SEOUL, Feb. 12 (Yonhap) -- North Korea is believed to have produced up to seven nuclear weapons worth of fissile materials even while the regime engaged in denuclearization negotiations with the United States last year, an American scientist said.
"Our analysis of open-source satellite imagery of the Yongbyon complex led us to estimate they may have added sufficient plutonium and highly enriched uranium for an additional 5 to 7 nuclear weapons on top of our 2017 estimate of approximately 30 weapons," Siegfried Hecker, a Stanford University professor, said in remarks posted on the school's website.
Yongbyon is where the North's main nuclear complex is located.
Hecker is considered a top expert on North Korea's nuclear program. He is also known for having had a first-hand look at North Korea's uranium-enrichment facility during his 2010 visit to the Yongbyon complex.
He was speaking about the latest report on North Korea's nuclear history research released by the university's Center for International Security and Cooperation.
Hecker said it is not surprising that the North has continued to produce fissile materials.
"We are not surprised that North Korea has not halted its fissile materials production in absence of formal negotiations," he said.
He also said that the denuclearization process has significantly reduced the threat posed by the North.
"The diplomatic initiatives have greatly reduced the threat," Hecker said. "Our study, which looks at the details of how the capabilities have changed in 2018, concludes that the rapid escalation of overall capabilities in 2017 and prior years was halted and in some cases rolled back."