Want to build a dirty bomb?

Do like the GAO did. Form a bogus company and get the NRC to give you a license to buy nuclear materials. The process to build the dirty bomb would contaminate you personally, most likely resulting in your death. However, if you're a terrorist, you probably won't care. Second, the purchase would be expensive and would only likely contaminate an area the size of a city block. An NRC official argued that it would be cheaper and more effective to blow up a train containing chlorine or other contaminants.

Maybe. But the psychological and economic damage that could be caused by detonating a dirty bomb in certain locations would be devastating. Think evacuation. Think quarantine and decontamination. Think about areas where lots of people gather or areas people have to pass through to get to work. I don't have to go too far here. I'll let your imagination do the rest. Besides, I don't need the FBI knocking on my door for suggesting where terrorists could place such a bomb. After all, most terrorists are uneducated morons with low intellect and couldn't figure such things out on their own. ... except for maybe the ones who are doctors.