The most likely scenario would be to extract and powder the weapons grade radioactive materials and put them into a conventional bomb. The explosion would scatter long half-life materials over a large area. Of course, not only would this expose suicidal terrorists to highly radioactive materials, giving them a highly deserved terminal dosage, the metals Uranium and Plutonium are themselves highly toxic. In the late 40s or early 50s, my dad along with a number of other machinists quit a company that got a contract to machine inert Uranium components. The company didn't want to provide the needed personal protective equipment. I heard that the guys who stayed and were stupid enough to do it the company's way were dead or otherwise incapacitated within a few years.
BTW, my dad did have experience machining Uranium with at least one company smart enough to provide proper PPE. Said it's a real challenge. Too high a tool feed rate, local temperaure goes up a little too high, the hardness changes, and the tool either goes dull fast, or breaks off in the casting.