Pyongyang's announcement yesterday was met with widespread skepticism, but whatever the North detonated in its fourth nuclear test, another round of tough international sanctions looms for the defiant, impoverished country.
The test likely pushed Pyongyang's scientists and engineers closer to their goal of building a warhead small enough to place on a missile that can reach the US mainland. But South Korea's spy agency thought the estimated explosive yield from the explosion was much smaller than what even a failed H-bomb detonation would produce.
The test was met with a burst of jubilation and pride in Pyongyang. A North Korean television anchor, reading a typically propaganda-heavy statement, said a test of a "miniaturised" hydrogen bomb had been a "perfect success" that elevated the country's "nuclear might to the next level".
North Korea's state media stood firm in saying the test was a self-defence measure against a potential US attack. "The [country's] access to H-bomb of justice, standing against the US, the chieftain of aggression ..., is the legitimate right of a sovereign state for self-defence and a very just step no one can slander."
In Seoul and elsewhere there was high-level worry. South Korean President Park Geun-hye ordered her military to bolster its combined defence posture with US forces. She called the test a "grave provocation" and "an act that threatens our lives and future". Japanese Prime Minister Shinzo Abe said: "We absolutely cannot allow this."
Washington and nuclear experts have been skeptical about past North Korean claims about H-bombs, which are much more powerful and much more difficult to make than atomic bombs. A confirmed test would further worsen already abysmal relations between Pyongyang and its neighbours and lead to a strong push for tougher sanctions on North Korea at the United Nations. The Security Council was holding an emergency meeting.
Britain's ambassador to the United Nations, Matthew Rycroft, says the meeting will aim to agree on a statement condemning the nuclear test and will follow up with a new resolution expanding sanctions against North Korea. He told reporters, "the Security Council needs to be clear in its condemnation and robust in its response."
A successful H-bomb test would be a big advance in North Korea's nuclear weapons programme. 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, radiation from a nuclear fission explosion sets off a fusion reaction responsible for a powerful blast and radioactivity.
A South Korean lawmaker said the country's spy agency told him that Pyongyang may not have conducted an H-bomb test given the relatively small size of the seismic wave reported. An estimated explosive yield of 6 kilotonnes and a quake with a magnitude of 4.8 (the US reported 5.1) were detected, lawmaker Lee Cheol Woo said the National Intelligence Service told him. That's smaller than the estimated explosive yield of 7.9 kilotonnes and 4.9-magnitude quake reported after the 2013 nuclear test, he said, and only a fraction of the hundreds of kilotonnes that a successful H-bomb test's explosion would usually yield. Even a failed H-bomb detonation typically yields tens of kilotonnes, the NIS told Lee, who sits on the parliament's intelligence committee. A miniaturized H-bomb can trigger a weak quake, but only the US and Russia have such H-bombs, Lee cited the NIS as saying.
"I'm pretty skeptical," said Melissa Hanham, senior researcher at the James Martin Centre for Nonproliferation Studies at the Middlebury Institute for International Studies in Monterey, California. "The seismic data indicates it would be very small for a hydrogen test. "It seems just too soon to have this big technical achievement," she said. "But North Korea has always defied expectations."
While also noting the quake was likely too small for an H-bomb test, Jaiki Lee, a professor of nuclear engineering at Seoul's Hanyang University, said the North could have experimented with a "boosted" hybrid bomb that uses some nuclear fusion fuel along with more conventional uranium or plutonium fuel.
Joel Wit, founder of the North Korea-focused 38 North website, said a boosted bomb "is the most likely option", while adding that he isn't surprised that North Korea has shifted focus to hydrogen weaponry. "Every nuclear power essentially moves down the same track as they develop nuclear weapons," he said. "And that track is miniaturisation, but also increasing the yield of nuclear weapons. That's what the Americans did; that's what the Russians did."
In Pyongyang the announcement was greeted with an expected rush of nationalistic pride, and some bewilderment. Kim Sok Chol, 32, said he doesn't know much about H-bombs, but added that "since we have it the US will not attack us".
University student Ri Sol Yong, 22, said: "If we didn't have powerful nuclear weapons, we would already have been turned into the slaves of the US."
It could take weeks before the true nature of the test is confirmed by outside experts - if they are able to do so at all. US Air Force aircraft designed to detect the evidence of a nuclear test, such as radioactive particulate matter and blast-related noble gases, could be deployed from a US base on the Japanese island of Okinawa. Japanese media said Tokyo mobilised its own reconnaissance aircraft for sorties over the Sea of Japan to try to collect atmospheric data. But North Korea goes to great lengths to conceal its tests by conducting them underground and tightly sealing off tunnels or other vents through which radioactive bomb residue could escape.
Ferenc Dalnoki-Veress, a physicist, scientist-in-residence and professor at the James Martin Centre, said it may not be possible for the monitors to ever determine if Wednesday's explosion was caused by a hydrogen bomb. "For that, you might need to have the particulates," he said. "But maybe we'll be lucky."
The test was unexpected in part because North Korea's last nuclear test was nearly three years ago and Kim Jong Un did not mention nuclear weapons in his annual New Year's speech. Some outside analysts had speculated Kim was worried about deteriorating ties with China, the North's last major ally, which has shown greater frustration at provocations and a possible willingness to allow stronger UN sanctions.
Chinese Foreign Ministry spokeswoman Hua Chunying told reporters that Beijing "firmly opposes" Pyongyang's purported test and is monitoring the environment on its border with North Korea near the test site.
Just how big a threat North Korea's nuclear programme 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 eventually carry smaller versions of those bombs.
Some analysts say the North hasn't likely achieved the technology needed to manufacture a miniaturised warhead that could fit on a long-range missile capable of hitting the US mainland. But the debate is growing on just how far the North has advanced.
North Korea needs fresh nuclear tests for practical military and political reasons. To build a credible nuclear programme, the North must explode new devices - and more advanced ones - so its scientists can continually improve their designs and technology. Nuclear-tipped missiles could then be used as deterrents, and diplomatic bargaining chips, against its enemies - and especially against the United States, which Pyongyang has long pushed to withdraw its troops from the region and to sign a peace treaty formally ending the Korean War.
"This is indeed a wake-up call," Lassina Zerbo, the head of the Vienna-based UN Comprehensive Test Ban Treaty Organisation, which has a worldwide network of monitoring stations to detect nuclear testing, said. "I am convinced it will have repercussions on North Korea and international peace and stability."
