On Fri, 24 Feb 2012 13:19:08 -0600, Tim Wescott
wrote:

On Fri, 24 Feb 2012 13:34:07 -0500, Ed Huntress wrote:

On Fri, 24 Feb 2012 12:25:24 -0600, Tim Wescott
wrote:

On Thu, 23 Feb 2012 23:53:17 -0500, Ed Huntress wrote:

On Thu, 23 Feb 2012 20:37:59 -0800, Paul Drahn
wrote:

On 2/23/2012 8:08 PM, Richard wrote:
Like most of us (baby boomers) I grew up in a world that already had
nuclear weapons. We did "Duck and Cover" drills in school, and I
thought I had a fair notion about how these things worked.

But this photo (second one on this page) of the PU core of the "Fat
Man" implosion device - it's so small...fit's in your hand.




http://simplethinking.com/home/nuclear_weapons.htm

The majority of the energy release is nearly instantaneous, the mean
time from neutron release to fission can be of the order of 10
nanoseconds, and the chain reaction builds exponentially. The result
is that greater than 99% of the very considerable energy released in
an atomic explosion is generated in the last few (typically 4-5)
generations of fission -- less than a tenth of a microsecond.*

This tremendous energy release in a small space over fantastically
short periods of time creates some unusual phenomena -- physical
conditions that have no equal on earth, no matter how much TNT is
stacked up.


A fascinating collection of high speed photos...

http://simplethinking.com/home/rapat...hotographs.htm

http://simplethinking.com/home/rapatronic_2.shtml

and the "rope tricks"...
http://simplethinking.com/home/rapatronic_3.shtml
Very impressive photos. I don't recall seeing them before.

Still the critical component is not described. It is the mechanism to
trigger the conventional explosive segments all at the identical time.
The key word being - identical. The failure of these devices in the
last North Korea test probably caused the dud. Until Iran is able
manufacture the devices and get them to work properly, all the uranium
in the world will not get them the bomb.

Paul

Well, that *is* really a problem. All of the plutonium in the world
won't get them a bomb, but the gun trigger for Little Man could
probably have been made in a good basement shop. That's why it's a lot
scarier for them to have enriched uranium than plutonium. Any goof can
make a gun-trigger uranium bomb, if they have a plan that gives them
useable dimensions.

The US made a few gun-trigger bombs, and the Brits made a few, and
then we both scrapped them. They're dangerous as hell: anything that
will set off the explosive charge, like lightening or a big spark,
can, theoretically, make them go "boom."

Gun triggers won't work with plutonium because the bomb will
self-destruct before the chain reaction is more than a fizzle. BTW. it
takes a much larger uranium pit than a plutonium pit to make a bomb.

Somewhere on the web -- I think I found it on Wikipedia -- there's a
discussion about early thoughts on gun-trigger plutonium bombs, along
with a picture of some ten prototypes that were scrapped. They were
very long and very skinny, to try to get the velocity up enough to avoid
the effect that you mention.

Yeah, in theory you can make a gun trigger for plutonium. I've seen
values of barrel length ranging from a few hundred meters to a
kilometer.

In fact, when N. Korea exploded its first bomb in a tunnel, I wondered
(and still do) if that's what they tried.


Apparently they originally thought that a gun-triggered Pu bomb would
work, and it was one of the big names (Oppenheimer or Feynmann) in
physics that re-did the calculations and figured out that it was a
no-go.

Right. They pursued both trigger mechanisms until they realized that gun
triggers weren't going to be practical with plutonium.

In talking with people about this over the years I find that few people
know that the Hiroshima bomb was an untried gun-trigger device, but that
the Nagasaki bomb was an implosion device based on the Gadget used in
the Trinity test.

And it disturbs me a bit that reporters often rely on stories about the
extreme difficulty of building a working spherical-implosion trigger,
not noticing whether it's plutonium or uranium that's being used for the
pit.

I find it scary that Iran went for uranium.

Well, I think the major difference between the nuclear programs of N.
Korea and Iran is than Iran wants to blow up Jews, but N. Korea just
wants to extort foreign aid from the West and convince their home-boys
that the leadership has really big balls.

So for N. Korea Pu is the best, because they can scare the hell out of
the dimwits with money, but the real experts won't be rattled. Whereas
for Iran U-235 is the best, because they may actually be able to vaporize
parts of Tel Aviv.

That's an interesting take. I'd like to hear it put that way from an
analyst on a TV show sometime. It would liven things up. g

--
Ed Huntress