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