The Teller-Ulam hydrogen bomb design is the most complex out of all previous examples of nuclear weapons designs. The roughly cylindrical bomb casing contains primarily two stages:
The Primary is simply an implosion-type fission device. When it explodes, the X-rays that escape fill the space between the bomb casing and the fusion capsule, called the radiation channel, with a photon gas. The radiation channel space is filled with plastic foam of carbon and hydrogen. The foam becomes complete ionized as the X-rays progress.
The uranium shield prevents the X-rays from penetrating and detonating the fuel prematurely. The inner casing and outer capsule surfaces become heated to very high temperatures. The heated tamper expands and burns, which exerts an extraordinary amount of pressure inward against the lithium deuterate.
The lithium deuterate is compressed to approximately 1/30 its original size and attains 1000 times its original density. The violent shock wave that results will heat the plutonium rod in the center but will not compress it significantly. This makes the plutonium rod supercritical and initiates nuclear fission. This rod acts as a "spark plug".
Heat and neutrons from the center plutonium rod enter the lithium deuterate. The neutrons combine with lithium to make tritium according to this reaction.
The high temperature and high pressure satisfies the conditions necessary for tritium-deuterium fusion.
The neutrons resulting from the fusion reactions induce fission in the uranium-238 tamper and shield, which produces even more heat. The bomb explodes with a tremendous force more than 700 times greater than the gun-type Little Boy with a yield in the order of megatons.
Below is the firing sequence as seen from split-second intervals .The entire process takes place in 600 nanoseconds--550 nanoseconds for the fission bomb implosion and 50 nanoseconds for the fusion.