At left: A plutonium bomb detonates in the first Soviet nuclear test, code-named “First Lightning,” conducted Aug. 29, 1949, at the Semipalatinsk test site in Kazakhstan. At right: The foothills of the Degelen Mountain complex at Semipalatinsk. (Left: Universal Images Group via Getty Images; Right: Courtesy of U.S. Department of Defense)

David E. Hoffman is a contributing editor at The Washington Post. Eben Harrell is an associate at the Project on Managing the Atom in the Belfer Center at Harvard University’s Kennedy School, which is publishing a detailed account of the Semipalatinsk operation. This report was supported by a grant from the Pulitzer Center on Crisis Reporting.

Last October, at the foot of a rocky hillside near here, at a spot known as Degelen Mountain, several dozen Kazakh, Russian and American nuclear scientists and engineers gathered for a ceremony. After a few speeches, they unveiled a three-sided stone monument, etched in English, Russian and Kazakh, which declared:

“1996-2012. The world has become safer.”

The modest ribbon-cutting marked the conclusion of one of the largest and most complex nuclear security operations since the Cold War. The secret mission was to secure plutonium — enough to build a dozen or more nuclear weapons — that Soviet authorities had buried at the testing site years before and forgotten, leaving it vulnerable to terrorists and rogue states.

The effort spanned 17 years, cost $150 million and involved a complex mix of intelligence, science, engineering, politics and sleuthing. This account is based on documents and interviews with Kazakh, Russian and U.S. participants, and reveals the scope of the operation for the first time. The effort was almost entirely conceived and implemented by scientists and government officials operating without formal agreements among the nations involved. Many of these scientists were veterans of Cold War nuclear-testing programs, but they overcame their mistrust and joined forces to clean up and secure the Semipalatinsk testing site, a dangerous legacy of the nuclear arms race.

They succeeded, but what they accomplished here may have to be done all over again if the walls of secrecy ever come down and reveal security vulnerabilities in other states that have developed the atomic bomb, including North Korea, Pakistan, China, India and Israel, or in countries that may develop weapons in the future, such as Iran.

During the Cold War, the Soviet Union carried out more than 450 nuclear explosive tests at the Semipalatinsk site, which sprawls over a portion of the Kazakh plains slightly larger than Connecticut. Most of the tests involved atomic explosions, while others were carried out to improve weapons safety, in part by examining the impact of conventional explosives on plutonium metal. A network of tunnels built under Degelen Mountain became the epicenter of these tests.

After the Soviet Union collapsed in 1991, the Russians gradually abandoned the site. Economic conditions in the main city near the testing grounds grew desperate, and residents began to search the tunnels for metal to sell. They used mining equipment to steal copper from the electrical wiring and to scavenge rails that once carried nuclear devices far underground for explosive testing.

In the 1990s, the United States, through an agency in the Pentagon dealing with nuclear security, funded a program to close off the entrances to the tunnels at Semipalatinsk so they could never again be used for nuclear tests. The tunnels were sealed at the portals but not explored to any depth. Plutonium from the earlier safety tests lay deep inside.

In 1995, two scientists from the Los Alamos National Laboratory visited Degelen Mountain and came away convinced that the site was a potential plutonium “mine” for thieves and terrorists. Then, in January 1998, Siegfried S. Hecker, who had just retired as the laboratory’s director, heard from a Kazakh scientist that the Degelen Mountain area was wide open, despite the U.S.-led tunnel-closing effort, and scavengers were searching it.

In April of that year, Hecker and two Los Alamos specialists went to Kazakhstan for nine days. At Semipalatinsk, Hecker found a lone, meager guard gate and no guards. He saw long trenches in the brown, dry land that could have been dug only by powerful excavating machines. “People on the site — no way to keep them off,” he wrote in his notes. In another location, Hecker saw one of the tunnels that had been closed. The front was still plugged, but scavengers broke in by drilling down from above and behind the barrier.

“I really thought these were guys digging a little bit of copper out. Instead, this was a major industrial enterprise,” Hecker said in a recent interview.

In a report he wrote after the trip, Hecker estimated that the total plutonium in the area could approach 440 pounds. A nuclear bomb would require only about 17 pounds, or even less in some designs. Hecker wrote that it was “material in reasonably concentrated form, easily picked up, completely open to whomever wants to come.”

