How do nuclear bombs work?

Answer 1

Depending on what type of nuclear bomb is being used:

"Atomic" bombs use the principle of nuclear fission. Fission requires a heavy element (one with a high atomic weight) atom to be split into two or more lighter (atomic weight) element atoms. This split process produces several extra neutrons and some energy as a by-product. The excess neutrons go on to cause fission in nearby heavy element atoms, resulting in a chain reaction where an enormous amount of energy is released quickly. In order to start this chain reaction (and explode the weapon), a sufficient amount of fissile material must be created. This amount of material is called the "supercritical" amount. Any amount less than this will simply not explode.

A typical atomic bomb uses Uranium-235 as heavy element. It is also possible to use Plutonium-239. Theoretically, it may be possible to use other trans-uranic elements, but so far, no one has succeeded in such a design.

Atomic bombs are generally of two designs: a "gun-type" and implosion weapon. In the Gun design, two lumps of fissile material (each about half the supercritical amount) are held at the end of a tube several feet long. An explosive charge is placed behind one of the lumps. When it explodes, the lump of fissile material is forced down the tube (in a manner identical to shooting a bullet from a gun) to smash into the other lump of fissile material. Together, they exceed the supercritical amount, and the atomic bomb goes off. In the Implosion design, a hollow sphere of sub-critical fissile material is surrounded by another sphere of chemical explosives. Detonating the explosives creates a shock wave of force, which compresses the fissile material to a point where it exceeds the density needed to achieve criticality. The atomic explosion then occurs. U-235 is generally used in a Gun design, while P-239 is the fuel for an Implosion device.

There are limits to the amount of energy released by an atomic bomb, for mechanical reasons. Thus, the largest (in terms of yield) pure atomic bomb is roughly 100kT.

"Thermonuclear" bombs (often called Hydrogen bombs) work on the principle of nuclear fusion, where two lightweight atoms are pushed together to form a heavier element (generally releasing a neutron and lots of energy). Nuclear fusion requires extremely high temperatures and pressures to occur. Currently, the only way to start a fusion explosion is to use an Atomic bomb.

Isotopes of Hydrogen (H-2 (deuterium) and H-3 (tritium)) are currently the fuel of choice for a thermonuclear weapon, with the resulting create element being standard Helium. A complex composition of exotic compounds containing H2 and H3 is placed next to a fully-working Atomic bomb. When the atomic bomb (the "trigger") is detonated, a shockwave of massive force is created. In the microseconds before this shockwave destroys the H2/H3 compound, it compresses this compound enough that nuclear fusion takes place. Even in this tiny fraction of a second, an enormous amount of energy is release, dwarfing even the atomic trigger's amount.

There is no upper limit to the size of a thermonuclear bomb. In practice, bombs bigger than 1MT are impractical. The largest H-bomb ever detonated was the USSR's "Tsara Bomb" at 50MT.

"Boosted" atomic bombs are those which use the Implosion design of the standard atomic bomb, but fill the hollow center of the P-239 sphere with gas of H2 or H3 just before exploding the weapon. This allows for a limited amount of fusion to occur at the center of the weapon, where the weapon has compressed the most. This boosts the yield of the weapon to up to 500kt, with only a little additional weight and complexity added.

"Hybrid" thermonuclear bombs (often called fission-fusion-fission weapons) are a standard thermonuclear bomb which is entirely wrapped in a spent-nuclear-fuel (i.e. mostly U-238) case. After explosion, the excess neutrons from the fusion reaction cause fission in the U-238 case. However, this particular type of weapon is very "dirty", causing a very large amount of radioactive by-products to be produced (mostly from the U-238 case, which is only partially fissioned). It will, however, increase the yield of a H-bomb by 3-5 times, with the only added issue of the weight of the U-238 case.

The above descriptions are of course conceptual. Actually building a nuclear weapon is very difficult, even if you have all the proper materials available - and, some of those materials are extremely difficult to obtain. There is a great deal of very high-tolerance machine tooling required (that is, many of the parts must be exactly a certain size, with no more than a few thousandths of an inch deviation), and there is very complex mathematics to be worked out to get the design right so that it will operate properly. Even on the very simple "gun-type" design, there is a significant chance that failure to properly calculate the right sizes, amounts, and shapes of the components will result in a dud weapon - one which, while the chemical explosive may fire, will fail to achieve a nuclear (fission and/or fusion) effect.