The probability of a nuclear weapon one day going off in an American city cannot be calculated, but it is larger than it was five years ago. Potential sources of bombs or the fissile materials to make them have proliferated in North Korea and Iran. Russia's arsenal remains incompletely secured 15 years after the end of the Soviet Union. And Pakistans nuclear technology, already put on the market once by Abdul Qadeer Khan, could go to terrorists if the president, Gen. Pervez Musharraf, cannot control radicals in that country.

In the same period, terrorism has surged into a mass global movement and seems to gather strength daily as extremism spills out of Iraq into the rest of the Middle East, Asia, Europe and even the Americas. More nuclear materials that can be lost or stolen plus more terrorists aspiring to mass destruction equals a greater chance of nuclear terrorism.

Former Senator Sam Nunn in 2005 framed the need for Washington to do better at changing this math with a provocative question: On the day after a nuclear weapon goes off in a American city, "what would we wish we had done to prevent it?" But in view of the increased risk we now face, it is time to add a second question to Mr. Nunn's: What will we actually do on the day after? That is, what actions should our government take?

It turns out that much could be done to save lives and ensure that civilization endures in such terrible circumstances. After all, the underlying equation would remain a few terrorists acting against all the rest of us, and even nuclear weapons need not undermine our strong societies if we prepare to act together and sensibly. Sadly, it is time to consider such contingency planning.

First and foremost, the scale of disaster would quickly overwhelm even the most prepared city and state governments. To avoid repeating the Hurricane Katrina fiasco on a much larger scale, Washington must stop pretending that its role would be to support local responders. State and local governments--though their actions to save lives and avoid panic in the first hours would be essential--must abandon the pretense that they could remain in charge. The federal government, led by the Department of Homeland Security, should plan to quickly step in and take full responsibility and devote all its resources, including those of the Department of Defense, to the crisis.

Only the federal government could help the country deal rationally with the problem of radiation, which is unique to nuclear terrorism and uniquely frightening to most people. For those within a two-mile circle of a Hiroshima-sized detonation (in Washington, that diameter is the length of the Mall; in New York, three-fourths the length of Central Park; in most cities, the downtown area) or just downwind, little could be done. People in this zone who were not killed by the blast itself, perhaps hundreds of thousands of them, would get radiation sickness, and many would die.

But most of a city's residents, being further away, would have more choices. What should they do as they watch a cloud of radioactive debris rise and float downwind like the dust from the twin towers on 9/11? Those lucky enough to be upwind could remain in their homes if they knew which way the fallout plume was blowing. (The federal government has the ability to determine that and to quickly broadcast the information.) But for those downwind and more than a few miles from ground zero, the best move would be to shelter in a basement for three days or so and only then leave the area.

This is a hard truth to absorb, since we all would have a strong instinct to flee. But walking toward the suburbs or sitting in long traffic jams would directly expose people to radiation, which would be the most intense on the day after the bomb goes off. After that, the amount would drop off day by day (one third as strong after three days, one fifth as strong after five days, and so on), because of the natural decay of the radioactive components of the fallout.

More tough decisions would arise later. People downwind could leave their homes or stay, leave for a while and then come back or leave and come back briefly to retrieve valuables. The choices would be determined by the dose of radiation they were willing to absorb. Except in the hot zone around the blast and a few miles downwind, even unsheltered people would not be exposed to enough radiation to make them die or even become sick. It would be enough only to raise their statistical chance of getting cancer later in life from 20 percent (the average chance we all have) to something greater--21 percent, 22 percent, up to 30 percent at the maximum survivable exposure.

Similar choices would face first responders and troops sent to the stricken area: how close to ground zero could they go, and for how long? Few would choose to have their risk of death from cancer go up to 30 percent. But in cases of smaller probabilities--an increase to 20.1 percent, for example--a first responder might be willing to go into the radiation zone, or a resident might want to return to pick up a beloved pet. These questions could be answered only by the individuals themselves, based on information about the explosion.

Next comes the unpleasant fact that the first nuclear bomb may well not be the last. If terrorists manage to obtain a weapon, or the fissile material to make one (which fits into a small suitcase), who's to say they wouldn' have two or three more? And even if they had no more weapons, the terrorists would most likely claim that they did. So people in other cities would want to evacuate on the day after, or at least move their children to the countryside, as happened in England during World War II.

The United States government, probably convened somewhere outside Washington by the day after, would be urgently trying to trace the source of the bombs. No doubt, the trail would lead back to some government--Russia, Pakistan, North Korea or other countries with nuclear arsenals or advanced nuclear power programs--because even the most sophisticated terrorist groups cannot make plutonium or enrich their own uranium; they would need to get their weapons or fissile materials from a government.

