Princeton scientists combine technical expertise and policy savvy to help prevent nuclear disaster
Researchers in Princeton’s Program in Science and Global Security: Sitting, from left, Chris Chyba, Frank von Hippel, Laura Kahn *02, and Alex Glaser. Standing, from left, Zia Mann and Harold Feiveson *72.
Researchers in Princeton’s Program in Science and Global Security: Sitting, from left, Chris Chyba, Frank von Hippel, Laura Kahn *02, and Alex Glaser. Standing, from left, Zia Mann and Harold Feiveson *72.
Ricardo Barros

Take, for example, the yellowing memo, framed in black, that hangs near the elevator on the second floor — the home of Princeton’s Program in Science and Global Security. The 1940 document, covered in the red strokes of the declassifier’s pen, was written by Otto Frisch and Rudolf Peierls, two scientists at the University of Birmingham in England. They had just made a disturbing discovery: It takes only a tiny chunk of uranium to make a nuclear bomb.  

At the time, physicists already had discovered chain reactions, the runaway process that propels a nuclear weapon’s massive explosions. But many top minds thought that the amount of the rare isotope uranium-235 needed for an explosive chain reaction was in the realm of many tons, and thus was a distant problem. Another famous letter, one in 1939 from Albert Einstein to President Franklin Roosevelt, had warned that uranium chain reactions could be used for a bomb, but suggested it would have to be hauled by ship, and would be too heavy to be carried in a plane.   

The Frisch and Peierls memo, with its typewritten calculations, showed otherwise. The amount of uranium-235 needed for a viable bomb, they argued, was just 1 kilogram — well within the reach of technology of the time. “They erred on the low side,” says Harold Feiveson *72, who founded Princeton’s global security program with physicist Frank von Hippel in 1974. But Frisch and Peierls weren’t off by much: A viable bomb requires only about 50 kilograms of uranium (the bomb that destroyed Hiroshima used 64 kilograms).   

Feiveson says the memo is famous less for its calculations — which were almost right — than for its political ramifications. The document created a sense of urgency that led to the Manhattan Project, and set science running squarely down the path that would lead to the arms race between the United States and the Soviet Union, with all its nuclear terrors. Indirectly, then, that memo also set the stage for the people who, 70 years later, sit in the second-floor offices that surround it. Like Frisch and Peierls, these Princeton scientists grapple with the frightening consequences of science run amok. “We are,” Feiveson says, “a combination of worriers and optimists.”  

Feiveson and his colleagues aim to offer ­policymakers in Washington independent and technically rigorous analysis of potentially devastating technology — and of the policies developed to combat those risks. Nuclear terrors are not the only worry, notes astrophysicist Christopher Chyba, who arrived in 2005 from Stanford University, where he led its Center for International Security and Cooperation, and became director of Princeton’s program in 2006. As a result, program researchers also are studying risks related to biological weapons and seeking ways to improve epidemiological surveillance (see PAW’s interview with Laura Kahn *02, a research scholar in the program, in the Nov. 4, 2009, issue). Chyba is particularly concerned about dual-use biotechnology, where the ability to manipulate genomes can be used for both good and evil. “There are half a dozen very powerful examples of that in the literature already, where people have made things much more dangerous than they had been before,” he says. “That technology is going to become much more widely available.”  

Historically, the bulk of the research group’s work centered on nuclear proliferation, a focus that continues today. The end of the Cold War left thousands of tons of weapons-grade uranium and plutonium lying in stockpiles, enough for tens of thousands of new bombs. Though the United States and Russia years ago stopped adding fresh materials to these stockpiles, they still present risks, since some of the depots are more secure than others. Meanwhile, a growing number of nations are pursuing the infrastructure needed to produce their own stockpiles of uranium and plutonium, either to power civilian reactors — or to make weapons. In 2006, North Korea became the ninth state to test a nuclear bomb; whether, or when, Iran becomes the 10th is one of the more unnerving questions rattling through Washington.   

These stockpiles of uranium and plutonium are called fissile materials, because they have the ability to undergo sustained fission reactions. In fission, a neutron splits the nucleus of an atom into pieces, releasing energy. When fissile materials are split, they emit more neutrons than they absorb, so a chain reaction can occur. There are many approaches to reducing nuclear threats — from treaties that reduce the number of weapons to agreements that discourage their being built in the first place. But many in the Princeton program are working toward establishing the basis for a treaty that would limit something more fundamental: the ingredients of a nuclear weapon. “We are all concerned that one day nuclear weapons may explode one way or another,” says Alex Glaser, who became an assistant professor in the program this year. “If you can control and manage fissile materials, you can solve all these problems at once.”  

