North Korea was celebratory in its claims that it detonated its first hydrogen bomb on Wednesday.
"Through the test conducted with indigenous wisdom, technology and efforts [North Korea] fully proved that the technological specifications of the newly developed H-bomb for the purpose of test were accurate and scientifically verified the power of smaller H-bomb," the country's official news agency reported.
But the White House, along with many others, isn't buying it.
"The initial analysis is not consistent with the North Korean claims of a successful hydrogen bomb test," White House Press Secretary Josh Earnest said in a briefing.
South Korean officials have also questioned the North Korean claim.
Independent experts share these doubts for one simple reason: The big bang from North Korea's test just isn't big enough.
Nuclear weapons come in two varieties: fission weapons and fusion weapons. These days, the fission weapons are the ones that usually make the headlines. They get their explosive power from the splitting, or fission, of heavy elements like uranium and plutonium. The fission releases energy that can be used in a bomb.
Fusion, on the other hand, gets its power from fusing light elements, such as hydrogen, together. The fusing of light atoms actually releases more energy than the splitting of heavy ones. So hydrogen bombs are far more powerful.
Rough estimates of North Korea's test on Wednesday range from a few kilotons of explosive power to a few tens of kilotons.
By contrast, the first U.S. test of a hydrogen bomb in 1952 was 10.4 megatons, roughly a thousand times as powerful.
"The apparent yield of the [North Korean] event is too small," says Jeffrey Lewis, of the Middlebury Institute of International Studies at Monterey.
So what could North Korea have done instead?
One possibility, suggests Lewis, is that it set off a "boosted" fission bomb. Such a weapon uses a small amount of hydrogen gas at its core to enhance the explosion of the uranium or the plutonium in the weapon. Since boosting technically involves the fusion of hydrogen, North Korea could call this test a "hydrogen bomb."
It may be possible to glean further clues about what exactly North Korea has done. The isolated nation is surrounded by International monitoring stations of the Comprehensive Test Ban Treaty Organization, which seeks to track nuclear testing worldwide.
Although the test was conducted underground, tiny quantities of radioactive xenon gas could be picked up by detectors hundreds or even thousands of miles away.
The exact composition of the xenon could yield clues about what kind of bomb it was, says Anders Ringbom, research director of the Swedish Defence Research Agency in Stockholm. "We believe in principle that it could be possible," he says.
Yet so far, he adds, the stations haven't seen anything.
UNIDENTIFIED WOMAN: (Speaking Korean).
AUDIE CORNISH, HOST:
That was an North Korean state television news reader announcing the country had tested a hydrogen bomb. If true, it would be a first for Pyongyang. Now, this afternoon, the White House cast doubt on the ability of the isolated nation to test such an advanced weapon. Here's Press Secretary Josh Earnest speaking to reporters.
(SOUNDBITE OF PRESS CONFERENCE)
JOSH EARNEST: The initial analysis that's been conducted of the events that were reported overnight is not consistent with North Korean claims of a successful hydrogen bomb test.
CORNISH: Coming up, we'll speak to a former State Department official about what North Korea hopes to gain by conducting the nuclear test. First, joining me in the studio is NPR science correspondent Geoff Brumfiel to talk about the claims from Pyongyang. Geoff, welcome to the studio.
GEOFF BRUMFIEL, BYLINE: Thank you.
CORNISH: The White House sounds fairly confident there that this was not a hydrogen bomb test. What are they basing this on?
BRUMFIEL: Well, when a nuclear weapon is tested, it sends off a shockwave through the earth. It's basically like an earthquake. And seismic stations around the world can pick that up, and they can make some sort of estimate about the size. Based on what was observed, the unofficial estimates by the Pentagon say this was under 10 kilotons. That's 10,000 tons of TNT-equivalent explosives. But the bottom line is that seems too small to be a hydrogen bomb. Just for comparison here, the first hydrogen bomb was 10 megatons. That's 10 million tons of TNT - a thousand times more powerful. It would make this thing look like a firecracker.
CORNISH: We should note that North Korea has, as far as what's known, tested three atom bombs. Now, what makes hydrogen bombs harder to build?
BRUMFIEL: There's basically two ways to make a nuclear explosion. You can either split apart heavy atoms like uranium and plutonium, and that splitting apart, called fission, releases energy. Or you can fuse together light atoms such as hydrogen. That's where the hydrogen bomb comes from. Causing those atoms to stick together is a lot harder than splitting them apart. In fact, you have to set off a fission bomb to make a hydrogen bomb work, so it's technically much more complicated.
CORNISH: So if this wasn't an H-bomb, what was it? I mean, could it have been a replica of the previous atom bomb test?
BRUMFIEL: Yeah, it could have been a standard nuclear bomb. And I don't want to - I say standard. That kind of downplays it. I mean, this would still be a bomb roughly the size of those used on Hiroshima and Nagasaki, so this is still a very serious thing. But there is another possibility, which is that North Korea put a little bit of hydrogen at the center of their bomb, and then they used that hydrogen to enhance the fission bomb. That would allow them to claim they had conducted an H-bomb test even though it wasn't quite what we would consider a hydrogen bomb.
CORNISH: We know what the White House thinks, but in the end, how will the international community verify what happened?
BRUMFIEL: Well, we may never be able to say much more, but there is a possibility that radioactive gases will leak out. These tests are conducted underground, and so we have to wait for that to happen. It could take days or even weeks. There are monitoring stations all around North Korea in Russia, China, places like that. And if they pick something up, if they catch a whiff, we may learn more about exactly what happened.
CORNISH: That's NPR science correspondent Geoff Brumfiel. Geoff, thanks.
BRUMFIEL: Thank you. Transcript provided by NPR, Copyright NPR.