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Trigger Device

For small plutonium bombs
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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!

Vernon, Jun 11 2005

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       I'd like to give a shout out to all the visiting Homeland Security agents that have no doubt just arrived.
waugsqueke, Jun 11 2005
  

       Vernon Gone to Guantanamo.
ldischler, Jun 12 2005
  

       LOL [waugsqueke].   

       + Congrats [Vernon] on a well-written piece. Very interesting, even if the idea turns out to be a damp squib.
Adze, Jun 12 2005
  

       Friend, you need a homemade vagina.
crater, Jun 12 2005
  

       Had to run off to work at both my jobs after posting the main text. As has been mentioned, very little of this stuff is not already in the public domain. You may recall actual cases where people managed to specify detailed designs based on mostly-public info, and I was not nearly that specific here. I might mention that BEFORE I posted this, I did look in the "Help" area to see if any such things were frowned-upon here, but found nada.   

       Anyway, I do know of some good reasons for doubting the technical ability of anyone (not merely terrorists) to make this trigger as described. See, the most important part, those bladed slicer things, are specified as must-be-made from a strong neutron absorber, such as a pure isotope. AND those same things have to each stay in-one-piece as they are EXPLOSIVELY expelled from the core mass. Are boron, cadmium, or gadolinium tough enough to do that? I think not! Which means diluting those neutron-absorbers with tougher stuff like iron--but will there then be enough neutron-absorbing material to do the job of preventing a premature Big Boom? These are very significant issues! I could almost desire that this Idea NOT be physically possible, just to see bad guys waste lots of time and effort trying to do it!
Vernon, Jun 12 2005
  

       [UnaBubba], worse, I found some time to refresh my memory of the Table of Isotopes in the Handbook of Chemistry and Physics, and found out that (1) Boron isn't as good a choice as say, Samarium, and (2) those neutron-capture figures which let elements like Cadmium and Gadolinium be good reactor-control-rod materials are for THERMAL neutrons (relatively slow, that is). This means that IN ADDITION to the slicer being made of neutron-absorber and strengthener, they ALSO have to be made of something like carbon, which is a neutron-moderator (slows them down to become absorbable). In the end, you may end up separating the plutonium slices so much that even if the slicers could be removed fast enough, the core may not have its parts near-enough to each other, to do anything but fizzle.
Vernon, Jun 13 2005
  

       Vernon, will all that Neutron slowing down business generate heat? <adds layer 5 for cooling, adds cooling system to bomb>
Ling, Jun 13 2005
  

       [Ling], my guess is that special cooling is not required. Some passive air-cooling fins on the outside will maybe be enough. After all, all those larger bombs sitting in storage each contain a critical mass of plutonium, and THAT is the real source of heat, regardless of the form that carriers of that heat take (neutrons, gamma rays, etc.).
Vernon, Jun 13 2005
  
      
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