elucidator: No “Fat Boy” - the bombs were called “Little Boy” and “Fat Man”.
I guess a “Little Boy” design should be achievable - it’s more or less the design that badtz maru describes, with a few tweaks, see below.
Constructing a “Fat Man” takes some people who are very good indeed with explosives, as it’s dependent on compressing the fissible material by a more or less spherical charge.
The problem for a terrorist organizaion is, among other things, that they can’t test. And if the conventional explosives go off, but the chain reaction doesn’t start, they’re in trouble. Deeply.
I’ll second Zenster’s recommendation of “The sum of all fears” for an idea of how it could be done and what problems must be overcome. IMHO, it’s the last readable novel of Clancy’s, anyway.
Replace “drop” with “shoot” and plutonium with U-235, and you’ve just described the Hiroshima bomb. Well, more or less:
I am not a Nuclear Scientist (IANANS ?), but I think the problem is: You might or might not get a chain reaction going. If you want to make sure thet the reaction starts, add a neutron source to your setup. The problem is, of course, that as energy levels rise, the explosion tears apart your expensive plutonium into a lot of subcritical masses. This makes for a lot of contamination with very little yield. Technically speaking, it’s a nuke, but if you could’ve achieved the same effect with 5 tons of Sempex, why bother ?
If you’re going to make a nuke, you want a ultrarapid change from subcritical to supercritical, you want to make sure that there’s an abundance of neutrons to start and boost the reaction (get some tritium in there), wou want to contain (if at all possible) the critical mass during the first reactions to expose as many nuclei to neutrons as possible.
The Hiroshima bomb was contained in a heavy steel canister to prevent the “nuclear firecracker” phenomenon, it did have a neutron source and it used an explosive charge to collide two subcritical masses to one supercritical.