For six years, management science and engineering professor Siegfried Hecker has offered Stanford, Washington and the U.S. scientific community a window into the secretive world of North Korea’s nuclear program. The 67-year-old former director of Los Alamos National Laboratory travels as a nonofficial diplomat, returning each time with insights, photographs and discussion points for U.S.-North Korea relations.

But on Nov. 12, during his seventh visit to the clandestine nation and his fourth to its Yongbyon nuclear complex, Hecker was “stunned” to find that the impoverished state had accelerated construction of a state-of-the-art uranium enrichment facility complete with 2,000 steel-rotor centrifuges producing low-enriched uranium.

If enriched further, the uranium could be used to produce nuclear weapons.

“If you enrich a little and have that capability, you can easily enrich more,” Hecker told an invite-only room Monday at Stanford’s Center for International Security and Cooperation (CISAC), where he serves as co-director. “That’s really the primary concern.”

Hecker was joined on the trip by visiting scholar Robert Carlin and emeritus professor John Lewis, both of CISAC. Though their guides at Yongbyon said the uranium will power a new light-water reactor to generate civilian nuclear energy, not produce weapons, the team’s findings captured international attention upon their return.

U.S. officials told The New York Times, which broke the findings in a front-page story on Nov. 20, that they think North Korea, like Iran, will say its reactor is for energy but will actually use it to develop weapons.

Hecker said construction of the reactor, which is in its early stages, is not a new finding, as was widely reported after the trip. “They told us in [February] 2009 they were going to build a light-water reactor,” he said.

The real shock came during the team’s visit to the enrichment facility, a structure formerly used for preparing nuclear fuel rods before North Korea dismantled it under a 2008 agreement with the Bush administration to end its weapons program. Between April 2009, when North Korea expelled U.N. inspectors from the site, and the team’s visit last month, the blue-roofed building was gutted, renovated and fitted with 2,000 centrifuges standing six feet tall, which enrich uranium so that it can fuel a reactor.

“I didn’t believe they would have an industrial-scale facility ready and available,” Hecker said. “We looked through the windows into the hall, and our jaws just dropped.”

Hecker said a North Korean official turned to the team and said, “No one believed us when we announced this in 2009,” before looking at him and adding, “including you, Dr. Hecker.”

Current assessments by Hecker and other top scientists who have visited Yongbyon say North Korea has between eight and 12 nuclear weapons. Despite the belief by U.S. officials and political pundits that the new uranium enrichment facility is the first stage of a full-fledged weapons program, Hecker isn’t worried that the facility will be used to make bombs.

Of the two paths to a nuclear weapon — obtaining plutonium from spent reactor fuel and enriching uranium to weapons grade — North Korea has already engaged in the former, and Hecker believes that if the country’s leadership were serious about producing more weapons, “they could resume all plutonium operations within approximately six months and make one bomb’s worth of plutonium per year for some time to come” instead of building a uranium enrichment plant. Yongbyon’s five-megawatt gas-graphite reactor, which was used to produce plutonium until it was shut down in July 2007, “appeared dormant,” Hecker wrote in a report following his visit.

Instead, Hecker’s worries are about North Korea’s ability to export nuclear technology to other rogue states — Iran and Syria, for example. Exporting centrifuge technology, he says, is easier to cloak than exporting reactor technology.

Hecker also worries about the safety of the new reactor. Yongbyon officials told him they conducted seismic tests on the location and have a regulatory agency on site, but Hecker said he’s “not so sure.”

John Lewis, who has visited North Korea 20 times since 1986, said Monday that regardless of the intent behind developments at Yongbyon, the U.S. must “stay the course on denuclearization.”

Lewis also said American diplomats and foreign-policy makers should not think of North Korea as “a weapon” but should mind the country’s modernization. Mobile phones, flat-screen TVs and personal computers are booming among the population, Lewis and Hecker said. Students are required to practice colloquial English, and one of the factories the team visited was even blaring American rock music.

“What is going on is mind-blowing,” Lewis said. “If you think sanctions are working, forget it. This is not a country cut off.”

Lewis and Hecker believe North Korea revealed its developments at Yongbyon not only because they would eventually be impossible to hide from satellite images, but also because the country wants to prove to the world that it is not about to collapse. “One of the things we learned is that we should not underestimate the North Koreans,” Hecker said.

And why reveal them to these men specifically? “They trust us,” Hecker said. “They trust us to give an honest report. Then they have to take their chances with what our assessment is.”