05000457/LER-2005-002
| Docket Number Sequential Revmonth Day Year Year Month Day Year N/A N/Anumber No. | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| 4572005002R00 - NRC Website | |
A. Plant Operating Conditions Before The Event:
Event Date: March 28, 2005�Event Time: 1246 Unit: 2 MODE: 1� Reactor Power: 100 percent Unit 2 Reactor Coolant System (RCS) [AB] Temperature: 581 degrees F, Pressure: 2236 psig
B. Description of Event:
There were no additional structures, systems or components inoperable at the beginning of the event that contributed to the severity of the event.
On March 28, 2005, at 1246, the Unit 2 main generator [TB] tripped as a result of a fault on the main generator "C" phase (T-3) bushing, followed by a turbine trip and reactor trip. An Engineered Safety Feature [JE] actuation of the auxiliary feedwater (AF) [BA] system occurred on low steam generator level. Operator response to the trip was proper and all safety equipment operated as expected, with the following minor exceptions:
- The tube side relief for the 27B heater lifted.
- Based on main control room indications, the 2AFOO5H, an air operated flow control valve for the 2D steam generator, failed full open when the AF system started following the reactor trip. Operator response was to control the steam generator level with the 2AF013H, the motor operated valve.
- 2CV121, the level control valve for the volume control tank [CB], was operating erratically in automatic after the Unit 2 reactor trip. Operator response was to place the valve in manual and stabilize flow and level. After a couple of hours, the 2CV121 controller was returned to Auto and the system was stable.
C. Cause of Event
The root cause of the Unit 2 trip was determined to be the lack of technical rigor applied during the redesign of the bushing in the 2000 time period. Contributing to this event was the unavailability of spare parts and lack of Original Equipment Manufacturer (OEM) expertise due to the sale of the bushing product line.
The bushing failed as a result of alterations made during refurbishment at a non-OEM repair facility.
The bushing was originally installed in the Byron Unit 2 main generator (T-3 position) and removed in 1999 due to signs of overheating. The bushing was sent to two non-OEM vendors for refurbishment. The first vendor could not disassemble the bushing. The bushing was then sent to a second vendor, where a problem in disassembly required changes to the design, including adding a second threaded connection and a solder joint to the bushing lower flange. The failure occurred at the second threaded connection.
After refurbishment, the bushing was accepted and returned. During the Braidwood fall 2003 refueling outage, the refurbished bushing was installed in the Braidwood Unit 2 main generator in the T-3 position as a replacement for an original bushing that had showed signs of overheating.
Inspections subsequent to the Unit 2 trip showed that the main generator T-3 bushing had suffered a catastrophic failure at a threaded connection between the bushing conductor and the bushing bottom flange. The bottom flange connection had been altered at the repair facility during bushing refurbishment. This failure caused the bottom flange and the bus adaptor to drop approximately six inches resulting in a ground fault and generator trip.
NRO FORM :IMA 3 During the bushing refurbishment process, some level of awareness existed by company personnel that a unique repair was being performed on this bushing. The company personnel defaulted to the repair vendor to provide the technical expertise due to not having sufficient experience in this area to render a technical judgment on the redesign. Because this component was non-safety related commercial grade, it was not subject to 10CFR50, Appendix B requirements for independent design review. This, coupled with the fact that no specific Exelon technical human performance process existed at that time, resulted in the bushing not being reviewed in more detail for design adequacy.
D. Safety Consequences:
There were no safety consequences impacting plant or public safety as a result of this event. The reactor trip occurred automatically due to the protective system actuation at the required setpoint by a ground over-current relay. Risk level remained low throughout this event. Following the reactor trip all safety equipment operated as required except for minor functions as noted in Section B of this report.
This event did not result in a safety system functional failure.
E. Corrective Actions:
Corrective Actions include:
- Implementation of a Technical Human Performance Program (Complete)
- Evaluating the addition of thermography windows to allow online monitoring of the bushings
- Formalization of the corporate high voltage bushing strategy
F. Previous Occurrences:
There have been no similar Licensee Event Report events at Braidwood Station in the last three years.
The extent of condition is limited to the failed bushing for Byron and Braidwood. The unique repair implemented in the bushing in the 2000 time period was a one-time event. There are no other bushings at Byron and Braidwood that have been modified from original design.
G. Component Failure Data:
Manufacturer Nomenclature Model Mfg. Part Number Westinghouse Main Generator Terminal Bushing NA 7344D09G01 NR(. FORM SFAA it.7nn11