This chronology of the North Korean nuclear program has its roots in the 1950s and begins in earnest in 1989 with the end of the Cold War and the collapse of the Soviet Union, the main economic ally of North Korea. The chronology mainly addresses the conflict between the United States and North Korea, while including the influences of the other members of the six-party talks: China, Russia, South Korea, and Japan.

The North Korean nuclear program can roughly be divided into four phases. Phase I (1956–1980) dealt primarily with training and gaining basic knowledge. Phase II (1980–1994) covers the growth and eventual suspension of North Korea's domestic plutonium production program. Phase III (1994–2002) covers the period of the "freeze" on North Korea's plutonium program (though North Korea pursued uranium enrichment in secret),David E. Sanger, North Korea Says It Has a Program on Nuclear Arms (), New York Times (17 October 2002), and phase IV (2002–present) covers the current period of renewed nuclear activities.

Nuclear weapons types

Nuclear weapon warhead designs fall into various categories. Fission devices (commonly called "atom bombs" or "A-bombs") rely on nuclear fission, the splitting of extremely heavy elements to release energy. The bombs used against Japan in World War 2 were of this type. At the other end of the scale, staged thermonuclear weapons (commonly called "hydrogen bombs" or "H-bombs") use one or more fission devices just as a first stage, to ignite a fusion warhead, in which extremely light elements fuse together releasing a great deal of energy. The most massive bombs exploded are of this kind and can be up to thousands of times more powerful than those used during World War 2. (All multi-megaton modern thermonuclear weapons are of this type) Finally, between the two are a variety of hybrids, such as "boosted" designs where a fission device is surrounded by (or contains) fusible material to boost its yield, and "fission–fusion–fission" devices. These can add anything from a moderate to a significant increase to a fission device.

Compared to fission weapons, thermonuclear designs are exceedingly complex, and staged weapons in particular are so complex that only five countries (USA, Russia, France, UK, China) have created them in more than 70 years of research. The fuels for an H-bomb are also far more difficult to create. Several countries with long-standing nuclear weapons programs, such as India and Pakistan, are suspected of striving towards a hybrid or "boosted" design instead, which is easier. Since both fusion weapons and hybrid designs can at times be referred to as "hydrogen bombs", it cannot be said with certainty at present, what type of weapon North Korea may have been referring to in any given test. At present, analysts are skeptical of the 2016 test being a staged thermonuclear design, while noting that the most recent test, in 2017, was considerably more powerful. In 2018, North Korea had offered and was reportedly preparing for inspections at nuclear and missile sites.

Phase I 1956: The Soviet Union begins training North Korean scientists and engineers, giving them "basic knowledge" to initiate a nuclear program. 1958: The U.S. deploys nuclear armed Honest John missiles and 280 mm atomic cannons to South Korea. 1959: North Korea and the USSR sign a nuclear cooperation agreement. 1962: The Yongbyon Nuclear Scientific Research Center opens. 1965: The Yongbyon IRT-2000 research reactor reaches a power rating of 2 MW. 1974: The Yongbyon IRT-2000 research reactor reaches a power rating of 4 MW. Between the late 1970s and early 1980s North Korea begins uranium mining operations at various locations near Sunchon and Pyongsan.

Phase II 1980–1985: North Korea builds a factory at Yongbyon to refine yellowcake and produce fuel for reactors. 1984: The DPRK completes construction of a "Radiochemical laboratory", which is actually a reprocessing plant used to separate plutonium from spent nuclear fuel at the Yongbyon site. 1984–1986: North Korea completes construction on a 5 MWe gas-cooled, graphite-moderated nuclear reactor for plutonium production. North Korea also commences with the construction of a second 50 MWe nuclear reactor. 1987: The Yongbyon IRT-2000 research reactor reaches a power rating of 8 MW. 1989: Soviet control of communist governments throughout Europe begins to weaken, and the Cold War comes to a close. Post-Soviet states emerge in Eastern Europe and Central Asia. As the USSR's power declines, North Korea loses the security guarantees and economic support that had sustained it for 45 years. Through satellite photos, the U.S. learns of new construction at a nuclear complex near the North Korean town of Yongbyon. U.S. intelligence analysts suspect that North Korea, which had signed the Nuclear Non-Proliferation Treaty (NPT) in 1985 but had not yet allowed inspections of its nuclear facilities, is in the early stages of building a nuclear bomb. In response, the U.S. pursues a strategy in which North Korea's full compliance with the NPT would lead to progress on other diplomatic issues, such as the normalization of relations. December 1990: North Korea conducts 70–80 high-explosives tests at its Yongbyon facility. 1991: The U.S. withdrew its last nuclear weapons from South Korea in December 1991, though U.S. affirmation of this action was not clear, resulting in rumors persisting that nuclear weapons remained in South Korea. The U.S. had deployed nuclear weapons in South Korea since January 1958, peaking in number at about 950 warheads in 1967. 1992: In May, for the first time, North Korea allows a team from the International Atomic Energy Agency (IAEA). The Agency inspection finds inconsistencies with North Korea declarations. Hans Blix, head of the IAEA, and the U.S. suspect that North Korea is secretly using its five-megawatt reactor and reprocessing facility at Yongbyon to turn spent fuel into weapons-grade plutonium. Before leaving, Blix arranges for fully equipped inspection teams to follow. The inspections do not go well. Over the next several months, the North Koreans repeatedly block inspectors from visiting two of Yongbyon's suspected nuclear waste sites, and IAEA inspectors find evidence that the country is not revealing the full extent of its plutonium production. 1993: In March, North Korea threatens to withdraw from the NPT. Facing heavy domestic pressure from Republicans who oppose negotiations with North Korea, President Bill Clinton appoints Robert Gallucci to start a new round of negotiations. After 89 days, North Korea announces it has suspended its withdrawal. (The NPT requires three months notice before a country can withdraw.) In December, IAEA Director-General Blix announces that the agency can no longer provide "any meaningful assurances" that North Korea is not producing nuclear weapons. 12 October 1994: the United States and North Korea signed the "Agreed Framework": North Korea agreed to freeze its plutonium production program in exchange for fuel oil, economic cooperation, and the construction of two modern light-water nuclear power plants. Eventually, North Korea's existing nuclear facilities were to be dismantled, and the spent reactor fuel taken out of the country. 26 October 1994: IAEA Chairman Hans Blix tells the British House of Commons' Foreign Affairs Select Committee the IAEA is "not very happy" with the Agreed Framework because it gives North Korea too much time to begin complying with the inspections regime.

