The Nuclear Regulation Authority on Monday provisionally evaluated the severity of last week's leak of radioactive substances at a Japan Atomic Energy Agency laboratory in the village of Tokai, Ibaraki Prefecture, as Level 1, within the bottom tier of the 7-scale international gauge.
The nuclear watchdog said the biggest factor in the rating was "the lack of safety culture."
Workers at the state-run lab vented exotic radioactive substances into the atmosphere even though they were aware of radioactive contamination in the facility, where their nuclear particle experiment went awry.
The accident occurred around noon last Thursday in the Japan Proton Accelerator Research Complex, one of JAEA's nuclear physics laboratories, causing at least 30 of the 55 at people in the facility to suffer internal radiation exposure.
The highest dose delivered was 1.7 millisieverts. The annual limit for nuclear workers in Japan is 50 millisieverts.
JAEA said Saturday it initially thought the leak was minor and had been confined to the laboratory when the alarm went off. Workers then switched on the ventilation system, sending radioactive contaminants into the air outside the building.
So far, according to prefectural officials, no abnormalities have been confirmed at seven radiation monitoring posts around Tokaimura.
Ibaraki Gov. Masaru Hashimoto criticized JAEA for failing to report the leak for more than 24 hours after the incident took place, saying at a press conference Monday, "It must be taking the matter lightly."
The exposure of researchers and other staff members to the leak was "very regrettable," he said.