To that end, the Princeton group administers the Inter­national Panel on Fissile Materials (IPFM), an independent group of arms-control and nonproliferation experts that makes nation-by-nation estimates of civilian and military stockpiles of fissile materials. The panel is co-chaired by von Hippel, and other Princeton program members contribute to the panel’s annual reports. (The most recent report appeared in October, and is posted on the panel’s Web site at Through their work with the IPFM, the researchers hope to illustrate the need for a treaty forbidding the use of fissile materials to create weapons and to show that such a treaty could be verified. The Princeton scientists know the odds are long — inertia has reigned since a 1993 United Nations consensus statement advocated such a treaty, commonly called the Fissile Material Cutoff Treaty.  

The five original nuclear-weapon states (the United States, Russia, China, the United Kingdom, and France) already have unofficially stopped adding to fissile-material stockpiles intended for weapons; one version of the treaty supported by the United States would turn those moratoria into binding commitments and, if possible, add Pakistan, India, and Israel to the mix. In its own version of a treaty, released in January, the panel argues for a more stringent alternative to the U.S.-supported draft, one that would ban not only production of new fissile materials for weapons, but also weapons production from existing stocks. That includes weapons production from existing non-weapon stocks — the reserve fissile materials used to power Navy submarines, for instance.

“The U.S. government has decided not to agree with us on that,” says von Hippel. “They don’t think it will fly with the other members of the permanent [U.N.] Security Council.” But if a treaty doesn’t bar the production of weapons from existing stocks, von Hippel says, then any progress in reducing the number of nuclear weapons could be reversed.  

Only a verifiable treaty has any chance of being signed — a formidable challenge. One problem: How to distinguish a newly created piece of weapons-grade uranium from uranium that had been created before the treaty was signed? The Princeton scientists have shown it is possible to date the uranium, based on the remaining quantities of certain radioactive decay products. Another approach that they have championed is the remote “interrogation” of a sample of uranium or plutonium. By looking for specific peaks and valleys in the spectrum of gamma-rays given off by a sample — even if it’s hidden in a container, as many nations would require for classification purposes — it could be determined whether the sample is the enriched form required for weapons. Finally, the scientists have shown that there could be ways to detect minute quantities of radioactive materials in the atmosphere that could indicate a leak from a clandestine production facility. Von Hippel says the panel has studied the challenges of verifying a Fissile Material Cutoff Treaty, and shown that the issues are very similar to those faced in verifying the Nuclear Nonproliferation Treaty, a landmark treaty signed in 1970 to limit the spread of nuclear weapons. “We’re not going to give up on this,” he says.  

Von Hippel came to physics naturally. His grandfather was James Franck, a Nobel Prize-winning physicist and member of the Manhattan Project; his father, Arthur von Hippel, left Germany in 1934 and went to the Massachusetts Institute of Technology, where he helped with the war effort to develop radar. Von Hippel’s middle name — Niels — recalls his parents’ friendship with Niels Bohr, the father of the modern picture of the atom and one of the most eminent physicists of the 20th century.  

As impressive as his grandfather’s achievements in physics were, von Hippel is proudest of a report that his grandfather compiled and took to Washington in 1945, one that correctly predicted that an arms race “will be on in earnest no later than the morning after our first demonstration of the existence of nuclear weapons.” Franck and his colleagues proposed that the first bomb be demonstrated in a desert or on an island, as a way to begin brokering an international agreement abolishing the weapons. Franck obviously wasn’t successful, but the seed of a new type of scientific career was sown in von Hippel’s mind. “There was this idea: You became a famous scientist so that people will listen to you,” von Hippel says.  

He did, following his father to MIT and earning a physics Ph.D. at Oxford University. When he arrived at Princeton to start the Science and Global Security Program with Feiveson, the oil crisis of 1973 was in full swing, and the Cold War was going strong. Then, as now, nuclear reactors were being considered as a way of ensuring a reliable domestic energy source; because the chain reactions that fuel both reactors and bombs rely on the same materials, that question raised high-stakes security issues.  

For three decades, von Hippel’s main mission has been to keep plutonium out of the civilian nuclear-power cycle. Many radioactive isotopes are produced as waste products in uranium reactors. Reprocessing plants allow plutonium-239 to be extracted from the waste, which can power a different breed of reactors. But the plutonium also can be siphoned off into weapons production. Believing that the security threat alone won’t halt reprocessing, von Hippel also has made an economic argument: His analyses have shown that plutonium reprocessing, given the overall cost of the reprocessing plants, would make economic sense only if the costs of raw uranium were to increase dramatically and drive up the the cost of a traditional uranium reactor. The administration of President Jimmy Carter was convinced, and the United States abandoned plutonium-reprocessing efforts at a time when it could have gone the other way. Once that happened, von Hippel says, it was easier to encourage other nations, such as the United Kingdom, to follow suit. But France, India, Japan, and Russia continue to reprocess plutonium, and the professor continues to try to prevent others from taking that route. Last spring, he traveled to China to meet with officials to convince them not to reprocess plutonium. But it’s an uphill battle. China already has a pilot reprocessing plant, and Pakistan and North Korea have committed to reprocessing as their nuclear-power route of choice.   

