The reason for the detonation is a primary factor, although I usnderstand why you want it out of consideration.
If it was an accident (in which case I'd like to know what a nuke was doing downtown) or stupidity (which could explaine the presence of a nuke downtown) the panic would be localized. Heads would roll and there might be lynch-mobs after the responsible party. The balance of the event would be somewhat Katrina-like.
If it was intentional, done by a known or unknown enemy, the chaos would be immense and national in scope. as mentioned, Air and rail transportation would shut down, and not for a day or two. Citizens would flee the cities where ever they were. The larger or more important the city, the greater the flight. If it was Chicago that was hit, San Francisco would empty. If it was Boston New Orleans would be a ghost town.
The flight from the cities would overwhelm smaller cities and towns and there would be refugee camps, official and non, nationwide.
The communication system would fail, overwhelmed as it was on 9/11. Huge logistical problems would arise as the powers that be tried to feed and care for people and at the same time try to get them to return home.
The Government would try to exercise authority and act, but, as is frequently the case, not be up to the task. the military would try to assist, with better results, but tactics that are understandably of dubious constitutionality. In any case, there are not enough of them to effectively calm the entire nation.
Wall street is sucking it's thumb in the corner.
If it is able, the nation would demand vengeance, If the culprit could be identified the government would have to take action in order to keep some semblance of control.
or maybe not...