U.N. Secretary General Ban Ki-moon calls North Korea's underground nuclear test "deeply troubling" and "profoundly destabilizing" at a news conference on Wednesday, Jan. 6. (Reuters)

North Korea's claim Wednesday that it has tested a hydrogen bomb alarmed Pyongyang's neighbors in Asia — and the rest of the world. We take a look at what's behind this new North Korean claim.

Is this a big deal? We already knew they had nukes, right?

It’s a big deal because a hydrogen, or thermonuclear, bomb is much, much more powerful.

The “Little Boy” atomic bomb that the United States dropped on the Japanese city of Hiroshima in 1945 had a yield of 15 kilotons, while the “Fat Man” dropped on Nagasaki a few days later had a yield of 20 kilotons. By comparison, the dry fuel hydrogen bomb that the United States tested at Bikini Atoll in 1954 had a yield of 15 megatons – making it more than 1,000 times as powerful as the Hiroshima bomb.

Here’s how Kim Du-yeon of the Carnegie Endowment for International Peace explains it:

It's basically a difference in the technical process by which an explosion is obtained and in the explosive power (measured in yield). An atomic bomb uses fission and an H-bomb uses fusion. An H-bomb (thermonuclear bomb) has an exponentially greater yield (thousands of times more powerful). It includes an atomic bomb inside its core that acts as a trigger.

It sounds like the end is nigh. Is it really? 

Hold on there. There’s still a considerable degree of skepticism about whether this really was a hydrogen bomb that was tested today, with lots of smart experts and the South Korean military saying that the size of the blast was not big enough for even a failed H-bomb.

South Korean lawmakers on the parliamentary intelligence committee told local reporters that the supposed hydrogen bomb that North Korea tested Wednesday had a yield of about six kilotons — smaller than North Korea’s 2013 atomic test and a much smaller explosion than usually associated with hydrogen.

Jeffrey Lewis, a nuclear expert at the Middlebury Institute of International Studies, says a few words of caution are in order on the technological front, too. Here’s what he wrote on 38 North after Kim’s claim in December.

Building a staged thermonuclear weapon — one in which the radiation from a fission primary compresses a secondary stage of thermonuclear fuel — seems to be a bit of a stretch for the North Koreans. That is the sort of device one normally thinks about when someone says “H-bomb.” Thermonuclear weapons are tricky; making one work requires a bit of test experience. While the North Koreans finally conducted an unambiguously successful nuclear test in 2013, the 2006 and 2009 tests were less so.

So how will we know?

Well, for starters, the tremor Wednesday was recorded at 5.1 magnitude, according to the U.S. Geological Survey — the same size as the 2013 nuclear test and slightly bigger than the ones in 2009 (4.5) and 2006 (4.1). But if it was a hydrogen bomb, the tremors would be much, much bigger, experts say. Seismologists will now be trying to figure out the exact depth of the quake so that they can work out the precise yield of the blast. That could come within a day or so.

Meanwhile, governments will be trying to capture any radioactive gases from around the site. The United States reportedly sent its WC-135 "sniffer plane" from the southern Japanese island of Okinawa on Wednesday, while Japan said it dispatched three aircraft. Gases can sometimes leak out for several weeks after a test, so it might take some time for this question to be answered.

After the 2006 test, a significant amount of gas was detected, enabling analysts to say it was a plutonium-based device, while in 2009, no gases were found. The residue from the 2013 test was very faint, meaning it was impossible to draw any conclusions, analysts say.

Mark Hibbs, a nuclear expert at the Carnegie Endowment for International Peace, notes:

If North Korea effectively used a small amount of fusion materials to boost the yield of a plutonium device to a level below that of a full-fledged thermonuclear blast, that could be revealed by isotopic relationships in fallout data from the explosion. But this was an underground test, and if North Korea successfully contained the fallout, foreign governments may not get that data.

North Korea said it had successfully conducted a test of a miniaturized hydrogen nuclear device on the morning of Jan. 6, in an announcement broadcast on state-run television. (Reuters)

Why did this test, whatever it was, happen now?

It's Kim Jong Un's birthday on Friday — probably his 33rd, although it could be his 32nd, such is the paucity of our knowledge about the "Great Successor" — so the launch could be an early gift for him. The leaders' birthdays are always celebrated with a lot of fanfare in North Korea, although this is more true of Kim Il Sung, the founding president and the current leader's grandfather, who died in 1994, and of Kim Jong Il, the second in the dynasty, who died in 2011.

More likely this is all about preparing for the much-awaited Seventh Congress of the Korean Workers' Party in May this year, the first in 36 years.

As Toshimitsu Shigemura, a North Korea specialist at Waseda University in Tokyo, puts it: "Kim Jong Un needs great results before the party congress in May. His father didn't test a hydrogen bomb, but now he can say that he has. That's a very great result for him."

And North Korea had hinted that this might be coming. In December, Kim Jong Un said his country was “a powerful nuclear weapons state ready to detonate self-reliant A-bomb and H-bomb to reliably defend its sovereignty and the dignity of the nation.”

Nuclear experts and the South Korean military said the size of the blast was consistent with an atomic explosion, not a hydrogen bomb with far greater explosive power. (The Washington Post)

So what happens next?

Regardless of whether the explosion was atomic or thermonuclear, it was a brazen provocation and a clear defiance of international treaties. So get ready for lots of international condemnation and some stern words at the United Nations. Already North Korea's neighbors — South Korea, Japan and China — have strongly criticized the test, and the United States is getting ready to do so, once it's confirmed. The U.N. Security Council is set to hold an emergency meeting.

The question is: Will the Security Council be able to pass a resolution with teeth?

The Security Council has adopted four major resolutions since North Korea's first nuclear test in 2006 — one after each of the 2006, 2009 and 2013 tests, and another after a satellite launch in December 2012. All have imposed sanctions on North Korea and sought to both stop it from getting the equipment it needs to develop its nuclear weapons program and to pressure it to give up the pursuit of nuclear capability. Clearly, none of these have had much, if any, impact.

Will this time be different?

It really depends how mad China is. In 2013, China — North Korea's closest ally and a veto-wielding permanent member of the council — was so angry with Pyongyang that it did support a resolution that expanded the sanctions regime, notably by making it more difficult for North Korea to transfer money.

But as mad as Beijing gets, it still has its eye on the bigger picture: It doesn't want North Korea to collapse and send millions of hungry refugees over the border into northeastern China; nor does it want the American troops currently in South Korea up on that border. So China's primary interest is stability. But don't expect Xi Jinping to be at the Congress in May.

After North Korea's claim that it tested a hydrogen bomb, The Post's Karen DeYoung explains why the U.S. response to the rogue nation is so different from how it treats Iran. (Jason Aldag,Gillian Brockell/The Washington Post)

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