Peter Trei:
The plutonium cores of thermonuclear devices have a limited shelf life - he claimed 6 years, which jibes with what I've heard from other open sources. Fission products build up in the cores which can poison a chain reaction. Thus all Pu based devices need to have the cores periodically removed and replaced with new ones, while the old ones have to go through a non-trivial reprocessing stage to remove the fission products.
Decay, rather than fission, I suppose. I believe there's a treaty prohibiting nuclear weapons in space. Not so surprising if they're inpractical - political points for nothing. Bill Frantz:
I think this comment is in error. Plutonium has a half life on the order of 250,000 years, so very little decay products would build up in 6 years. The tritium used in thermonuclear weapons has a much shorter half life, and would need to be replaced about that often.
Replacement of tritium is certainly the dominant need. As for the decay products - it depends how close they get to pure 239. Half-lives: (years) Pu 238 89 Pu 239 24000 Pu 240 6500 Pu 241 15 Pu 242 400000 D 0.015 T 12.3 Paul Pomes wrote:
Even a fizzle with a yield in the hundreds of tons equivalent is respectable. Plutonium decay products have a high neutron cross-section and steal the fast neutrons necessary for the chain reaction to build. Sufficient amounts can kill off the last three or more re-doublings which is where most of the explosive power comes from.
The only books I have to hand contain _thermal_ cross-sections. :( If it's an H-bomb I was under the impression you don't care that desperately about the size of the plute yield - only that it is enough to start the fusion. Anybody know what happened to the proposed fissionless H-bomb of the '60s ? Presumambly it never got working. Warm&ComfyMonger