U235 is available in nature only to a limited extent of 0.7% of natural Uranium. To acquire weapons grade U235, Uranium must be enriched to 90%. In order to fuel a bomb, the U-235 must be separated from the more abundant U-238 isotope in a process called enrichment. But because these two uranium isotopes are almost identical chemically, they cannot be readily separated by using a simple chemical reaction. They must be separated by exploiting the slight difference in their weights.

There are several methods developed for this: two are commercial - gaseous diffusion process and the centrifuge process.
USNRC Uranium Enrichment
Uranium Information Center

Electro Magnetic Separation

This is a schematic of the method developed by E.O.Lawrence. Passing ions of Uranium threw a magnetic field causes the U235 ions to take a different path than the U238. Collectors at the other end of the semi-circle collect the separated U235.

Fission of U-235

The minimum amount for a critical mass for U-235 is 52Kg, for Pu-239 it is 10Kg.

Neutron fired into a U-235 atom creating a U-236 atom.

The resulting U-236 is unstable and will rapidly decay.

Decay

The decay of U-236 into an atom of Ba-141 (barium), an atom of Kr-92 (krypton) and three neutrons releases a significant amount of energy.

Resulting Chain Reaction

The three available neutrons collide with other U-235 atoms causing them to fission producing more free neutrons. This process continues creating a chain reaction in the (critical) mass of the U-235 Isotope. The mass can fission quickly as the number of split atoms grows exponentially.

Energy Released

The left side of the equation has a mass of 236.045563 and the right side totals to 233.849298. The difference between the masses (2.196265) must, by Einstein's equivalence principle, appear as energy released in the reaction. This energy is represented as intense heat. 200MeV with each fissioned molecule. 1 MeV (million electron volts) = 1.609 x 10 -13 joules

Though the mass is small, the energy is calculated by multiplying the speed of light squared, then multiplied by the number of atoms that fission during the process!

More on Nuclear Fission

USAF Academy online lesson in atomic bomb physics

Nuclear Weapons Frequently Asked Questions

Nuclear Weapons

Little Boy

Considered a gun-type bomb. A disc of uranium 235 is shaped with a center section missing. The center bullet, is place down the barrel from the larger ring mass. A conventional explosive is used to propel the center section into the larger ring section. The two sections then come together forming a critical mass and starting the reaction. The barometric sensor detects the height from the surface of the Earth by measuring air pressure. This allowed the bomb to be exploded above the ground reulting in more damage.

Fat Man

A Beryllium/Polonium mixture, which is radioactive elements that release neutrons, is placed in the center of a sphere. The sphere is made up of equally spaced and shaped plutonium sections. The sphere looked a lot like a soccer ball. When the bomb was detonated, the sphere implodes, or collapses inward, causing all the plutonium to fuse together thus reaching supercritical mass, and starting the chain reaction. The initial explosion, which caused the implosion, was made by conventional explosive placed evenly on the outside of the sphere.