The blaze broke out in a hallway of the Oskarshamn plant in southwestern Sweden and triggered a shutdown of one of the site's three nuclear reactors, the TT news agency reported.
Power authority spokesman Anders Osterberg told the news agency the fire hadn't posed any threat to the reactor.
"This was a relatively minor incident," he said.
Fire and nuclear officials didn't divulge a cause for the fire.
The reactor shutdown didn't affect the country's power supply. The Oskarshamn facility's three nuclear reactors produce about 10 percent of the country's electricity, the report said.