Fire crews raced to a decommissioned nuclear reactor complex 350km west of Tokyo yesterday after a small explosion at a building housing an incinerator, officials said.
Officials said there were no reports of radiation leaks at the 165,000-kilowatt Fugen experimental reactor, which stopped operating in March so it could be scrapped.
An official at the Nuclear and Industrial Safety Agency said firefighters at the scene determined there had been no fire.
"It's been confirmed that this wasn't a fire ... It seems there was a small explosion," said Masahiro Yagi, a deputy director at the agency.
Yagi said the agency was investigating the cause of the explosion, which broke a window at a building housing an incinerator that burns items such as protective clothing worn by workers at the reactor complex.
There were no signs of radiation leaks and the situation seemed to be under control, he said.
"There's no change in the figures on the 24-hour [radiation] monitors ... so there was no leak to the outside and there are no injuries," Yagi said.
The incinerator building was kept at negative pressure so no radioactive material would escape, Yagi added.
The incident followed a string of accidents over the past decade that have dented public confidence in Japan's nuclear industry.
Japan's worst nuclear accident occurred in September 1999, when two workers at a uranium-processing plant at Tokaimura, northeast of Tokyo, died and hundreds of residents, plant workers and emergency personnel were exposed to radiation.