That summer, on a trip to Russia, Hecker met with Radi Ilkaev, the director of Arzamas-16, one of the two leading Soviet-era nuclear weapons labs, which continued to operate in Russia. One evening, Hecker quietly pressed Ilkaev about Semipalatinsk: Did the Soviet Union leave nuclear materials buried there? Ilkaev responded cautiously, Hecker recalled. He said Russia was finished at Semipalatinsk and never wanted to go back, but could not afford the environmental cleanup.

Hecker pulled out the photos he had taken at Semipalatinsk. He showed Ilkaev evidence that huge earth-cutting machines had sliced through the ground. “Radi, that’s your test site. Those are the copper cable thieves,” he said.

Ilkaev looked at the photos and finally said, according to Hecker, “I’ll have someone to talk to you in the morning.’”

The next day, Ilkaev introduced Hecker to a pair of scientists, Yuri Styazhkin and Viktor Stepanyuk, who had worked on the test site. Styazhkin knew the whole story, but he did not reveal it all at once. “There are a lot of things we did out there,” he told Hecker.

Once back in the United States, Hecker gave a series of briefings in Washington about what he had discovered. He showed the photographs of the trenches and warned officials at the departments of Energy and Defense that the amount of recoverable plutonium was perhaps enough for a dozen nuclear weapons. Maybe more.

At a June 1999 seminar with U.S. officials in Almaty, Kazakhstan’s largest city, Ilkaev and other Russian scientists revealed that the problem at Semipalatinsk was bigger than just Degelen Mountain. In a field near the mountain, Soviet experiments in vertical shafts (or “bore holes”) had left plutonium residue in shallow holes. Kazakh scientists reported that scavenging was occurring there, as well.

At first, any cooperation seemed unlikely. Officials from the Russian Atomic Energy Agency, then known as Minatom, were suspicious that the United States was trying to collect intelligence about Russian nuclear weapons. Russia was also going through a tumultuous period after an economic collapse in 1998, the outbreak of a second war in Chechnya in 1999 and President Boris Yeltsin’s sudden resignation that December, with Vladimir Putin ascending as his handpicked successor. At the time, Russian officials were making progress toward securing nuclear facilities and reducing weapons stockpiles, but resentments toward Washington lingered. Eventually, they agreed to move ahead on Semipalatinsk but, as a condition, refused to take back any nuclear material. All of it would have to be secured in place, in Kazakhstan.

By contrast, officials in Kazakhstan were eager to get started on the project. President Nursultan Nazarbayev, disturbed by the remnants of Cold War-era Soviet nuclear, biological and chemical weapons programs that had been carried out in his country, actively backed nonproliferation efforts. All three countries agreed not to officially notify the International Atomic Energy Agency of the Semipalatinsk operation, in part because they feared leaks. As a ­non-nuclear-weapons state, Kazakhstan is required to report all weapons-usable materials on its territory to the IAEA, but in the case of the plutonium, it did not.

To secure the plutonium at the bore holes, the scientists and engineers borrowed a method from the 1986 Chernobyl nuclear accident. The Soviet Union had built a concrete containment dome around the destroyed Chernobyl reactor. The Russians pointed out that such an approach could also work at Semipalatinsk; scavengers would be deterred from breaking into a giant concrete sarcophagus.

The project to build the dome was called Operation Groundhog. The funding came from a program approved by Congress in late 1991, sponsored by Sens. Sam Nunn (D-Ga.) and Richard Lugar (R-Ind.), to cope with risks posed by nuclear weapons and materials in the former Soviet Union. With the operation, the effort shifted to official government cooperation: The United States would provide the money; Russia would provide the data; Kazakhstan would do most of the work.

Conceived in 2000, Operation Groundhog suffered repeated delays, including work stoppages during the frigid winters. But with the nuclear ambitions of al-Qaeda coming into clearer view in documents seized during the invasion of Afghanistan, U.S. officials felt the urgency of preventing plutonium from falling into the wrong hands. The concrete dome over the bore holes was completed in August 2003.

