Science Can Tell if North Korea’s Test Was Really an H-Bomb
It was the whomp felt ’round the world. Last night, at around 1:30 AM UTC, monitoring stations—over two dozen total in North America, Asia, and Europe—picked up a telltale seismic rattle. They triangulated the signal back to its source in North Korea’s northeastern highlands at almost the exact moment that the rogue nation distributed a triumphal press release declaring that it had successfully tested its first hydrogen bomb.
Which might not be true. North Korea has a history of exaggerating its military claims to achieve its political ends. (South Korea, the US, and Japan are typically named, but for practical purposes everybody besides Dennis Rodman and some Chinese political factions count as North Korea’s enemies). And because North Korea’s leader Kim Jong Un is unlikely to let international inspectors anywhere near the test site, the only real way to tell whether North Korea’s big boom was the big H is by analyzing data collected from a suite of global sensors.
North Korea has detonated nuclear weapons at least three times in the past. Based on analysis, most experts believe that those previous tests were atomic bombs, but not hydrogen bombs. What’s the difference? Well, “normal” atomic bombs rely solely on fission—that is, splitting an atom (typically plutonium or enriched uranium), which releases a bunch of energy and creates a big boom. Big enough to level the Japanese cities of Nagasaki and Hiroshima in 1945, killing over 200,000 civilians and military personnel.
Hydrogen bombs, on the other hand, use nuclear fusion—melding atoms together—to release way more explosive energy. These “thermonuclear” weapons are so powerful that they actually need atomic fission to kickstart the fusion process. That’s right, H-bombs use an A-bomb just to get going. American scientists detonated the first H-bomb in 1952, on a Pacific atoll. It was over 500 times more powerful than the bomb the US dropped on Nagasaki. Modern H-bombs are at least twice as powerful. Which is why everyone is so freaked out about whether North Korea, the world’s most famous renegade nation, has a hydrogen bomb.
But whether there’s fury behind the sound is still in question. Whatever North Korea blew up, it did so underground. Which is why the specific characteristics of seismic energy are among the most important analytical factors in finding out exactly what made the earth move. “When you squeeze or stretch a rock, it propagates just like sound does,” says Terry Wallace, a Principal Associate Director for Global Security. Practically speaking, Wallace is a forensic seismologist, solving geopolitical mysteries by looking at signatures in the earth.
Explosions, volcanic eruptions, and underground collapses predominantly compress rock, creating what’s called a P-wave. Earthquakes, which usually happen when two pieces of rock slip past each other, cause shearing and twisting that create S-waves. “Imagine you have a slinky. If you hit just one end of it, the slinky compresses, then releases in a wave from source to receiver,” says Wallace, describing a P-wave. “In an S-wave, you are actually going to shake the slinky from side to side.” Seismograph wiggles record shaking in three dimensions, which give telltale signs of what kind of waves come out.
A big underground explosion pushes a bunch of rock away all at once, and therefore makes mostly S-waves. But the signal can get distorted—bent, reflected, refracted—by discontinuities in the earth. Even changes in underground temperature can muck up the readings. “A piece of stuff you’d see around Old Faithful is going to be much less efficient at transmitting energy than the tough granite of Mt. Rushmore,” says Wallace.
Which is why seismologists take recordings from multiple sensors. The agency responsible for monitoring atomic blasts, the Comprehensive Nuclear-Test-Ban Treaty Organization, currently has 42 certified seismic stations distributed around the globe (plus over 100 auxiliary stations). Because seismic signals bounce through the Earth, not only did Russia and Japan pick up North Korea’s event, but so did the US.
Besides looking at the seismic signal’s strength—today’s was a 5.1 magnitude—scientists have a hard time telling whether an explosion was chemical or nuclear. “Nuclear energy does get released over a shorter amount of time,” says Wallace, which would show up on the seismograph. But with enough TNT or other conventional explosive, he says a country could make a blast that looks a lot like nuclear.
And that’s not even getting at the difference between atomic and hydrogen. The smoking gun can only really come by detecting radioactive material. To that end, CTBTO has radionuclide detection stations scattered throughout the globe. These come in two flavors. The first looks for radioactive dust—fallout. These systems use suction pumps to pull air through a filter, which then goes through a radiation counter. The types of particles present, and their radioactivity, would give a lot of clues as to the bomb’s type. Let’s say you have a typical atom bomb: Its fallout particles would be decayed bits of uranium or plutonium.
