The site's five-megawatt reactor, experts believe, has produced enough plutonium from its spent fuel rods for possibly up to a dozen small atomic weapons since it began operating in 1987. Two larger reactors are at the same site but are not thought to be operational, along with a plutonium reprocessing plant several storeys high. Moves in 1994 to remove spent fuel rods from Yongbyon triggered the first nuclear crisis with the United States.

The Pentagon drew up plans to bomb the facility but diplomacy involving former US president Jimmy Carter and others averted a clash and led to an eight-year shutdown. Under a 1994 "Agreed Framework" deal with the United States, an international consortium started work on two proliferation-resistant light-water reactors. The United States also provided an interim 500,000 tonnes a year of heavy fuel oil, although shipments were often delayed. The deal collapsed in 2002 when Washington accused the North of running a secret highly enriched uranium program. North Korea denied the charge but restarted Yongbyon, expelled UN atomic inspectors and announced it was leaving the nuclear Non-Proliferation Treaty. In October 2006 the North staged its first nuclear weapons test.

Just four months later it reached a six-nation deal which promised energy aid and major diplomatic and security benefits in return for full denuclearisation. Yongbyon was shut down in July 2007 and Pyongyang began disabling key plants there. But six-party negotiations stalled last December because of disputes about ways to verify the North's declared nuclear activities. If talks ever resume, assessing the size of the North's plutonium stockpile will be a key part of verification.

This was estimated by the Washington-based Institute for Science and International Security at 46-64 kilos (101-141 pounds). Some 28-50 kilos of this is estimated to have been separated, enough for about five to 12 nuclear weapons, the institute said in February 2007. The North reportedly put the size of its plutonium stockpile at 31 kilos when it handed over a nuclear declaration in June 2008. US expert Selig S. Harrison, who visited Pyongyang in January, says he was told that this has "already been weaponised."