Fission is when a nucleus splits into many small fragments called fission products.  These fission products become nearly half the mass of the original nucleus and the nuclues generally emits two to three neutrons.

   U²³⁵ and  Plutonium each have the inherent ability to fission making them perfect candidates for the atomic bomb.  Plutonium 239 has a very high spontaneous fission rate making it easy to for the bomb to accidently fizzle before production is complete.  While Uranium 235 has a lower rate of spontaneous fission making it easier to deal with when building an atom bomb.

 

Bomb Design
There were two types of atom bombs created one was called the "Little Boy" and the other was titled the "Fat Man."

The Little Boy
The "little boy" atom bomb was designed to be a gun type bomb.   They call it a gun type bomb due to the fact that the bomb shoots a mass of uranium 235 into another mass of uranium 235 to create a supercritical mass.

                       
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Once the supercritical mass is formed the initiator emits a burst of neutrons creating the fission chain reactions.  These chain reactions continue until the energy released becomes so big that the bomb blows itself up.

                            

The Fat Man:

                                  


The Fat Man atom bomb was created due to the fact that plutonium 239 has such a fast spontaneous fission rate.  A new design was needed to facilitate such an isotope.  The Little Boy is too slow to work with such an isotope.  So, Seth Neddermeyer created an idea to use explosive charges to compress a small sphere of plutonium quickly to a required density that will make the plutonium go critical and produce a nuclear explosion.  Thus, the Fat Man atom bomb was born.

Building the Bomb

Fission: The Atomic Bomb

The atomic bombs that were used on Hiroshima and Nagasaki derived their explosive power via a fission chain reaction. The fuel used for Little Boy, the bomb used on Hiroshima, was enriched uranium. Fat Man, used on Nagasaki, was fueled by plutonium.

Below is a sketch of the kind of uranium bomb used on Hiroshima. The explosive drives the uranium "bullet" into the secondary mass of uranium to create a SuperCritical mass.

             

Fusion: The Nuclear Bomb

"In thermonuclear weapons, radiation from a fission explosive can be contained and used to transfer energy to compress and ignite a physically separate component containing thermonuclear fuel."

The 3 basic concepts of thermonuclear devices, U.S. DOE, Sept 1980, Duane Sewell, Assistant Secretary of Energy for Defense, Official Declassification Act.

Fusion Fuel

The forms of Hydrogen used in a fusion reaction -- H2 (deuterium) and H3 (tritium) -- are gases at room temperature, so their storage in a weapon would be difficult. Instead, a substance called lithium deuteride (Li6D or Li7D) is used.

Lithium

The lightest of metals, lithium is only half as dense as water. Lithium is found combined with other elements in igneous rocks and mineral spring water. Lithium has several isotopes, ranging from Li5 to Li9. Li6 and Li7 are used in weapons, and are naturally occuring. Li5, Li8, and Li9 are man-made radioisotopes.

Li6 is present as 7.5% of all naturally occuring Li. Separation methods include electrolysis, distillation, chemical exchange, or EM methods. Li bonds with H to form the solid Li6D. This material is a whitish, slightly-blue powdery light salt-solid (which is extremely hygroscopic) at room temperature. It is made by heating metal lithium in a vessel, into which deuterium gas is injected. It is then pressed and shaped into a ceramic.

Li6D in the Fusion Reaction

When a neutron is absorbed by a molecule of litium deuteride (Li6D), the molecule breaks up into a He, H2 (deuterium) and H3 (tritium). The deuterium can then react with the tritium in fusion. This releases enormous amounts of energy, much greater than you would get in a fission reaction. The end products include a free neutron and a helium atom. Schematically:

Li6 + n -> He4 + T + 4.7 MeV

then

D + T -> He4 + n + 17.6 MeV.

n + U-238 -> neutrons + fission products + energy

These reactions occur in under 1/10-6 secs.

Li6D in the Bomb

In order to begin a fusion reaction in a thermonuclear weapon, the lithium deuteride solid must be compressed to 15-30 times its original uncompressed density at RTP (15lbs/foot^3).

Compression is achieved by a smaller fission reaction, which forces the Li6D fuel together.

[image: showing fission reaction forcing fusion fuel together]

Compression is needed to:

(1) increase fusion probability

(2) increase fusion rate

By packing the molecules closer together, you increase the probability of fusion occuring. Compressing the molecules paves the way to overcoming the electrostatic repulsion of the H atoms in the Li6D.

A faster rate of fusion produces a larger chain reaction. The reaction rate is proportional to the square of the fuel density -- in other words, the more compressed the fuel is, the shorter the reaction time will be. Increase the density by a factor of 30, for example, and the rate of fusion increases by a factor of 900.