in the event of any major disruption of society, most of the nuclear power plants can be expected to melt down
Source? I'm given to understand that they have a lot of fail safes. Wouldn't they all just turn off? Also, it seems like the disruption would have to be extremely sudden for even that to happen. Otherwise, they'd just turn the power plant off.
Even 1 Chernobyl corresponds to awful lot of nukes.
It may correspond to an awful lot of nukes in terms of fallout, but it corresponds to exactly zero in terms of sending dust into the air and messing with the climate.
1: One does not simply turn the reactor all the way off (picture of Aragorn, err, Boromir, or who ever). There's the decay heat, several megawatts of it even after months of shutdown (google decay heat), that can't be turned off, and virtually all reactors require intervention to keep that cooled. Likewise for spent fuel pools, that boil itself out over course of a week or two. That's what happened in Fukushima - the reactors did shut down correctly but all powered decay heat removal systems failed when tsunami water flooded the basements (in which they ke...
Just a reminder that some of the old threats are still around (and hence that AI is not only something that can go hideously badly, but also some thing that could help us with the other existential risks as well):
http://blog.practicalethics.ox.ac.uk/2012/03/old-threats-never-die-they-fade-away-from-our-minds-nuclear-winter/
EDIT: as should have been made clear in that post (but wasn't!), the existential risks doesn't come from the full fledged nuclear winter directly, but from the collapse of human society and fragmentation of the species into small, vulnerable subgroups, with no guarantee that they'd survive or ever climb back to a technological society.