TIME Nuclear Weapons

Here’s the Difference Between a Hydrogen Bomb and an Atom Bomb

One is bad, the other's worse, and now North Korea may have both

In some ways it’s a shame human beings ever figured out how the sun works. If we hadn’t figured that out, we never would have learned how nuclear fusion works. And if we hadn’t learned that, we wouldn’t be worried about the latest weapon North Korean leader Kim Jong Un may have just added to his nuclear toy box.

Pyongyang had already successfully tested fission bombs (more popularly known as atom bombs). But the news that the hermit kingdom might have just detonated a fusion—or hydrogen—bomb, is a much bigger cause for worry. Here’s why.

Fission bombs, as their name suggests, work by splitting unstable radioactive atoms and setting off a nuclear chain reaction that releases enormous amounts of destructive energy instantaneously. In the case of “Little Boy,” the bomb the U.S. dropped on Hiroshima near the end of World War II in 1945, fission was achieved by firing what amounted to a bullet of uranium 235 into a larger sphere of the same material. The nuclei in some of the atoms in the target mass absorbed neutrons from the incoming projectile. This caused them to split into smaller atoms, emit gamma rays, and release more neutrons, which then collided with more U235 atoms, setting off the chain reaction that is, in effect, the city-leveling explosion.

“Fat Man,” dropped on Nagasaki a few days later, was also a fission bomb, but in this case its explosive fuel was plutonium 239. More significantly, it was what is known as an implosion fission device, with its nuclear core surrounded by a few thousand pounds of conventional high explosives that used sudden, violent compression to get the nuclear chain reaction going. Fat Man produced a blast equivalent to 21 kilotons—or thousand tons—of TNT. Little Boy unleashed 15 kilotons.

A fusion bomb, better known as a hydrogen bomb like the one North Korea says it successfully tested, is a much scarier beast. It too involves a fission explosion—but it uses that fearsome blast merely as a trigger. The enormous heat released when the fusion explosion goes off within the bomb compresses a core fuel of hydrogen—specifically the hydrogen isotopes deuterium and tritium. That sets off the same kind of fusion reaction that takes place in the center of the sun, where hydrogen atoms fuse into helium, giving off light, heat and other kinds of energy.

Hydrogen bombs are much more powerful than atomic bombs. The largest ballistic missile in the U.S. arsenal is a fusion weapon packing a punch of 455 kilotons, according to The Brookings Institute. The Castle Bravo hydrogen bomb the U.S. dropped on Bikini Atoll in its 1945 test was 15,000 kilotons.

It’s not clear yet if North Korea has indeed succeeded in cracking H-bomb science or if the blast detected by seismic sensors around the world was caused by a high-power fission device instead. Either way, the worry over what Pyongyang does have in its arsenal is very real indeed. Nuclear weapons are scary no matter what. Nuclear weapons in the hands of an unstable state like North Korea don’t bear contemplating.

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