Imagine this worst-case scenario: a 10-kiloton terrorist nuclear device, constructed with stolen Russian uranium, parked in a small van in one of the busiest intersections in America.
In the moments after the blast, everything within a half-mile radius would simply cease to exist.
That's the scenario documented in an unpublished Department of Homeland Security document, first reported by the National Journal, and obtained by "Primetime."
The report describes an attack in Washington, D.C. But in New York City, according to the report's formulas, the damage would be even more devastating.
Within a half mile -- 10 blocks -- "several hundred thousand people would be killed either by burns, by debris, by the blast, or by exposure to radiation," said Jerry Hauer, the former director of New York's Office of Emergency Management and now an ABC consultant.
However, Hauer adds that if such a blast were to take place in New York, there is also some good news. "If you look up and you see it, you've got a chance. You're not part of that initial blast," he said.
According to experts, Midtown Manhattan's massive steel-and-concrete skyscrapers would absorb and contain the blast. In the next 10-block ring, there would be far less destruction and death. And outside that ring -- the city would be eerily intact.
At detonation plus 15 seconds, the aftermath would look chillingly similar to 9/11.
But rising up five miles into the air above the city would be an iconic -- instantly recognizable and completely deadly image: a mushroom cloud of radioactive dust.
Within 15 minutes, that deadly plume would start falling back to Earth -- and begin drifting with the wind. At that point, time would determine the difference between those who live and those who die.
"Ten to 15 minutes later, you're going to be exposed to a huge amount of radiation," said Dr. Irwin Redlener of the National Center for Disaster Preparedness.
For more than 7 million people, continued survival would depend on what they do next. The problem is many of their first instincts may be a waste of time.
Turn on the television for instructions? There probably won't be any electricity.
Pick up the phone? If the electrical grids are out, the phone grids are probably out, too.
Drive away? "There's no question there's going to be gridlock," Hauer said. And the car won't provide any shielding from radiation.
But while most people may not know what to do, the good news is the people in charge say they do.
Here's their plan: Within minutes of a nuclear detonation, a sophisticated computer program would track the path of that nuclear plume.
First responders would evacuate people out of the path of the fallout. Sept. 11 offered a lesson in the importance of beefed-up communications: Even if ordinary channels of communication fail, help can still be on the way.
And first responders will likely have drilled to prepare for treating the injured, said Ken Raske, president of the Greater New York Hospital Association.
"9/11 burned it into our brains on what level of protection we actually need," he said.
The broadest task they will have is decontamination. Radiation destroys the body's ability to create white and red blood cells, so hospitals in the New York area have set up over a hundred facilities.