So I’ve been thinking about this for a while now, and I haven’t really been able to answer my question no matter how much I look into it, I was wondering if you guys could help.
So I was wondering, the critical mass of nuclear material is easier to achieve if the material is denser. I’m also aware that this is kinda how some nuclear bombs work, by launching a chunk of material at more material to cause the explosion. (I think?)
So in that case, what would happen if you grabbed a ball of Uranium or Einsteinium and threw it at the ground hard enough? Would it go critical? If so, how hard would you have to throw it and how big would the ball need to be?
As an optional bonus if all the aforementioned is possible, if the mass of Uranium were say, the size of a bullet, how fast would you need to fire the bullet in order to achieve criticality by kinetic impact alone?
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Throwing or shooting a blob of metal at the ground doesn’t increase the density in any meaningful amount. Plus it would at best only increase the density at one particular spot while decreasing it around the edges. In other words it splats like an egg. At some speed then yes you could trigger an explosion, but at the expense of blowing apart the bullet in the process. And the little bit of fuel that did compress enough would blow apart the rest of the fuel before it had a chance to explode.
The trick with some nuclear weapon designs then is to compress the metal from all directions at once using very carefully shaped plastic explosives so that the density increases evenly. Think of a baseball suddenly shrunk to a golf ball. The Nagasaki bomb for example compressed its plutonium core to double its normal density. Even so the majority of the bomb blew apart before it could undergo fission, and it was very inefficient.
The other way, the bullet or gun method which was used over Hiroshima, does not require compression. Sometimes just having enough radioactive metal in one spot is enough to set off a chain reaction . You could set off the bomb simply by slapping the two halves off a sphere together. No compression needed. It was inherently more dangerous however and because there was No explosion to compress the fuel , only about 2% of the uranium actually fizzed . The other 98% blew apart uselessly
The bomb they dropped on Hiroshima I believe was a bullet/gun design. A bullet of fissile material was suspended at the other end of a barrel, with a big explosive charge behind it. Infront of the bullet was a sub-critical mass of fissile material, when the bomb gets detonated, the bullet is fired into the sub critical mass with enough force to initiate the full chain reaction almost instantly, resulting in a bigger, much bigger boom.
However, that involves shooting a fissile bullet into a fissile mass. If you’re talking about firing a single bullet of Uranium or Plutonium at a flat surface, the bullet would have to be huge in order to achieve any kind of chain reaction – the energy you would need to achieve that may also cause the bullet to shred and disintegrate before it even hits the target.
No. Whilst you can theoretically achieve criticality by compressing a sub critical mass, there’s two reasons this wouldn’t work just by throwing a blob
The obvious one is that no human is even remotely in the neighborhood of being strong enough to through anything that hard
The second reason, even if you were strong enough, it you threw a lump of plutonium against something that hard (and assuming the thing you throw it against doesn’t just break away), you still won’t achieve compression, because the lump of plutonium will heat the object and then just disintegrate, sort of like throwing a tomato at a wall really hard: you don’t get a compressed tomato when you do that, you just get chunks of tomato everywhere. To actually achieve criticality, the pressure has to be coming very evenly from all sides to prevent the material from scattering.
I don’t think throwing at the ground will increase density enough before the shape becomes very suboptimal (it’s not only about density, it’s about how many neutrons escape the blob and how many are absorbed).
If nuclear fuel is malleable (I have no clue), you could probably design a non-spherical shape (like a teardrop pointing down maybe?) so that when you shoot it very hard against a concrete floor it becomes closer to a sphere. Not sure how much the specific properties of the concrete can vary without affecting the result strongly.
Then you can start from a mass that is subcritical when it’s teardrop-shaped, but supercritical when it’s a sphere. If I remember correctly, one of the problems with such “simpler” designs (like the gun-type Little Boy) is they’re less efficient than spherical designs.
The answers you’ve gotten in this thread so far are very specific to nuclear *fission* which wouldn’t happen, *nuclear fusion* however could, depending on how strong your arm is.
See obligatory [XKCD](https://what-if.xkcd.com/1/)
Yes it would possibly work, but the force you’d have to throw it with is so insanely high that it’s impractical to attempt without explosive propulsion.
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