[wildmusings]

It's unlikely that a nuke falls into terrorist hands, but its not unimaginable. Pakistan is a half-failed state with nuclear weapons and it's easy to imagine that government collapsing completely. North Korea is another unpredictable actor. US officials have warned about lax security around smaller nuclear weapons in Russia. You are right that they would face further obstacles in trying to detonate it.

There are many more-likely but still catastrophic scenarios. A bigger Paris/Mumbai-style attack could kill thousands. An attack on the Hoover Dam or a chemical plant could also kill thousands and cause billions of dollars of economic damage.