SIEGFRIED HECKER, a professor who used to run America’s nuclear laboratory at Los Alamos, recalls the most recent of the seven trips he has made to North Korea, in 2010. His hosts were showing off their sprawling Yongbyon atomic-energy complex. With a blend of shyness and defiance, they displayed an astonishing spectacle: a hall with 2,000 brand-new centrifuges, machines that enrich uranium, either for electricity or nuclear bombs.

Apparently assembled in another, unsuspected site, they had appeared in Yongbyon since Mr Hecker’s previous trip in 2008. This implied that, besides its existing plutonium-based technology, the country could make nuclear bombs from uranium. He was also shown the beginnings of a light-water reactor that could produce more plutonium. The message: “We have more nuclear capacity than you think, and you’ll never know how much…”

North Korea’s arsenal has since grown. Estimates range from 20 to 60 warheads, and its latest test was apparently of a hydrogen bomb, 100 times bigger than the earliest devices. It has also made strides in developing missiles; one tested last year could have reached America. The whole nuclear and military complex may involve 100 sites besides Yongbyon. The world’s knowledge is sketchy. A well-connected American think-tank, the Institute for Science and International Security, recently spelled out a long-mooted suspicion. As well as Yongbyon, it said, there seemed to be an older, undisclosed uranium-enrichment site, which it named as Kangsong.

So diplomats and nuclear scientists take a deep breath as they contemplate the “denuclearisation” of North Korea, an undefined goal reaffirmed at the summit between Kim Jong Un and President Donald Trump on June 12th. In Pyongyang this week Mike Pompeo, America’s secretary of state, is trying to put flesh on those flimsy bones. Even if the two sides can agree on a definition of denuclearisation, it will present bigger challenges than any previous exercise in managed disarmament.

Larger arsenals have been dismantled elsewhere, but in kinder political climates. And well-run programmes have monitored pariah states suspected of coveting the deadliest of weapons. But none involved an arsenal or a nuclear-fuel cycle as lethal, big or elusive as North Korea’s.

The world has a well-tried set of mechanisms for coping with such situations. They include the Nuclear Non-Proliferation Treaty (NPT) and the Vienna-based nuclear inspectorate, the International Atomic Energy Agency (IAEA). These structures would be tested by any deal with North Korea; but history suggests they can morph in surprising ways if the will exists.

Broadly, the NPT is a deal between five recognised nuclear powers and other signatories. In exchange for abjuring nuclear weapons, these other signatories are promised access to civil nuclear power, if they can show (with the IAEA’s help) that it will not leak to military uses. North Korea left the NPT in 2003, after America accused it of pursuing a secret nuclear-arms effort. It cannot rejoin until all its atomic weapons have been dismantled. So one dilemma is whether any rewards should be offered for steps along that road.

The only country to have built nuclear weapons and then renounced them, gaining NPT membership and respectability, is South Africa. But it had just six bombs and its state was transparent compared with North Korea’s. Also, it faces no external threats. With apartheid crumbling, its white rulers were eager to renounce illicit weapons. They began dismantling in early 1990 and by September 1991 they were ready to call in the IAEA for tough inspections, which were completed in two years.

An inspector calls

North Korea, by contrast, already has a dismal history with the IAEA. Its inspectors began working there in 1992, to be thrown out the following year and readmitted a year later. Co-operation ceased in 2009, after multilateral negotiations broke down and North Korea started producing plutonium again. The IAEA’s director, Yukiya Amano, says his people stand ready to go back to Pyongyang. But they will need far greater access than they ever enjoyed before if their efforts are to be meaningful.

During the years of intrusive inspection which followed the 1991 Gulf war in Iraq, the agency also played a vital role (see article). And under the deal struck in 2015 to curb Iran’s nuclear activities, from which America has just pulled out, IAEA inspectors have enjoyed access to the Iranian nuclear cycle of which Korea-watchers only dream. That deal also allows for short-notice inspections; the IAEA says that at least until America’s withdrawal, the Iranians were co-operating quite well. But the agreement had noisy critics; some military sites were out of bounds, and it did not touch delivery systems, as any deal with North Korea must.

And there is another snag. The IAEA can trace the flow of nuclear fuel from one facility to another and pinpoint where materials might have been diverted to bomb-making. But once nuclear weapons are made, the Vienna agency bows out. Only the five authorised nuclear states can help dismantle and remove nuclear weapons.

The most ambitious effort of that kind was the destruction and evacuation, after 1991, of parts of the former Soviet nuclear arsenal in Ukraine, Belarus and Kazakhstan. This was an initiative by the world’s nuclear giants: America and newly democratic Russia. They agreed that the Soviet arsenal must be slashed and regrouped in Russia. America acquiesced in its monopoly over the Soviet strategic legacy.

Hundreds of long-range missiles and silos were destroyed. The warheads were taken to Russia and neutralised. Uranium was extracted and much of it sold to America. The Soviet system for transporting sensitive materials by rail made this easier. By 1996, the effort was complete; an arsenal of 1,800 warheads was no longer in Ukraine.

Two American senators, Sam Nunn and Richard Lugar, masterminded the legislation which helped neutralise the residual threat from the Soviet weapons of mass destruction (WMD): chemical and biological as well as nuclear. Among their best ideas was an initiative to find benign jobs for re-employed weapon scientists.

Vice-President Mike Pence conferred last month with Mr Nunn and Mr Lugar. The American-Russian amity that underpinned their work has gone. An obvious partner in any effort to neutralise and evacuate North Korea’s nuclear bombs and material would be China.

A less auspicious precedent has also been mentioned as a template for North Korea. The dismantling of Libya’s ramshackle nuclear programme (along with chemical weapons and efforts to make biological ones) started in December 2003 and was completed in 2004. Libya’s leader, Muammar Qaddafi, craved respectability and sanctions relief. American aircraft carried documents and equipment, including parts of centrifuges and missiles, to a laboratory in Tennessee. But Libya was years away from making any complete bombs. John Bolton, America’s national-security adviser, caused consternation in Pyongyang when he suggested in April that Libya might be a useful model. He since added that again, components from a lethal arsenal might be flown to Tennessee.

Nuclear scientists say that airlifting Korean bombs could be acutely risky. Disassembling and transporting nuclear warheads could trigger explosions, albeit probably not nuclear ones. As a minimum it will need close co-operation between the scientists who made them and at least one member of the existing nuclear club.

The other reason why talk of Libya horrifies North Korea is the fate of Qaddafi. Overthrown in 2011 with the help of Western air power, he was captured, raped with a bayonet and shot by Western-backed rebels. The pariahs of the world took note. Still, Mr Bolton made clear on July 1st that one aspect, at least, of the Libyan operation was worth copying: its speed. He said Mr Pompeo would be talking to the North Koreans about dismantling all their WMD and ballistic-missile programmes “within a year”. Mr Pompeo has mooted a slightly longer timetable: by January 2021.

If “dismantling programmes” means renouncing the capacity for further development, this could indeed be done in a year or two. For example, reactors that produce plutonium could be paralysed; and missile-testing sites destroyed, as Mr Trump (wrongly) claimed was happening already. But verifiably dismantling the existing arsenal and deploying inspectors across all sites would take much longer; Mr Hecker and others have suggested a decade.

Mr Bolton’s brisk approach leaves some analysts gasping. “If you knew everything they had, if they were fully committed and if you had unlimited resources, something might be achieved quite rapidly,” says Tom Plant of RUSI, a London think-tank. “But all those conditions are hypothetical.”