Nicholas D. Kristof:

If a 10-kiloton nuclear weapon, a midget even smaller than the one that destroyed Hiroshima, exploded in Times Square, the fireball would reach tens of millions of degrees Fahrenheit.

It would vaporize or destroy the theater district, Madison Square Garden, the Empire State Building, Grand Central Terminal and Carnegie Hall (along with me and my building). The blast would partly destroy a much larger area, including the United Nations. On a weekday some 500,000 people would be killed.

Could this happen?

Unfortunately, it could -- and many experts believe that such an attack, somewhere, is likely. The Aspen Strategy Group, a bipartisan assortment of policy mavens, focused on nuclear risks at its annual meeting here last week, and the consensus was twofold: The danger of nuclear terrorism is much greater than the public believes, and our government hasn't done nearly enough to reduce it.

Graham Allison, a Harvard professor whose terrifying new book, "Nuclear Terrorism," offers the example cited above, notes that he did not pluck it from thin air. He writes that on Oct. 11, 2001, exactly a month after 9/11, aides told President Bush that a CIA source code-named Dragonfire had reported that al-Qaida had obtained a 10-kiloton nuclear weapon and smuggled it into New York City.

The CIA found the report plausible. The weapon had supposedly been stolen from Russia, which indeed has many 10-kiloton weapons. Russia is reported to have lost some of its nuclear materials, and al-Qaida has mounted a determined effort to get or make such a weapon. And the CIA had picked up al-Qaida chatter about an "American Hiroshima."

President Bush dispatched nuclear experts to New York to search for the weapon and sent Dick Cheney and other officials out of town to ensure the continuity of government in case a weapon exploded in Washington instead. But to avoid panic, the White House told no one in New York City, not even Mayor Rudy Giuliani.

Dragonfire's report was wrong, but similar reports -- that al-Qaida has its hands on a nuclear weapon from the former Soviet Union -- have regularly surfaced in the intelligence community, even though such a report has never been confirmed. We do know several troubling things: Al-Qaida negotiated for the $1.5 million purchase of uranium (apparently of South African origin) from a retired Sudanese Cabinet minister; its envoys traveled repeatedly to Central Asia to buy weapons-grade nuclear materials; and Osama bin Laden's top deputy, Ayman al-Zawahiri, boasted, "We sent our people to Moscow, to Tashkent, to other Central Asian states, and they negotiated, and we purchased some suitcase ((nuclear)) bombs."

Allison offers a standing bet, at 51-to-49 odds, that, barring radical new anti-proliferation steps, a terrorist nuclear strike will occur somewhere in the world in the next 10 years. So I took his bet. If there is no such nuclear attack by August 2014, he owes me $5.10. If there is an attack, I owe him $4.90.

I took the bet because I don't think the odds of nuclear terror are quite as great as he does. If I were guessing wildly, I would say a 20 percent risk over 10 years. In any case, if I lose the bet, then I'll probably be vaporized and won't have much use for money.

Unfortunately, plenty of smart people think I've made a bad bet. William Perry, the former secretary of defense, says there is an even chance of a nuclear terror strike within this decade -- that is, in the next six years.

"We're racing toward unprecedented catastrophe," Perry warns. "This is preventable, but we're not doing the things that could prevent it."

That is what I find baffling: an utter failure of the political process. The Bush administration responded aggressively on military fronts after 9/11, and in November 2003, Bush observed, "The greatest threat of our age is nuclear, chemical or biological weapons in the hands of terrorists, and the dictators who aid them." But the White House has insisted on tackling the most peripheral elements of the WMD threat, like Iraq, while largely ignoring the central threat, nuclear proliferation. The upshot is that the risk that a nuclear explosion will devastate an American city is greater now than it was during the Cold War, and it's growing.

In my next column, I'll explain how we can reduce the risk of an American Hiroshima.

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