Hollywood has destroyed Washington -- or New York or Los Angeles -- lots of times with nuclear bombs detonated by terrorists. It turns out to be harder in real life than in the movies.

Thinking about the unthinkable, a U.S. government study analyzed the likely effects from terrorists setting off a 10-kiloton nuclear device a few blocks north of the White House. It predicted terrible devastation for roughly 800 meters in every direction, with buildings reduced to rubble the way that World War II bombing raids destroyed parts of Berlin. But outside that blast zone, the study concluded, even such a nuclear explosion would be pretty survivable.

“It's not the end of the world,” said Randy Larsen, a retired Air Force colonel and founding director of the Institute for Homeland Security. “It's not a Cold War scenario.”

The little-noticed, 120-page study by the Federal Emergency Management Agency was hardly a summer blockbuster. The study, “Key Response Planning Factors for the Aftermath of Nuclear Terrorism,” was produced in November by the Homeland Security Department and the National Nuclear Security Administration. Even though the government considers it “for official use only” and never published it online, the study circulated months later on scientific and government watchdog websites.

The report estimated the blast zone would extend just past the south lawn of the White House and as far east as the FBI headquarters. “Few, if any, above ground buildings are expected to remain structurally sound or even standing, and few people would survive,” it predicted. It described the blast area as a “no-go zone” for days afterward due to radiation. But the U.S. Capitol, Supreme Court, the Washington Monument, the Lincoln and Jefferson memorials, and the Pentagon across the Potomac River were all in areas described as “light damage” with some broken windows and mostly minor injuries.

The government study predicted 323,000 injuries, with more than 45,000 dead. A 10-kiloton nuclear explosion would be roughly 5,000 times more powerful than the truck bomb that destroyed the federal building in Oklahoma City in 1995.

The flash from the explosion would be seen for hundreds of kilometers, but the mushroom cloud -- up to eight kilometers tall -- would only keep its shape for a few minutes. The flash would be so bright it could temporarily blind people up to 19 kilometers away, including drivers on Washington's Beltway. At least four area hospitals would be heavily damaged or couldn't function, and four others would experience dangerous radiation fallout. The government said it expects to send warnings afterward by television, radio, e-mail, text message and social media services like Twitter and Facebook.

It predicted the seriousness of radioactive fallout, which would drift with prevailing winds that vary depending on the season and expose victims closest to the explosion to 300 to 800 Roentgens in the first two hours, or enough to kill nearly all of them. In the spring, fallout would drift mostly to the north and west of downtown Washington. But in the summer, it would drift mostly southeast. After two hours, the radioactive cloud would move over Baltimore with far less exposure.

“Unfortunately, our instincts can be our own worst enemy,” the report said. After the bright flash of a nuclear explosion, people would rush toward windows to see but the resulting blast could break glass as far as five kilometers away just 10 seconds later and cause injuries.

Terrified victims would try to flee the area, but going outside could expose them to deadly amounts of radiation within a few minutes. A car offers no protection. The government's advice for everyone within 80 kilometers: Head downstairs into a parking garage or basement. Anyone caught outside who heads indoors should remove shirts or jackets and shoes and brush their hair to remove large fallout particles.

The blast zone could be smaller or larger, depending on the city. In more dense cities, including New York, towering buildings could help confine how far debris flies, though the radioactive fallout cloud would still drift over a larger area.

The key is to quickly head underground to parking garages or sturdy basements and wait, Larsen said. After about seven hours, radiation begins to disperse significantly, he said.

The government's study did not examine the plausibility that terrorists could build a nuclear bomb or smuggle one into Washington, which is protected with radiation sensors and other technology designed to thwart such an attack. It didn't say why it chose the intersection -- 16th and K streets -- as the epicenter for its fictional nuclear bomb.

The biggest difference between the disaster that the government studied and the nightmares of incoming ICBMs from the former Soviet Union is the size of the explosion. Cold War-era fears imagined massive hydrogen bombs detonated in the sky, not a smaller device -- one that might fit inside a parked van -- exploding on the street.

“Our images of nuclear war are either of Hiroshima or Nagasaki or what we saw in the movies during the Cold War,” said Brian Michael Jenkins, a senior adviser to the president of the RAND Corporation. “If you are thinking about (a city) being wiped off the face of the earth, that's not what happens.”