If it happened, it is unlikely to happen the way it is outlined in the OP. It's much harder to trigger a nuke than one would think.
It is, however, more likely to be a dirty bomb. Not so dramatic, but seriously deadly. In fact, our security services have - on at least one occasion - intercepted the components for such a devise coming into the United States.
Hysterical over dramatization, such as that of the OP, is ridiculous. But that does not mean that there is not a very real threat to our country.