The temptation would be to retaliate against that government. But it might not even be aware that its bombs were stolen or sold, let alone have deliberately provided them to terrorists. Retaliating against Russia or Pakistan would therefore be counterproductive. Their cooperation would be needed to find out who got the bombs and how many there were, and to put an end to the campaign of nuclear terrorism. It is important to continue to develop the ability to trace any bomb by analyzing its residues. Any government that did not cooperate in the search should of course face possible retaliation.

Finally, as buildings and lives were destroyed, so would the sense of safety and well-being of survivors, and this in turn could lead to panic. Contingency plans for the day after a nuclear blast should demonstrate to Americans that all three branches of government can work in unison and under the Constitution to respond to the crisis and prevent further destruction.

A council of, say, the president, the vice president, the speaker of the House and the majority leader of the Senate, with the chief justice of the Supreme Court present as an observer, could consider certain aspects of the government's response, like increased surveillance. Any emergency measures instituted on the day after should be temporary, to be reviewed and curtailed as soon as the crisis ends.

Forceful efforts to prevent a nuclear attack--more forceful than we have seen in recent years--may keep the day from coming. But as long as there is no way to be sure it will not, it is important to formulate contingency plans that can save thousands of lives and billions of dollars, prevent panic and promote recovery. They can also help us preserve our constitutional government, something that terrorists, even if armed with nuclear weapons, should never be allowed to take away.

William J. Perry, a professor at Stanford, and Ashton B. Carter, a professor at Harvard, were, respectively, the secretary and an assistant secretary of defense in the Clinton administration. Michael M. May, also a professor at Stanford, is a former director of the Lawrence Livermore National Laboratory.

Siegfried S. Hecker, a prominent U.S. expert on nuclear technology and policy, was appointed co-director of the Center for International Security and Cooperation, in the Freeman Spogli Institute for International Studies at Stanford University on Jan. 16. He also assumed positions as a professor (research) in the Stanford School of Engineering's Department of Management Science and Engineering and a senior fellow at FSI.

Hecker's "scientific achievements as a metallurgist, his leadership and talent as the head of a renowned U.S. Department of Energy laboratory and his decades-long dedication to improving global security make him an extraordinary choice to help direct CISAC in the years ahead," FSI Director Coit D. Blacker said, announcing the appointment.

Political science Professor Scott Sagan, whom Hecker joins as a co-director of CISAC, said he is "thrilled to have Sig Hecker as a partner" in leading the center. "Hecker follows in a long line of distinguished scientists--Sidney Drell, William Perry, Michael May, and Christopher Chyba--who have become leaders of CISAC's efforts to produce cutting edge policy-relevant research," Sagan noted. "Stanford University is extremely fortunate to be able to have a scholar-practitioner of Sig Hecker's stature coming to CISAC to help guide our multidisciplinary efforts to address the tough security challenges facing the world right now."

The center, traditionally co-directed by a scientist and social scientist since its founding in 1983 by physicist Drell and political scientist John Lewis, draws from a range of disciplines to focus on current problems in international security.

An emeritus director of Los Alamos National Laboratory, Hecker has fostered U.S. cooperation with Russian nuclear laboratories for 15 years to secure the vast stockpile of former Soviet nuclear weapons and materials. At CISAC, where he has been a visiting professor since fall 2005, Hecker has contributed to international projects to prevent the spread of nuclear weapons and secure materials for making them.

Looking forward to the new assignments, Hecker said, "I have enjoyed the Stanford environment--the students, faculty, and the great range of international issues being examined. I look forward to the new challenge of leading CISAC with Scott Sagan, as well as teaching and research in management science and engineering."

With Lewis, Hecker has made three visits to the Democratic People's Republic of Korea in the last three years, gaining rare access to and expertise on North Korea's nuclear weapons program. His reports on the program's status provide valuable insights to U.S. diplomats and scholars seeking to resolve the nuclear crisis on the Korean peninsula. With Sagan, Hecker has participated in meetings with security experts from China, India, Pakistan, Russia, and the United States to secure nuclear weapons and materials and lessen tensions in South Asia.

Last fall, Hecker co-taught Stanford's popular management science and engineering course, Technology and National Security, with CISAC and MS&E colleague Perry. Hecker lectured on nuclear weapons history and technical fundamentals, nuclear terrorism, and North Korea.

"Dr. Hecker has added 'outstanding professor' to his list of many accomplishments," Perry said. "I am delighted he has accepted this appointment and look forward to working with him."