Just a few miles away, however, Degelen Mountain was still unattended, and scavengers continued to burrow in close proximity to weapons-grade plutonium. When a senior Pentagon official, Andy Weber, met with Russian and Kazakh officials in mid-2003 to discuss extending projects to the mountain, the Russians were still ambivalent and did not reveal all they knew. They offered the locations of three more experiments, at two sites. If work at these sample locales went well, and if the Russians felt confident that the Americans were not committing espionage, Minatom would consider sharing more information.

As it turned out, these sample locations weren’t in Degelen Mountain but in a nearby bunker. They involved three “kolbas” — large metal cylinders, about 8 by 24 feet, insulated with Kevlar and fiberglass and designed to contain explosions equivalent to the force of 440 pounds of dynamite. They were most often placed deep within Degelen Mountain for plutonium tests, but three had been used above ground and were stored in the bunker.

The U.S. Defense Threat Reduction Agency agreed to work on the three kolbas, one of which had been pried open by scavengers, and to defer action on Degelen Mountain. Operation Matchbox, begun in 2004, secured the kolbas by filling them with a concrete mixture.

In the spring of 2005, U.S. scientists finally got the breakthrough they’d been waiting for when Russia released all the remaining information about Degelen Mountain. But it wasn’t pretty. The mountain contained about 220 pounds of recoverable plutonium — enough for more than a dozen nuclear bombs. Even more surprising, Russia revealed that at one location, the Soviets had left behind some high-purity plutonium and equipment that could be used to build a nuclear weapon.

This disclosure alarmed U.S. officials, but the Russians were extremely cautious. In their reports to the U.S. side, they used code names for 16 sites in and around Degelen Mountain, ranking them according to proliferation risk. Three of the sites were found to present the “maximum risk” if they fell into the wrong hands and were given the code names X, Y and Z. One day, while crews were drilling a hole at the Y site, a concrete retaining wall collapsed, exposing the plutonium and equipment. Eventually, material from two of the sites was sent back to Russia, and the third was entombed in concrete.

Scavengers continued to raid the tunnels until 2008, when Kazakhstan finally declared Degelen Mountain an “exclusion zone” — which allowed U.S. officials to erect warning signs — and when Kazakh security forces got the authority to expel the scavengers. The following year, the United States funded and helped install an elaborate security system at the site.

Still, the work remained slow. In a 2010 summit in Washington that included 47 nations, President Obama arranged a personal meeting with Nazarbayev. Officials of the two nations then met with their Russian counterparts. The United States, Russia and Kazakhstan agreed in confidence to complete the work at Semipalatinsk by the next summit, scheduled for March 2012 in Seoul.

This high-level commitment galvanized the operation. For the first time, Kazakh crews worked through the winters, and American officials stayed on site in Semipalatinsk with them, while increased U.S. funding meant four crews could work simultaneously instead of one. Obama, Nazarbayev and Russian President Dmitri Medvedevannounced the completion of the work in Seoul, though the news was overshadowed by Obama’s “open mike” incident with Medvedev.

The Semipalatinsk operation succeeded: It secured the plutonium, reducing the threat that it could fall into the hands of scavengers, terrorists or a state with malevolent intentions. The operation showed once again that funding for nuclear security can pay large dividends. But it was a close call. Had scientists not prodded the governments of the United States, Russia and Kazakhstan, the cleanup might never have been launched, or traffickers might have arrived before the materials could be secured.

Questions also remain over the long term. Plutonium’s half-life is 24,110 years. Will someone, someday attempt to recover the material from the cemented tunnels and holes? Will it ever pose an environmental risk? While Nazarbayev’s commitment to nonproliferation has been strong, he is now 73 years old and has not designated a successor. What kind of leadership will prevail in Kazakhstan a decade or a century from now? How will the U.S.-Russian relationship evolve in the years ahead?

The operation highlighted the role of unofficial collaboration and contact among scientists and others who are devoted to getting results without cumbersome cross-government negotiations. But securing the plutonium in Kazakhstan proved to be a laborious undertaking spanning 17 years, an effort requiring scientists willing to work together across countries and time zones, united only by a determination to get results.

Such hidden repositories might be found elsewhere, wherever nations have tested nuclear weapons or carried out other research on fissile materials such as plutonium. Will all that scientific collaboration and goodwill be readily available? It is true, as the plaque at Degelen Mountain attests, that the world is safer thanks to this operation. But it is also true that the scars left by nuclear weapons testing during the Cold War will last for millennia.

david.hoffman@washpost.com

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