How did North Korea’s nuclear tests become more powerful between 2006 and 2017?

Between 2006 and 2017, North Korea conducted six underground nuclear tests at the Punggye-ri site, with each successive event demonstrating greater technical sophistication and explosive yield.1

The progression of these tests reflects the regime's advancement in weapon engineering and design:

• 2006–2009: The initial 2006 test was only partially successful, yielding less than two kilotons. By 2009, the regime achieved a more powerful test with an estimated yield of 2–6 kilotons, signaling improvements in design.1
• 2013–2016: The 2013 test yielded roughly double that of the 2009 event. In January 2016, North Korea claimed to have tested a hydrogen bomb; however, experts suggested the yield was more consistent with a boosted-fission device. By September 2016, a subsequent test exceeded 10 kilotons.1
• 2017: The final test in this period, conducted in September 2017, was the country's most powerful to date, with estimates ranging from 50 to 160 kilotons.1

These tests were part of a broader national priority to develop a credible nuclear weapons deterrent, supported by parallel pathways in plutonium production and uranium enrichment at the Yongbyon facility.1