On September 30, 1999, an accident occurred at the Tokaimura nuclear fuel processing plant in Japan, resulting in one of the most severe radiation exposures in history. Hisashi Ouchi, along with his colleagues, was exposed to a massive dose of radiation while handling enriched uranium. The radioactive accident triggered an uncontrolled nuclear chain reaction, leading to Hisashi Ouchi being exposed to radiation.

Hisashi Ouchi was exposed to 17 Sieverts (unit to calculate radiation dosage), which is a lethal amount of radiation, and despite extensive medical efforts, he suffered immensely for 83 days from radiation sickness before succumbing to multiple organ failure. This tragic incident underscores the consequences of inadequate safety measures in nuclear facilities.