[tt] [Beowulf] Re: "hobbyists"

Tue Aug 19 15:55:06 UTC 2008

----- Forwarded message from "Robert G. Brown" <rgb at phy.duke.edu> -----

From: "Robert G. Brown" <rgb at phy.duke.edu>
Date: Thu, 19 Jun 2008 13:51:15 -0400 (EDT)
To: Jim Lux <James.P.Lux at jpl.nasa.gov>
Cc: Beowulf Mailing List <beowulf at beowulf.org>,
	Jakob Oestergaard <jakob at unthought.net>
Subject: Re: [Beowulf] Re:  "hobbyists"

On Thu, 19 Jun 2008, Jim Lux wrote:

>Uh.. probably not.. That's the "alarm clock" or "layer cake" design and has 
>some issues.
>I refer interested readers to Morland's 1977 article in "The Progressive" 
>for more information.

Fusion boosting of fission, OTOH, is relatively easy to accomplish and a
common practice, and recall that the yield range I was speaking of was
LESS than a MT, which is the upper bound of layer cakes in any event.
But my point was that throwing in a bunch of deuterium and/or tritium is
almost certainly going to increase your yield, moreso for a "bad design"
(near fizzle) than for a really good implosion device.  The extra
neutrons basically fission a lot of stuff that would otherwise be blown
apart without fissioning, and you do get SOME increased yield from
fusion, although it may well be small unless you do the "good" designs.

For strategic weapons, this matters.  For tactical weapons, it totally
matters.  For terrorist-class weapons or homemade weapons, getting a
"good fusion design" doesn't matter, because boosting a KT to 5 KT or 10
KT might as well be infinity -- a blast capable of causing kilodeaths
and gigabuck damages is more important than details of the yield.

>The study done a couple years ago that postulated a "nominal yield"(e.g. 
>20kT) fission device in Los Angeles/Long Beach port showed that the 
>majority of damage and death resulted from things like traffic jams and 
>accidents in the crowds fleeing and overloading hospitals.  The actual 
>damage radius is fairly small (in the context of Los Angeles, which is 
>>100km across) and the fallout (from a particularly dirty surface burst.. 
>i.e. the shipping container on the dock sucking up the dirt) wasn't all 
>that bad, in terms of dose.  The panic, on the other hand, is lethal.

Sure, maybe.  OTOH Hiroshima with a 20KT device killed order of 10^5
people.  20KT in a dense population of any sort surrounded by wood frame
dwellings that can play in a fire storm might not make a megadeath, but
it would dwarf 9/11.

>Hmm. Mcrit for good quality U235 is fairly high, especially unreflected. I 
>don't know if your shotgun has sufficient oomph to assemble it quickly 
>enough without predetonation or a fizzle.  I've seen numbers for required 
>assy speed in the 1000 m/sec sort of range (i.e. you've got to move from a 
>noncrit to a crit configuration in the amount of time between spontaneous 
>neutrons appearing to get things rolling). Accelerating 10kg, say, to 1000 
>m/sec, takes a heap o'joules

I can only quote Oppenheimer, that if you drop a subcrit piece of U235
off of a table to assemble with one on the floor you are likely to get a
creditable nuclear blast.  I don't know what he meant by 'creditable',
but I'm assuming KT range, about the same as the davy crockett (nice
movie:-), and that even if he was exaggerating about the table, a
shotgun is better than a table;-)

But sure, real explosives would be better, a real gun barrel would be
useful, and both are readily available.  I would argue that a U235
device is within "garage" assembly either way.  "Pure" Pu-239 gun
designs are likely to be a lot trickier, but still fairly accessible.
It's getting rid fo the Pu-240 that is the problem -- it puts you right
back in the isotope separation game only worse, as the mass difference
is even smaller.

>why worry about ICBMs when DHL/FedEx will deliver it to your selected 
>doorstep?

Well, even a small bomb would be pretty heavy.  Kind of at the boundary
of what FedEx will delivery;-)

   rgb

-- 
Robert G. Brown                            Phone(cell): 1-919-280-8443
Duke University Physics Dept, Box 90305
Durham, N.C. 27708-0305
Web: http://www.phy.duke.edu/~rgb
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