Five things science can (and can’t) tell us about North Korea’s nuclear test

Blast waves rippling through the ground from North Korea and picked up by seismic sensors suggest that the country has tested yet another nuclear weapon (site marked with star). But the North Korean government's claim that the device was a powerful hydrogen bomb doesn't line up with the scientific evidence.

North Korea sent political shock waves around the world on January 6 when it claimed to have carried out a successful test of a hydrogen bomb, which, if true, would be a substantially more powerful and sophisticated class of weaponry than the country’s previous efforts. The underground test generated a magnitude 5.1 earthquake, according to the U.S. Geological Survey.

Verifying the country’s boast will be tricky, however. Here’s what experts have sussed out so far and which questions are still up in the air.

The quake wasn’t naturally caused.

A natural earthquake kicks off when rock under intense stress suddenly breaks and slides. These quakes occur over a wide section of a seismic fault, whereas a nuclear blast can be pinpointed to a single location. Natural quakes also start off slow and then ramp up to full strength. Artificial blasts, on the other hand, start strong and fade away.

Based on the seismic waves picked up by nearby seismometers, the tremors emanating from North Korea didn’t originate from a natural earthquake, experts say. The shaking also came from very close to Earth’s surface, whereas many strong earthquakes originate from deeper down.

A recent burst of tremors picked up by seismometers in China is consistent with a North Korean nuclear bomb test, rather than a natural earthquake.

The explosion appears nuclear.

Nuclear bombs cause much bigger booms than their nonnuclear counterparts. A 2013 nuclear test in North Koreareleased energy equivalent to the detonation of around 6 to 9 kilotons of TNT. For comparison, the 1917 Halifax Explosion in Nova Scotia, the largest artificial explosion until the advent of nuclear weapons, only clocked in at the equivalent of about 2.9 kilotons of TNT.

The blast originated from North Korea’s nuclear testing site.

By measuring the time it took blast waves from the test to reach various locations, seismologists were able to triangulate the source back to the country’s Punggye-ri Nuclear Test Site — the site of the country’s 2006, 2009 and 2013 nuclear tests.

North Korea probably did not successfully detonate a hydrogen bomb.

While North Korea declares that it tested a hydrogen bomb, a lot of evidence doesn’t fit that claim, experts say.

H-bombs, also called thermonuclear bombs, are much more powerful than the plutonium-powered weaponry that North Korea has tested in the past. The country’s early devices broke apart plutonium atoms in a process called fission, releasing large amounts of energy.

A hydrogen bomb, however, fuses hydrogen atoms together to produce its boom, similar to nuclear reactions in the sun. That atomic assembly requires colossal amounts of energy, however, so H-bombs include a primary fission bomb that feeds energy into a secondary fusion bomb.

The two-stage explosive process generates a much more powerful blast than a fission-only device. North Korea’s recent test, however, produced an explosion of roughly the same intensity as its 2013 fission bomb test. A full-scale thermonuclear detonation should have generated a blast at least 10 times bigger, experts say.

The test’s relatively puny size suggests that either the North Korean government exaggerated its bomb’s capabilities, or the bomb tore itself apart without successfully igniting its secondary fusion stage, nuclear experts contend.

Experts can’t rule out an H-bomb, but that’s partly semantics.

The United States and other countries have launched aircraft to patrol the skies near North Korea’s border, sniffing the air for bits of radioactive debris from the test. Which radioactive isotopes show up in that debris could help scientists determine whether they came from fusion or fission reactions. Getting any debris might be difficult, however, because North Korea detonated the bomb underground. Similar attempts following the 2009 nuclear test failed to turn up any radioactive debris.

North Korea may have tested a hybrid bomb that included some nuclear fusion fuel alongside a primary stage made up of uranium or plutonium. This kind of “boosted” bomb would result in an explosion less powerful than a full thermonuclear device, but more potent than a purely fission-powered bomb of similar size. While not an H-bomb by strict definition, a similar device was tested by the United States in 1951 during the country’s development of thermonuclear weapons. A test of a boosted bomb could similarly be an important step toward North Korea becoming a thermonuclear nation.

sciencenews.org