Nuclear Experts Assess the Threat of a "Backyard Bomb"

Peter D. Zimmerman (left) and Jeffrey G. Lewis

With the correct expertise and access to highly enriched uranium, terrorists could build a crude atomic bomb in the United States for less than $10 million, according to two nuclear specialists.

During a briefing for reporters co-sponsored by AAAS's Center for Science, Technology and Security Policy and the Center for Media and Security, Peter D. Zimmerman and Jeffrey G. Lewis outlined a disturbing scenario in which terrorists build a nuclear device on a 150-acre ranch in a remote section of the United States rather than attempting to smuggle it fully assembled into the country. The bomb, once assembled, is likely to be less than nine feet long. They then transport the device in a van or small panel truck to a heavily populated target area such as lower Manhattan or Washington, D.C.

Zimmerman, formerly chief scientist for the U.S. Senate Foreign Relations Committee, is now with the Department of War Studies at King's College, London. Lewis is director of the Managing the Atom Project at Harvard University's Belfer Center for Science and International Affairs. They first detailed the steps in the November/December issue of Foreign Policy magazine.

Al Qaeda already has made at least one attempt to purchase highly enriched uranium from a military officer in Sudan for $1.5 million, Lewis said at the 7 December briefing, but the material turned out to be bogus. When asked about the rationale for the purchase, one of the al Qaeda henchmen explained: "It's easy to kill more people with uranium." His remark was cited in the Final Report of the 9/11 Commission.

A "backyard atomic bomb" is a low-probability event, but Zimmerman said there is at least a 10 per cent chance of it happening in the years to come. He and Lewis say the job of building such a device would not be easy or inexpensive. Nevertheless, they said it would be "conceptually simple" once terrorists obtain the key ingredient—fissile material such as highly enriched uranium. Zimmerman and Lewis said there is still much work to be done to secure bomb-grade nuclear materials in the former Soviet Union and elsewhere and to prevent those with nuclear expertise from offering their services to terrorist groups.

Zimmerman said he is optimistic that the nuclear powers could secure the material, though he and Lewis acknowledged the threat that a couple of insiders could help smuggle materials out of a weapons facility.

In the "backyard bomb" scenario, terrorists choose a crude but well-known design concept in which a gun fires a bullet of highly enriched uranium into a target, also made of highly enriched uranium. The design is similar to the "Little Boy" bomb that the United States used to destroy Hiroshima in 1945. Zimmerman estimates that a successful backyard bomb could kill 100,000 people or more if exploded in a crowded city.

In their cost estimate, Zimmerman and Lewis assume that most of the money—$4 million—would go to acquiring highly enriched uranium on the black market, though terrorists might also try to steal the material from a poorly guarded facility or buy it from a rogue nation that is a willing supplier.

The building of a backyard bomb would not require large numbers of technicians, according to Zimmerman and Lewis. They argue that no more than 19 people—the same number who hijacked planes on 9/11—could accomplish the feat over the course of a year. Most of them would be technicians with skills in areas such as metallurgy and casting, machining and electronics. The team leader, a senior physicist, would design the bomb assembly with the help of two post-doctoral students.

The core of the weapon could be fabricated quickly once enough highly enriched uranium is obtained. "When China built its first nuclear bomb in 1964," Zimmerman and Lewis write in their Foreign Policy account, "a single technician named Yuan Gongfu used a lathe to shape the highly enriched uranium in just one night. New or used lathes large enough to properly finish the roughly cast pit can be bought on the Internet, even on eBay, for $10,000."

Some critics have expressed skepticism about Zimmerman and Lewis's scenario. Milton Leitenberg, a chemical and biological weapons specialist at the University of Maryland, told a reporter for United Press International that the authors had underestimated the difficulty of finding the kinds of highly qualified specialists the project would require. The small size of the proposed team also would mean that in each of a dozen or more categories of expertise, "you would have to find someone with the absolute optimal skills," Leitenberg argued.

Another weapons specialist told UPI there is a good chance that a crude device would be subject to pre-detonation, in which the nuclear chain reaction starts too soon. The result would be a low-yield "fizzle."

Zimmerman said that even a fizzle could kill thousands. He also disputed Leitenberg's contention that each of the bomb builders would have to be exceptionally talented. "These are solved problems," Zimmerman said. The engineering is straight-forward and so is the design work, he said. "You need a few good people, but not very many," he said.

In their article, Zimmerman and Lewis note that "only a very small number of terrorist groups fit the profile we have outlined: interested in mass casualties, well financed, and organizationally sophisticated. We have identified only two groups in recent history with all these qualities: al Qaeda and Aum Shinrikyo," the Japanese religious group that carried out a sarin gas attack in the Tokyo subways in 1995. To date, they say, al Qaeda does not seem to have developed the necessary technical expertise to obtain the right bomb material and the right expertise. "Bin Laden, perhaps, has yet to find his Robert Oppenheimer," they wrote.

To prevent him from doing so, governments must continue to restrict funding to terrorist groups and use good intelligence methods to break up terrorist plots, Zimmerman and Lewis say. But groups intent on killing as many people as possible for as little money as possible will still find the allure of the atomic bomb irresistible, they say. "Just because a nuclear terrorist attack hasn't happened shouldn't give us the false comfort of thinking it won't," they conclude.

Earl Lane

19 December 2006