Trigger Device

In theory, this trigger device lets the size of the bomb be almost as small as the critical mass of the plutonium.

In practice, real plutonium bombs are somewhat large, because they use distance to keep the mass of material from being able to go boom. That is, the critical mass can only explode when it is all together. So, you have this hollow shell of plutonium surrounded by explosives, and when they go off, the shell collapses into a blob that achieves criticality for the Main Event. It takes VERY INTENSE ordinary explosives and most-excellent timing for this to work.

As background, a classic design for a uranium bomb is the "gun". Here you have two big subcritical pieces, one of which is explosively shot at the other. When they collide, the Main Event happens. But this cannot work for plutonium; the stuff is TOO reactive. The Main Event will start to happen even before the picecs get together, and the result is more of a fizzle than a Bang. So that's why they use the hollow shell, so that higher velocities are involved in getting the critical mass together.

Once upon a time I did an analysis of fission, and came to the conclusion that a mere "critical mass" isn't really the main requirement for the Main Event. A "critical DENSITY" is a more fundamental idea. See, any density you might care to specify can be associated with a distance-between-atoms. When fission is concerned, the bigger this distance, the more easily the neutrons can miss hitting/splitting atoms. This can be compensated by using a bigger volume of material, and in general a larger volume of less-dense material might have about the same total critical mass as a smaller volume of denser material, but I came to the conclusion that the match wasn't perfect. At small-enough distances-betwen-atoms (high-enough density), the result is skewed such that a surprisingly smaller mass can still do a Big Boom.

But the key to that is achieving a rather-high density. I'm not sure that you CAN take a safely-less-than-critical mass (at ordinary density), and surround it with ordinary explosives, and expect it to scrunch-down ENOUGH for the Main Event, when those mere ordinary explosives are set off. Unless you use a LOT of those explosives, and thereby still end up with a largish bomb.

(Ok, I do know about using beryllium to enhance the numbers of neutrons and allow some bomb shrinkage, but that's somebody else's Idea and so I can't post it here on the HalfBakery.)

This Idea qualifies as Half-Baked because it might not work as intended. CAN the things I'm about to describe be moved fast enough? Certainly I'm in no position to make one to find out! (And, fortunately, neither is any terrorist on the planet, as you will see.)

As some more background information, think about the "control rods" in a nuclear reactor. These depend on certain elements (more precisely, certain isotopes of those elements) which absorb neutrons strongly. The usual elements are boron and cadmium and gadolinium. For this Idea we want to extract only the particular isotopes that strongly absorb neutrons. I don't think that such extraction is being done anywhere, because so far as I know, in reactors the ordinary chemical element is good enough. And since the desired pure isotopes are basically not available anywhere, we need not worry about terrorists and this Idea.

Now consider the kitchen gadget you use to slice up a hard-boiled egg. It consists of a bunch of wires stretched between a hinge and a handle; you set the egg under it and push down, and the wires all slice the egg up in one swoop.

Suppose you had a similar device that used blades instead of wires. Further suppose you had four of them. You set one of them down with the blades facing upward, and place the egg on it and press down. The egg slices, with its pieces fitting between the blades.
1___1___1___1___1___1
Now take the second slicer of the four, and press it down into the egg from above. Every slice can now be further sliced in half.
1_2_1_2_1_2_1_2_1_2_1_2
Now take the other two slicers and push them in from two of the sides; the sliced egg is subdivided even more.
132413241324132413241324 (the numbers represent the THIN blades of particular slicers; the egg is between the blades, of course.)

Now think about making those slicers out of neutron-absorbing isotopes, and replacing the egg slices with plutonium slices. It has to be manufactured very carefully, of course (you DON'T start with a critical-mass sphere that you want to slice up!), from layers of plutonium and absorber. When done, it should be inert and pretty small.

Now redesign that slightly to include some explosives between the plutonium and the "base" part of each set of neutron-absorbing blades. Not a lot, and so the bomb isn't much bigger than before. When those explosives go off, they have the purpose of REMOVING the slicers' blades from the plutonium, in all four directions. IMMEDIATELY the Main Event happens, because all the pieces of that critical mass are quite-close-enough-together, thank you. It will fizzle only if the blades can't be expelled fast enough. To be determined!