The United States Defense Department has paid for aerial drones to spot intruders, and for motion detectors that signal when a person, or a horse or a car, crosses into restricted territory. The classified project aims to keep terrorists away from what the Soviets left behind in patches of earth and a warren of tunnels that they used for atomic testing: among other things, plutonium and highly enriched uranium that Western scientists fear could be used to build an improvised nuclear device.
Protecting this material has meant teasing out nuclear secrets that have been kept for decades. Russia is warily sharing archival material about Soviet-era tests, and the United States is paying to remove or secure weapons-grade material. Kazakhstan is providing the labor, but because it is not a nuclear power, its officials are forbidden from learning exactly what it is that they are guarding.
“People ask me, are we doing the right thing, closing access to the tunnels?” said Kairat K. Kadyrzhanov, general director of Kazakhstan’s National Nuclear Center, which manages the test site. “And I say I do not know what is there, and I do not have the right to know.”
The edge of the Semipalatinsk Test Site is a two-hour drive from the nearest large city, across an expanse of dun-colored, featureless steppe.
In 1948, racing to break the American monopoly on nuclear weapons, the Soviet Union selected this stretch of land to test their own. Local villagers knew only that the earth quaked, making china shudder on the shelf; years later, charts showed radioactive plumes that settled over population centers.
With the collapse of the Soviet Union, 20,000 to 30,000 troops withdrew from their positions, leaving 500 Kazakh soldiers to guard the site, Mr. Kadyrzhanov said.
Ever since then, the test site — or, specifically, the fissile material and fission products left there — has been a concern for the United States. The project has continued behind a curtain of secrecy, with a few exceptions; in 2003 Kazakh officials told a reporter for Science magazine about “Operation Groundhog,” in which plutonium-contaminated earth was paved with a two-meter-thick slab of steel-reinforced concrete to protect it from terrorists who might cart it away to use in a dirty bomb.
Officials from the Defense Department and the State Department would not comment. But cables published by WikiLeaks last year describe an urgent push to “prevent nuclear residue material from falling into terrorists’ hands,” as a top defense official put it in 2009. One describes it as “the most critical” of all the American-financed projects to secure nuclear materials in the former Soviet Union.
After above-ground testing was banned, the Soviets detonated 295 devices in 181 tunnels under the Degelen Mountains, according to a study published last year by Kazakhstan’s Institute for Radiation Security and Ecology. Each explosion consumed between 1 and 30 percent of the device’s fissile material, leaving the remaining fuel mixed with debris and melted rock underground, according to the study.
President Obama’s inauguration marked a “radical change of attitude” toward security at the site, and American officials demanded a fivefold acceleration in the work, Mr. Kadyrzhanov said. Meanwhile, Russia, which for years refused to share Soviet documents about the site, has been more forthcoming, he said.
“The danger that Russia is keeping something from us has been diminished,” he said.
With the Soviet collapse, poverty and disorder lapped at the edges of the Semipalatinsk Test Site, still littered with radioactive hazards.
Activity ended so abruptly that a nuclear device lowered into one tunnel in preparation for a test sat unexploded until 1995, when technicians managed to destroy it without creating a nuclear reaction, according to the National Nuclear Center.
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