2 of 3 Fixed some typos, corrected grammar and spelling, improved formatting while I was at it ;)

As both @AlanSE and @Philipp said, Pu-238 won't go boom no matter what you do to them. However, lets pretend it was Pu-239 and see what happens.

First, the battery puts out only about 1/2% of the power of a Pu-238 battery, Galileo doesn't work at all.

Second, while it is just barely possible to make a mass supercritical with pressure, you would have to start with a mass that's just a hair below critical and then subject it to some really extreme pressure. While we think of metal as incompressible, it will squash a bit under planetary core type pressures.

Finally, if you manage to do this, you end up with a criticality accident, not a mushroom cloud. The one I recall reading about knocked a nearby worker off a catwalk – he lived long enough to run out of the building. If you want a boom, you have to convert the subcritical mass to a very supercritical mass very quickly – which is why all the complex mess with explosives and very precise detonators.