[neurobama]

A highly-enriched uranium fission bomb is conceptually and practically simple to construct. The hard part is getting enough HEU. That part isn't even hard to achieve (North Korea managed it after all), just moderately expensive and hard to conceal. This process and the bomb design is explained in detail in the nuclear non-proliferation documentary, "Countdown to Zero" (2010): https://www.imdb.com/title/tt1572769

The amount of HEU needed for a device of this type with a yield comparable to Little Boy could fit in a shoebox. Seal it in some lead pipe, stick it in a shipping container and no one would know. The above documentary illustrates this powerfully with animations and expert testimony. Strongly recommended, especially as a companion to "Command and Control" (2016): https://www.imdb.com/title/tt5598206/