TOKYO (Kyodo) -- The Nuclear Regulation Authority said Wednesday that it assessed the severity of a nuclear exposure accident in June in eastern Japan provisionally as level 2 on the zero-to-seven international scale.
The June 6 accident at the Japan Atomic Energy Agency's Oarai Research and Development Center in Ibaraki Prefecture, east of Tokyo, left five workers internally exposed to radiation although no harmful consequences were detected for the surrounding environment.
Level 2 on the International Nuclear and Radiological Event Scale is a stage defined as an "incident." The Fukushima nuclear accident, triggered by the huge earthquake and tsunami in March 2011, was rated at the maximum level 7, on a par with the 1986 Chernobyl disaster.
When one of the five workers, a male in his 50s, opened a container in a storage room at the facility, a plastic bag inside it with plutonium and uranium powder samples ruptured.
The worker continued the check even when the plastic bag swelled, resulting in the inhalation of radioactive substances. Tests have found a small amount of radioactive materials -- plutonium and americium -- in the urine of the five workers, confirming they suffered internal radiation exposure.
It was estimated one of the workers will be internally exposed to a radiation dose from 100 to 200 millisieverts in total over 50 years, a level that could slightly increase the risk of cancer.
NRA Chairman Shunichi Tanaka urged the JAEA to take measures to prevent further accidents, saying it is "responsible for ensuring the safety of workers."
Japan previously assessed as level 2 a critical-state accident at the No. 1 reactor of Hokuriku Electric Power Co.'s Shika nuclear plant in 1999. The utility hid the accident, which occurred while the unit's operation was suspended for regular checkups, until 2007.
Tokyo also rated as level 2 a 1991 accident at the No. 2 reactor in the Mihama plant run by Kansai Electric Power Co., in which one of a steam generator's tubes snapped, activating the emergency core-cooling system for the first time in the country.