HANOI (Reuters) - One of the most visible parts of North Korea’s nuclear program, the Yongbyon reactor complex is a central point of contention for diplomats trying to resurrect a deal after U.S. President Donald Trump and Kim Jong Un’s Vietnam summit ended without agreement.

U.S. President Donald Trump and North Korean leader Kim Jong Un shake hands before their one-on-one chat during the second U.S.-North Korea summit at the Metropole Hotel in Hanoi, Vietnam February 27, 2019. REUTERS/Leah Millis

Buried in low-lying hills about 100 km (60 miles) north of North Korea’s capital, Pyongyang, Yongbyon is home to nuclear reactors, fuel re-processing plants and uranium enrichment facilities.

In a rare press conference held in Hanoi late on Thursday, North Korean foreign ministry officials said they had made a “historically unprecedented offer” to close all of Yongbyon together with U.S. experts.

U.S. negotiators confirmed North Korea had made the offer, but said talks broke down over exactly which facilities were included, and the scope of sanctions relief that Pyongyang demanded in return.

Here are some key facts about Yongbyon, and a summary of what it would mean and take to demolish the complex.

WHAT’S THERE?

Built in the late 1950s with Soviet aid, the Yongbyon complex houses at least three reactors which North Korea says are intended to produce electricity.

It also has a fuel fabrication facility and a plutonium reprocessing plant, where weapons-grade materials can be extracted from spent fuel rods, according to the Nuclear Threat Initiative (NTI), a Washington-based think-tank.

A five-megawatt reactor there produces weapons-grade plutonium, a major source of fuel for the nuclear program.

North Korea promised to disable Yongbyon as part of a 2005 disarmament-for-aid deal that emerged from so-called six-party talks, involving the United States, China, Japan, Russia and the two Koreas.

In 2008, the North publicly demolished a cooling tower as an initial step to implement the 2005 accord, but those efforts fell apart after the North barred international inspectors from accessing the facilities. The reactor was reactivated in 2013.

A larger, 50-megawatt reactor was under construction in the 1980s but work was suspended after a 1994 deal with the United States.

There is also a facility to produce highly enriched uranium (HEU) used to make atomic bombs, experts say, and an experimental light water reactor has been under construction.

Siegfried Hecker, a nuclear scientist who previously headed the Los Alamos weapons laboratory in the United States, said North Korea told him they had 2,000 centrifuges, to produce weapons-grade uranium to build bombs, in operation during his visit to Yongbyon in 2010.

Kim Dong-yub, a military expert at Kyungnam University’s Institute of Far Eastern Studies in Seoul, said there could be up to 6,000 centrifuges in North Korea, and 4,000 of them in Yongbyon, based on studies by Hecker and other agencies.

Assessments of the scale of North Korea’s nuclear stockpile vary, with the U.S. Defence Intelligence Agency at the high end with an estimate of about 50 nuclear warheads.

The Stimson Centre’s 38 North think-tank in Washington estimates the North has 50 to 60 nuclear warheads. It said in 2017 the 5MWe Yongbyon reactor was capable of producing about 6 kg of plutonium every year, enough to make about two bombs.

HOW IMPORTANT IS IT?

Estimates of how significant Yongbyon is in the North’s nuclear program also vary.

One South Korean official said dismantling Yongbyon would take a comprehensive plan involving a declaration of arms and facilities, samplings of fissile materials and equipment, and ad hoc inspections by outside experts.

“Skeptics would be tempted to downplay the significance of Yongbyon in favor of a more comprehensive roadmap for denuclearization, but if it’s properly done, inspecting Yongbyon would have most of the necessary elements of a bigger deal,” the official told Reuters on condition of anonymity due to the sensitivity of the issue.

Inspectors, for example, would know whether North Korea runs a covert facility elsewhere by sampling and analyzing radioactive isotopes from Yongbyon which could indicate any mismatch between what was generated, and used in tests or weapons production, the official said.

Pyongyang has denied the existence of other secret sites, but U.S. intelligence sources cited by media last year said the North has been operating at least one clandestine uranium enrichment facility just outside of Pyongyang, known as Kangson.

Neither U.S. or North Korean officials identified the extra areas that Washington was seeking as part of the summit talks, but in a news conference after ending the talks on Thursday Trump said he had raised the issue of facilities beyond Yongbyon.

“I think they were surprised we knew,” Trump said.

Yongbyon would only be the beginning of any nuclear deal, however, because its destruction would still leave North Korea with a nuclear weapons stockpile and its ballistic missiles, and the capability to produce weapons-grade uranium at secondary sites, 38 North said in a February report.

CHALLENGES, SUSPICIONS

Dismantling Yongbyon might slow the production of plutonium and highly enriched uranium, but would not reduce the current stockpile of material, said Kim Sung-han, a former South Korean vice foreign minister who teaches at Korea University in Seoul.

Citing satellite imagery, Hecker told Reuters that spent fuel generated by the reactor at Yongbyon from 2016-18 appeared to have been reprocessed starting last May, enabling North Korea to boost its nuclear arsenal by 5-7 weapons.

In the 38 North report, the authors said implementing an agreement to dismantle Yongbyon would pose “enormous political, technical, and financial challenges”.

Among the challenges would be physically dismantling and decontaminating the plutonium and uranium enrichment plants and the 5MWe reactor, and safely disposing of the radioactive fuel and nuclear waste, the report said.

Based on previous experiences at other facilities, all options for disablement and dismantlement of the complex will require “years of work and millions if not billions of dollars,” the report concluded.

A senior State Department official explained on Friday that the complex includes “many institutions, building, outbuildings” and North Korean officials had not provided details on exactly what they were willing to dismantle.

“It’s a substantial set of facilities on a single property,” the official said. “And it is important to be very precise about that. And the North Koreans struggled to give us a precise definition of what that was.”