NEW YORK (NYTIMES) - As the Trump administration races to start talks with North Korea on what it calls "rapid denuclearisation," a top federal government adviser who has repeatedly visited the North's sprawling atomic complex is warning that the disarmament process could take far longer, up to 15 years.

The adviser, Siegfried S. Hecker, a former director of the Los Alamos weapons laboratory in New Mexico, and now a Stanford professor, argues that the best the United States can hope for is a phased denuclearisation that goes after the most dangerous parts of the North's programme first.

The disarmament steps and timetable are laid out in a new report, circulated recently in Washington, that Hecker compiled with two colleagues at Stanford's Centre for International Security and Cooperation.

Hecker has toured that nation's secretive labyrinth of nuclear plants four times and remains the only US scientist to see its facility for enriching uranium, a bomb fuel. US intelligence agencies had missed the plant's construction.

Hecker's time frame stands in stark contrast with what the United States initially demanded, on what could be a key sticking point in any summit meeting between President Donald Trump and Kim Jong Un, the North Korean leader.

Two US delegations, one in Singapore and one in North Korea, are attempting to work out a meeting between the two leaders. Trump cancelled the meeting in a letter to Kim on Thursday (May 24) but has been working to reconstitute it ever since, posting Twitter messages that say he is confident the North Korean economy will prosper if an accord is reached.

The delegation in Singapore is discussing the logistics of a meeting, to be held June 12 or afterward. The other, led by Sung Kim, a US diplomat with long North Korea experience, is meeting senior officials of the North Korean Foreign Ministry at the Demilitarised Zone to work on the wording of what kind of communique might be issued by the two leaders. But the White House and State Department have said nothing about the details of those discussions.

In an interview, Hecker said he was making the Stanford study public to advance discussion of a complicated topic that will be at the heart of Trump's encounter with Kim in Singapore, if that meeting happens. So far, the denuclearisation agenda has been a mix of bold claims by the administration about what it will demand, and vague generalities from the North.

"We're talking about dozens of sites, hundreds of buildings, and thousands of people," Hecker said on Friday (May 25).

The key to dismantling the sprawling atomic complex, begun six decades ago, Hecker added, "is to establish a different relationship with North Korea where its security rests on something other than nuclear weapons."

Hecker cautioned that his team's road map left room for many knotty points of negotiation - such as where to draw the line between civilian and military nuclear activities. At first, the Trump administration said the North must give up all enrichment of uranium, which can fuel not only bombs but reactors that illuminate cities.

Last week, US Secretary of State Mike Pompeo, testifying before the Senate Foreign Relations Committee, said for the first time that he needed some "negotiating space" on that question.

But Trump exited the Iran nuclear deal this month because it allowed the country to produce atomic fuel after 2030, which he said was an unacceptable risk. It is unclear how he could ban Iran from peaceful production, yet allow North Korea to do the same.

Hecker said a similar open question was whether to let the North's rocket engineers, now making long-range missiles, redirect their skills into a peaceful space programme.

"They're not going to eliminate everything, and there're some things that aren't a problem," Hecker said. "Some of the risks are manageable."

In its report, the Stanford team sees three overlapping phases of denuclearisation activity that, in total, would take 10 years. The initial phase, taking up to a year, is the halt of military, industrial and personnel operations. The second, taking up to five years, is the winding down of sites, facilities and weapons. The final and hardest phase, taking up to 10 years, is the elimination or limiting of factories and programmes.

Hecker noted that the decontamination and decommissioning of a single plant that handles radioactive materials could take a decade or more.

In an interview on Sunday, Hecker said his personal denuclearisation estimate ran to 15 years given the tangle of political and technical uncertainties that the United States and North Korea would face if they went ahead and sought a historic accord.

The road map, which was posted late Monday on a Stanford website and was circulated to some administration officials and members of Congress, underscores the complexity of the task at hand: While politicians and cable news commentators use the shorthand of the North surrendering its nuclear arms, the road map makes clear that denuclearisation would be a vast undertaking that involved the shuttering of large industrial plants and decades of detailed inspections.

The Trump administration has made public no details of what particular steps it sees for the North's denuclearisation, or what it intends to demand if Trump meets with Kim. Its bottom line is that denuclearization must be complete, verifiable and irreversible.

Trump's hawkish national security adviser, John R. Bolton, argued before joining the administration in April that the president should use a summit meeting exclusively to tell North Korea to dismantle and deliver up all its nuclear arms and equipment, saying only then should the United States discuss easing sanctions and participating in the North's economic development.

In recent television and radio interviews, Bolton has advocated quick denuclearisation in which the North would send its weapons and equipment to the Oak Ridge National Laboratory in Tennessee, where nuclear inspectors in 2004 shipped some of Libya's gear for enriching uranium. Bolton has repeatedly cited Libya as a role model for the North's atomic disarmament.

In the interview, Hecker argued that the only safe way to disassemble the North's nuclear warheads was to have the job done by the same North Korean engineers who built them.

Trump, in contrast to Bolton's public stance, twice last week opened the door to phased denuclearisation, saying the North might find it impossible to dismantle its entire nuclear programme in one step.

Hecker comes to the issue with decades of experience in learning about foreign nuclear programmes and managing their phased reductions.

After the Cold War, as the Los Alamos director, he fostered wide cooperation between US and Russian nuclear laboratories to secure and safeguard vast stockpiles of ex-Soviet nuclear materials. His 2016 book, Doomed To Cooperate, details the long collaboration.

Hecker made his first visit to the North's sprawling Yongbyon nuclear site in 2004, with follow-up visits in 2007, 2008 and 2010, learning more than any other Western expert about the North's secretive atomic doings. Since then, he has emerged publicly as one of the world's most knowledgeable experts on its nuclear programme.

His co-author Robert L. Carlin, a former CIA analyst and State Department intelligence official who has travelled to North Korea more than 30 times, is frequently cited as an expert on the North Korean leadership. The third author is Elliot A. Serbin, a senior fellow at the Stanford centre and a research assistant to Hecker.

The team divides up the North's nuclear programme into eight general categories and 22 subgroups. The range is wide. It includes not just plants and facilities but related issues such as ending the North's missile and nuclear exports and redirecting its technical experts from military to civilian work.

Plutonium fuel for atom bombs is especially frequently mentioned. The radioactive metal is considered the founding step for aggressive programmes set on making a variety of nuclear arms.

Producing it is easier than purifying uranium, and it takes far less plutonium to make a blast of equal size. Atop a missile, all else being equal, the reduced weight means warheads fuelled by plutonium can fly longer distances, making them more threatening. Plutonium is also ideal for igniting the thermonuclear fuel of H-bombs.

The Stanford team recommends six ways to curb the North's plutonium complex, targeting Yongbyon, the secretive site that Hecker has repeatedly visited.

For instance, the team calls for the dismantlement of the North's 5-megawatt reactor for making plutonium. It began operating in 1986, and Western experts say it produced the fuel for the North's first atom bombs.

The team is less categorical in recommendations for a large new reactor, known as the experimental light-water reactor, being started up at Yongbyon. Since the plant can make electrical power for civilians, the team suggests the reactor needs to be closely inspected before its fate is negotiated.

The team calls for the North to join two global accords meant to halt the making of nuclear arms and the means of delivering them. The pacts are the Nuclear Nonproliferation Treaty, which the North once observed, and the Missile Technology Control Regime. Its member states coordinate export licensing to curb the spread of long-range missiles that can deliver weapons of mass destruction.

"We're going to have some people argue with us," Hecker said of how technical experts were likely to react to the team's recommendations. "That's OK."