None

[nobody@REPLAY.COM (Anonymous)]

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.