What are the dangers from a nuclear power accident?

The worst case scenario for dangers of Nuclear Power is a Meltdown. A Meltdown occurs when the core of the nuclear reactor reaches unstable temperatures usually related to a severe failure of the reactors cooling system. The effects of a nuclear meltdown depends on the safety features designed into the reactor, newer reactors SHOULD be designed to make a meltdown highly unlikely and should be able to contain one should it happen.

The worst documented nuclear meltdown is probably Chernobyl, in April of 1986 reactor number 4 suffered a catastrophic power increase, leading to explosions in the reactor core. This expelled large amounts of radioactive fuel and core materials into the atmosphere and ignited the combustible graphite moderator which increased the emissions of radioactive particles into the atmosphere.

The radiation levels in the worst hit areas of the reactor building have been estimated to be 5.6 roentgens per second (R/s), which is equivalent to more then 20,000 roentgens per hour. A lethal dose is around 500 roentgens over 5 hours, so in some cases unprotected workers received a fatal dose within minutes.