At 06:58 AM 6/19/2008, Robert G. Brown wrote:
On Wed, 18 Jun 2008, Jim Lux wrote:
So.. if your (foreign person) buddy is designing thermonuclear
devices in their garage, and they complain about how slow it is to
run the hydrocodes to simulate stuff, better not hand them that old
copy of Sterling, et al., or even worse, give them rgb's website.
(the latter would be too suspicious, since rgb *is* a physicist,
doing monte carlo simulations no less, while Tom Sterling is *just
a computer scientist*)
Ah, taking my name in vain I see...;-)
hardly in vain..
(hoping you actually get to read this, in some venue other than your
detention hearing, <grin>)
Remember, building nuclear bombs ranges from VERY easy to not terribly
difficult, until you want to build a BIG one, or a small one, or one
that has to be "precise" in its performance. Terrorists need none of
these things -- sloppy to the point of being a fizzle of sorts is still
more than good enough.
Nicholas Freeling, "Gadget" is a thriller/detective story about
someone (like rgb) being kidnapped and forced to design a smallish
device for some nefarious sorts. A better story, in my opinion,
than, say, Clancy's "sum of all fears".
Building a plutonium bomb (given the plutonium) is considerably more
difficult and not a project for MY garage even with a lathe. Plutonium
is downright dangerous to handle, and the construction requires shaped
lensing charges, which in turn requires an ability to make precision
casts of at least two different explosives with differential burn rates
and to set them off with high speed triggers at exactly the same time.
One can look at the dual-use list at ORNL and see what sort of
precision is considered "dangerous" to bound your design effort.
However, "exactly the same time" very probably means something quite
different now from what it did in 1945. Again, my PC contains
nanosecond clocks; over the counter electronics can probably provide
enough switching speed and power to get within the range that will
suffice for an implosion device, especially one slightly overengineered
in other respects. Sure, the government controls known fast switches,
but I very much doubt that they control the knowledge of how to make
them, and I doubt that they are that hard to make.
Simultaneous is easy.. equal length coax, for instance. It's the
switches that are tough (and export controlled)
A bit trickier than you might think, to get the jitter down in the
subnanosecond range, and to have the high peak currents needed. And
no, you can't use spare xenon flashtubes you've scavenged out of
disposable cameras.
If you're worried about small (always an issue when thinking about
nanoseconds.. 30cm/nanosecond for free space propagation), there's
also the energy storage to consider.
To optimize the yield, too, the timing of your zipper relative to
your compression is important.
Thermonuclear fusion and the 100+ KT to MT range are similarly
straightforward. From what I recall, one can just monkey around with
building bigger bombs surrounded by more fissile material and get close
to the latter, adding fusile material such as tritium (expensive and
dangerous) or deuterium (plentiful and harmless) and lithium into spaces
between trigger and a U-238 casing.
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.
To get to MT, one has to build a
proper dual implosion device so that the trigger causes both heating,
compression, and neutrons to all happen at the same time to a
significant volume of fusile material. The NWFAQ doesn't contain
engineering specs (as it proudly and irrelevantly announces). So it is
entirely possible that one could try for MT and only end up with a 100
KT or so. However, range of total destruction scales like the 1/3 power
or thereabouts of the energy, so a 1 MT device only does roughly twice
the damage of a 100 KT device anyway. Code and my laptop might up the
odds of making a MT+ device work the first time, but...
cube scaling is the rule in explosives (all the way down to sub kilo
amounts), however, for very large devices, where the "point source"
assumption is invalid (e.g. that 10km fireball), it does break
down. There's also the issue of the propagation of a shock wave in
air and air not being a linear medium.
...from the point of view of strategic war or tactical war these
differences matter, I suppose. A 100 KT "fizzle" might let a hard
shelter survive where a 1 MT non-fizzle would kill it. Getting too big
or two small an explosion can either kill your own troops or not kill
all of the enemy on an actual battlefield.
Or, knocking the lid off a silo, which requires both precision
targeting AND suitable yield AND something that fits in a package
that can be appropriately delivered.
Tactical devices like
neutron bombs require significant engineering and experimentation to
achieve and are not garage projects, I suspect -- get them wrong on the
one side and they're thermonuclear devices that are far more powerful
than you anticipate, get them wrong on the other and they don't make a
significant flux of neutrons and the enemy soldiers overrun your
position.
Hence the interest in things like the National Ignition Facility.
However, from the point of view of terrorist bombs NOBODY CARES -- or
should care -- a 1 KT "near fizzle" bomb is the moral equivalent of two
million pounds of TNT, 100 panel trucks loaded full of TNT and set off
all at once. Set off in the right place, it would do billions of
dollars in damage and kill as many as hundreds of thousands of people,
especially if it were surrounded by e.g. a ton or so of cobalt.
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.
I could
do that with my shotgun.
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
Bad as it is, this beats the hell out of the condition I grew up in --
living just outside of DC but well within the radius of TD expected for
a 10 MT airburst over the Washington Monument, with MIRV'd ICBMs
targeted on both sides and a single moment of insanity away from MAD --
but it isn't terribly desireable. No matter what the holding action, no
matter what the defenses, it is just too easy. Put a shotgun-bomb into
a freighter as machine parts or a nameless lump of concrete down in the
bilge, sail it up the Saint Clair river, there goes Detroit, or maybe
Chicago. SF, NY, Miami, New Orleans, Washington, Baltimore, Boston --
all vulnerable. Or unload it, put it on a truck, and anyplace is a
target.
why worry about ICBMs when DHL/FedEx will deliver it to your selected doorstep?
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