I read about that guy they arrested ( I can't recall all the details) recently who was charged with attempting to build a dirty bomb. He had some uranium as well but it was the low enriched kind which is more like lead than anything else (what uranium turns into eventually). the article went on to say that the uranium he had was worthless for his purposes but that didn't make him any less guilty of "attempting" the crime.

It turned out the dirty bomb was a bit dirty because he had filled it up with Chinese fire crackers and kept it in the pig pen on his farm.