Chain reactions in modern american thermonuclear devices
It is commonly understood that a thermonuclear device is typically a two-stage device comprising a primary and a secondary. (1)
Most people think the primary stage is a fission device that ignites fusion in the secondary stage.
What is not as commonly understood is that each stage of a modern boosted weapon (also known as dial-a-yield) is itself a coupled fission-fusion-fission chain reaction for a total of fission-fusion-fission-fission-fusion-fission.
The primary stage is typically a hollow plutonium-239 pit (2), as its critical mass is much less than uranium-235. After conventional explosives implode the pit and cause it to go critical, a small amount of fusion fuel (usually 50/50% deuterium/tritium gas) at the center ignites and generates a large number of neutrons which increases the fission rate and thus induces a more complete fission reaction in the plutonium before the core explosively disassembles. Without boosting, a large portion of the fissile material will remain unreacted; the Fat Man bomb had an efficiency of only 17%, because it was unboosted. (3) The energy released by the fusion reaction in the primary is not significant, only about 1% of the total energy from the primary, it is the neutrons generated by fusion that are important.
Boosting is mainly responsible for the remarkable increase in the efficiency of fission weapons since 1945.
The secondary stage is typically a cylinder and comprises a heavy tamper surrounding the fusion fuel (lithium deuteride). At the center is a fissile plutonium sparkplug. It is this sparkplug that ignites first when triggered by the primary. The sparkplug in turns ignites the fusion fuel which is confined by the tamper. Fusion generates a copious number of neutrons and can cause the tamper to fission if it is made of an appropriate material such as uranium.
Although un-enriched uranium can be used, the increased yield of enriched uranium makes it the preferred tamper material in light weight strategic weapons. One could argue that weight is very important in the highest priority application: submarine launched ICBMs. Weight and volume are precious and must be balanced against other essential performance parameters like maximum dive depth and missile range. Even land based ICBMs benefit from lower weight because it allows more re-entry vehicles to be carried. A bomber is more likely to be limited in the number of targets it can service but again, light weight could allow a smaller aircraft to perform the mission. Also, stealth aircraft are more survivable but then the weapons must be carried internally and volume is likely to be a limiting factor.
Fission of the tamper (if it occurs) provides considerable energy yield, as much as half of the total yield in large devices. It also creates far more fallout than a pure fusion device (assuming airburst).
1. It is believed that the W-53 9 Mt warhead carried by Titan II ICBMs was so large, 3 stages were required. The Mk-53 bomb was carried by B-52 bombers and in that role has been involved in several accidents.
2. Composite plutonium/uranium pits have been used when uranium was more plentiful than plutonium but a pure plutonium pit is smaller and is the current standard.
3. The Little Boy gun type device had an efficiency of 1.38% of the fissile material reacted.
A good resource on the subject:http://www.sciencemadness.org/lan