WASHINGTON • North Korea has continued to produce bomb fuel while in denuclearisation talks with the United States and may have produced enough in the past year to add as many as seven nuclear weapons to its arsenal, according to a study released just weeks before a planned second summit between leaders of the two countries.
However, Pyongyang's freeze in nuclear and missile testing since 2017 means that North Korea's weapons programme probably poses less of a threat than it did at the end of that year, the report by Stanford University's Centre for International Security and Cooperation found.
Dr Siegfried Hecker, a former director of the US Los Alamos weapons laboratory in New Mexico who is now at Stanford and was one of the report's authors, told Reuters analysis of satellite imagery showed North Korea's production of bomb fuel continued last year.
He said spent fuel generated from operation of the 5MW reactor at North Korea's main nuclear plant at Yongbyon in 2016-18 appeared to have been reprocessed starting in May and would have produced an estimated 5-8kg of weapons-grade plutonium.
This combined with production of perhaps 150kg of highly enriched uranium may have allowed North Korea to increase the number of weapons in its arsenal by between five and seven, the Stanford report said.
Dr Hecker's team had estimated the size of North Korea's arsenal in 2017 at 30, bringing a possible current total of 37 weapons. US intelligence is not certain how many nuclear warheads North Korea has.
Last year, the US Defence Intelligence Agency's estimate of about 50 nuclear warheads was at the high end, while analysts have given a range of 20 to 60.
The Stanford report said that while North Korea was likely to have continued work on warhead miniaturisation and to ensure they can stand up to delivery via intercontinental ballistic missiles, the halt in testing greatly limited its ability to make improvements.
"They have continued the machinery to turn out plutonium and highly enriched uranium," Dr Hecker said. "But it also depends on weaponisation - the design, build and test, and then the delivery.
"When they ended missile testing, those things rolled backwards. So when I look at the whole spectrum, to me North Korea ... is less dangerous today than it was at the end of 2017, in spite of the fact that they may have made another five to seven weapons worth of nuclear material."
The Stanford experts said it was their assessment that "North Korea cannot deliver a nuclear warhead with any measure of confidence to the US mainland", although Dr Hecker said its nuclear weapons were a real threat to Japan and South Korea.
Dr Hecker said it was understandable that North Korea should have continued its weapons work, given that it had reached no specific agreement in the latest talks with the US to stop that work.
North Korean leader Kim Jong Un pledged during an unprecedented first summit with President Donald Trump in Singapore last June to work towards denuclearisation of the Korean peninsula.
There has been little concrete progress since, but in September, Mr Kim expressed willingness to take steps, including the permanent dismantlement of nuclear facilities at Yongbyon, in return for "corresponding measures" by the US.
US Special Representative for North Korea Stephen Biegun held three days of talks in Pyongyang last week to prepare for a second Trump-Kim summit due to be held in Hanoi on Feb 27 and 28.