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The question "Did NASA nuke Jupiter?" while debunked immediately (non-fissile isotope was used) arose many what-if questions.

What would happen if a subcritical chunk of a fissile isotope, like Pu-239 was dumped down the atmosphere of Jupiter?

We know fission bombs use a big conventional charge to compress the fissile material into supercritical mass, primarily to release a lot of energy in very short time, and because that's the only viable way to reach these pressures in portable devices, but also because the moment chain reaction starts, the whole device "disassembles itself" (in extremely explosive manner) and the pressure causing the supercriticality vanishes.

Now what would happen if the pressure was applied continuously growing, and in a way where even the nuclear explosion is not able to remove it - like in the case of descent through a gas giant?

Would the remaining material be taken by sudden increase of pressure from explosion? Or would it dissipate, stopping the reaction at mere "Los Alamos event" level? Or would something entirely different happen?

(and by the way, what are the pressures caused by conventional charge of an implosion-type nuclear device? Is the 3000-4500GPa I found quoted for Jupiter core even sufficient?)

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Now that's on-topic :) The thing that matters with implosion is the speed of reactivity insertion. So, somebody has to estimate that for a particular descent profile and phase of the plutonium in question (the shape of the chunk would matter quite a lot). –  Deer Hunter Aug 1 '13 at 8:20
    
@DeerHunter: What is the phase/shape of plutonium in RTG batteries? As for the speed, we can assume quite slow, probably order of 1m/s; the surrounding matter will be liquid metallic hydrogen (~0.6g/cm^3) so its viscosity will prevent any rapid movement. –  SF. Aug 1 '13 at 8:45
    
SF, only weapons scientists can calculate that. My bets are on a fizzle, but I have no idea about the yield. –  Deer Hunter Aug 1 '13 at 8:51
    
@DeerHunter: My bets go between "sustained fizzle" (something quite similar to nuclear reactor) if the core balances on criticality level, with its own temperature expansion making it subcritical, cooling and contracting back to supercritical, and "self-destruct", the core melts and currents/disturbances/diffusion simply dissolve it. That would depend on whether liquid plutonium can mix with liquid metal hydrogen or do they stay separate. –  SF. Aug 1 '13 at 8:59
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From nuclear weapons FAQ

A high performance explosive can generate shock wave pressures of 400 kilobars (four hundred thousand atmospheres), implosion convergence and other concentration techniques can boost this to several megabars. This pressure can squeeze atom closer together and boost density to twice normal or even more (the theoretical limit for a shock wave in an ideal monatomic gas is a four-fold compression, the practical limit is always lower).

So Jupiter pressure in its center and even pressures at which hydrogen becomes metallic (which probably be practical boundary to which object dropped from space fall) are enough to compress plutonium at least twice. That means slightly subcritical chunk of fissile material sinking into Jupiter will become critical at some point in its descent. However the timescales of this process would be much to slow for proper nuclear explosion to occur because as soon as criticality is achieved the energy released expands fissile material producing fizzle with much lower energy release than could be expected from plutonium bomb, because neutron multiplication factor never raises much over 1.

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Note unlike with nuclear devices where fizzling obliterates the ifrastructure here the pressure doesn't vanish. Could the plutonium keep "boiling" at the border of criticality, expending considerable amounts of energy over a long time, or would it just dissipate? –  SF. Aug 1 '13 at 8:51
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@SF: Probably with clever design of the device, such as shell keeping fissile material from escaping, it is indeed possible to achieve balance of criticality between thermal expansion and external compression. That would be fast spectrum molten plutonium reactor. –  user23660 Aug 1 '13 at 9:20
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