What is in an atom bomb?

Answer 1

There are several types of nuclear weapons, the most common being a fission bomb (atomic bomb) containing uranium and/or plutonium. The more powerful type is a fission-fusion bomb (thermonuclear bomb) that uses the fission energy to fuse isotopes of hydrogen into helium. Fission bombs and radioactive "dirty bombs" (no fission involved) can be much smaller than fusion bombs.

Fission bombs obtain their energy by breaking up (fissioning) very large atoms (uranium or plutonium). Fusion bombs obtain their energy by combining (fusing) very small atoms (hydrogen). The most modern bombs are not clearly separable into pure fission or fusion, but use some combination of both.

For fuel, fission bombs use fissile isotopes: Uranium-235, Plutonium-239, or Uranium-233. In order to be fissile the material must be very pure (Oralloy, the codename for weapons grade Uranium is 93.5% Uranium-235. Natural uranium has only 0.72% Uranium-235. Anything less than 20% Uranium-235 is not fissile, but is only fissionable. Too much Plutonium-240 or 241 in plutonium makes it unusable in bombs.) This purity is very expensive to obtain.

For fuel, fusion bombs use easily fused isotopes of hydrogen: deuterium and tritium. The first fusion bomb test Ivy Mike used liquid Deuterium and Tritium in an 80 foot tall, 20 foot diameter, 2 foot thick steel case with a giant triple thermos bottle suspended in it -- to keep them cold enough to stay liquid until detonated. The first "practical" fusion bomb test Castle Bravo used solid Lithium-Deutride, the Lithium being split to make Tritium at detonation by neutrons from a fission bomb built into the fusion bomb.

A fission bomb works by rapid assembly with conventional explosives. In rapid assembly, you take a very subcritical configuration of fissile material and assemble it suddenly to a very supercritical configuration, then hit it with a precisely timed pulse of neutrons. There are 2 methods of rapid assembly: gun and implosion. In gun assembly the fissile material is divided into 2 pieces: target and bullet; each is subcritical but if brought together they are supercritical. They are placed at opposite ends of a gun barrel and the bullet is fired into the target. Assembly takes roughly 10ms, detonation takes roughly 1us. In implosion the fissile material is one subcritical piece, explosive shaped charges crush it to about 30 times normal density making it very supercritical. Assembly takes roughly 1 microsecond, detonation takes roughly 1 picosecond.

A fusion bomb works by the "Teller-Ulam Principle" and operates in stages. The primary (or 1st stage) is a fission bomb. The secondary (or 2nd stage) is composed of a radiation channel (to capture x-rays from the previous stage and distribute them over this stage, to drive a radiation implosion much faster than possible with any conventional shaped charge explosive), a dense cylinder of tamper (to contain the explosion), fusion fuel, a plutonium "sparkplug" rod. When the primary detonates, its x-rays implode the secondary, crushing the tamper, fusion fuel, and "sparkplug". Neutrons from the primary then hit the supercritical "sparkplug" detonating it causing it to push out against the inward push of the tamper, compressing and heating the fusion fuel. Neutrons from the "sparkplug" split the Lithium in the fusion fuel making Tritium. The secondary fusion fuel then detonates. To raise the yield as many additional stages as you want can be added, each driven by the x-rays and neutrons of the previous stage.

Practical military nuclear bombs also need a method of delivery, arm/safe mechanism, fusing mechanism, etc. like any conventional military bomb.