Working in top secret over a period of 17 years, Russian and U.S. scientists collaborated to remove hundreds of pounds of plutonium and highly enriched uranium — enough to construct at least a dozen nuclear weapons — from a remote Soviet-era nuclear test site in Semipalatinsk, Kazakhstan that had been overrun by impoverished metal scavengers, according to a report released last week by the Belfer Center for Science and International Affairs at Harvard.

The report sheds light on a mysterious $150-million cleanup operation paid for in large part by the U.S. Defence Department, whose nuclear scientists feared that terrorists would discover the fissile material and use it to build a dirty bomb.

Over the years, hints emerged that something extraordinarily dangerous had been left behind in a warren of underground tunnels — like the U.S. aerial drones that circled over the site, looking for intruders, or the two-metre-thick slab of steel-reinforced concrete that was poured over the earth.

Among the report’s new revelations is that the Soviet testers left behind components, including high-purity plutonium, that could have been used to build not just a dirty bomb but a sophisticated nuclear device, a U.S. official told the report’s authors.

U.S. scientists spent years trying to coax information about Soviet-era testing from their counterparts in Russia. It was a white-knuckled effort at times, since local scavengers in search of scrap metal were regularly digging “within yards” of the fissile material.

Siegfried S. Hecker, a former director of the Los Alamos National Laboratory, described his astonishment when he first visited the site in 1998, cruising past unattended gates.

“When I went out there, I was expecting to see guys on camels, pulling on copper cables,” Mr. Hecker told the authors. Instead, he saw miles of trenches that had been dug by teams with excavating machines — evidence of what he called “a major industrial enterprise,” which dug up miles of irradiated copper cable and sold it to Chinese dealers.

The report describes a project driven forward almost entirely by Russian and U.S. scientists, without the “elaborate, negotiated state-to-state agreements of the kind used for arms control during the Cold War.”

Mr. Hecker said he understood his counterparts’ reluctance; the mixture of plutonium isotopes used in Russian nuclear bombs was a closely guarded secret that Americans might discover if they gained access to the tunnels.

In 1999, on the margins of an international nonproliferation conference, the United States agreed to finance the cleanup effort; Russia agreed to provide information and scientists; and Kazakhstan to do the fieldwork. Step by step, the team began encasing the fissile material in cement, rendering it virtually unusable in a bomb.

In October 2012, a small group of scientists — American, Russian and Kazakh — gathered for a picnic to mark the completion of the cleanup. A monument now stands at the site, with the inscription: “1996-2012. The world has become safer.” — New York Times News Service

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