What is the difference between atomic bombs and hydrogen bombs?

Answer 1

"Atomic bomb" (or "Atom bomb" or "A-bomb") is an ambiguous term and could mean either a fission bomb specifically or any nuclear bomb, including a fusion (Hydrogen) bomb. However, the term "Atom bomb" was popularized before the invention of the second major class of nuclear weapons, the fusion bomb or "hydrogen bomb" (a.k.a., the "H-bomb" or "thermonuclear bomb"), and so it is often taken to mean a "fission" (non-hydrogen) bomb. (The term nuclear bomb is the most neutral, embracing both classes ob weapons.)

The first major type of nuclear weapon is the fission bomb. A fission bomb uses a conventional chemical explosive (like TNT) to create a supercritical mass of certain metals that have unstable nuclei (like Uranium 235 or Plutonium 239). It usually does this by "imploding" a sub-critical mass of the metal and crushing it to such a density that it becomes super-critical (because the critical mass is smaller when density is higher). When a supercritical mass of the metal is achieved, neutrons start a chain reaction that splits the atoms in the metal releasing both large amounts of energy and several additional neutrons that will, in turn, split more atoms, and so on, with more and more energy being released until the bomb finally blows itself apart.

The other main type of nuclear weapon is the fusion bomb. Commonly called the "H-bomb," the "hydrogen bomb," or the "thermonuclear bomb," the fusion bomb relies on the fusion of light isotopes (usually of hydrogen and sometimes helium) to create a large amount of its energy. This is different from fission bombs, which release energy by inducing a neutron chain reaction that splits large atoms in metals like Uranium 235 and Plutonium 329. The fusion bomb was invented in the decade after the first nuclear weapons were designed in the early 1940's.

However, in practice, most modern nuclear weapons make use both fission and fusion in varying degrees. Indeed, all "hydrogen" bombs (fusion bombs, thermonuclear bombs) in use today rely on a fission bomb first stage (called a "primary") in order to compress and heat a second fusion stage (called a "secondary"). The second stage has a thick shell of dense metal (which can be a fissionable metal, but need not be) on the outside and is filled with fusion fuel (hydrogen isotopes, or more usually a solid lithium-hydrogen compound [LiD]). It is usually round. In the center of the fusion fuel is another piece of fissile metal (usually Plutonium 239) called a "spark plug." The primary and secondary stages are themselves placed inside a case of dense metal, usually shaped like a peanut, with one stage at each end.

When the fission primary goes off, x-ray radiation floods down around the fusion secondary instantly heating its metal shell and causing it to implode inwards as it outer layers explode away. This is called "radiation implosion." As the shell of the secondary implodes, it compresses both the fusion fuel and the "spark plug." The "spark plug" quickly is crushed to such a density that it is supercritical and it fissions and explodes against the fusion fuel which is still being crushed inward by the radiation implosion. The effect is that the fission primary is pushing inward on the secondary while the spark plug (basically another fission bomb) explodes outward--the fusion fuel is caught between. That fuel is heated and compressed (and any lithium transmuted by neutrons into more hydrogen fuel) to such a degree that fusion can finally occur. The light isotopes fuse and some mass is converted in to huge amounts of energy. A large number of fast neutrons are also produced. If the casing of the bomb or the metal shell of the secondary are made of uranium or thorium or a similar fissionable metal, these neutrons will fission the metal producing even more energy (this can almost double the yield in such designs as well as increasing fallout dramatically.) Because of these fission components in the their design, up to a half or more of the explosive yield from many so-called "fusion" or "hydrogen" bombs actually comes from the fission of heavy atoms and not from the fusion of lighter atoms. This shows the imperfectness of the terms "fusion bomb" or "hydrogen bomb" to describe modern thermonuclear weapons.

It is possible to add additional fusion stages after the second stage. This has been done in practice, producing three stage bombs, and any number of additional, ever-larger stages is hypothetically possible. Thus, theoretically, a thermonuclear ("hydrogen") bomb of unlimited yield can be build by adding a third, forth, and fifth, etc., fusion stages of increasing size, each imploding the next, larger stage. While most nuclear weapons existing today are thermonuclear ("hydrogen") designs, most of them are intentionally made no more powerful than the largest pure fission bomb (500kt), since military needs actually favor multiple smaller yield weapons over fewer larger ones.

All of the most powerful nuclear bombs ever built have been thermonuclear ("hydrogen") bombs. The most powerful bomb ever detonated was a thermonuclear bomb that was equivalent to 50 million tons of TNT. The largest fission bomb tested was only one 100th as powerful, yielding 500 kilotons (half a million tons of TNT), which is still more than 20 times more powerful than the fission weapon dropped on Nagasaki, Japan in 1945.