 10 inches in length, in the same general location and similar configuration as the Unit 3 crack. These cracks resulted in a change in the Main Turbine Generator Rotor shaft stiffness, which caused the increasing trend in bearing vibration.
These events are being reported as a Voluntary Licensee Event Report (LER) in accordance with the guidance contained in NUREG 1022, Revision 2, "Event Reporting Guidelines 10 CFR 50.72 and 50.73.
C. Cause of Event:
The root cause of the rotor cracks was determined to be intermittent oscillating torsional loading on the generator rotor, which produced a torsional fatigue failure mode. The cause of the intermittent oscillating torsional loading is indeterminate.
Dresden Unit 2 and Unit 3 have identical Main Turbine Generators manufactured by General Electric Company. The Main Turbine Generator Rotors are NiMoV alloy steel forgings that were fabricated to GE Specification B50A375A70-S4 in the mid-1960's.
The Unit 2 and Unit 3 cracks initiated in one of the two generator rotor shaft keyways under the turbine end coupling. The coupling is an interference fit to the shaft. The assessment of the metallurgical examination identified that each crack propagated at a 45-degree angle in a spiral fashion around the shaft with approximately 200 beach marks. The beach marks are indications where the cracks stopped and started again. The assessment concluded that intermittent oscillating torsional loads above the material fatigue endurance limit caused the crack propagation.
Dresden sent both Main Turbine Generator Rotors offsite for the inspections and to have the cracked end of the rotor shaft replaced with a new stub-shaft. A contributing cause to the reduced shaft material fatigue endurance limit was fretting. The ability of the shaft material to withstand fretting has been significantly increased by a redesign of the shaft keyway to eliminate stress risers and to increase shaft torsional capacity by an improved coupling shrink fit.
D. Safety Analysis:
The safety significance of the event is minimal. The Main Turbine Generator is not a safety related component and it is not credited in any Dresden accident analyses. Additionally, an engineering assessment concluded that Main Turbine Generator vibration levels would have exceeded operational limits prior to reaching critical crack size and the generator would have been removed from service prior to potential rotor failure. Therefore, the consequences of this event had minimal impact on the health and safety of the public and reactor safety.
E. Corrective Actions:
Dresden sent both Main Turbine Generator Rotors offsite for the inspections and to have the cracked end of the rotor shaft replaced with a new stub-shaft. A contributing cause to the reduced shaft material fatigue endurance limit was fretting. The ability of the shaft material to withstand fretting has been significantly increased by a redesign of the shaft keyway to eliminate stress risers and to increase shaft torsional capacity by an improved coupling shrink fit.
F. Previous Occurrences:
A review of Dresden Nuclear Power Station LERs identified no similar events. Additionally, a review of LERs from other nuclear plants did not identify any similar events.
G. Component Failure Data:
GE Main Turbine Generator Rotor shaft