A hydrogen bomb also uses those materials, but they’d be mostly burned away by the super hot fusion reaction. According to this 1991 analysis of a Chinese explosion published in Science and Global Security, an H-bomb’s radioactive particulate signature would have a lot less decayed plutonium and uranium, and also different ratios of their various decayed isotopes. But if someone knew the exact particles found after an H-bomb went off, they could use that knowledge to build their own H-bomb (that’s probably one of the ways the Soviets copied the US’s weapon). Which is why Wallace told me the details of the analysis are secret. But if the blast is underground, as this one seems to have been, radionuclide detection is little help—the particles get contained.
The other type of detector looks for radioactive gases, rather than particles. Xenon gas is the most potent of these, partly because it is a noble gas that doesn’t interact with other substances. Xenon can, however, decay. And the rate of decay tells scientists the gas atoms’ exact age. For instance, after North Korea’s 2013 test, a Japanese sensor picked up xenon isotopes that scientists deduced were exactly 55 days old. The exact same day as North Korea’s test.
Xenon doesn’t just come in one flavor. Xenon has four different varieties associated with nuclear activity, and not all of it from weapons. Xenon can leak during medical isotope production, or from nuclear power plants (alarming for different reasons). “It’s not so much whether one type of radionuclide is present. People look at ratios between different radionuclides,” says Randy Bell, Director of International Data Center for CTBTO. Of course, hydrogen bombs would have different gas signatures than atomic bombs, but those differences are state secrets.
Time isn’t the only predictor. Analysts can look at weather models to see how particles might have traveled in the wind. When the Japanese detector spiked in 2013, atmospheric transport modeling traced the xenon molecules back to Punggye-ri, the North Korean source of today’s seismic rattling. And that technology runs forward, too. CTBTO’s sensors are stationary, but you can bet the US and other countries have planes flying downwind of Punggye-ri.
What they find might be something that’s neither strictly atomic nor hydrogen. These days an old fashioned fission bomb can come in “boosted” flavor, a sort of hybrid between fission and fusion weapons. Rather than use hydrogen, boosted fission bombs use tritium and deuterium fusion to ratchet the bomb’s temperature up, increasing its explosive energy. Worse than a straight up A-bomb, but not nearly as brow-sweatingly bad as an H-bomb. Which is in fact what US officials are hinting at.
What’s at Risk
Of course, according to international treaties—of which North Korea is not a signatory—no one is supposed to be detonating either kind of bomb. Bob Menendez, a Democratic senator from New Jersey and the ranking member of the Senate Foreign Relations Committee (and author of the 2015 North Korea Sanctions Act), called for tough new sanctions against the regime in a statement. “North Korea’s nuclear detonation claims are a grave provocation and threat to international peace and security, and must be met with firm action and a clear effort to halt Pyongyang’s nuclear ambitions,” Melendez’s office wrote, pushing yet more sanctions against what might be the most sanctioned country on Earth.
The real concerns, of course, are whether North Korea could drop an atomic bomb on someone else. That takes a delivery vehicle, probably a missile. And the country doesn’t seem to have one of those. Even if they built an H-bomb, North Korea’s engineers still haven’t tested a “re-entry vehicle” that can protect the nuclear warhead from atmospheric heat that builds up as the rocket sails down to its target, says Hans M. Kristensen, director of the Nuclear Information Project at the Federation of American Scientists. “The device they are testing underground is different from the way it would look and fit into a ballistic missile,” Kristensen says. “They have only conducted four [Ed: technically three, at this point] tests. It would take quite a few to advance the weapon to fly it in a missile.” (For what it’s worth, Kristensen doesn’t think it was an H-bomb, either.) Though he adds that the country could certainly put a bomb on a ship and float it into someone else’s harbor.
Kingston Reif, Director for Disarmament and Threat Reduction Policy at the Arms Control Association in Washington, says North Korea is working on an intercontinental ballistic missile—the KN-08—that could fly across the Pacific. But so far it has only appeared in military parades. “It’s not believed to be operational,” Reif says. “The best estimate I have seen is early 2020s, at the earliest.” The country does have other, shorter range missiles, though.
That’s why it matters not just what kind of bomb North Korea detonated, but that the country detonated one at all.
With additional reporting by Eric Niiler and Sarah Zhang.