So one guy builds the bomb, another transports it.
Even that may not be enough...
If you want your RDD to cause radiation casualties, it needs to be
really hot, even after it's dispersed. You typically need to absorb 1,000 mSv before even the vulnerable start showing acute symptoms; 2,000 mSv is what you need before a majority will show symptoms. How long is it going to take to evacuate the site after the bomb goes off? Ten minutes? Let's be generous and call it an hour. So your material needs to have a 2,000 mSv/hr dose rate, even after it's widely dispersed by the explosion. Its dose rate is going to be at least an order of magnitude higher pre-dispersal. That's enough to cause death in 100% of people within fifteen minutes.
You can shield the device, of course, but that shielding is going to be
heavy. You saw the pics upthread. And good luck getting this sucker, shielded or not, onto an airplane, even pre-9/11.
All in all, while it's not quite
impossible to make this work, it's pretty tough.