Graham Allison, in the Bulletin of Atomic Scientists, discusses what is needed to stop a nuclear version of 9/11. Read the whole thing.

On a normal workday, half a million people crowd the area within a half-mile radius of New York City’s Times Square. If terrorists detonated a 10-kiloton nuclear weapon in the heart of midtown Manhattan, the blast would kill them all instantly. Hundreds of thousands of others would die from collapsing buildings, fire, and fallout in the hours and days thereafter.

The blast would instantly vaporize Times Square, Grand Central Terminal, and every other structure within half a mile of the point of detonation. Buildings three-quarters of a mile from ground zero would be fractured husks.

Lest this seem too hypothetical, recall an actual incident that occurred in New York City one month to the day after the 9/11 attacks on the World Trade Center and Pentagon. A CIA agent, code-named Dragonfire, reported that Al Qaeda had acquired a live nuclear weapon produced by the former Soviet Union and had successfully smuggled it into New York City. A top-secret Nuclear Emergency Support Team was dispatched to the city. Under a cloak of secrecy that excluded even Mayor Rudolph Giuliani, these nuclear ninjas searched for the 10-kiloton bomb whose blast could have obliterated a significant portion of Manhattan. Fortunately, Dragonfire’s report turned out to be a false alarm. But the central takeaway from the Dragonfire case is this: The U.S. government had no grounds in science or in logic to dismiss the warning.

A nuclear terrorist attack on the United States would have catastrophic consequences even for other countries. After the nuclear detonation, the immediate reaction would be to block all entry points to prevent another bomb from reaching its target, resulting in the disruption of the global “just-in-time” flow of goods and raw materials. Vital markets for international products would disappear, and closely linked financial markets would crash. Researchers at RAND, a U.S.-government-funded think tank, estimated that a nuclear explosion at the Port of Long Beach in California would cause immediate indirect costs worldwide of more than $3 trillion and that shutting down U.S. ports would cut world trade by 10 percent.

The negative economic repercussions would reverberate well beyond the developed world. As U.N. Secretary-General Kofi Annan has warned, “Were a nuclear terrorist attack to occur, it would cause not only widespread death and destruction, but would stagger the world economy and thrust tens of millions of people into dire poverty.”

Hat tip to Karen Myers.