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.


Its so big because it is hollow. The chemical implosion
crushes it and mixes it with the neutron trigger in the center,
starting the chain reaction.

Modern devices have a solid 239 Pu sphere in the center that
is compressed to about 2 X normal density to create the
critical mass. The reaction is started by a pulse linac
shooting protons at a target timed just right so the neutrons
get there at the instant of maximum compression. The Swan device
uses only two exploding wire triggers and 2 explosive lenses
of varying propagation rate that form a spherical explosion
front. They have a "driver" and a tamper to enhance the compression
and delay the flying apart of the compressed pit.

Some of these Swan devices are only a little bigger than that
"pit" in your picture!

Jon