It wasn't immediately clear what triggered the accident at the experimental plant near the town of Tsuruga, about 200 miles west of Tokyo. Workers in the plant's control room said they heard an explosion coming from the nuclear complex's incinerator just before noon and the incinerator shut off automatically after it began overheating and smoking.
Firefighters rushed to the scene, but there was no sign of fire from the outside and the smoke died down without firefighters turning on their hoses, a local fire 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 generated power since its reactors were shut in March.
Although the accident didn't appear to be serious, it could rekindle doubts about whether Japan's nuclear energy policies are sound.
In March, Tokyo Electric Power Co., which runs a separate network of 17 nuclear power plants in Japan, took its plants off line for emergency inspections ordered by authorities following revelations it covered up structural problems a decade ago. It reopened one plant in May and a second in June, after getting the go-ahead from local authorities.
The shutdown had stoked concerns that heavy electricity use during Japan's balmy summer months could mean Tokyo's first major blackouts in 20 years. Nuclear power accounts for about 30 percent of Japan's electricity needs.
On Friday, Economy, Trade and Industry Minister Takeo Hiranuma said Japan's utilities need to work harder to ensure that nuclear reactors are safe.
``The government is indeed very concerned,'' he told reporters, adding that officials were trying to reassure people while conducting strict monitoring of plants.
Many Japanese have been nervous about possible nuclear mishaps since 1999, when two workers trying to save time at a reprocessing plant north of Tokyo set off an uncontrolled nuclear reaction while they were mixing uranium in buckets instead of in mechanized tanks.
The radiation leak forced 161 people to evacuate their homes and another 310,000 to stay indoors for 18 hours as a precaution. A total of 439 people were exposed to radiation. The two workers died from extreme exposure.
Chikara Gunji, a spokesman at the Tsuruga facility's operator, Japan Nuclear Cycle Development Institute, said authorities were still investigating the cause of Friday's incident.
It appeared to have started when a viewing window on an incinerator duct broke, possibly due to high temperatures or wear, said a plant official who gave reporters a tour of the incinerator control room.
The broken window may have allowed too much air into the incinerator, stirring up ash inside the burning chamber and causing the smoke that triggered an alarm, said the official, who declined to be identified. About 100 people were in the complex at the time.
