With modern weapons-grade uranium, the background neutron rate is so low that terrorists, if they have such material, would have a good chance of setting off a high yield explosion simply by dropping one half of the material onto the other half. Most people seem unaware that if separated HEU is at hand it’s a trivial job to set off a nuclear explosion … even a high school kid could make a bomb in short order.
(the book is actually quoting Luis Alvarez there)
A US government sponsored experiment in the 1960s suggests that several physics graduates without prior experience with nuclear weapons and with access to only unclassified information could design a workable implosion type bomb. The participants in the experiment pursued an implosion design because they decided a gun-type device was too simple and not enough of a challenge (Stober, 2003).
I’m far from an expert, but Global Catastrophic Risks makes it sound like that’s not the case:
(the book is actually quoting Luis Alvarez there)
I stand corrected. I think those quotes overstate matters a decent amount, but indeed the security of fissile material is a significantly more important barrier to misuse.