Plant spokesman Buddy Eller said the fire began at 4:42 p.m. Tuesday at the South Texas Project Electric Generating System, about 90 miles southwest of Houston. Eller said the fire was extinguished within 15 minutes.
The San Antonio Express-News said no workers were injured and the fire did not threaten the reactor.
Eller did not know when the reactor will be restarted.
The Express-News said each reactor at the plant produces 1,280 megawatts of electricity. One megawatt is enough to power 500 homes during mild winter conditions.