Right Reason asks its readers: how likely is a nuke-attack?
Another probability must be considered: What is the likelihood a prospective nuclear attack will be used as a scare-tactic by irrational, though (for argument's sake) well-meaning political and journalistic actors? Can we any longer tell a fearmongerer from a prudent man?
As I have been over-fond of asking: Is it safe to panic?
The hypothetical of a nuke cloud over DC was used to short-circuit argument all the time in the run-up to the Iraq invasion. Jack Bauer situations are even now used to try to silence rational objections to torture. The abusive effect of such hypotheticals is lasting, grave, and certain.
The panic-button potential of the question is at least as weighty a matter as an actual attack.
My own opinion, as ill-formed as anyone's: an attack using nothing more than box cutters, a few hours of flight training, and surprise is an operation on a totally different scale than that required to acquire and detonate a nuclear weapon. I gauge it more likely that my panic-button will be pressed for evil ends than that a nuclear incident will take place.