While von Hippel concentrates on the problems of plutonium, Glaser, the global security program’s newest faculty member, ponders what to do with the other problematic material: uranium-235. A nuclear bomb made with uranium-235 can be detonated simply by smashing two large pieces of it together. Manhattan Project scientists were so confident with the easy design that the uranium bomb dropped on Hiroshima never was tested, Glaser points out. (The Trinity test was used for the more complicated implosion design of the plutonium bomb dropped at Nagasaki.)  

Thankfully, nature presents a roadblock. Uranium-235 exists as only 0.7 percent of natural uranium ore, which is made up almost entirely of the heavier, and non-fissile, isotope uranium-238. Uranium-235 has to be enriched — no easy task. During World War II, the United States used a laborious gas-diffusion process in which a gaseous form of uranium is pushed through porous barriers; the lighter uranium-235 is slightly more likely to cross. Nowadays, enriched uranium is created through thousands of interconnected, quickly spinning centrifuges that separate the heavier uranium isotope from the lighter one.   

Commercial nuclear-power reactors have no need for the highly enriched uranium, which means that even if their non-weapons-grade material were stolen, it would be of no use for weapons unless it was enriched further. To Glaser’s dismay, however, there are a set of research reactors — operated primarily by universities (not Princeton) — that operate on bomb-grade uranium. “You want to completely avoid the use of weapons-grade materials in the civilian nuclear-fuel cycle,” he says. The research reactors are not used to produce energy but, rather, a beam of neutrons that are then used to study the structure of materials, or to produce special radioactive isotopes useful in medical procedures.

Glaser says that these reactors, even though they use materials of the highest concern, are much less secure than commercial reactors. An ABC News investigation a few years ago pointed out security lapses at the research reactor at MIT, for example. And yet conversion to using low-enriched uranium is possible; Glaser has created a design that would allow research reactors to use low-enriched uranium with only a negligible loss in performance. Still, a 2004 study from the U.S. Govern­ment Accountability Office — the most recent available — found that the Department of Energy was making slow progress: Of 105 research reactors targeted for conversion in the United States and abroad, only 39 had been converted.   

Ted Postol, director of the Science, Technology, and Global Security Group at MIT — one of the few universities that has a program like Princeton’s — says scientists like von Hippel and Glaser have done a service in raising awareness about the dangers of fissile materials. “I think the Princeton group can properly take credit for enormous influence in the general thinking among policymakers about the potential for the diversion of nuclear energy-oriented technology toward weapons production,” he says.   


Chyba, who in the 1990s served on the White House national security staff, understands well that the likelihood of taking even incremental steps to curtail fissile materials depends not just on the strength of the technical analyses, but also on how well those assessments are communicated to policymakers. In June, he was appointed to the President’s Council of Advisers on Science and Technology, a group of 18 scientists who meet with the president every few months. And indeed, undermining stereotypes of scientists in labs, most of the Princeton researchers are well-versed in the political arts. They are extroverts, not at all averse to donning suits and trekking to Washington, D.C. “Making a difference is the aspiration,” says Chyba. “The proof of that is in the pudding.”  

Recently, there have been some encouraging signs. Glaser says he has been receiving more requests for reports by the fissile-materials panel since President Obama, in an April speech in Prague, called for a cutoff treaty to be signed. “Suddenly, you get phone calls from D.C. saying, we need this report,” Glaser notes. “Suddenly, you give the same briefings and the room is packed.”    

The researchers are the latest in a long line of what von Hippel refers to as “citizen scientists” — a lineage born of necessity after the Frisch-Peierls memo made clear the deadly reach of nuclear technology, a threat that grows with the ever-expanding sphere of potentially dangerous technology. The lineage includes that most famous scientist of the 20th century, Albert Einstein, who became politically involved, especially once he set up shop in Princeton at the Institute for Advanced Study. There, he found many pacifist causes to support — including one that resulted in another framed document that hangs in the second-floor lobby of 221 Nassau St., next to the Peierls and Frisch uranium memo.  

In 1955, just a few days before his death, Einstein, along with philosopher Bertrand Russell, signed what’s now known as the Russell-Einstein Manifesto in which the two thinkers ruminated on the horrific power of the thermonuclear H-bombs that recently had been invented. “We have found that the men who know most are the most gloomy,” they wrote.  

So what prompts the researchers on the second floor to continue to plod on? You’d think that von Hippel, a warrior in the movement against nuclear proliferation since the 1970s, would feel glum about the state of affairs. But he describes himself as a “glass is half-full person,” and says that though the world certainly is scary, there are reasons for hope. For every nation that has ignored treaties and pursued nuclear weapons (North Korea, Pakistan), there are many more — Sweden, Switzerland, South Korea, Australia — who have considered developing nuclear weapons but ignored that siren call. That’s why von Hippel says the Fissile Material Cutoff Treaty is so important.  

The world’s stockpiles of fissile materials are certainly not empty — and may never be. But as they become a bit emptier, the world becomes a little bit safer — making von Hippel’s glass that much more full. 

Eric Hand ’97 is a writer for the science journal Nature.