I'm scared of someone with a nuclear weapon. I'm more scared of someone who can assemble ten very large FAEs (tanker truck size) and stick a few boxes of medical radioactive waste in with each one.

It will look a lot like a nuke, and the giegers will click. And you know the media will be all over the guy with the clicking box- a live exclusive on every network and news channel. That's all it would take to start a full fledged panic, which would effect locations other than the targetted ones, and even in the target cities the reaction of paniced, terrified people would probably rival the damage of a few thousand gallons of propane and a couple tons of explosives.

And it would just take time and money to legally procure and possition the components. You could move them by truck, by train, by boat, and so long as you have the proper manifests, you can ship the explosives in one lot, the propane tank(s) in an another, and assemble it in an evening.