The nuclear regime in place since the Nonproliferation Treaty (NPT) took effect in 1970 is broken and needs to be fixed, the world's highest-ranking nuclear official told a half-full Memorial Auditorium in a wide-ranging lecture about the future of nuclear energy and weapons yesterday afternoon.

"We have a dysfunctional system -- system that cannot endure," said Mohamed ElBaradei, the director general of the International Atomic Energy Agency (IAEA). "We're reaching the fork in the road. Events in the last few years have made it clear that we need to change course."

The big news from ElBaradei's speech was his support for American entreaties to Iran. But the 2005 Nobel Peace Prize recipient also commented on North Korea, India, Pakistan, Iraq, terrorism, disarmament and the future of nuclear energy.

He said there are probably eight current nuclear states, excluding North Korea. He worried aloud that countries which can currently produce nuclear energy peacefully are only six months away from developing nuclear weapons for military purposes.

"Acquiring the technology to enrich uranium or reprocess uranium basically is the key to develop nuclear weapons as we have seen in Asia and Iran," he said. "They are virtually weapons states because in six months time if they decide for security reasons to develop their own weapon, they are there."

Iraq

While not a household name, ElBaradei was a prominent figure in the news as the lead weapons inspector in Iraq during the run-up to the 2003 American invasion. He said at the time that he could not find evidence of Iraqi weapons of mass destruction, but he would not conclude that there were no weapons in the country or that Saddam Hussein did not have a program.

ElBaradei asked for more time to complete inspections, but the Bush administration declined his request and decided to invade. The U.S. never found the nuclear and biological material that some had promised existed.

"Luckily...well I'm not sure luckily...we were proven right that there was no nuclear or any weapon of mass destruction in Iraq," he said. "But I hope that all of us have learned from the Iraq experience that we cannot just jump the gun. You have to be absolutely sure of the facts."

India

ElBaradei surprised observers when he supported the U.S. agreement with India earlier this year, which allowed the country to continue developing nuclear weapons and energy. He said the agreement with India did not endorse its proliferation activities but was indicative of the kind of outside-the-box thinking the international community needs when considering the spread of nuclear weapons and material.

"The end result is India coming closer and working with the rest of the world," he said. "It is not a perfect agreement, but it has a lot of advantages. From the safety, security and nonproliferation perspective, I see that agreement as a win-win situation."

Pakistan

Pakistan developed nuclear weapons as a response to India. Some have criticized Pakistan for its poor stewardship and control of the bomb, pointing out that weapons were almost fired during a skirmish over the disputed Kashmir region.

AQ Kahn, a senior nuclear scientist who helped Pakistan join the exclusive nuclear club, was caught selling compact discs and other information about bombs to several other countries.

"How much damage was done in the process we don't know," ElBaradei said.

The release of this nuclear material demonstrates the need for a "more robust verification system," he said, adding that Pakistan has come closer to the international community in recent years.

North Korea

Kim Jong Il expelled all IAEA inspectors in Dec. 2002, withdrew from the NPT in Jan. 2003 and announced in February 2005 that his military had a nuclear deterrent.

"North Korea is still a major problem," ElBaradei said. "We don't talk about it enough, but North Korea is declaring right now that they have a nuclear weapon. And the longer that they continue to be in that status, the more it is accepted in the collective conscious. This would be terrible because it will have a lot of negative ramifications in South Korea and Japan."

ElBaradei said ongoing negotiations are an important development but more needs to be done.

"What we see with the current six-party talks should have taken place years ago," he said.

Nuclear Proliferation

ElBaradei stressed that he understands the value of nuclear power, which produces much of the developed world's energy. Reducing its use would create more dependence on greenhouse gas-creating fossil fuels, he said.

"We need to use nuclear energy responsibly to maximize benefit and minimize risk," he said.

He said his "number one nightmare scenario" is a terrorist group acquiring nuclear technology since terrorists are not deterred by the possibility of reprisal.

In the post-Cold War world, ElBaradei said he could see no justification for the U.S. and Russia to maintain their nuclear arsenals on ready alert to fire with thirty minutes notice. He called on America to lead by example and continue to disarm its nuclear stockpile.

"Rather than pass judgment, I'd definitely like to say the U.S. should do more in leading by example in terms of nuclear disarmament," he said.

In September 2005, ElBaradei was reappointed to a third term as director general of IAEA. The United States had considered holding up his nomination but dropped its objections under pressure from European allies, who admire the former law professor from New York University.

Diplomat to the Core

The Egyptian native's sometimes broken English was interspersed with self-corrections and careful legalese nuance. ElBaradei answered questions posed by Political Science Prof. Scott Sagan, the director of the Center for International Security and Cooperation.

