A fire alarm went off at 11:52 am (0252 GMT) at a waste-disposal facility in the Tsuruga plant, some 350 km west of Tokyo.
|No radiation leaks were reported|
in Tsuruga plant
Firefighters raced to the scene but found that there was no fire, according to an official at the Nuclear and Industrial Safety Agency.
"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 also said that there were no signs of radiation contamination and that the situation seemed to be under control.
"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.
Investigations are underway to know the cause of the explosion, according to Yagi.
The explosion broke a window at a building housing an incinerator that burns items such as protective clothing worn by workers at the reactor complex.
The reactor is owned by the government-funded Japan Nuclear Cycle Development Institute.
It has stopped operating in March so it would be completely dismantled after about three decades.
The incident followed several accidents over the past decade that have harmed public confidence in Japan's nuclear industry.
The country’s worst nuclear accident occurred in September 1999 when two workers at a uranium-processing plant at Tokaimura, northeast of Tokyo, died.
Hundreds of residents, plant workers and emergency personnel were exposed to radiation in the same incident.