A fire in the main transformer of the South Texas Project (STP) Electric Generating Station’s Unit 2 on Tuesday evening automatically tripped the reactor in Bay City, Texas, and powered on emergency diesel generators.
The fire that started at about 4:40 p.m. CST in the transformer was attributed to a "failure of the Unit 2 Main Transformer," according to a filing with the Nuclear Regulatory Commission (NRC). An onsite fire brigade that responded to the fire declared it under control within 10 minutes and extinguished the fire at 4:56 p.m. CST. No offsite assistance was required, and no injuries or radiological releases were reported.
Unit 2 tripped from 100% power to 0% power in Mode 3. Auxiliary feedwater pumps were started, as required, and decay heat is being removed through the steam generator relief valves. Unit 1 was unaffected by the event.
"Due to the site electrical lineup at the time, the loss of the main transformer resulted in a loss of power to 4160 ESF buses 2A and 2C, and associated Standby Diesel Generators 21 and 23 started as required and loaded on to their respective buses. 4160 ESF bus 2B remained energized from offsite power during this event and Standby Diesel Generator 22 did not start since an undervoltage condition did not exist on its ESF bus," STP told the NRC.
As part of its ongoing oversight, the NRC will monitor the licensee’s follow-up actions. These include identification of the cause of the transformer fire; a review of the behavior of the plant’s electrical protection systems; and various repair activities.
“Overall, from what we now know, plant operators responded well to the event,” said Acting Deputy Regional Administrator Steve Reynolds. “The NRC will conduct an independent and comprehensive assessment of this incident as part of its oversight process.”
The two-reactor 2.7-GW STP plant comprises two Westinghouse pressurized water reactors that began operation in 1988. The reactors are operated by the STP Nuclear Operating Co., whose owners include NRG Energy, CPS Energy, and Austin Energy.
Unit 1 set a plantwide record last October when it ran for 530 consecutive days after its last refueling outage in spring 2011. The previous record for consecutive days online was held by Unit 2 for 525 days, ending in 2008.
Sources: POWERnews, NRC, STP
—Sonal Patel, Senior Writer (@POWERmagazine)