Science

I would think many precocious undergraduates could produce a workable design.

The problem lies with getting the materials and equipment to do it. Isotopic purity is a big deal in this case, still a nontrivial big deal.

It is relatively straight forward to make actinides unsuitable for weapons use.

I covered the point on another website: On Plutonium, Nuclear War, and Nuclear Peace

In the early years of nuclear weapons production, scientists were killed in criticality accidents, most notably, Louis Slotin, who invented the dollar and cents nomenclature still used in reactivity. His knowledge of reactivity did not prevent him from being killed by a failure to control it.