Reaction of Pu-239 to gradual increase of pressure 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?)
 A: 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.