Phase III 18 March 1996: Hans Blix tells the IAEA's Board of Governors North Korea has still not made its initial declaration of the amount of plutonium they possess, as required under the Agreed Framework, and warned that without the declaration IAEA would lose the ability to verify North Korea was not using its plutonium to develop weapons. October 1997: spent nuclear fuel rods were encased in steel containers, under IAEA inspection. 31 August 1998: North Korea launched a Paektusan-1 space launch vehicle in a launch attempt of its Kwangmyŏngsŏng-1 satellite. U.S. military analysts suspect satellite launch is a ruse for the testing of an ICBM. This missile flew over Japan causing the Japanese government to retract 1 billion in aid for two civilian light-water reactors.

2002 Early in the year construction of a suspected uranium enrichment facility at the Kangson enrichment site started. 29 January: U.S. President George W. Bush in his State of the Union Address named North Korea as part of the axis of evil, aiming to threaten the peace of the world and posing a grave danger. 7 August: "First Concrete" pouring at the construction site of the light-water nuclear power plants being built by the Korean Peninsula Energy Development Organization under the 1994 Agreed Framework. Construction of both reactors was many years behind the agreement's target completion date of 2003. 17 September: Japanese Prime Minister Junichiro Koizumi becomes the first Japanese prime minister to visit Pyongyang, making a number of political and cooperative offers. 3–5 October: On a visit to the North Korean capital Pyongyang, US Assistant Secretary of State James Kelly presses the North on suspicions that it is continuing to pursue a nuclear energy and missiles programme. Mr Kelly says he has evidence of a secret uranium-enriching program carried out in defiance of the 1994 Agreed Framework. Under this deal, North Korea agreed to forsake nuclear ambitions in return for the construction of two safer light water nuclear power reactors and oil shipments from the US. 16 October: The US announces that North Korea admitted in their talks to a "clandestine nuclear-weapons" program. 17 October: Initially the North appears conciliatory. Leader Kim Jong-il says he will allow international weapons inspectors to check that nuclear facilities are out of use. 20 October: North-South Korea talks in Pyongyang are undermined by the North's nuclear program "admission". US Secretary of State Colin Powell says further US aid to North Korea is now in doubt. The North adopts a mercurial stance, at one moment defiantly defending its "right" to weapons development and at the next offering to halt nuclear program in return for aid and the signing of a non-aggression pact with the US. It argues that the US has not kept to its side of the Agreed Framework, as the construction of the light water reactors—due to be completed in 2003—is now years behind schedule. 14 November: US President George W Bush declares November oil shipments to the North will be the last if the North does not agree to put a halt to its weapons ambitions. 18 November: Confusion clouds a statement by North Korea in which it initially appears to acknowledge having nuclear weapons. A key Korean phrase understood to mean the North does have nuclear weapons could have been mistaken for the phrase "entitled to have", Seoul says. 27 November: The North accuses the US of deliberately misinterpreting its contested statement, twisting an assertion of its "right" to possess weapons into an "admission" of possession. December: South Korean presidential election. The Grand National Party, who opposed the Sunshine Policy which was conceived in 1998, made much of the North Korean situation, although it eventually lost the election. 4 December: The North rejects a call to open its nuclear facilities to inspection. 11 December: North Korean-made Scud missiles are found aboard a ship bound for Yemen. The US illegally detains the ship, but is later forced to allow the ship to go, conceding that neither country has broken any law. 12 December: The North pledges to reactivate nuclear facilities for energy generation, saying the Americans' decision to halt oil shipments leaves it with no choice. It claims the US wrecked the 1994 pact.