Tokaimura Criticality Accident, Japan (1999)

Tokaimura's criticality accident killed two workers over agonizing 83-day periods, suffering that outpaces the instant deaths of SL-1's operators and underscores Japan's worst nuclear processing disaster. On September 30, 1999, workers at a uranium processing plant in Ibaraki Prefecture manually poured 16 kilograms of enriched uranium solution into a precipitation tank, causing an uncontrolled chain reaction that released 2.5 times the permissible neutron exposure and harmed 667 people. One victim received 17 sieverts of radiation—10 times the lethal dose—yet survived for 83 days. The accident exposed deeper systemic failures than Goiânia's scavenger incident, as it involved trained personnel ignoring procedures.