Re: russia_1.html
From: Jonathan Wienke <JonWienk@ix.netcom.com> At 09:37 AM 10/6/97 -0700, Bill Frantz wrote:
At 3:01 AM -0700 10/6/97, Peter Trei wrote:
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.
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.
One of the decay products of tritium (half-life: 12.5 years) is Helium-3, which aggressively absorbs neutrons and poisons the fission reaction of the plutonium. If one has access to more tritium, which is commercially available for about US $50,000 per gram, the existing tritium can be purified and combined with the new tritium to bring the bomb back to full power. If someone has the resources to obtain the bomb in the first place, refreshing the tritium probably wouldn't be a major problem.
Jonathan Wienke
I conceed that it may not be the Pu decay products that cause the limited shelf life of thermonuclear weapons; I suspect that it may be components of the 'pit', which produces the initial burst of neutrons to kick of the reaction (the pit is effectively a small fission bomb). This 'pit' may contain tritium (pit design is a very well guarded secret, and I've seen very little about it in the open literature).
If someone has the resources to obtain the bomb in the first place, refreshing the tritium probably wouldn't be a major problem.
I disagree. A terrorist could obtain a bomb by being given it, stealing it, or buying it as a turnkey system. However, I suspect that anyone with the facilities to recondition one of these weapons could also build one from scratch. I don't know if it's comforting or worrying knowing that these devices degrade - any in the hands of terrorists have a limited time that they are a threat, but that fact may pressure a terrorist to 'use it or lose it'. Peter Trei
Peter Trei wrote:
I conceed that it may not be the Pu decay products that cause the limited shelf life of thermonuclear weapons; I suspect that it may be components of the 'pit', which produces the initial burst of neutrons to kick of the reaction (the pit is effectively a small fission bomb). This 'pit' may contain tritium (pit design is a very well guarded secret, and I've seen very little about it in the open literature).
The pit usually contains beryllium-9 and an alpha source. When beryllium-9 is struck by an alpha particle it will eject a neutron. Tritium is rather useless for this purpose as it is a beta emitter and betas do not have enough energy to eject neutrons from anything. The reason the pit goes bad is because you need to be able to produce a large number of neutrons in a short period of time, so you need a very strong alpha emitter (stronger than the Pu239) which means it will decay more rapidly. Furthermore, those neutrons are going to find their way into various nuclei and transmutate them into things you don't want. The reason the pit is necessary is that Pu is too fast - the critical mass will blow itself apart just as the reaction is getting underway. To avoid this "fizzle", an implosion bomb is used which squeezes the Pu core and holds it there long enough for it to react. Once the critical mass is assembled you want to make sure it actually blows. If there aren't any free neutrons during that split-second then it won't explode. So that's what the 'pit' is for.
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