Oh Dear...
when I said:-
While we're at it, Johannes, why is it so much more difficult to build an Uranium bomb than a plutonium bomb?
I wasn't actually asking a question in nuclear physics, but in basic chemistry.
Don't worry about half-lives (except in the most adstruse sense), fission yield, or by-products.
The key word is BUILD. i.e. find and purify the component elements and assemble the materials, design etc.
The answer is in the Plutonium breeder cycle, Uranium enrichment (a "hot topic" today - ironic pun richly intended), and chemical separation.
Beyond this it gets into painful territory where the modern state believes that common-sense and scientific knowledge are state secrets that must be suppressed to reduce the risk of nuclear terror.
It is (unfortunately) within the capacity of any decent physics graduate and a team of engineers to build a crude fission bomb, if they have access to the relevant nuclear materials (fortunately including a couple of difficult ones) and are careless of the lives of their workers and agents.
I choose to collude with the state.