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Hydrogen bombs versus atomic bombs, explained

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Brian Resnick is Vox’s science and health editor, and is the co-creator of Unexplainable, Vox's podcast about unanswered questions in science. Previously, Brian was a reporter at Vox and at National Journal.

North Korea's government claims it has successfully conducted a test of a hydrogen bomb. For now experts are very skeptical of that claim. They agree that North Korea likely tested some sort of atomic weapon on Tuesday, but it remains to be seen what type.

This question matters greatly. North Korea already has atomic bombs, similar to the ones used in World War II. But hydrogen bombs can be thousands of times more powerful — they're the most terrifyingly destructive inventions humans have ever assembled. A short overview of the difference is below.

The difference between an atomic bomb and a hydrogen bomb

Atomic bombs — like the two the United States used against Japan in World War II — rely on a process known as nuclear fission.

Isotopes like uranium-235 and plutonium-239 easily undergo fission — when a neutron hits their nucleus, the nucleus splits, releasing more neutrons and a tremendous amount of energy.

And when you have a large critical mass of uranium-235 or plutonium-239, all that splitting and neutron creation leads to a runaway chain reaction. Each time an atom is split apart, it releases more neutrons and energy, which split apart other atoms and release even more energy:


To create an atomic bomb, engineers typically design explosives that can force together pieces of uranium-235 or plutonium-239 into a critical mass. Once that happens, boom.

These bombs are incredibly powerful: The two dropped on Hiroshima and Nagasaki completely leveled those cities, exploding with forces of 15,000 and 20,000 tons of TNT, respectively. The most powerful fission bombs ever built can produce explosions equal to 500,000 tons of TNT.

But they're not the biggest bombs out there. Hydrogen bombs are thousands of times more powerful than their atomic predecessors. The first hydrogen bomb the United States ever tested in the Marshall Islands in 1952, called Ivy Mike, had the force of 10 million tons of TNT (or 10 megatons).

The most powerful hydrogen bomb ever — a Russian nuke called Tsar Bomba, literally "king of bombs" — had a yield of 50 megatons of TNT. A blast from Tsar Bomba could cause radiation burns as far as 62 miles away. Windows more than 500 miles away shattered during the Tsar Bomba test.

Hydrogen bombs combine both nuclear fission and a different process known as nuclear fusion to produce a far, far more powerful blast.

How a hydrogen bomb works

The first stage of a hydrogen bomb involves a fission explosion, as described above. That explosion, in turn, leads to a second stage — fusion.

The extreme heat and pressure from the initial atomic blast force together deuterium and tritium (two light gases made of hydrogen). When they are forced together, some of the hydrogen atoms fuse to one another, creating helium.

This process of fusion releases even more energy per unit of mass than fission does, and the energy released from the fusion reaction also feeds back into the fission reaction, increasing its output. This all happens nearly instantaneously.

This diagram shows a very simplified design of a hydrogen bomb.


Science historian Alex Wellerstein has created an online tool for comparing the impact of the different types of nuclear weapons.

For the sake of a demonstration, let's drop "Little Boy," the bomb used in Hiroshima, on Lower Manhattan. This is the resulting blast radius.


Now let's look at a hydrogen bomb. This is the blast radius of Ivy Mike — the first (but not the most powerful) hydrogen bomb ever tested:


The first is a catastrophe. The second is so horrifying it's unimaginable.

How to tell if North Korea tested a hydrogen bomb

Right now, experts seem skeptical that North Korea has actually produced a working hydrogen bomb, which is a much harder technological feat to pull off.

The blast recorded in North Korea on Tuesday was of a similar magnitude as the country's last nuclear test in 2013, which suggests the North Koreans have not upped their nuclear capabilities with a more powerful weapon. (Or perhaps they tested a hydrogen bomb design and the second fusion phase failed.) As the Korea Herald reports, North Korea has yet to produce a bomb that even matches the power of the two bombs dropped on Japan in World War II (as best we know).

But ultimately, we may not know for sure for days or weeks what North Korea tested. Scientists will have to examine the radioisotopes released into the atmosphere from the blast, to see whether those match the profile of an atomic bomb or a hydrogen bomb.

The Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization — CTBTO for short — maintains a worldwide network to detect nuclear explosions whether they occur underground, in the air, or underwater. The process is described below: