How does one class a nuclear accident?

The events in Japan have led the NSA to classify the nuclear disaster at level 6 / 7 from around the world.

• With 34 Reactors in At Risk Zones, France Wonders If It's Hostage to Nuclear Power (14/03/2011)
• French greens call for end to nuclear energy - Reuters (12/03/2011)
• French greens call for end to nuclear energy - Reuters (12/03/2011)
• UPDATE 1-French greens call for end to nuclear energy (12/03/2011)
• French greens call for end to nuclear energy (13/03/2011)