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A fire in the main transformer at a nuclear power plant in Bay City, TX, near Houston shut down one of the plant’s two reactors, utility spokesmen said.

The fire started at 4:42 p.m. Tuesday at the South Texas Project Electric Generating System, about 90 miles southwest of Houston, said plant spokesman Buddy Eller, who added they extinguished the fire within 15 minutes.

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The fire that started was the result of a “failure of the Unit 2 Main Transformer,” according to a filing with the Nuclear Regulatory Commission (NRC). An onsite fire brigade responded to the fire declared it under control within 10 minutes and extinguished the fire at 4:56 p.m. The utility did not require offsite fire assistance and there were no injuries or radiological releases.

Unit 2 tripped from 100 percent power to 0 percent power in Mode 3. Auxiliary feedwater pumps started, as required, and decay heat removed through the steam generator relief valves. Unit 1 remained unaffected by the event.

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“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,” plant officials told the NRC.

As part of its ongoing oversight, the NRC will monitor the plant’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 consists of two Westinghouse pressurized water reactors that began operation in 1988. STP Nuclear Operating Co., whose owners include NRG Energy, CPS Energy, and Austin Energy, operate the reactors.

Unit 1 set a plantwide record last October when it ran for 530 consecutive days after its last refueling outage in spring 2011. Unit 2 held the previous record for consecutive days online with 525 days, ending in 2008.

Eller did not know when the Unit 2 reactor will restart.

Each reactor at the plant produces 1,280 megawatts of electricity. One megawatt is enough to power 500 homes during mild winter conditions.

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