What do scientists have to do with the uranium before using in a nuclear bomb?

Answer 1

To use natural uranium in a bomb either of 2 things must be done first, both are expensive and require large infrastructure investment to do them:

1. Enrich the uranium from 0.72% uranium-235 (natural) to 93.5% uranium-235 (Oralloy or HEU).
2. Process the uranium to turn some of the uranium-238 to plutonium-239 in a reactor then chemicallly separate the plutonium from the rest of the irradiated material chemically.

In WW2 the US did step 1 at Oak Ridge, TN using a gigantic gaseous diffusion enrichment plant and an electromagnetic separation plant; and step 2 at Hanford, WA using several graphite moderated reactors and large chemical separation plants called "canyons".

Answer 2

Scientists need to enrich the uranium by increasing the concentration of the fissile isotope uranium-235. This is typically done using centrifuges to separate the uranium isotopes. Once enriched, the uranium can be used in a nuclear bomb.