Not as far-fetched an idea as you might think. I saw a story last night on The Rachel Maddow Show about a guy who built a replica of Little Boy, the bomb dropped on Hiroshima. Apparently there is a documentary about him called "Atomic Trucker," since that's what this guy does. One of the men who worked on the actual bomb was amazed at what the guy had done, since its design is still a secret.

One thing the US tries hard to do is keep nuclear material out of the hands of terrorists, although the Tea Party members in Congress are trying to cut most of the funding for the agency that does this. It's called the National Nuclear Security Agency.
http://nnsa.energy.gov/

Cutting this agency's budget is another really stupid right wing idea.