This is known by everybody who's read about the Manhattan Project. The hard problem was separating uranium isotopes. The Hiroshima bomb was a nuclear IED. It could be built today by a good auto racing shop if they had the enriched uranium. Even machining uranium isn't that hard. There's a how-to guide from Union Carbide's tools unit. It takes some precautions, but not remote manipulators or anything like what's needed for plutonium.