Since the US has gone ballistic over Iran's potential to develop nuclear weapons I also thought it would be useful to find out how fission weapons are made.
There were two different tracks to achieving an explosive fission process. One depends on only using U-235 and refining or enriching it up to about 80% to 90% purity. The second track uses plutonium. Fission from neutron bombardment of U-235 releases about half as much energy as fission from neutron bombardment of Pu-239, an isotope of plutonium.
Recall from the first post, U-238 is stable and absorbs bombarding neutrons and does not break apart. When U-238 absorbs a neutron in the nucleus it becomes U-239, Z=92 gives off a gamma particle (x-ray or high energy photon), and in about twenty minutes decays by releasing a beta particle (electron) and becomes Np-239 or neptunium, a transuranium element with atomic number Z=93 (Z = number of protons, different from atomic weight)
When U-238 is bombarded with deuterons or a heavy hydrogen nucleus (one proton, one neutron) it becomes Np-238. In about two days Np-238 decays by releasing a beta particle to become plutonium, Pu-238. By moderating the bombardment speed of neutrons hitting U-238, Pu-239 can be produced. In turn when Pu-239 is bombarded with neutrons, it undergoes fission, breaking up into U-235 and lighter elements and releases an alpha particle (helium nuclei or 2 protons, 2 neutrons).
Plutonium can be made with a particle accelerator bombarding small samples of U-238 or it can be produced through a controlled chain reaction in a reactor, where afterward plutonium can be chemically separated from the partially used uranium fuel.
In the Manhattan Project the basic nuclear physics and chemistry for plutonium was done in Berkeley using the cyclotron. The controlled chain reaction method for producing plutonium was done in Chicago. The Chicago chain reaction reactor based process was transformed into a production system at Oak Ridge, which also performed the chemical refining of Pu-239.
In addition to plutonium production, Oak Ridge also developed several different processes to enrich U-235 purity from UF4, including gas diffusion, centrifugation (covered in Nuke'm1) and electromagnetic separation.
For the first atomic bomb the electromagnetic isotope separation system (EMIS) was use as a final stage. EMIS relies on the principle that the different isotopes of uranium, U-235 and U-238 have different electromagnetic properties and will separate from each other when moved through a magnetic field. The UF4 containing both isotopes is vaporized, ionized by bombardment with electrons and then accelerated through a magnetic field where the two isotopes separate and U-235 is collected out.
To get to weapons grade, about 85% U-235, at Oak Ridge, all the above processes were used in series. First natural uranium was run through thermal diffusion for U-235 .86%, then UF6 was run through gaseous diffusion for U-235 7%, then the first stage of EMIS using UF4 achieved 15% and the second stage got to UF4 with U-235 85% which was further processed by reduction into uranium metal.
The Hiroshima bomb used U-235 fission. The test bomb set off at Alamogordo and the Nagasaki bomb used Pu-239 fission.
To gain some idea of the scale of these production systems go here:
http://www.atomicheritage.org/oakridge.htm
and look at the aerial view of just the gas diffusion plant.
Consider that Pu-239 can be produced from a reactor and then purified through relatively straight forward chemical extraction methods, and plutonium fission releases twice the energy of U-235 fission. It's little wonder plutonium became the main product line for nuclear weapons.
In the hydrogen bomb or fusion weapon, Pu-239 fission is the starter process of implosion that produces the high energies, high pressures, and high temperatures necessary for thermonuclear fusion of compressed nuclei of a solid lithium-6 deuteride core. The lithium deuteride core is temperarily held together and compressed by a rapidly collapsing U-238 metal casting that will, a few billionths of second later blow-off inward or ablate to compressed the core into a plasma fusion and pretty soon...the whole Teller-Ulam-Sakharov contraption goes off.
Amazingly, the whole interior is filled with polystyrene (styrefoam?) which vaporizes into a carbon-hydrogen plasma and acts as a temporary reflective shield so the U-238 casting will implode inward. Go here for a diagram:
http://nuclearweaponarchive.org/Library/Teller.html
CG