When Sagan made bold pronouncements about different country's nuclear activities, including the United States' "colossal failure" when North Korea violated the NPT, ElBaradei seemed careful not to point fingers, play the blame game or make enemies. Nonetheless, for a senior United Nations official, his speech was notably blunt.

"There's no international public servant whose integrity and work I admire more than yours," Sagan told ElBaradei.

Accompanied by his wife, ElBaradei spent the day at the University visiting with faculty and students. He spoke at a lunch sponsored by the Freeman Spogli Institute for International Studies and traveled to Sagan's home for a dinner with invited guests. He left the area at 8:30 p.m.

Before attacking Iraq in March 2003, the United States told the world that Saddam Hussein had reconstituted his nuclear weapons program in defiance of the United Nations. That claim, used to justify the war, was based on assessments provided by the United States intelligence community. But as everyone now knows, those assessments were wrong. So Jeffrey T. Richelson's history of American nuclear intelligence, including our attempts to learn about Iraq's nuclear program, could hardly be more timely.

In "Spying on the Bomb," Richelson, the author of several books on American intelligence, has brought together a huge amount of information about Washington's efforts to track the nuclear weapons projects of other countries. He examines the nuclear projects of Nazi Germany, the Soviet Union, China, France, Israel, India, South Africa, Taiwan, Libya, Pakistan, Iran and North Korea, as well as Iraq. Through interviews and declassified documents as well as secondary works, he sets out briefly what we currently know about those projects and compares that with assessments of the time.

This may sound like heavy going, but Richelson writes with admirable clarity. And along the way he has fascinating stories to tell: about plans to assassinate the German physicist Werner Heisenberg during World War II; about discussions in the Kennedy and Johnson administrations on the possibility of attacking Chinese nuclear installations; about Indian measures to evade the gaze of American reconnaissance satellites; and about the bureaucratic infighting over the estimates on Iraq.

The United States has put an enormous effort into gathering information about the nuclear projects of other countries. After World War II it equipped aircraft with special filters to pick up radioactive debris from nuclear tests for isotopic analysis. It created a network of stations around the world to register the seismic effects of nuclear explosions. Most important, in 1960 it began to launch reconnaissance satellites that could take detailed photographs of nuclear sites in the Soviet Union and China. Richelson occasionally speculates about the role of communications intercepts and of spies, but these appear from his account to have been much less important than the other methods of collecting information.

Through these means the United States has gathered a vast quantity of data, sometimes to surprising effect. Intelligence played a crucial role in the cold war, for instance, by reducing uncertainty about Soviet nuclear forces. Alongside such successes, however, there have been failures. One notable example concerned the first Soviet test, which took place in August 1949, much sooner than the C.I.A. had predicted. Another was the failure to detect Indian preparations for tests in May 1998, even though at an earlier time the United States, with the help of satellite intelligence, had managed to learn about preparations the Indians were making and to head off their tests.

But the most serious failure of all was in Iraq in 2003, because in no other case did the intelligence assessments serve as justification for the use of military force. The information needed for avoiding political surprise is one thing. That needed for preventive war is quite another, if only because of the consequences of making a mistake.

Beyond making the uncontroversial recommendation that "aggressive and inventive intelligence collection and analysis" should continue, Richelson draws no general conclusions. That is a pity, because his rich material points to issues that cry out for further analysis. He suggests in one or two cases that failures sprang from the mind-set of the intelligence community, but he does not elaborate on this point. He has little to say about relations between policy makers and the intelligence community, even though the quality of intelligence and the use made of it depend heavily on that relationship.

His focus is no less narrow in his discussion of foreign nuclear projects. He concentrates on the programs themselves, paying very little attention to their political context. Does that reflect a technological bias in nuclear intelligence? Would, for example, the prewar assessment of Iraqi nuclear capabilities have been more accurate if it had paid more attention to the broader political and economic circumstances of Hussein's regime?

The task of intelligence has become more complex than it was during the cold war. A single dominant nuclear opponent has now been replaced by a number of nuclear states, along with states and stateless terrorists that are aiming to get their hands on nuclear weapons. Meanwhile, the technology needed for producing nuclear weapons has become easier to acquire.

Many critics believe the recent performance of the intelligence community shows it has not responded adequately to this new situation. Richelson does not have much to say on this question; nor does he discuss the likely impact of the current reforms, initiated in response to the Iraq war, on the quality of intelligence. His reticence may imply that he does not think reform is necessary. Still, it is disappointing that he does not draw on his historical survey to discuss whether new approaches are needed for dealing with nuclear threats, and, if so, what those new approaches might be.