Timeline of the North Korean nuclear program
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 (1956–80) dealt primarily with training and  gaining basic knowledge. Phase II (1980–94) 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) and Phase IV (2002–present) covers the current period of renewed nuclear activities. Nuclear weapons types. Nuclear weapon warhead designs fall into various categories. 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.