The underground explosion that rocked North Korea’s northeastern coast Wednesday triggered seismic shock waves that measured 5.1 magnitude, suggesting that the device was substantially more powerful than Pyongyang’s first claimed atomic test a decade ago.
But nuclear experts saw plenty of reason to doubt the country’s assertion that it had successfully detonated a hydrogen bomb — just before leader Kim Jong Un’s 33rd birthday.
The evidence so far, they said, points to something well short of a true H-bomb, or thermonuclear device, a class of weapons that is more powerful and technically challenging than any the North Koreans have ever built.
“On balance, it is not believed that North Korea tested a two-stage H-bomb,” said David Albright, a former U.N. weapons inspector and president of the Institute for Science and International Security, a Washington, D.C., organization that analyzes nuclear weapons programs around the world.
Not enough is known to say definitely what blew up inside the Punggye-ri nuclear test site, Albright and other experts acknowledged. But although North Korea’s scientists have managed to overcome many technical hurdles needed to develop relatively simple atomic weapons, a hydrogen bomb “is assessed as being beyond North Korea’s capabilities,” Albright said.
And nothing in the data being analyzed by U.S. officials and independent analysts suggests that Kim has crossed that line.
The nuclear test, if verified, would be North Korea’s fourth since that country began to test atomic weapons in 2006. The first, on Oct. 10, 2006, was a relatively small explosion that independent analysts deemed a “fizzle,” or partial detonation. Radioactive traces from that test were picked up by sensors around the world, and seismic data suggested that the device’s explosive yield was equal to about a half-kiloton of TNT. By comparison, the U.S. atomic bomb dropped on Nagasaki, Japan, in 1945 had a yield of about 20 kilotons.
The second test, in 2009, appeared to be more successful, with an estimated yield of 2 to 7 kilotons, while the third, in 2013, was measured at up to 10 kilotons.
Although it is technically possible that Pyongyang designed a hydrogen bomb that either fizzled or produced a very low yield, the more likely explanation is that the device was a “boosted fission” bomb, said Karl Dewey, an analyst with IHS Jane’s.
