Few things concentrate the mind like the prospect of a nuclear mushroom cloud in your own neighborhood. So please concentrate on this: I asked Ambassador Thomas Graham Jr., a sober, respected, retired career arms control official who was President Clinton’s special representative for nonproliferation and disarmament from 1994-97, to quantify the risk of nuclear terrorism. Here is what he said, from Moscow, via cell phone:

"Any judgment like this is a guess. But my judgment is that in the next year, there is perhaps a 10 percent risk of a major nuclear event in a large city, and in the next five years, perhaps a 50 percent risk. This risk would include the theft and use of an actual nuclear weapon, the fabrication and detonation of a crude nuclear device from fissile material, as well as a radiological bomb possibly based on fissile material."

Five years. Fifty percent. Maybe several cities destroyed. Hundreds of thousands or millions dead. The nation in chaos. Worse than our worst nightmares.

Even the least of these threats, a radiological "dirty bomb" using a conventional (non-nuclear) explosion to scatter fallout over a large area, could-if laced with highly lethal fissile (bomb-making) material such as plutonium-kill thousands of people through radiation sickness and "make the city of Washington unlivable for thousands of years," Graham says.

To be sure, Graham has no access to the latest intelligence secrets. Some counter-terrorism specialists suggest that the obstacles facing would-be nuclear terrorists remain formidable. Other experts differ in other respects. "There is simply no basis for quantification" of the nuclear risk, says Philip Bobbitt, a former Clinton National Security Council official. Adds his former colleague Daniel Benjamin, now of the Center for Strategic and International Studies: "For my money, the biggest threat of all would be a smallpox attack, which would cause unimaginable destruction." Some current officials say there’s no evidence that Al Qaeda has a nuclear bomb. Yet.

"You shouldn’t talk only about missiles and bombs," notes former White House Counsel Lloyd Cutler, co-chair (with Howard Baker) of an Energy Department task force that issued a chilling report in January stressing the "urgent" need to improve the "dire state" of security in Russia’s vast nuclear complex. "You should also talk about efforts to trigger the spent fuel rods in poorly secured nuclear power plants." That form of sabotage, Cutler said, could send a nuclear cloud into the atmosphere that "might kill hundreds of thousands of Americans and other people around the world."

But in assessing the danger of Al Qaeda terrorists stealing or buying atomic materials or some of the 40,000 nuclear weapons stored at more than 100 sites across Russia, or at other sites in Pakistan, Thomas Graham knows whereof he speaks. Now president of the Lawyers Alliance for World Security, he has long been steeped in the dangers of proliferation and "loose nukes." And most or all experts share his sense of urgency about the nuclear threat.

Graham’s analysis draws credence from various studies and news accounts, such as the October 26 report in The Times of London that "Osama bin Laden and his Al Qaeda network have acquired nuclear materials [illegally from Pakistan] for possible use in their terrorism war against the West, intelligence sources have disclosed." The article added that "the Western sources say that [bin Laden] does not have the capability to mount a nuclear attack." Yet.

"It’s the most dangerous threat we face," asserts Graham. "We can find a way to deal with biological terror. But if these guys acquire enough nuclear weapons and blow up four or five major cities, it wouldn’t end civilization as we know it, but it would come pretty close." Nor would the rest of the world be exempt from the jihad: "They would go after London and Paris and Russia, as well as the United States, if they had enough weapons."

President Bush said on November 6, "We will not wait for the authors of mass murder to gain the weapons of mass destruction." But it’s possible that they already have such weapons. One who says they do is Yossef Bodansky, an Israeli-born congressional terrorism investigator. "There’s a tremendous amount of evidence from both Middle Eastern and Russian sources, Arabian senior officials included, that bin Laden has acquired several handfuls, according to the Russians-up to 20, according to the Arabs-of the suitcase bombs that the Russians have lost," Bodansky asserted in a recent broadcast interview. In a 1999 book about bin Laden, Bodansky claimed that Al Qaeda had paid Chechen rebels $30 million and 2 tons of heroin.

Counters Daniel Benjamin, with what must pass for optimism these days: "If they had a nuclear bomb, I think they would have used it by now. They will certainly keep trying to acquire them."

Why dwell on such horrors? Because it’s past time to focus our sense of urgency on the gravest dangers we face. At best, Bobbitt stresses, "we are in a race against time." We will lose that race unless we work much harder at averting nuclear catastrophes.

That means using as much military force as necessary to destroy Al Qaeda before it can destroy us. It means spending as many billions as it takes to secure or eliminate Russia’s nuclear stockpiles and our own spent nuclear fuel rods. It means avoiding a disastrous fundamentalist takeover in nuclear-armed Pakistan. It means launching pre-emptive strikes or covert operations to disarm (and when possible to kill) potential nuclear terrorists abroad. It means developing and deploying better sensors to detect hidden nuclear material and undertaking a massive effort to screen millions of shipping containers, trucks, cars, even suitcases. It means aggressive use and broad interpretation of the government’s powers to search for doomsday weapons and spy on suspected terrorists and their associates. And much more.

In the long run, the threat will grow, Graham stresses, unless the Bush Administration abandons the unilateralism and the efforts "to selectively eviscerate multilateral treaty regimes" that were hallmarks of its foreign policy before September 11: "We need to look on this as a war between the civilized world and barbarism, and pursue the closest possible cooperation and indeed integration with the rest of the world community…. The United States has not been willing to take the lead in reducing the number of weapons, in avoiding proliferation, in supporting the Comprehensive Test Ban Treaty, in giving up the first-use option…. We’ve just been drifting for years."

Other experts vigorously dispute some of Graham’s policy prescriptions. Whoever is right, such debates must no longer be the province of policy wonks. The survival of our civilization is at stake. Among other reasons for special concern on the nuclear front are these:

In a 1998 statement titled "The Nuclear Bomb of Islam," bin Laden asserted: "It is the duty of Muslims to prepare as much force as possible to terrorize the enemies of God." Al Qaeda’s efforts to buy nuclear weapons, bomb-making materials, and Russian scientists have been convincingly documented. In Graham’s words, the terrorists seem to seek "the destruction of all civilizations except their own, and they do not seem to be deterrable."

The International Atomic Energy Agency has reported 175 cases of attempted trafficking in nuclear material from sites in Russia since 1993, including 18 instances that involved bomb-making plutonium or highly enriched uranium. Those are the ones who got caught-sometimes after making off with the nuclear material.

"It’s certainly reasonable to speculate," says Cutler, that there may also have been successful efforts. "In the worst-case scenario," notes the Cutler-Baker report, "a nuclear engineer graduate with … an orange-sized lump of plutonium, together with material otherwise readily available in commercial markets, could fashion a nuclear device that would fit in a van [and] would destroy every building in the Wall Street financial area and would level Lower Manhattan."

In 1997, former Russian national security adviser Alexander Lebed told visiting members of Congress that more than 50 out of some 100 "nuclear suitcases" were not accounted for. Lebed later retreated when incumbent Russian officials insisted that no nuclear weapons were missing and that no suitcase bombs had ever been made.

The Administration’s October 30 global attack alert was reportedly spurred by the interception of Al Qaeda discussions of a planned attack "even bigger" than September 11. What could be bigger than that, if not a large-scale nuclear, biological, or chemical attack?

All this is at least as disagreeable to write as to read. We are beset by so many crises that it is hard to summon the emotional and intellectual energy to focus on the remote possibility of a catastrophe that would dwarf September 11. But the possibility is not remote.