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Comment by pfdietz | Hacker News Reader

Fission reactions shut down when the core expands enough to become subcritical. Boosting can help some, but most of the fissionable material will still fail to fission.

Thermonuclear assemblies don't have a chain reaction, so they can burn most of their fuel, once they get hot enough.

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I see some downvotes on that. Would any of the downvoters explain their objection, so I can address it?

Not one of them... But were you talking about explosive assemblies or reactor assemblies? Cuz fission "reactor assemblies" going subcritical has very little to do with expansion in the long term, but rather with consumption of excess reactivity and buildup of poisons.

Also, you interestingly reminded me that fusion is unlike fission, the latter being a chain or cascading reaction (the self-sustaining kind), while former isn't (or is it? :M.Stevens face:)

The person I was responding to said "bombs", so I was talking about that. A fission bomb's chain reaction shuts down as soon as the assembly expands enough to become a bit subcritical. The thermonuclear bomb's hot burning fuel, on the other hand, continues to react (albeit at a slower rate) as it expands; there is no such sharp cutoff. The latter fact can be used to improve fission bombs some by "boosting": including a small amount of DT gas in the fission bomb, to provide a source of neutrons that induce some fission even after k drops below 1. If k is (say) 0.9, then each DT neutron will produce 10 fission events, for a large energy return, roughly 100x the energy from the fusion itself.

I'll add another observation: the secondary in a thermonuclear bomb is a lot like the target in inertial confinement fusion, only much larger. The expansion time of these systems is proportional to their radius, so if (say) the bomb has a million times the fuel, it takes 100 times longer to expand. So for a given density and temperature of fuel, there is 100x as much time for the fusion reactions to occur. In a real sense, thermonuclear bombs become easier to build as they get bigger.

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