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 reactor, and so it is fisable.
How it could happened that you more beleive Sandberg when calculations of Szillrd?
Actually, when I did my calculations my appreciation of Szilard increased. He was playing a very clever game.
Basically, in order to make a cobalt bomb you need 50 tons of neutrons absorbed into cobalt. The only way of doing that requires a humongous hydrogen bomb. Note when Szilard did his talk: before the official announcement of the hydrogen bomb. The people who could point out the problem with the design would be revealing quite sensitive nuclear secrets if they said anything - the neutron yield of hydrogen bombs was very closely guarded, and was only e...
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.