My bigger fear is someone like Iran or Pakistan supplying one to a terrorist group to give them plausible deniability. Iran gives one to a terror group. It gets put on container ship, maybe switch ships at sea a couple times to hinder tracing. Then launch from 300 miles offshore high over East Coast, sink ship immediately so no ones knows. EMP attack. Maybe another over West Coast, one over Europe. World in a serious hurt.
yes, it will be few rogue people high up in an extremist country that gain access to a nuclear weapon of some sort and figure out a way to fire it.
Much bigger fear is computer attack. as that can be done anywhere. already see it happening across the world with hacking into things and holding them for ransom. that is only going to become a larger issue moving forward.