Rating Nuclear Accidents and Incidents: Which Were the Worst?

Level 7 Major Accident: A major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures.

Incidents:

1986 -- Chernobyl, Ukraine A nuclear reactor located at a power plant in Chernobyl experienced a steam explosion and fire that caused a meltdown, releasing massive quantities of radioactive material. A significant fraction of the reactor core inventory was released, and contaminated areas of Belarus, Ukraine and the Russian Federation. There were widespread health and environmental effects.

Nuclear Energy Agency reports there were 31 fatalities immediately following the incident, with latent deaths estimated at between 9,000 and 33,000 over the 70 years after, based on current radiation dose risks.

More than five million people received low whole-body doses of radiation, about 1,000 emergency workers received the highest doses of radiation, some of them fatal. By 2002, more than 4,000 thyroid cancer cases were diagnosed among people who were children at the time and ingested radioactive iodine, often through contaminated milk.