It wasn't immediately clear what happened inside the experimental plant, near the town of Tsuruga. City spokesman Yoshihiro Kadono said the accident occurred at an incinerator in the nuclear complex.
Firefighters rushed to the scene after reports of an explosion, a local fire official said. There was no sign of fire from the outside, however, and the smoke subsided without firefighters turning on their hoses, he said.
The incinerator is used to burn items contaminated with low-level radiation, such as workers' protective clothing, an industry and trade ministry official said.
A spokesman at the Nuclear and Industrial Safety Agency said on condition of anonymity there was no danger of leaking radiation because the plant had not been generating power since March. It was shut down because of high operating costs.
Chikara Gunji, a spokesman at the facility's operator, Japan Nuclear Cycle Development Institute, said the incinerator had shut down automatically early Friday after it began to overheat. Plant officials tried to restart the incinerator about a half-hour later, and the fire alarm was triggered shortly thereafter.
It wasn't immediately clear what triggered the overheating, Gunji said.
Officials later found that a viewing window attached to the incinerator duct was broken, and that wind had probably stirred up ash inside the burning chamber, Gunji said. The flying ash may have triggered the fire alarm, he speculated.
Gunji said some workers inside the compound also reported hearing an explosion. About 100 people were in the complex, which is being dismantled.
The Japanese government decided in 1995 to close down the Fugen plant at the end of March due to high operating costs. It had an output of 165,000 kilowatts of electricity.
Fugen was designed to burn a mixture of uranium and plutonium as a transition to more advanced fast-breeder reactors, which use plutonium fuel instead of uranium and produce more plutonium that can be used as fuel.
Fugen began operation in 1979 in Tsuruga, 200 miles west of Tokyo. Dismantling it will take 40 years, according to its operators.
By Mari Yamaguchi