How much kg of plutonium did you need to make a sizable bomb?

Fat man used about 6.4kg of plutonium in a solid sphere split in half with a small hollow in the center and gold foil separating the spheres. (the hollow was for the neutron initiator - the Urchin). read more

With all fissile materials, you need a “critical mass” of the substance to make a bomb. The critical mass for plutonium is about 11 kg and would make a sphere about 4″ in diameter. Other materials have much larger critical masses. read more

You need a fully functional nuclear reactor to irradiate the U238 to make the Pu239, and you have to know how long to soak the U238 so you don’t start getting Pu240, which is poison for a plutonium bomb. You simply can’t have but a trace of Pu240 and the Pu239 is worthless. read more

Trace amounts of plutonium-238, plutonium-239, plutonium-240, and plutonium-244 can be found in nature. Small traces of plutonium-239, a few parts per trillion, and its decay products are naturally found in some concentrated ores of uranium, such as the natural nuclear fission reactor in Oklo, Gabon. read more

Plutonium is much more common on Earth since 1945 as a product of neutron capture and beta decay, where some of the neutrons released by the fission process convert uranium-238 nuclei into plutonium-239. read more