NEW YORK • 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, Dr Siegfried 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 Dr Hecker compiled with two colleagues at Stanford's Centre for International Security and Cooperation.
Dr 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.
Dr Hecker's timeframe stands in stark contrast with what the US initially demanded, in what could be a key sticking point in any summit meeting between US President Donald Trump and North Korean leader Kim Jong Un.
In an interview, Dr Hecker said he was making the Stanford study public to advance discussion of a complicated topic that will be at the heart of Mr Trump's encounter with Mr 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," Mr Hecker said last Friday.
The key to dismantling the sprawling atomic complex, begun six decades ago, Dr Hecker added, "is to establish a different relationship with North Korea where its security rests on something other than nuclear weapons".
He 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 also rectors 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 Mr 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.
Dr 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," he 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.
Dr Hecker noted that the decontamination and decommissioning of a single plant that handles radioactive materials could take a decade or more.
He said his personal denuclearisation estimate ran to 15 years, given the tangle of political and technical uncertainties that the US and North Korea would face if they went ahead and sought a historic accord.
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 Mr Trump meets Mr Kim. Its bottom line is that denuclearisation must be complete, verifiable and irreversible.