Lol at. "Nuclear Terrorism". There hasn't been a significant terrorist attack in a First World country using conventional explosives since 7/7 in the UK, and that was back in 2005. The Boston Bombers only managed to kill two people with their IED's. Tragic, of course, but hardly Apocalyptic. Nowadays, terrorists are reduced to stabbing lone off-duty soldiers to death with knives and trying to shoot cops with guns that blow up in their hands.
Getting hold of even a small, low yield Nuclear Weapon would be extremely expensive and difficult. You would need to buy it from tier one Organised Crime, and even most of those groups would be unwilling to risk the shit storm that would follow the detonation of a Nuke by terrorists if it could be traced back to them.
If a Nuke exploded on American soil, the response of the US Military could be summed up as Wrath of God, with the Knobs Turned Up to 11. Anyone and anything connected to the plot would be wiped from the face of the earth.