The blaze started shortly after 9am at Ringhals’ Reactor 1 and was extinguished less than an hour later.
“The smoke came from oil inside the insulation on one or several of the pipes in the turbine hall,” emergency services spokesman Roger Banck told the TT news agency.
However, the reactor continues to operate at half capacity and it remains unclear how long it will continue to do so.
“Now we have to disassemble certain parts in order to access where the fire took place and see what the damage is and we don’t know how long that will take,” Ringshals spokesman Gösta Larsen told TT.
Ringhals’ Reactor 1 was restarted on Tuesday after having been shut down the day before due to a broken meter.
The reactor had been closed for inspection for the previous five weeks and it was undergoing a test run when the meter malfunction was discovered.
Reactor 4, which also remains shuttered for a safety review, is supposed to be restarted on Sunday.
On Tuesday, the Swedish Radiation Safety Authority (Strålsäkerhetsmyndigheten, SSM) announced Ringhals would no longer be held under special observation, a status implemented for Ringhals by the agency back in 2009 following a series of safety lapses.