Anders Sandberg has done calculations for cobalt bombs.
He said: «However, when I calculated the necessary amount of cobalt and from that the necessary yield of the bomb I found that they were definitely in the very, very impractical range (many thousands of tons of metal, at least 960 megatons of yield).« http://www.overcomingbias.com/2009/10/self-assured-destruction.html
But it is only 10 times more when Tzar bomb and it could and should be made as a stationery device, not a transportable bomb. Tipical nuclear reactor weight several thosand tones. So one cobalt bomb will be as heavy and complex as nuclear reac...
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.