On the desolate steppe of eastern Kazakhstan, the Soviet Union carried out 456 nuclear explosive tests during the Cold War at the Semipalatinsk Test Site, which sprawls over an area approximately the size of Belgium. Of these, the Soviet Union performed 116 tests in the atmosphere, and 340 underground. While some of the nuclear tests at Semipalatinsk involved atomic explosions, other experiments were designed to study the impact of conventional explosives on plutonium and highly enriched uranium (HEU), the fissile materials used in nuclear bombs, or to ensure the safety of nuclear weapons during a simulated accident such as a fire or nearby explosion.

Some of these tests—particularly tests involving plutonium—did not vaporize the material in a nuclear blast. It remained in tunnels and containers, in forms that could be recovered and recycled into a bomb. In addition, the Soviet Union discarded equipment that included high-purity plutonium that would have provided materials and information that could lead to a relatively sophisticated nuclear device if it had been found.

When scientists and military personnel withdrew from Kazakhstan following the collapse of the Soviet Union, they abandoned tunnels and bore holes filled with plutonium residue—enough plutonium, if fully reclaimed, for terrorists or a state to construct dozens of nuclear bombs. Between 1991 and 2012, scavengers looking for valuable metal and equipment from the former Soviet test site came within yards of the unguarded fissile material; in two cases the scavengers broke into the vessels used to contain some of the experiments, although there is no evidence that they removed any plutonium.

In October, 2012, at the foot of a rocky hillside, at a spot known as Degelen Mountain, several dozen Kazakh, Russian, and American nuclear scientists and engineers gathered for a small ceremony that marked the completion of a 17-year, $150 million operation to secure the plutonium in the tunnels of Degelen Mountain and in surrounding bore holes by filling portions of the tunnels and holes with a special concrete, greatly reducing one of the largest nuclear security threats since the collapse of the Soviet Union. They unveiled a three-sided stone monument, etched in English, Russian, and Kazakh, which declared: “1996-2012. The world has become safer."

The story of the operation at Semipalatinsk is a tale of scientists working together to achieve real results in reducing nuclear threats. It began in 1995, after the collapse of the Soviet Union, when experts from the Los Alamos National Laboratory were told during a visit to Kazakhstan that plutonium residue in recoverable form was likely to have been abandoned at the test site. Two years later, in 1997, Siegfried S. Hecker, just retiring as director of Los Alamos, decided to look more closely. Hecker, who helped pioneer cooperation with his counterparts in the Soviet and later Russian nuclear weapons laboratories, used personal connections to push for action. He succeeded in enlisting the cooperation of Russian nuclear scientists who had been involved in the testing program in Kazakhstan before the Soviet collapse.

Scientists and engineers from the United States, Russia, and Kazakhstan overcame deep-rooted suspicions in their governments to find technical solutions to the plutonium threat at Degelen Mountain. The operation was based almost entirely on ad-hoc agreements struck at a lower level, and was carried to a conclusion without elaborate, negotiated state-to-state agreements of the kind used for arms control during the Cold War. Each country played a crucial role.

The operation took 17 years to complete, a period in history that saw the rise of al-Qaeda and its nuclear ambitions, the 9/11 attacks, and spanned three different U.S. administrations, the latter two of which proclaimed nuclear terrorism the greatest threat to U.S. security and spent billions of dollars to prevent it. Serious logistical difficulties were just one reason the Semipalatinsk operation took so long. Another was lingering mistrust, inertia and excessive secrecy. Until prodded by the scientists, the governments had done very little about the plutonium abandoned at the test site.

The informal approach was effective, but slow, and it left some issues unresolved or partly unexplored. For example, although Kazakhstan signed the 1968 Nuclear Non-proliferation Treaty in 1993 and had a legal obligation to declare all the fissile material on its territory, the three countries overseeing the operation decided not to formally notify or involve the International Atomic Energy Agency. Each country felt it had good reasons for excluding the IAEA. They shared a concern that information leaks from the agency would compromise the secrecy of the operation. Now, the IAEA and Kazakhstan face long-term uncertainties about whether the material can be safely left in its current form forever, or whether it may pose a risk that someone might seek to recover it someday, or that it could cause environmental contamination beyond the radiation already polluting some portions of Semipalatinsk.

This paper begins with a narrative reconstruction of the threat reduction work at Semipalatinsk. It is based on interviews with U.S., Russian, and Kazakh scientists and officials—conducted in the United States and Kazakhstan—and is supported by documents provided to the authors. It concludes with observations about the lessons for future efforts to reduce and ultimately eliminate the threat posed by nuclear weapons and the essential nuclear materials needed to construct them.