How bad could Chernobyl get?
I guess somebody important seeing a mushroom cloud over a major Soviet facility during a peak period of cold war tension, and getting the wrong idea about the cause, could make things get really very bad indeed.
Thermonuclear reaction if the molten fuel material sinks all the way to the underground reservoir beneath the reactor. The resultant explosion levels Kiev (along with pretty much everything else along the way) and renders swathes of Europe uninhabitable.
Change the location of the accident - if it happened in a major state of the US - it could make major cities unlivable.
Although the original explosion itself was may be already worst-possible, the worst possible scenario of entire disaster would be:How bad could Chernobyl get?