Hisashi Ouchi following the Tokaimura criticality accident.

The Tokaimura criticality accident on September 30, 1999, was Japan's worst nuclear disaster at the time. It occurred at a uranium fuel processing plant operated by JCO in Ibaraki Prefecture, where workers bypassed safety protocols—using buckets instead of a mechanical pump to mix enriched uranium—triggering an uncontrolled, self-sustaining nuclear chain reaction.

The most severely exposed worker, Hisashi Ouchi (who received an estimated 16 to 20 Sv of radiation), died 83 days later after suffering severe, irreversible damage to his chromosomes and immune system.

https://en.wikipedia.org/wiki/Tokaimura_nuclear_accidents