05000423/LER-2009-002
| Millstone Power Station - | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4232009002R00 - NRC Website | |
EVENT DESCRIPTION:
On December 19, 2009 an automatic reactor trip occurred on Millstone Power Station Unit 3 (MPS3) while the unit was in Mode 1, at 100 % power, due to a turbine trip caused by a generator output electrical fault. All control rods fully inserted into the reactor. The auxiliary feedwater pump started as designed and maintained steam generator level. All other post trip actions were standard and all systems operated as expected. A post trip walk-down identified damage to 'C' phase main generator breaker and isophase ducting.
There were no radiological challenges as a result of the event.
This event is being reported in accordance. with 10 CFR 50.73(a)(2)(iv)(A) as an event that resulted in manual or automatic actuation of systems listed in 10 CFR 50.73(a)(2)(iv)(B).
CAUSE:
The Root Cause Team determined the most probable cause of the ground fault, which resulted in the turbine trip, was a manufacturing defect in the main generator [EL] breaker [BKR], 15G-3U-2, 'C' pole moving contact. This defect was a failure of a "punch mark" to secure a screw-in flange to the moving contact piston. This allowed the flange to unscrew over time. When the breaker closed in November, 2009, the flange had unscrewed sufficiently such that the moving contact was not fully engaged in the fixed contact. This condition increased contact resistance, causing temperatures to increase, eventually leading the breaker failure, the ground fault, and the turbine trip.
ASSESSMENT OF SAFETY CONSEQUENCES:
The operating crew responded to the reactor trip by entering Emergency Operating Procedure (EOP) 35 E-0, Reactor Trip or Safety Injection. The reactor trip was not complicated by a safety injection allowing transition to EOP ES-0.1, Reactor Trip Response. The electric plant fast transferred from the normal station services transformer (NSST) [XFMR] to the reserve station services transformer (RSST) [XFMR] due to the loss of the NSST on the generator breaker fault. Initial primary plant cool down was greater than expected. The operating crew took action in accordance with plant procedures, stopping the cool-down by turning off the steam dump valves and isolating steam traps and inactive steam dump valves. The normal heat removal path via the main condenser was intact and available.
The auxiliary feedwater system started automatically on the trip as expected and restored the steam generator levels to their normal operating band. Heat removal capability was maintained.
The operator actions and plant mitigating equipment responded as expected with no safety system failures. There were no challenges to any fission product barrier. Therefore, there were no safety consequences due to the reactor trip.
CORRECTIVE ACTION:
The 'C' pole of the main generator breaker was repaired. All three poles of the breaker will be inspected during the next refueling outage (3R13) scheduled in the spring of 2010 to verify the proper geometry of the contacts (moving and fixed) as well as the security of the fasteners holding the moving contact piston flange in place. Additional corrective actions will be evaluated in accordance with the station's corrective action program.
There is no other use of this type breaker at Millstone.
PREVIOUS OCCURRENCES:
No previous similar events/conditions were identified.
Energy Industry Identification System (EIIS) codes are identified in the text as [XX].