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(Some Guy)   The five most extreme nuclear experiments ever...including the nuclear reactor powered bomber   ( divider line
    More: Cool, nuclear reactors, experiments, Freeman Dyson, liquid metal, General Atomics, natural gas field, South Atlantic, compressed air  
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8541 clicks; posted to Main » on 16 Aug 2013 at 12:18 PM (3 years ago)   |   Favorite    |   share:  Share on Twitter share via Email Share on Facebook   more»

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2013-08-17 07:36:42 AM  

erewhon: Generally single or double point triggers.
What a double set trigger might look like.

Added Pacific Rim bonus:  It's for a jaeger.
2013-08-17 10:02:04 AM  

stevarooni: KarmicDisaster: FTFA, "No airplane was ever built that was actually powered by a nuclear reactor "

They did, however, fly planes with reactors that exuded the estimated radiation levels of a nuclear-powered plane.  The pilots, if I heard correctly, weren't legally permitted to stay in the cockpits more than a few hours.

Yeah, I know, they would pull over a hole in the ground and lower in the reactor as soon as they landed. Just pointing out the problem with reading comprehension and the headline.
2013-08-17 02:19:34 PM  
The hell is a "topology changer"?

Nuke goes off and everything switches coordinate systems?
2013-08-17 05:32:46 PM  

studebaker hoch: The hell is a "topology changer"?

Nuke goes off and everything switches coordinate systems?

Well, what you do is, you look at the equations for mean free paths of neutrons in your working mass, and you diddle with anything that will decrease that. Anything that does, is a possible way to build a weapon. So you can play with reflectors, reflector efficiency, density, topology and so on, and anything that'll allow you to rapidly change the MFP is fair game. From the MFP comes your 'critical mass'. CM is not a set number like most people think, it's derived from the MFP. So, for, say, a solid sphere without reflectors at STP, you'll have one CM figure. For a sphere with reflectors of a particular efficiency number, you'll have another and so on. The game is to take a lump of fissiles and suddenly do something to it that changes the MFP so that your previously sub-critical lump very quickly becomes a super-critical lump. You can't change the lump size (although I guess a gun type device sort of does), so you diddle the other factors in the equation to get this to happen.

The weapons designs you would normally see fiddle with density. That's mostly because the density term comes with an exponent, so it pays off more, effort for effort. It's generally about a power of two, although that changes depending on other stuff like your reflector efficiency and so on. But for rules of thumb, you can take it as a square. So if I double the density, more or less the CM drops by a factor of four. That's why most designs increase the fissile density using an explosive compression technique.

That's not the only thing you can screw around with, though. Density changes pay off handsomely in terms of CM delta, but they're tough to manage in terms of basic hydrodynamics, and Rayleigh-Taylor instabilities are always a big bugaboo, as are Munroe effect jet formations from your timing and materials not being exquisitely perfect. How you get past Munroe jets with an aspheric pit such as the W88 is left as an exercise for the astute student.

So, if you're someone with fissiles and maybe not so much on the precision machining and hydro calculations, you do it with one of the other terms. Another way to go is to change the topology of the pit. Each pit shape has a CM associated with it, given that all other factors are equal. That's generally for unshielded masses at STP. There's an equation that takes your material characteristics and you integrate it over the shape of the mass to see if the MFP ends inside the mass or outside, which is where the CM comes from. If too many of the neutron's mean free paths end outside the mass, it doesn't go bang. So there's a nice table of pre-computed CMs for various topologies, mostly shapes of rotation. So you can get the CM of an infinite plane, an infinite cylinder, spheres, obrounds, rotated ellipses and so on out of a book, if you've got the book. Or you can do the integration yourself, although it's not trivial. At least it's not for me, I have done a few for 'fun' and now I do it with MathCad. At any rate, the shape of the thing matters. If it's a random-ish shape, you have to do it with numerical methods. And the equation, while it ought to be perfect, isn't in practice (although it's close) and there are a number of numerical method modeling programs that they use. Mostly written in Fortran for some god-awful reason. And they often diverge for weird topologies, although I digress.

At any rate, simple shapes of rotation have solutions for CM that are generally pretty close for well-known materials. So, what you can do if you can't manage a density change reliably, is to take a sub-critical topology and change it to a super-critical one for that mass. In practice, you generally start with an obround, which has a high CM, and compress it into a sphere, which has a lower one. Some designs, like the howitzer shell, compress the obround into a partial reflector "anvil", reflectors get you gain on that exponent term on the density. So if you take a shape that's like a frankfurter with one end in a neutron reflector cup, and compress the thing into a sphere that's mostly in the cup, then you get reflector gain increases at the same time you're getting topology changes. That's important, because you don't get that exponent thing with a topology change like you do with density.

Density changers go from way sub-critical to very super-critical in a trice by increasing the density, because you get that big exponent driven alteration. A topo will get you a bit better than linear. So you have to design just under criticality going to somewhat super-criticality. That makes for a low-yield, crappy, inefficient, dirty weapon that doesn't get you all the bang for your buck in terms of fissile mass, but it's easy to do. Except you don't want to pile a lot of them up together (because they're unreflected and close to critical anyway) and you have fratricide issues (a weapon going off nearby can trigger a partial detonation because you're close to critical) and you've got that single point detonation thing, which is GREAT from an engineering standpoint. No lensing, no wave forming, and all you need is some decent explosive and a flyer plate, and you're home free. So it's a third world country sort of thing that works for crappy fissiles and bad engineering.
2013-08-17 06:16:29 PM  
s /neutron's/neutrons'/
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