Like to know how that's going to happen.
Saw a show on cable called "The Dam Busters" about bombing German dams and how difficult it was. A couple of truck bombs on top of the dam won't do it. They'll have to drop a bomb of MOAB power that would detonate and a precise location and even that may not cut it.
I agree: it would be very difficult technically and also hard to penetrate security. But they've had lots of time to think about it. It the terrorists are in fact in possession of a fission weapon the dam would be an obvious target. A delivery vehicle, especially from a utility or other that frequently visits might be able to slip through the inspections with a small nuke. Airborne and water-born (surface and sub) delivery paths also exist. Internal sabotage is a possibility. An ambush on dam security by machine gun-toting Jihadis on hillsides can't be ruled out. There are possibilities, however remote they seem.