The US government prepares for all sorts of threats, but none match the pomp and circumstance they'll display in the event of a nuclear explosion.
Prepared scripts for many disaster scenarios are already written ranging from biowarfare and chemical weapons to volcanoes and wildfires. The good news is that the Cold War is over and a limited nuclear strike or a terrorist attack can be survivable (a direct hit notwithstanding). The bad news is that the government's plans for survival don't really include you anyway. Couple that with a new arms race which is already underway, albeit shrouded in secrecy, and one that may add small, portable nuclear weapons to the global stockpile. Lawmakers and experts fear such "tactical" or battlefield-ready devices (and their parts) may be easier for terrorists to obtain via theft or sale.
And even a small nuclear weapon on the ground can create a stadium-size fireball, unleash a city-crippling blastwave, and sprinkle radioactive fallout hundreds of miles away. In this case, it's important to be prepared for this scenario, even if it's unlikely to play out.