Biggest nuclear disasters Biggest nuclear accidents Today marks the 25th anniversary of the nuclear accident at Chernobyl. In light of that, and the story continuing to unfold at Japan's Fukishima plant, we'll look at three of the worst nuclear plant disasters. First, to give some context, here's how the International Nuclear and Radiological Event Scale (INES) classifies each type of event: Level 7 disaster Major release of radioactive material; widespread health and environmental effects Level 6 accident Significant release; requires planned countermeasures Level 5 accident Limited release; several deaths from radiation Level 4 accident Minor release Level 3 incident Exposure in excess of 10 times the annual limit for workers; possible burns from exposure Level 2 incident Exposure of statutory annual limits Level 1: Anomaly Definition Skip ahead to see: * Three Mile Island * Chernobyl * Fukushima Chernobyl Date: April 26, 1986 Level: 7 on INES ...