The feat instantly erased lingering skepticism about Pyongyang's technical capabilities and brought the prospect of nuclear-tipped North Korean intercontinental ballistic missiles a step closer to reality, U.S. analysts and weapons experts said. Many predicted that a miniaturized version of the presumed thermonuclear bomb would soon be in North Korea's grasp, and that it probably already exists.
"North Korea has achieved a capability to wipe out a big chunk of any major city," said Sue Mi Terry, a former senior analyst on North Korea at the CIA and now managing director for Korea at the Bower Group Asia. "If the North didn't test a hydrogen bomb, as they said they did this time around, they will get there very soon."
The blast, at exactly noon local time in the country's northeastern mountains, produced seismic waves equivalent to a 6.3-magnitude earthquake, or 10 times as strong as the country's last nuclear test, which occurred a year ago this week. A conclusive analysis will take days or weeks, but weapons experts said the sheer force of the explosion is highly suggestive of a thermonuclear bomb. Sometimes called hydrogen bombs or H-bombs, these second-generation nuclear devices entered U.S. and Soviet arsenals in the 1950s, threatening adversaries with a vastly greater destructive force compared with atomic bombs dropped on Japan in the final days of World War II.
Because of the H-bomb's relatively complex two-stage design, many experts thought it would be months, or perhaps years, before North Korea's scientists could master the necessary technology. When Pyongyang boasted last year that it had tested a thermonuclear device, many U.S. experts dismissed the claim as propaganda.
By early Sunday, Washington time, the skepticism had mostly evaporated.
"There's little doubt in my mind," said James M. Acton, a physicist and co-director of the Nuclear Policy Program at the Carnegie Endowment for International Peace, a Washington think tank. "North Korea has been hinting for a while that it was working on an H-bomb — even apart from the photos it released last night — so this should not come as a huge surprise. But it does represent a significant technological advance."
The apparently successful test came hours after leader Kim Jong Un appeared on state-run television with what appeared to be a prototype of a new North Korean thermonuclear bomb, in a remarkable display of his confidence in the capabilities of his country's weapons engineers. Given other recent technical gains in producing long-range missiles and miniaturized warheads, U.S. experts said there is little doubt about North Korea's ability to eventually master all the steps needed to send a nuclear-tipped missile halfway around the world.
Although it is not known for certain that North Korea can build a miniaturized thermonuclear warhead that can fit on a missile, Acton said, "I believe we have to assume it can."
Peter Zimmerman, a nuclear physicist and former chief scientist for the Senate Foreign Relations Committee, said initial calculations based on seismic readings suggested a device with a yield of up to 200 kilotons — a destructive force 13 times as powerful as the bomb dropped on Hiroshima, Japan, in 1945, and probably "too big for a pure fission bomb." Moreover, the prototype displayed by Kim on the eve of the test "pretty well shows they know the essentials of a thermonuclear device design," he said.
Several other nuclear experts noted that the vaguely peanut-shaped metallic device shown on North Korean television bore features that were broadly consistent with a two-stage hydrogen bomb, although it did not resemble any weapon in past or current U.S. arsenals.
"This was a major step forward for the [North Korean] scientists and engineers," Zimmerman said. "Their first test was a dud; the next couple were very low yield. Since then, their yields have steadily gone up. But this is a discontinuity indicating the introduction of new technology."
The technical hurdles appear to be falling at a surprising clip, considering North Korea's economic backwardness and diplomatic isolation. Yet, Pyongyang's progress with nuclear weapons roughly parallels that of other countries that developed the same weapons decades ago, said Jeffrey Lewis, founder of Arms Control Wonk, an influential blog focused on nuclear weapons proliferation.
"If you look at the United States, the Soviet Union and China, by their fifth nuclear tests they were all well on their way to thermonuclear weapons," Lewis said. "There is no reason to think that North Korea couldn't do this. The materials are pretty straightforward, so that's not a problem. In the past, the trick was the concept. But you also need tests and data to understand how the materials behave."
For longtime North Korea watchers, Sunday's test was another in a succession of technical surprises. Exactly two months earlier, on July 4, Pyongyang launched what many experts think was the country's first intercontinental ballistic missile, capable of reaching Alaska and perhaps cities in the Midwestern United States. Weeks later, U.S. intelligence officials formally concluded that North Korea is able to build miniaturized nuclear warheads that can fit inside the country's long-range missiles.
In each case, North Koreans announced the achievement of the new milestone well in advance — often eliciting scoffs from experts — before they offered proof. Given the rapid pace of North Korea's advances, analysts can no longer afford to dismiss Kim's claims as mere propaganda or empty boasts, said Joshua Pollack, an expert on nuclear and missile proliferation.
"What they show us is going to be the real deal, or very true to life," said Pollack, a senior researcher with the James Martin Center for Nonproliferation Studies at the Middlebury Institute for International Studies in Monterey, Calif. "Given the closeness in time between the exhibition of the device and the actual test, I wouldn't be surprised if that was the actual device" shown on state-run television the day before the blast.
"They have very good reasons to show exactly what they've got, because they're trying to enhance their credibility," Pollack added. "I give them the benefit of the doubt on this one."