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.
--
Ed Huntress