The Nuclear Regulation Authority said Wednesday that it provisionally assessed the severity of a nuclear exposure accident that occurred in June to be 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 caused internal radiation exposure in five workers, although no harmful consequences were detected in the surrounding environment.

Level 2 on the International Nuclear and Radiological Event Scale is 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.