NORTH KOREA is working to restart a reactor that would produce enough plutonium to make 10 atomic bombs a year, a leading American nuclear scientist has revealed.

Siegfried Hecker, former director of the US government’s top secret Los Alamos laboratory, also said the North Koreans reprocessed 8,000 fuel rods to make up to 14kg (30lb) of plutonium last summer, despite taking part in six-party talks hosted by China to end their weapons programme.

“They have the plutonium,” he said. “We have to assume the North Koreans can and have made a few nuclear devices.”

Hecker’s revelations were based on information gleaned during two visits to North Korea, the last in August 2005, in which he met physicists and, in a pure moment from spy fiction, was handed a specimen of weapons-grade plutonium, stored in a marmalade jar.

His findings are being studied with increasing concern in Washington. North Korea further hardened its defiant stance this weekend by ending all United Nations food distribution to its people and by ordering out aid workers, including a British team from the charity Save the Children.

Thousands will be put at risk by the decision. The UN’s World Food Programme had been feeding up to 6.4m of North Korea’s 23m people since the famine of the 1990s.

Diplomats believe the expulsions are part of a clampdown ordered by Kim Jong-il, North Korea’s “Dear Leader”, as he prepares to stand alongside Iran to confront the Americans over their right to have nuclear weapons.

Hecker, who was at Los Alamos from 1973-97, gave his warning at a recent conference in Washington as one of the most authoritative observers of North Korea’s programme.

On his first visit to North Korea in 2004, Hecker was taken to the Yongbyon nuclear research centre to meet its director. His hosts brought a small steel container into the conference, containing a wooden box.

“They slid open the box and inside were two glass jars — two marmalade jars, actually — with screw-on tops,” he said. One contained powder, the other a thin scrap of metal — the “ stuff you would use for the bombs”.

“I held the plutonium and it passed the test,” he added. When he told the director it was not very warm, the latter replied: “Well Dr Hecker, that’s because the plutonium 240 content is low, which means that it’s good bomb-grade plutonium.”

On his second visit, Hecker met the director in Pyongyang, the capital, to learn that while the North Koreans had been negotiating in China, they had also been making up to 14kg of plutonium, taking their total stock to an estimated 43kg, enough for about eight bombs.

Hecker said his main fear is that North Korea’s impoverished regime might sell material to terrorists. “Forty kilograms of plutonium, some number of briefcases anywhere in basements, in one of the 15,000 tunnels in North Korea — nobody will find it,” he said.

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