h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

In the text you state:

>
>The Design of the first A-Bombs
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>There are two basic design concepts for atomic weapon construction. >The implosion device;

and then neglect to mention or discuss the second design concept. Ink-wiring minds want to know.

My guess is the second design concept is the what I call the cannon concept. It was used for the "Little John" bomb that was detonated over Hiroshima. I don't know all of the details, but in a nutshell it works like this.

You place a sub-critical mass of Uranium 235 at one end of a sealed tube. At the other you place a second sub-critical mass, backed by some high explosive, and seal the tube. At the appropriate time, you detonate the high explosive when then forces the two sub-critical masses of Uranium together to form a super-critical mass, which then, promptly, explodes. There may or may not have been a neutron source to goose things.

From what I've read on the subject, a neutron source isn't really necessary to ignite a runaway chain reaction in either Uranium 235 (U-235) or Plutonium 239 (Pl-239), although it helps since it reduces the amount of fissile material required. Both U-235 and Pl-239 will go into spontaneous fission as a primary mode of decay if you get enough of it together in one spot. In other words, if you form a super-critical mass of either, it explodes without any further ado, thank you very much.

The main function of the conventional explosive is to keep the supercritcal mass of fissile material together long enough to create a massive explosion as opposed to smallish one. But even so, the yeild will be very low if measured as a percentage of material that actually fissions to the initial mass. That's because no conventional explosive can overpower the effect of nuclear explosion for very long, microseconds at best.

However, as you pointed out, this all became moot in 1953 when hydrogen bombs were developed.

Oops, the Hirsohima bomb was "Little Boy" not "Little John". My bad . . .