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Radioactive substances were released into the atmosphere Thursday outside the controlled area at one of the Japan Atomic Energy Agency's nuclear physics laboratories in Ibaraki Prefecture and at least six people at the facility have been exposed to leaked radiation, officials said Saturday.
The JAEA said six people were confirmed to have been exposed by Saturday night and a quick checkup suggested possible exposure of 24 others.
The highest radiation dose detected was 1.6 millisieverts, compared with the annual exposure limit for nuclear facility workers of 50 millisieverts, the Nuclear Regulation Authority's secretariat said Saturday.
Of 55 researchers and others who were engaged in experiments and other work at the laboratory, 14 did not suffer any radiation exposure, but 11 others have yet to undergo a checkup, the secretariat said. None of them were taken to the hospital, it said.
No impact from the radiation is expected beyond the premises of the accelerator laboratory in Tokaimura.
The NRA secretariat said it received a report on the incident at the laboratory of the Nuclear Science Research Institute from the JAEA at around 9 p.m. Friday. The incident occurred at 11:55 a.m. Thursday.
Officials of the JAEA said in a press conference early Saturday in Tokyo that it failed to report the incident to the NRA immediately because it believed the leak had been confined to the laboratory.
The Ibaraki Prefectural government officials conducted an inspection of the facility. One of them told reporters the laboratory has a structural flaw from the standpoint of preventing a radiation leak.
The Ibaraki government said it was not notified of the incident until around 9:40 p.m. Friday.
The latest incident follows the resignation of the president of the state-run JAEA last week. Atsuyuki Suzuki resigned over the agency's failure to conduct a proper inspection of its Monju prototype fast-breeder reactor in Tsuruga, Fukui Prefecture.
The researchers at the lab in Ibaraki were engaged in an experiment to generate particles by applying a proton beam to gold. The equipment they were using suffered a malfunction, causing it to overheat, which resulted in the evaporation and release of radioactivated gold, the authorities said.
The JAEA initially thought the leak had been confined to the lab area and that the radiation was within acceptable levels. As a result, workers switched on the ventilation fan, which eventually caused radioactive substances to escape into the outside atmosphere.
All work at the facility has been suspended, they said.