# Fissionable's Starting Mass

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Supposing the critical mass of a fissionable material is Mc, what do you suppose the "starting" mass would be in one of the stock-piled devices of the privileged nations?

0.8 Mc, 0.9 Mc? Too close, it gets a little hairy, too far away it can't be "squeezed" enough.

IOW, does anyone know just how much can the mass of a ball of metal be increased, practically, by subjecting it to enormously high pressure? imp

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IOW, does anyone know just how much can the mass of a ball of metal be increased, practically, by subjecting it to enormously high pressure? imp

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Funny!

I think you mean density, and the answer is quite a lot. IIRC a softball sized core can be compressed to roughly the size of a walnut (maybe significantly more). The density can be calculated, just assume that 239Pu has a density (uncompressed) similar to W, which you can look up (it is surprisingly large).

Check out "coin shrinkers" (these are nuts, but demonstrate the idea).

The idea is to reach a density where the neutrons from SF intiate more fission than (neutrons) can escape. If the density is very high and there is a concomitant sprinkling of neutrons, away you go.

Cheers,

O3

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The relevant factor is the bulk modulus, and for metals it's usually in the 100's of GPa.

http://en.wikipedia.org/wiki/Bulk_modulus

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IIRC a softball sized core can be compressed to roughly the size of a walnut (maybe significantly more).

O3

Please enlighten me as to the source of the above information..... we are not talking about plastic deformation here, but rather actual change in volume of the original hunk of metal.

Perhaps the means used for your example could be notched-up a bit more, and simply compress the core so small, that it disappears? imp

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There is a great deal of space between atoms (which, when close enough, will repel VERY strongly thus preventing your disappearing core) in a solid. And, the core is probably hollow, with the "pit" inside.

OK, maybe not that small, but I thought that would get the idea across. The core deformation is significant (appx. density would be 2-3 over normal, for a high-yield device, which approaches our walnut). I don't recall the reference off-hand, but I will dig out the old notes and look for it.

Cheers,

O3

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I concede that, yes, atoms are made up of largely empty space (relatively large distance between atoms). However, this has no bearing on the fact that solid materials, as opposed to gases, are very incompressible, so much so, that in hydraulics, for example, oil compresses a fraction of a percent at several thousand psi. (forgive the use of antiquated units, I am old-school). Metals, like Plutonium, are even less compressible.

The fact that a bomb's core might be a hollow shere also has no bearing: it is the TOTAL VOLUME of material which must be compressed to bring it's MASS to criticality, yes?

My next question would be what order of magnitude of pressure applied is necessary to sufficiently compress to criticality? The process is probably impossible by conventional mechanical means, like squeezing in a press of some sort. So, high explosives create a very high over-pressure wave, that must still be focussed and directed toward the core from all directions.

Practically, this does not appear to me to be an easy task. imp

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the compression is not so great as ozone says, pressures experienced are usually on the orger of a few million atmospheres though.

but it is the density that is increased to cause criticality by lowering the critical mass which depends on density.

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