Being radioactive doesn't mean that it's immune, or unharmed, by radiation. Not to mention that besides radiation, a reactor runs at extremely high temperatures.
@Mezzo: Just to let you know, I'm not trying to be a dick: it just happens that, due to my profession, there's a few subjects I actually know a bit about, and "Hollywood science" can be quite painful to see :)
>>Even more not to mention the most obvious problem of all: you can't see the clouds from within a nuclear reactor! (Unless it's Chernobyl...) Black raven, black raven, circling above the grave.