North Korea’s Nuclear Test Explained

Early Sunday morning, North Korean state TV announced that it had tested a hydrogen bomb designed to be mounted on an intercontinental ballistic missile (ICBM), declaring that it was a “perfect success.” The test came hours after North Korean media released photos of what if claimed to be a hydrogen bomb ready to be used on a missile. It’s not known whether this was a working device. For all we know, it could be just a mock up filled with confetti.

Early indications are that it could indeed be a hydrogen bomb, also known as a thermonuclear bomb — or at least a boosted fission device, a regular atom bomb that gets some additional oompf from a small fusion reaction. This is significant because the fission devices tested by North Korea were likely too large to be delivered by a missile. A thermonuclear weapon, on the other hand, can be miniaturized to fit atop a missile. The test confirms that if North Korea doesn’t already have a nuclear warhead that can be deployed on an ICBM, it soon will.

Rumblings in the Ground

Whatever North Korea tested was huge compared to its five previous tests. The first indications came Saturday night in a bulletin from the US Geological Survey (USGS) reporting a 6.3 magnitude seismic event near the surface centered at North Korea’s Punggye-ri test site. The signature was unmistakeable. This was North Korea’s sixth nuclear test — and it was far larger than those of the past.

Based on the seismic data, NORSAR, a Norwegian nuclear monitoring group estimated the yield of the device as equivalent to 120 kilotons of TNT. This would make it 8 times more powerful than the 15 kiloton weapon dropped on Hiroshima.

Basics of the Teller–Ulam configuration. The X-rays produced by a directed primary fission explosion at one end of a chamber heat and compress fuel material at the other end, triggering the secondary fusion reaction. (Photo: Wikimedia Commons)

How a Hydrogen Bomb Works

A staged thermonuclear nuclear device, or hydrogen bomb, is different from an atomic bomb. The nuclear weapons previously tested by North Korea were simpler fission devices similar to the bombs dropped on Hiroshima and Nagasaki. These sorts of weapons derive their power from splitting atoms of uranium or plutonium in a process called fission. By contrast, thermonuclear weapons fuse atoms together to release far more power through nuclear fusion, the same process that produces energy in stars.

A basic thermonuclear device contains two two stages: a primary fission stage that serves as a trigger for a much larger secondary fusion stage. It’s unknown exactly what specific bomb design North Korea is using.

In a typical thermonuclear bomb configuration, the primary is an implosion type boosted fission device. This will usually consist of a multi-layer sphere with an outer shell or “tamper,” often made of Uranium-238, a void filled with fusion fuel (likely a hydrogen isotope such as tritium or deuterium gas) and a core of Plutonium-239 or Uranium-235. High explosives are used to initiate a fission chain reaction in the core. This compresses the hydrogen isotope fuel, causing its atoms to undergo fusion. This bombards the fissile material with high-energy neutrons, which are reflected back by the tamper to cause further fission. While the fission reaction itself does not produce much energy, it dramatically increases the energy released from the fission reaction. It’s possible the test was only of this first stage.

The second stage consists of a Uranium-238 or lead tamper containing the fusion fuel — typically lithium deuteride. When detonated, the hot plasma and radiation from the primary boosted fission stage is channeled into the secondary stage, compressing the fuel and initiating a much larger fusion reaction.

Most of the weapons deployed by the five acknowledged nuclear powers are thermonuclear devices. Such devices  the advantage of being both more powerful and more compact.

A staged thermonuclear device  is far more difficult to build than a simple atom bomb. It is possibly that what North Korea actually tested is only the primary boosted fission first stage. But, if so, it still is pretty alarming. A boosted fission device is a more powerful weapon on its own and a big step towards a full hydrogen bomb.

The Missile Threat

The missile test late last month demonstrated that North Korea’s ICBMs had the range to reach the United States. Whether it is capable of successfully delivering a warhead intact is still an open question. Video from a Japanese weather station of the ICBM test seemed to show the re-entry vehicle burning up as it came back into earth’s atmosphere. This suggests that the missile system may not yet be capable or delivering a weapon. But, this is hardly the most challenging part of designing a missile system. With a workable miniaturized warhead, the rest is just a matter of time.

No Good Options

The U.S. strongly condemned the test, as did many countries including China and Russia. The question is what to do about it. None of the options are good.

Defense Secretary Jim Mattis appearing, in the Rose Garden at the White House on Sunday, said that “any threat to the United States or its territories including Guam or our allies will be met with a massive military response.” It’s notably tough rhetoric coming from Secretary Mattis, a man not known for hyperbole.

The military options for halting North Korea’s nuclear development are all terrible. Taking out their nukes with a surgical strike with conventional weapons would be nearly impossible. North Korea’s nukes are hidden in caves and bunkers throughout the country where they are hard to find and even harder to bomb. North Koreas conventional military capabilities, including 14,000 artillery pieces, thousands of which are in range of Seoul and the 25,000 U.S. troops stationed on the Korean Peninsula, are sufficient to inflict an unacceptable cost if Pyongyang decides to retaliate.

The diplomatic options aren’t very good either. President Trump suggested that the U.S. was considering cutting off all trade with countries that do business with North Korea, which means China. That would be a complete disaster.

Additional sanctions will surely be on the table. But, how effective they are will depend heavily on China, Pyongyang’s ally and largest trading partner. And it’s unclear that North Korea will respond to pressure, even from China at this point.

It seems more likely that North Korea’s leader Kim Jung Un has decided that the benefits of a deliverable nuclear weapon outweigh the risks. It would be a game-changer that would allow Kim to come to the table from a position of strength. It would provide him an insurance policy to guarantee his regime’s survival and leverage to achieve policy goals – chief among them forcing the U.S. off the Korean Peninsula. It will also help the young leader consolidate his power. Kim’s frequent killing of political rivals suggests that his internal political position may not be fully solidified. Refusing to back down in the face of pressure from the west and winning will go along way to securing his internal grip on power.

Kim is gambling that preemptive military action by the U.S. is not a substantial risk. After all, North Korea got away with building a bomb in the first place. Kim might concluded that he can get away with further development of it too. With diplomatic options running out and military contingencies all bad, the U.S. may be stumbling its way towards a third strategy — just live with it.

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