Why North Korea's latest nuclear weapons test may have been a game-changing explosion

• North Korea on Sunday set off a nuclear blast under a mountain.
• The test was part of leader Kim Jong Un's campaign to make a "city busting" weapon small enough to fit on a rocket.
• Differing seismic measurements and other factors make it difficult to pin down the strength of the blast from afar.
• A thermonuclear bomb may have caused the explosion, as North Korea claims, but other types of atomic bombs may explain its strength.
• Experts are concerned North Korea may soon have a deliverable nuclear weapon if a diplomatic agreement isn't created.

Deep inside a North Korean mountain on September 3, the government of leader Kim Jong Un set off its largest-ever nuclear test blast.

The explosion occurred at the Punggye-ri Nuclear Test Site, a series of tunnels located within Mount Mantap where the so-called Democratic People's Republic of Korea has detonated at least five other nuclear devices.

Early estimates suggest the blast yielded the energy of at least 60 kilotons' worth of TNT, according to NK News, though perhaps more than 100 kilotons' worth, according to 38 North. Other estimates suggest the blast yielded 300 kilotons or perhaps up to 500 kilotons, though that's less likely.

For reference, 100 kilotons is at least twice as energetic as North Korea's previous and formerly most-powerful nuclear test. It's also seven times the yield of the US bomb detonated over Hiroshima that caused some 150,000 casualties.

While 100 kilotons may not seem like much today — both the US and the then-Soviet Union have detonated thermonuclear bombs devices that exploded with millions or megatons' worth of TNT — experts believe it shows North Korea is at least on the path to building such powerful weapons.

"[T]his test demonstrated that North Korea has attained the technical capability to produce a nuclear weapon whose yield corroborates their claims of having a thermonuclear weapon," Dave Schmerler, a researcher at the James Martin Center for Nonproliferation Studies, wrote for NK News on Tuesday.

What's more, the test suggests North Korea is one step closer to fitting a usable warhead on top of an intercontinental ballistic missile — a rocket that's capable of reaching the continental US, and which the nation is expected to test-launch this weekend.

How to measure a nuclear blast deep inside a mountain

Subterranean blasts can trigger landslides visible from space, an effect made clear by historical footage of subterranean nuclear tests. (Nearly all nuclear tests moved underground in the 1960s due to concerns about radioactive fallout.) Rare gases created during a blast can also leak out of the ground and be "sniffed" by sensors at a distance.

But these lines of evidence don't betray an explosion's size.

To do that, scientists rely on the small earthquakes hidden nuclear blasts cause. These tremors move through the Earth's rock, are detectable all over the world, and get picked up by vibration-sensing devices called seismometers.

That doesn't mean coming up with an agreed-upon estimate of an underground test blast's strength is easy.

One issue is that seismic measurements of a blast aren't the same from one site to the next. For example, the South Korean government measured a magnitude 5.6 earthquake from the blast, while seismometers operated by Japan's Meteorological Agency detected a magnitude 6.1 earthquake.

That means Japan detected an earthquake three times more powerful than the one detected by South Korea, even though Japan is located hundreds of miles further away from the Punggye-ri Nuclear Test Site. The US Geological Survey, meanwhile, detected a magnitude-6.3 earthquake — five times stronger than a magnitude-5.6 quake — from thousands of miles away.

A few things explain these differences.

Earthquakes travel through different types of rock between the source of a blast and a seismometer, which can give different magnitudes, as can the noise of mining operations and natural earthquakes. (Small earthquakes occur hundreds of times a day.) And monitoring stations aren't all the same; some use just a few seismometers, while others use many in an array.

Also, researchers can use several earthquake signals and plug the data into different formulas, each of which give different answers. Given the secrecy of Kim Jong Un's regime, exact information about a blast's depth, the size of the rock chamber it blew up inside, and the surrounding geology aren't readily available (at least publicly).

This leads to guessing some variables and a sometimes wild range of estimates that researchers don't always agree on.

Merging many readings leads to better estimates, however, and the Comprehensive Nuclear Test Ban Treaty Organization operates a network of 170 seismic monitoring stations around the world. Fifty stations continuously monitor for nuclear-test earthquakes and beam that data to the organization's headquarters in Vienna, Austria, via satellite.

The CTBTO settled on a roughly magnitude-6.0 tremor this week.

"A magnitude 6 compared to a magnitude 5.1, in 2016, if you take it and look at it in scale, and just use Hiroshima in comparison, I think we're talking about a detonation that is 10 to 20 times bigger," Lassina Zerbo, the executive secretary of the CTBTO's Preparatory Commission, said in a United Nations podcast.

This puts the yield of the blast somewhere around 150 kilotons, or perhaps as high as 300 kilotons. "This is a key indication that things are progressing," Zerbo added.

North Korean super nukes?

But was it a "city-busting" thermonuclear blast, as North Korea has claimed?

There are a few main types of nuclear weapons, any of which could explain the explosion's wide-ranging size: fission, boosted fission, and thermonuclear (also called hydrogen or fusion) bombs.

Fission bombs — the type dropped on Japan in 1945 — yield the smallest explosions. Yet building a very large fission bomb can yield a larger explosion, perhaps around 500 kilotons, as was the case in the US military's 1952 Ivy King test.

However, it's unlikely North Korea blew up a large pure-fission bomb. The sanction-shackled country has a limited ability to produce weapons-grade nuclear material, so every ounce of it is precious and unlikely to be wasted on a test, even though production has ramped up in recent years.

What's more likely is that North Korea detonated a "boosted" fission device, as experts believe the nation detonated in January 2016.

Boosted weapons amplify a fission bomb's yield by packing it with some "heavy" hydrogen. During an explosion, this hydrogen partly undergoes fusion — the same process that powers the sun — and increases the amount of energy released by weapons-grade plutonium and uranium.

Boosted bombs can also be built large to increase yield (up to 720 kilotons, as with Great Britain's "Orange Herald" test), but again: North Korea is not swimming in a supply of weapons-ready uranium and plutonium.

Thermonuclear bombs can yield tens of kilotons to hundreds of megatons of energy, but are extremely difficult to make correctly. This is because they're essentially two bombs in one: a "boosted" fission bomb, which releases a blast of powerful X-ray radiation, and a fusion bomb that's set off by the first explosion's X-rays. (Since X-rays travel at light-speed, the second bomb can blow up before the first's slower-moving shockwave can rip it apart.)

A yield of 60 to 300 kilotons on September 3 suggests at least a boosted fission device, but perhaps a small, missile-ready thermonuclear warhead, as North Korea has claimed and showed alleged photos of.

Determining the type of bomb North Korea detonated and its yield is important, but what the government's latest and greatest test represents is enough to worry many experts — thermonuclear weapon or not.

"What is important is that North Korea isn't content to settle for a crude nuclear program," Jeffrey Lewis, director of the East Asia Nonproliferation Program at the Middlebury Institute of International Studies at Monterey, told Tech Insider via email in 2016. "They want an arsenal of relatively advanced nuclear weapons."

Zerbo thinks a diplomatic effort like the one that the Obama administration used to shutter Iran's nuclear weapons program is worth trying.

"[W]e cannot let this program go on. It would jeopardize the whole arms control and non-proliferation regime as a whole," he said. "People will say, 'If North Korea can do it, and then nothing happens, then why not us?' That's a risk that we might see in this world, and that's a risk that is unacceptable."

SEE ALSO: Never-before-seen videos show nuclear weapons being secretly detonated in the Nevada desert

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