“It’s a big, big deal,” said Siegfried Hecker, a leading nuclear scientist at Stanford University who has visited the complex four times. “I view Yongbyon as the heart of their nuclear program.”

Yongbyon is home to North Korea’s nuclear reactors and is its only source of plutonium for potential nuclear weapons. It is also believed to be the only source of tritium to build powerful thermonuclear devices.

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Finally, it is one site — but not the only one — at which North Korea produces highly enriched uranium capable of use in warheads, experts say.

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Closing Yongbyon would do nothing to reduce North Korea’s current arsenal of nuclear bombs and missiles. And it remains unclear whether North Korean leader Kim Jong Un is prepared to surrender that arsenal.

But a complete and verifiable closure of Yongbyon would significantly slow the pace at which North Korea could build nuclear weapons and would be a concrete step in the direction of turning the broad principles expressed at last year’s Singapore summit into measurable progress, experts and diplomats say.

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Expectations were raised when Kim told South Korean President Moon Jae-in in September that he is prepared to permanently close Yongbyon if the United States takes “corresponding measures.”

Last month, U.S. envoy Stephen E. Biegun said Kim had gone significantly further, pledging to dismantle and destroy all of North Korea’s plutonium and uranium-enrichment facilities at “a complex of sites” extending beyond Yongbyon, if the United States reciprocates with measures of its own.

The next question is whether the two sides can agree on those “corresponding measures.”

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It’s clear that North Korea wants sanctions relief. Yet the United States is reluctant to ease the pressure significantly. It is far from clear if they will bridge that gap in Hanoi next week.

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Even if they do, Melissa Hanham at the One Earth Future foundation says the devil is in the details.

“During the summit, the thing that will make me most perk my ears is which parts of Yongbyon may or may not be frozen, or dismantled or suspended,” she said. “I am curious about which facilities and whether they are irreversibly dismantled.”

The reason? We’ve been down this road twice before.

Yongbyon, a complex of 390 buildings stretching over 3.4 square miles, is home to 10 nuclear research institutes. It is also home to three reactors: the country’s main nuclear reactor that was opened in 1986 and is the source of its plutonium for nuclear weapons, a less important research reactor built by the Soviet Union and an experimental light water reactor that is still thought to be under construction.

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The main reactor is old and was shut down in the 1990s under an agreement with the United States, only to be started up again in 2003 after the breakdown of the deal.

In 2007, another deal was reached to shut down the reactor, and the cooling tower was demolished the following year. But when that deal fell apart, North Korea simply pumped in river water to cool the reactor and fired it up again.

Twice bitten, three times shy. This time around the main reactor would have to be disabled quickly and irreversibly to impress anyone on the U.S. side or among its allies.

In 2010, Hecker was also shown into a building at Yongbyon containing around 2,000 centrifuges to produce highly enriched uranium for weapons. Since his visit, satellite imagery shows the floor space of that building has doubled. Closing it down and surrendering or destroying the centrifuges would be an essential element of any deal.

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The problem is that Yongbyon is not the only place where North Korea is thought to enrich uranium to make nuclear weapons.

Jeffrey Lewis, a scholar at the Middlebury Institute of International Studies at Monterey, identified a covert facility at Kangson, just outside Pyongyang, that became operational in the 2000s. He says there is probably at least one more facility.

It remains unclear whether Biegun is right to suggest that North Korea is prepared to close down all its uranium enrichment facilities.

North Korea is thought to have anywhere between 20 and 60 nuclear warheads, and the capability to produce perhaps half a dozen more every year, experts say. North Korea has never given figures on its arsenal.

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Thae Yong Ho, a former North Korean diplomat who defected in 2016, says the process of dismantling and inspecting Yongbyon could also take years, delaying real progress on real denuclearization. Thae told reporters that Kim has no intention of surrendering his nuclear weapons but will extract what benefits he can by giving away things he no longer really needs.

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“North Korea has already produced enough material to churn out nuclear weapons,” he said, adding in reference to Yongbyon: “North Korea’s intention is to repaint their broken down car and sell it to the United States.”

On Jan. 1, 2018, Kim told his defense industry to “mass-produce nuclear warheads and ballistic missiles.” Lewis says satellite evidence suggests that process has indeed been happening, in parallel with negotiations.

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Satellite images and intelligence reports suggest liquid-fueled intercontinental ballistic missiles (ICBMs) are being built at a complex at Sanumdong, on the outskirts of Pyongyang. A plant producing solid-fueled, shorter-range missiles has been expanded near the city of Hamhung, while a missile base near the Chinese border also appears to be under expansion to accommodate ICBMs, Lewis said.

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“Completing their deterrent and increasing their leverage — exactly what they said they’d do and what they should do,” he said. “Not that I want them to do bad things, but why would they give these things up?”

Nevertheless, closing Yongbyon — and allowing in international inspectors for a credible program of meaningful dismantlement — would be a big step forward, and add significantly to the outside world’s knowledge of North Korea’s nuclear program, most experts say.

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“So when people ask me, they say, ‘This guy is never going to give up his nuclear weapons. He is never going to give them up,’ ” Hecker said. “I say, ‘Is he going to give them up? I don't know — but it’s time to find out.’ ”