The North’s 2013 test produced an estimated yield of 6-7 kilotons of explosives, according to South Korean officials.
Fusion is the main principle behind the hydrogen bomb, which can be hundreds of times more powerful than atomic bombs that use fission. In a hydrogen bomb, a nuclear fission explosion sets off a fusion reaction responsible for a powerful blast and radioactivity.
The hydrogen bomb already is the global standard for the five nations with the greatest nuclear capabilities: the U.S., Russia, France, the U.K. and China. Other nations may either have it or are working on it, despite a worldwide effort to contain such proliferation.
Just how big a threat North Korea’s nuclear program poses is a mystery. North Korea is thought to have a handful of rudimentary nuclear bombs and has spent decades trying to perfect a multistage, long-range missile to carry smaller versions of those bombs.
Some analysts say the North probably hasn’t achieved the technology needed to make a miniaturized warhead that could fit on a long-range missile capable of hitting the U.S. mainland. But debate is growing on just how far the North has advanced.
To build its nuclear program, the North must explode new and more advanced devices so scientists can improve their designs and technology. Nuclear-tipped missiles could then be used as deterrents and diplomatic bargaining chips — especially against the U.S., which Pyongyang has long pushed to withdraw its troops from the region and to sign a peace treaty formally ending the Korean War.
U.S. aircraft designed to detect evidence of a nuclear test, such as radioactive particulate matter and blast-related noble gases, could be deployed from a U.S. base on the Japanese island of Okinawa. Japanese media said Tokyo mobilized its own reconnaissance aircraft over the Sea of Japan to try to collect atmospheric data.
Associated Press writers Kim Tong-hyung and Youkyung Lee contributed to this report
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