Can't provide a cogent explanation, but I believe the terrorists can both hire talent and procure special materials from agents of the former Soviet Union. Also, the original weapon can be reconfigured as a lower-yield fission device without the need for the fancy enhancers. Labs in Iran, NK, and (formerly) Libya would be more than happy to lend expertise. The Hiroshima device was much less sophisticated than these portable weapons but was effective nonetheless (ouch!).