Dirty bombs are cold war FUD. You could build a dirty bomb out of some smoke detectors. You could build a dirty bomb from the countless orphan sources scattered around the world with far less fuss than attacking a ship and causing an international incident. You could also build much more potent terror weapons without using any nuclear materials.
The biggest threat a dirty bomb poses is fear. Meanwhile shipping pollution is estimated to be responsible for hundreds of thousands of deaths every year which we just take as a fact of life.
