Does North Korea really have an H-bomb?

Courtesy of Andy Frassetto, Incorporated Research Institutions for Seismology
Seismograms of North Korea’s four declared nuclear tests that were recorded in Mudanjiang, China, approximately 370 kilometers from the test site.

North Korea claims to have detonated its first hydrogen bomb yesterday. But experts are skeptical that the pariah state detonated—not an ordinary atomic device—but a much more powerful “H-bomb of justice,” as state media is now calling it. So what kind of device did the reclusive regime test? And how can nuclear jockeys make such a determination from afar?

There’s no doubt that North Korea detonated something near where it conducted nuclear tests in 2006, 2009, and 2013. Seismic stations yesterday recorded a magnitude-5.1 earthquake with a waveform nearly identical to those registered after North Korea’s earlier tests, supporting its claim. The waveform confirms that an explosion triggered yesterday’s earthquake, says Brian Stump, a seismologist at Southern Methodist University in Dallas, Texas. “It could be a chemical or nuclear explosion, but because of the magnitude it is likely a nuclear explosion,” he says. Researchers are now “chewing through the waveforms” registered by seismometers in the region “to see what’s different from 2013,” says Andy Frassetto, a seismologist with the Incorporated Research Institutions for Seismology consortium in Washington, D.C.

The estimated magnitude of yesterday’s detonation, 7 to 10 kilotons, equates to a small fission bomb. Compared to standard H-bombs, which get most of their ferocity from fusing hydrogen, that’s downright puny. The most powerful H-bomb ever tested had a yield of 50 megatons, around 2000 times more powerful than the 21-kiloton bomb dropped on Nagasaki at the end of World War II.

There are “multiple explanations for North Korea’s consistently low weapon yields,” says R. Scott Kemp, a nuclear physicist at the Massachusetts Institute of Technology in Cambridge. “My best guess is that nobody really knows, even in the darkest corners of the CIA.” One line of thinking is that North Korea yesterday may have tested a “primary:” a miniaturized atomic bomb used to initiate a hydrogen bomb. In this case, the hydrogen bomb part might not have existed, or might have failed. Alternatively, the test could have been a standard fission bomb that relies primarily on plutonium or uranium for its explosive yield.

An H-bomb would require much more sophistication. But it’s something North Korea clearly covets. State media in recent years have touted progress toward a fusion device, and last month the nation’s leader, Kim Jong Un, stated that his country has an H-bomb.

So could yesterday’s blast plausibly have been an H-bomb? “First,” Kemp says, “you need to distinguish between a legitimate H-bomb and a fusion-boosted device,” the latter being a kind of turbocharged fission bomb that uses a small thermonuclear reaction to increase yield. By comparison, a traditional H-bomb—the sort of device that comprises most weapons stockpiled by the United States and Russia—is a two-stage device with a dedicated thermonuclear secondary. The first stage is a fission explosion. It releases x-rays that heat and implode a hydrogen-based secondary, causing the atoms to fuse and release massive quantities of energy. “The secondary is how you get to very large yields,” Kemp says.

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