Path: utzoo!attcan!uunet!cs.utexas.edu!tut.cis.ohio-state.edu!att!cbnews!cbnews!military
From: henry@zoo.toronto.edu (Henry Spencer)
Newsgroups: sci.military
Subject: Re: Question about Nuclear Weapons
Message-ID: <1990Oct29.032444.9881@cbnews.att.com>
Date: 29 Oct 90 03:24:44 GMT
References: <1990Oct23.190943.7623@cbnews.att.com> <1990Oct24.114913.4072@cbnews.att.com> <1990Oct25.151832.2153@cbnews.att.com>
Sender: military@cbnews.att.com (William B. Thacker)
Organization: U of Toronto Zoology
Lines: 34
Approved: military@att.att.com



From: henry@zoo.toronto.edu (Henry Spencer)
>From: ntaib@silver.ucs.indiana.edu (Nur Iskandar Taib)
>... I wonder if the 
>explosives are really necessary: if one simply moves all
>the pieces together into a big lump ... won't it ex-
>ceed critical mass and go off? ...

No, because timing is critical.  The chain reaction is not all or nothing.
It starts to happen as the pieces *approach* each other (or as a single
piece's density increases, which is actually what most modern bombs do).
The graph of activity versus distance is not a step function at zero; it
gradually rises in a near-asymptotic increase as distance drops.  The
significance of this is that if you move the pieces together slowly,
the beginnings of the chain reaction will melt them down into radioactive
slag (in the process generating enough radiation to kill you) long before
they can come into contact.  Move them together rather more quickly, and
you will actually get an explosion equivalent to a modest charge of TNT,
just enough to splatter radioactive debris around the immediate vicinity.
If you want them to go "bang" instead of "splut", they have to go from
separate pieces to one big lump ***QUICKLY***.

In fact, the problems don't end when you get them together, because the
critical mass will still tend to blow itself apart before it can react
fully.  Modern bombs have a massive casing that is imploded inward at
very high speed, striking the fissionable core, compressing it almost
instantaneously to very high densities, and delaying its expansion by
sheer inertia to give it more time to fission.

(The above is all public information, by the way.)
-- 
The type syntax for C is essentially   | Henry Spencer at U of Toronto Zoology
unparsable.             --Rob Pike     |  henry@zoo.toronto.edu   utzoo!henry