Question

Assuming you can obtain all the materials and fuel, which is more difficult to build: a plutonium bomb or a uranium bomb? Why?

Answer 1

The arrangement of high explosives around the plutonium pit has to be very precise in order to compress the pit to a supercritical state via the tamper. In uranium bombs, like Little Boy, the "bullet" is simply shot into the uranium pit, and by bringing the two subcritical pieces close together you get supercriticality. This is quite easy to do, which is why the US government didn't even bother to test one before they dropped it on Japan. The "Gadget" used at Trinity was a uranium-based "Fat Man" style bomb