in my opinion, the amount of latent heat in some room temperature plutonium is the key reason why you can't make it cause a fission chain reaction of any usable size.

i'm not sure what its latent heat capacity is, i know that water has the greatest latent heat capacity of all materials known, and additionally, metals are very heat conductive, which is another way of saying that when you put heat in, it comes out quite easily and quickly.

however, if you break the structure, as must happen when you break a nucleus, all the electrons are liberated and all the latent heat (which is basically captured infrared photons) are also liberated, and are absorbed and conducted well by the remainder of the solid, and rapidly turning into liquid, plutonium metal.

when a metal becomes liquid, right off the bat it expans by about 3x or more in volume, which is another way of saying that your fission nucleus strike precision now has to rise by this factor of expansion

add a few more successful hits and there is a certain point at which the amount of heat, which presumably is all trapped inside the implosion device of an alleged plutonium based explosive charge, by both the mechanical feature of it being a metallic sealed vessel and by the implosion force of your shape charges, the entire mass of plutonium is going to reach above the triple point and be ready to turn into gas the moment that the compression and containment cease to hold.

i would estimate that this point is probably a pretty small percentage of fissioned material because the strongest implosion vessel i can conceive of would have to be made out of solid, inch or more thick tungsten, at which point you know that if the thermal yield of the fission so far has achieved the point where this metal starts to melt, just so happens to also be in the range of the boiling point of plutonium

so, if you can explain where the kinetic energy (not heat) is coming from to rupture this violently instead of melting it like a baloon lit by a lighter (or like a hindenberg, hissing a violent burst of combusting hydrogen out the first crack that opens) and providing your kinetic force (not just heat) then i'm sorry but your plutonium is going to be no more than 10-20% fissioned, and the rest is going to be boiled, and even the strongest conceivable containment vessel around it, surrounded by the delicate temperature regulation gear that is required because of the micron tolerance of the whole apparatus to failing to achieve even this pissy amount of fission, is just going to be a nasty dirty bomb that fortunately is only a few kilos of metal boiled rapidly into vapor, and there certainly isn't going to be a great flash and much wailing and gnashing of teeth. maybe some cancer. a little.

anyway, do the math yourself. there is a good text you can read that goes through every sordid detail of the alleged nuclear fission chain reaction-and-hypothetical-explosion story, it's got more holes in it than grandma's magnum opus doily.