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Lakeshore
An author of no particular popularity

Jay Lake
Date: 2007-09-26 06:41
Subject: [science] Question re primitive nuclear weapons
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Location:Nuevo Rancho Lake
Mood:awake
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Tags:help, science, tech
Question for you science and technical types out there: Can the fissiles in a "gun type fission" weapon such as Little Boy be incited to criticality by something as mundane as a gunshot?

This sounds silly to me, but apparently this design was abandoned quite early on for fears that something as prosaic as a plane crash could set off the nuclear explosion. That quite clearly implies that some level of kinetic energy input is a sufficient trigger.
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(no subject) - (Anonymous)
Marty
User: martang
Date: 2007-09-28 16:52 (UTC)
Subject: Re: Sorta
ooh, that makes me think of the Howard Waldrop story where they do that...pretty chilling storytelling.
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Arrow In A Stream
User: jabber
Date: 2007-09-26 14:22 (UTC)
Subject: (no subject)
The name "gun type" implies that there is a fissile "bullet" that is added to an fissile "target" in a mechanical way to create the necessary critical mass.

Criticality can be achieved very easily if one is not careful. It has a great deal to do with the amount of the material, but also with the geometry of it. You may recall an accident in a Japanese nuclear plant a few years ago, where workers handling Uranium in solution were accidentally exposed to a great deal of radiation. The reason behind that accident was geometry.

Typically, Uranium solution is transported in shallow tubs of narrow cylinders so that escaping neutrons leave the solution quickly and do not excite the rest of the Uranium to free more neutrons. By accident, the solution was placed into a squat, broad container (bucket) which made for a geometry such that escaping neutrons had the opportunity to hit many more Uranium nuclei than they should have for a safe situation. A low volume chain reaction resulted.

As with liquid, also with solid. If you have Uranium of the right density in an alloy, certain shapes of the necessary volume of the alloy will be critical. Imagine two half-spheres of it. Separately, they're safe. Put them together, and BOOM.

I imagine the "gun-type" of device consists of a sphere of fissile material of proportions necessary to result in criticality, but with the core slug removed. This slug is kept separate on the far end of a "barrel" and is fired into the sphere, completing it, when the device is triggered.

Such a device is necessarily mechanical, and so adequate shock to it could cause the slug to come loose from the firing chamber and enter the target. BOOM.
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User: dirkcjelli
Date: 2007-09-26 14:45 (UTC)
Subject: (no subject)
The problem with the gun-type, which is largely as you describe, is that the fissile slug produces neutrons as it moves along the barrel... and this produces heat in the target. If you don't move the slug very, very fast you don't get the desired pop.
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User: dirkcjelli
Date: 2007-09-26 14:41 (UTC)
Subject: (no subject)
The velocity of the smaller mass is very important. Lower speed collisions would lead to a sub-critical meltdown... some radiation, great big mess... probably something along the lines of a dirty bomb.

They might have been concerned about premature detonation of the charge. Unlike the symmetric device, premature detonation could result in a full-on nuclear reaction.
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User: dirkcjelli
Date: 2007-09-26 14:43 (UTC)
Subject: (no subject)
sorry, should have said "critical or sub-critical" above. One wants "super-critical" for a nuke to do its pretty 'splosion.
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Jay Lake
User: jaylake
Date: 2007-09-26 14:46 (UTC)
Subject: (no subject)
So if I shot a dismounted bomb-ready hemisphere of U235 with a .38, for example, I'm going to get aradioactive burst at a minumum, and possible a dirty bomb effect, but not a critical reaction? If I shot a pile of U235 shards (from a smashed hemisphere, for example), I'm not going to get much except kicked up dust with some incidental radiation? (Not that you'd want to be around for that, either, I'm thinking.)
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User: dirkcjelli
Date: 2007-09-26 14:49 (UTC)
Subject: (no subject)
Yeah. This is sort of the angry-monkey-with-crowbar, Die-Hard-with-no-sequel way to diffuse a nuke.
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Jay Lake
User: jaylake
Date: 2007-09-26 14:53 (UTC)
Subject: (no subject)
You got it in one...
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Autopope
User: autopope
Date: 2007-09-26 15:18 (UTC)
Subject: (no subject)
The flip side is, the bomb is basically a pre-loaded gun barrel with a sealed end. All you need to do to make a mushroom cloud is to detonate the charge in the breech of the gun (sending the "bullet" into the "target") -- although it may well need other preparations (e.g. a polonium initiator that needs to be inserted prior to detonation), or have mechanical safety interlocks. Nuke designers are keen on mechanical interlocks with "weak links" in the firing chain designed to break if a situation arises in which the bomb might detonate accidentally. For example, the charge in a uranium gun design is triggered by a detonator. There might be a physical barrier between the detonator and the charge, which has to be physically removed before the bomb can fire -- for example, by being pulled out by a wire fastened to the airframe of the bomber that's dropping the bomb. And so on.

You can find a whole lot more interesting stuff here (and indeed, I've been strip-mining it today for my own WIP).
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Kevin Roche
User: kproche
Date: 2007-09-26 16:33 (UTC)
Subject: (no subject)
As in RAH's The Long Watch...
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User: n5red
Date: 2007-09-27 01:02 (UTC)
Subject: (no subject)
The book "Steam Bird" does talk about the possibility of a nuclear powered aircraft causing a weak and very dirty explosion in certain types of crashes. I think there are some actual cites in the back of the book. My copy is currently packed or I would go look it up for you.
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