There is two things that make nuclear war x-risks.
One is that nuclear weapon could be used unconventially - to disturb magmatic chambers of supevolcanos or (and) bomb existing nuclear reactors and storages of nuclear waste.
Another is possibility of radial improvment of nuclear bomb technology. First thing is new cheap ways of uranium enrichment via laser enrichment. Another is building of hydrogen thermonuclear bombs without uraniun fuse, by using somekimd of electric-magnectic pinch, laser or cold fusion.
And I forget a cobalt bomb as stationery doomsday device which could be built with price tag 10-100 billions USD.
Classicical nuclear winter is survivable in my oinion.
Anders Sandberg has done calculations for cobalt bombs.
The FHI's mini advent calendar: counting down through the big five existential risks. The first one is an old favourite, forgotten but not gone: nuclear war.
Nuclear War
Current understanding: medium-high
Most worrying aspect: the missiles and bombs are already out there
It was a great fear during the fifties and sixties; but the weapons that could destroy our species lie dormant, not destroyed.
But nuclear weapons still remain the easiest method for our species to destroy itself. Recent modelling have confirmed the old idea of nuclear winter: soot rising from burning human cities destroyed by nuclear weapons could envelop the world in a dark cloud, disrupting agriculture and the food supplies, and causing mass starvation and death far beyond the areas directly hit. And a creeping proliferation has spread these weapons to smaller states in unstable areas of the world, increasing the probability that nuclear weapons could get used, leading to potential escalation. The risks are not new, and several times (the Cuban missile crisis, the Petrov incident) our species has been saved from annihilation by the slimmest of margins. And yet the risk seems to have slipped off the radar for many governments: emergency food and fuel reserves are diminishing, and we have few “refuges” designed to ensure that the human species could endure a major nuclear conflict.