Washington

Speeding along a deserted highway towards North Korea's secret city of Yongbyon earlier this month, Siegfried Hecker, former director of the Los Alamos nuclear-weapons laboratory in New Mexico, wasn't sure what to expect.

“I had heard that the North Koreans were less than cooperative, to put it kindly,” he says. In the end, Hecker, who was invited to make the semi-official trip by North Korea's nuclear-weapons researchers, came away impressed by their technical expertise, but unsure of the status of their weapons programme.

Hecker, who told a closed Senate hearing on 21 January about his visit and later discussed it with reporters, says he engaged in a surprisingly frank and even humorous discussion with his North Korean counterparts. Ri Hong Sop, the director of the Yongbyon facility, gave Hecker and four other US visitors a guided tour on 8 January and sought to convince them that North Korea had successfully assembled nuclear weapons.

The visitors were shown a cooling pond where 8,000 spent fuel rods had been housed in steel canisters. The North Koreans claimed that all of them had been reprocessed to extract plutonium, but many of the canisters looked to be intact. “I said, ‘I can't go back home and report that all the fuel rods are missing, because I didn't see them all empty’,” Heckern recalls. So Sop suggested that he test the claim by choosing a canister to be opened at random.When he opened a random canister and found it empty, he concluded that the fuel rods were indeed missing.

Later, Hecker was allowed to handle a mildly warm metal sample, which the North Koreans claimed was bomb-grade plutonium alloy. Hecker, a metallurgist who knows plutonium and its alloys rather well, asked Sop what he had used to create the alloy.“He said,‘I'm not authorized to tell you, but you know, it's the same stuff that you use’.” Hecker says he is reasonably convinced that what he held could have been weapons-grade plutonium.

But Hecker is less sure that Nort Korea has turned its plutonium into a bomb.A plutonium-based weapon is more difficult to build than the enricheduranium device that some analysts assumed North Korea would try to build. But Hecker was impressed by the technical abilities of some of the scientists he met. “The people were very competent, no question about it,”he says.