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.
A boosted device is simpler. Heavy hydrogen—deuterium and tritium gas—are injected into the center of a plutonium pit, Kemp says. The heat of the fission reaction ignites a short-lived fusion reaction that enhances—or boosts—the yield, but not to the degree of a two-stage bomb. Such a device is much more plausible than an H-bomb, says Li Bin, a nuclear analyst at the Carnegie Endowment for International Peace in Washington, D.C. Yesterday’s test, he says, “could have been a boosted bomb.”
Assuming North Korea doesn’t invite nuclear inspectors to its test site, the only way outsiders could determine whether it exploded an H-bomb or a boosted bomb would be through a careful analysis of the fission products released in the blast. North Korea’s test was underground, but unless it was exceptionally well-contained, radioactive noble gases—primarily radioxenon isotopes—could start leaking into the atmosphere in the next couple days, says Anders Ringbom, a nuclear physicist at the Swedish Defence Research Agency in Stockholm. “If you detect the xenon isotopes, that's the smoking gun that proves the detonation was nuclear in nature.”
The neutrons generated in a fusion reaction are about seven times as energetic as neutrons generated in a fission reaction, which means that the species of radioisotopes spawned by an H-bomb are different from that of a fission bomb. “You should be able to detect this,” Kemp says, depending on how quickly the radioactive gases are able to escape into the atmosphere and get sampled. “It becomes more and more difficult the longer it takes to leak.”
The Comprehensive Test Ban Treaty Organization (CTBTO) operates a global network of radioactive noble gas detectors. Its network picked up on xenon-133 and xenon-131m leaking several weeks after the 2013 test. The late sightings did not provide enough information to determine whether Korea used plutonium or uranium as the fissile material that time. Japan, South Korea, and the United States also have airborne noble gas detectors thought to be even more sensitive than the CTBTO detectors. But whatever they learned from previous North Korean tests has not been made public. And that’s likely to be the case this time around, too, which means we might have no choice but take North Korea’s word—or leave it—about the new “H-bomb of justice.”