But none match the specter of a nuclear explosion.
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.
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: A new arms race is likely underway — 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.
"Terrorist use of an actual nuclear bomb is a low-probability event — but the immensity of the consequences means that even a small chance is enough to justify an intensive effort to reduce the risk," the Bulletin of the Atomic Scientists said in a September 2017 article, which outlines what might happen after terrorists detonate a crude device that yields a 10-kiloton, near-Hiroshima-size explosion in a city.
A nuclear terrorist attack of this magnitude is one of 15 major disaster scenarios planned for by FEMA and other US agencies. (The same scenario also includes a dirty bomb explosion, though such an event would be dramatically less harmful.)
As part of the planning effort, the Environmental Protection Agency maintains a series of manuals about how state and local governments should respond. A companion document anticipates 99 likely questions during a radiation emergency — and scripted messages that officials can copy or adapt.
"Ideally, these messages never will be needed," the EPA says in its messaging document. "[N]evertheless, we have a responsibility to be prepared to empower the public by effectively communicating how people can protect themselves and their families in the event of a radiological or nuclear emergency."
Here are a handful of the questions the EPA anticipates in the event of a nuclear emergency, parts of statements you might hear or see in response, and why officials would say them.
"What will happen to people in the affected neighborhoods?"
What they'll say: "As appropriate: Lives have been lost, people have been injured, and homes and businesses have been destroyed. All levels of government are coordinating their efforts to do everything possible to help the people affected by this emergency. As lifesaving activities continue, follow the instructions from emergency responders... The instructions are based on the best information we have right now; the instructions will be updated as more information becomes available."
Why: The worst thing to do in an emergency is panic, make rash decisions, and endanger your life and the lives of others. However, it's also incumbent on officials to be truthful. The first messages will aim to keep people calm yet informed and as safe as possible.
"What is radioactive material?"
What they'll say: "Radioactive material is a substance that gives off radiation in the form of energy waves or energized particles."
Why: Nuclear bombs split countless atoms in an instant to unleash a terrifying amount of energy. About 15% of the energy is nuclear radiation, and too much exposure can damage the body's cells and healing ability, leading to a life-threatening condition called acute radiation sickness.
Without advanced warning, people can do little about the energy waves, also called gamma radiation, which are invisible and travel at light-speed. But the energized particles — including radioactive fission products or fallout — travel more slowly, giving people time to seek shelter. The particles can also be washed off.
"Where is the radioactive material located?"
What they'll say: "Radiation and environmental health experts are checking air, water and ground conditions in and around the release site to locate the areas with radioactive contamination. Stay tuned to radio or television, or visit [INSERT AGENCY WEBSITE HERE] for the latest information."
Why: If a nuclear bomb goes off near the ground (which is likely in a terrorist attack), the explosion will suck up debris, irradiate it, and spread it around as fallout. Some of this material rapidly decays, emitting gamma and other forms of radiation in the process.
Fallout is most concentrated near a blast site. However, hot air from a nuclear fireball pushes finer-grade material high into the atmosphere, where strong winds can blow it more than 100 miles away. It may take days for radiation workers to track where all of it went, to what extent, and which food and water supplies it possibly contaminated.