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?)