Korea hydrogen bomb: What is a hydrogen bomb and how is it different to an atomic bomb?

State media in North Korea at the end of August claimed the country’s leader Kim Jong-un recently carried out a “perfect” test of a hydrogen bomb at a nuclear test site in Punggye-ri. Since this initial test, South Korea has warned that its neighbour appears to be gearing up for more missile launches and possibly the launch of an intercontinental ballistic missile.

Korea hydrogen bomb: What is a hydrogen bomb and how is it different to an atomic bomb?

The reported hydrogen bomb trial marked the sixth test by Kim Jong-un in recent years and was condemned by world leaders as well as the UN.

Hydrogen bombs are said to be 1,000 times more powerful than the atomic bombs dropped during the Second World War and represent a significant escalation in North Korea’s nuclear plans – but how do they work? 

What is a hydrogen bomb?

A hydrogen bomb is what’s known as a thermonuclear bomb. It is classed as a second-generation nuclear weapon as it uses the processes seen in atomic bombs and advances them to make a more powerful detonation.

In particular, it uses the fission chain reaction seen in an atomic bomb to bombard a fusion bomb with energy that results in a devastating explosion.  

The dictionary definition of a hydrogen bomb is “an immensely powerful bomb whose destructive power comes from the rapid release of energy during the nuclear fusion of isotopes of hydrogen (deuterium and tritium), using an atom bomb as a trigger.”

READ NEXT: Nuclear fission versus nuclear fusion

The US was the first country to develop nuclear weapons, followed by Russia in 1949. The first thermonuclear test, code-named Ivy Mike, detonated on 1 November 1952 on the island of Elugelab in Enewetak Atoll, in the Pacific Ocean, as part of Operation Ivy.  

It marked the first full-scale device that created an explosion using nuclear fusion. You can read more about the differences between nuclear fission and nuclear fusion in our nuclear energy explainer.

How does a hydrogen bomb work?

All nuclear weapons use a process called nuclear fission to generate the energy needed for their explosions. Early weapons, including the Little Boy dropped on Hiroshima, created the critical mass needed to kickstart a fission reaction by firing a hollow uranium-235 cylinder at a target made from the same material.

 This technique creates a blast that implodes on itself first, forcing atoms together. Neutrons are then used to create a chain reaction which leads to the outward atomic explosion.

Hydrogen bombs take things a step further and use a process called nuclear fusion to force the atoms together, similar to the extreme process that powers our sun. To create a fusion reaction, you need a vast amount of energy, and in hydrogen bombs, this comes from a fission reaction which means a hydrogen bomb is effectively a fusion bomb and a fission bomb rolled into one. 

Fusion takes place when the nuclei of two atoms combine to form a single heavier atom. At extremely high temperatures, the nuclei of hydrogen isotopes deuterium and tritium fuse together relatively easily (compared to other isotopes) to release energy. 

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The fission bomb releases a blast of powerful radiation, using the fission method, and this radiation is then aimed at the fusion bomb. The energy from this radiation is enough to trigger the chain reaction needed for atoms to merge inside the fusion bomb. As the atoms merge, they generate more energy which triggers the second of the two bombs and leads to a more powerful explosion.

There are issues with these forms of bombs, though. The fuels needed for fusion to take place are difficult to store, tritium, in particular, has a short half-life. Secondly, fuel inside the bomb needs to be regularly topped up. To overcome these issues, scientists use lithium-deuterate, which is easier to store because it doesn’t decay at room temperatures, as the main thermonuclear material. The fission part of the reaction additionally helps produce tritium from lithium and the X-rays produced in the fission reaction creates the high temperatures needed to kickstart fusion.  

All modern-day thermonuclear weapons in the United States, specifically, use what’s known as the Teller–Ulam configuration after scientists Edward Teller and Stanislaw Ulam.

What’s the difference between a hydrogen bomb and an atomic bomb?


Atomic bombs use nuclear fission which splits the nuclei of plutonium and/or uranium into smaller atoms. When neutrons, or neutral particles, of these atoms are split, they hit the nuclei of other nearby atoms, which in turn causes them to split. This generates a chain reaction that releases massive amounts of energy. They are also typically large devices – the Fat Man atomic bomb dropped on Nagasaki in 1945 weighed around 4,700 kilograms.

READ NEXT: A guide to Kim Jong-un’s nuclear weapons

By contrast, as explained above, hydrogen bombs use a similar fission technique to create the initial chain reaction to provide the ‘fuel’ needed to generate the second chain reaction and cause the fusion bomb to explode. Scientists are working to make hydrogen bombs small enough to sit on nuclear missiles.

The atomic bombs dropped on Hiroshima and Nagasaki exploded with the yield of around 15 kilotons and 20 kilotons of TNT, respectively, according to the Union of Concerned Scientists. During the Ivy Mike hydrogen bomb test, this yield was closer to 10,000 kilotons of TNT.

Images: Wikimedia/Reuters

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