From: utzoo!decvax!decwrl!sun!megatest!fortune!hpda!hplabs!hplabsb!soreff
Newsgroups: net.physics
Title: Re: Any nuke designers out there?
Article-I.D.: hplabsb.1522
Posted: Wed Apr  6 11:52:57 1983
Received: Fri Apr  8 02:45:29 1983
References: sri-arpa.832

Also not a nuke designer, but I was under the impression that the
assembly time of a plutonium weapon must be much shorter than for
a U-235 weapon, not for fast fission cross section reasons, but because
of neutron backround reasons.  Depending on how long the uranium used to
make Pu-239 is irradiated, there will be varying amounts of Pu-240 mixed
in with it, and this has a substantial spontaneous fission rate, thus
raising the neutron backround.  Does anyone out there have any numbers?
If a hollow sphere with a radius of ~10 cm is being imploded at a few
km/sec (typical for high explosives), the implosion time is going to be
10-100 usec or so.  Does anyone know what the typical fraction of Pu-240
in weapons-grade plutonium would be?  That, together with the spontaneous
fission half life should show how important this effect is.  I've been
told that natural backround neutron flux is very low, of the order of
one per square cm per minute, so if that were the only problem it would
be possible to assemble a very supercritical assembly slowly and wait many
seconds till the first neutron came along to trigger it.  Does anyone
out there know if this is true, or completely off the wall?
	-Jeffrey Soreff (hplabs!hplabsb!soreff)