[skorpeon87]

Pu-239 is an alpha emitter, but that's not the end of the story. All plutonium weapon cores are also neutron emitters because all weapons grade plutonium is impure and contains 2-7% Pu-240. Pu-240 is very prone to undergoing spontaneous fission which emits neutrons. So the presence of neutron radiation around a plutonium weapon core is unavoidable.

Furthermore, when Pu-239 is struck by neutrons (from the Pu-240 contaminant, for instance), it has a chance of undergoing fission and that results in the release of more neutrons which can cause additional Pu-239 to undergo fission. This chain reaction is how Pu-239 bombs work. A properly stored core will not sustain this chain reaction because the core is in a sub-critical configuration.

So there is in fact a very serious critical mass concern with plutonium bomb cores.

[not2b]

Yes, alpha particles aren't relevant to the fission reaction, so the fact that it's easy to shield against alpha emission isn't relevant. It's all about the neutrons.