It is all true, but one needs to take into account that because of the different properties of the materials, the critical mass for uranium-235 is intrinsically

It is all true, but one needs to take into account that because of the different properties of the materials, the critical mass for uranium-235 is intrinsically much greater than that for plutonium-239. For a bare sphere, it is about 10 kg for plutonium and 50 kg for uranium.