Path: utzoo!attcan!uunet!tut.cis.ohio-state.edu!att!cbnews!cbnews!military
From: fiddler%concertina@Sun.COM (Steve Hix)
Newsgroups: sci.military
Subject: Re: Question about Nuclear Weapons
Keywords: MRVs
Message-ID: <1990Oct29.032214.9600@cbnews.att.com>
Date: 29 Oct 90 03:22:14 GMT
References: <1990Oct23.190943.7623@cbnews.att.com> <1990Oct25.151832.2153@cbnews.att.com>
Sender: military@cbnews.att.com (William B. Thacker)
Organization: AT&T Bell Laboratories
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From: fiddler%concertina@Sun.COM (Steve Hix)

In article <1990Oct25.151832.2153@cbnews.att.com>, ntaib@silver.ucs.indiana.edu (Nur Iskandar Taib) writes:
> 
> I always thought the fissionable material is stored in
> the bomb in several sub-critical mass pieces, which are 
> slammed together by explosive charges. 

That's one way to do it.

> I wonder if the 
> explosives are really necessary: if one simply moves all
> the pieces together into a big lump (do neutrons really 
> care if there is 0.1mm of air in the way?) won't it ex-
> ceed critical mass and go off? 

Doing that might get you a "squib" explosion.  Very little
of the mass would fission, but the generated heat would
blow the mass apart and scatter pieces around.  You have
to slam it together very quickly, and in just the right shape,
or you don't get full yield.

This was one of the tricky technical problems solved by the
Manhatten project team; getting the explosives to all go off
at the right time in the right direction(s) to properly
implode the reactive mass.

Just in case, they developed a second approach that fired a
piece of (darn...one used Pu, the other used U) stuff at a
just-less-than-critical mass.

Both were used.  Both worked.

> I heard that the first 
> atomic bomb scientists determined the critical mass for 
> ^235 U by dropping various size spheres through donuts,
> and watching for neutrons. This was supposedly very risky.. 

"Tickling the dragon's tail".  At least one researcher (Louis
Slotin) died after one such experiment went wrong.

> if the two together exceeded critical mass, the whole 
> thing could have gone off. 

The problem was not so much one of an explosion killing you,
as it was getting fried by the resulting high neutron flux.

--
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  The only drawback with morning is that it comes 
    at such an inconvenient time of day.
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