Radioactive substances were released into the atmosphere Thursday, May 23, outside the controlled area at one of the Japan Atomic Energy Agency's nuclear physics laboratories in Ibaraki Prefecture, the Nuclear Regulation Authority's secretariat said early Saturday morning, Kyodo news agency reported.

Fifty-five researchers and others who were engaged in experiments and other work at the laboratory may have been exposed to radiation as a result of inhaling the substances, but none were taken to hospital, the government body said. Four have so far undergone checkups and the highest radiation dose detected was 1.6 millisievert.

No impact from the radiation is expected beyond the premises of the accelerator laboratory in Tokaimura.