What is the difference between uranium atom bomb and a hydrogen bomb?

Answer 1

A uranium bomb is a kind of fission bomb. A fission bomb uses a conventional chemical explosive 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 supercritical (the critical mass is smaller when density is higher). When a supercritical mass is of the metal is achieved, neutrons start a chain reaction that splits the atoms in the metal releasing 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 (the hydrogen isotopes deuterium and tritium) to create a large amount of its energy. This is different from fission bombs, that release energy but inducing a neutron chain reaction to split 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.

The fusion bombs in use today all rely on a fission bomb first stage (called a "primary") 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 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." These two stages are 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 to tritium) to such a degree that fusion can finally occur. The light isotopes fuse and some mass it 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 of a similar fissionable metal, these neutrons will fission the metal producing even more energy. This important technique can yield up to 90% of the total yield in thermonuclear designs that use it, even though the weapon's size and weight remain unaffected. The only other effect is a disproportionate increase in fallout. As a result, this has become the most common type of thermonuclear weapon design in use.

It is possible to add additional fusion stages, (which has been done in practice), and any number of additional ever-larger stages is possible. Thus, theoretically, a fusion bomb of unlimited size can be build. While most nuclear weapons existing today are fusion designs, most of them are no larger than the largest fission bomb (500kt), since military needs actually favor smaller weapons over big yields.

All of the biggest nuclear bombs ever built have been fusion bombs. The largest bomb detonated was a fusion bomb that was equivalent to 50 million tons of TNT. It used non-fissionable tampers and generated almost all (97%) of its yield from fusion. The largest fission bomb tested (Ivy King) 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 weapon dropped on Nagasaki.

Edward Teller once proposed building multistage hydrogen bombs with gigaton range yields, but calculations showed that most of the blast would simply be directed upward blowing the top of the atmosphere above the explosion off into space with only a fraction of the yield producing damage to the target below. The military had no interest in that and rejected his proposal.

[It should be noted that modern fission bombs, including the primaries in thermonuclear weapons, incorporate a minor thermonuclear effect called "boosting." This involves placing a small amount of fusionable material (usually deuterium and tritium gases) into the core of the bomb. During the initial detonation of the bomb, these elements fuse under heat and pressure releasing a burst of neutrons. This functions like a second, more powerful, neutron initiator, contributing many neutrons to start multiple chain reactions at once and promoting very swift, efficient, and complete fission of the fissile material. The fission yield is greatly increased as a result. The contribution of fusion itself to the yield is negligible, however, and these weapons are not "thermonuclear," "hydrogen" or "fusion" bombs in the usual sense.]