Hydrogen Bomb vs. Atomic Bomb: What's the Difference?

North Korea claims to have tested a hydrogen bomb on Wednesday (Jan. 6), a weapon powerful than the bombs that devastated the Japanese cities of Nagasaki and Hiroshima during World War II.

Hydrogen bombs, or thermonuclear bombs, are powerful than atomic or "fission" bombs, so the similarly sized seismic events cast doubt on North Korea's claims, experts say. The difference between thermonuclear bombs and fission bombs begins at the atomic level. Fission bombs, like those used in Nagasaki and Hiroshima, work by splitting the nucleus of an atom. When the neutrons, or neutral particles, of the atom's nucleus split, some hit the nuclei of nearby atoms,causing them to split too. The result is an highly explosive chain reaction. The bombs dropped on Hiroshima and Nagasaki exploded with the yield of 15 kilotons and 20 kilotons of TNT, respectively, according to the Union of Concerned Scientists. [The 10 Greatest Explosions Ever]

In comparison, the first test of a thermonuclear weapon, or hydrogen bomb, in the United States in November 1952 yielded an explosion of the order of 10,000 kilotons of TNT. Thermonuclear bombs start with the same fission reaction that powers atomic bombs — but the majority of the uranium or plutonium in atomic bombs actually goes unused. In a thermonuclear bomb, an additional step means that more of the bomb's explosive power is useable.

First, an igniting explosion compresses a sphere of plutonium-239, the material that will then undergo fission. Inside this pit of plutonium-239 is a chamber of hydrogen gas. The high temperatures and pressures created by the plutonium-239 fission causes the hydrogen atoms to join together. This fusion process releases neutrons, which feed back into the plutonium-239, splitting more atoms and boosting the fission chain reaction.

Governments around the world use global monitoring systems to detect nuclear tests as part of the effort to enforce the 1996 Comprehensive Test Ban Treaty (CTBT). There are 183 signatories to this treaty, but it is not in force because key nations, including the United States, did not ratify it. Since 1996, Pakistan, India and North Korea have carried out nuclear tests. Nevertheless, the treaty put in place a system of seismic monitoring that can differentiate a nuclear explosion from an earthquake. The CTBT International Monitoring System also includes stations that detect the infrasound — sound whose frequency is too low for human ears to detect — from explosions. Eighty radionuclide monitoring stations around the globe measure atmospheric fallout, which can prove that an explosion detected by other monitoring systems was, actually  nuclear.