Nuclear weapon design

The first nuclear explosive devices provided the basic building blocks of future weapons. Pictured is the Gadget device being prepared for the Trinity nuclear test.

Nuclear weapons design means the physical, chemical, and engineering arrangements that cause the physics package of a nuclear weapon to detonate. There are three existing basic design types:

1. Pure fission weapons are the simplest, least technically demanding, were the first nuclear weapons built, and so far the only type ever used in warfare, by the United States on Japan in World War II. Pure fission weapons have been the first type to be built by new nuclear powers, as a step toward building more advanced weapons.
2. Boosted fission weapons are fission weapons that use nuclear fusion reactions to generate high-energy neutrons that accelerate the fission chain reaction and increase its efficiency. Boosting can more than double the weapon's fission energy yield.
3. Staged thermonuclear weapons are arrangements of two or more "stages", most usually two, where the weapon derives a significant fraction of its energy from nuclear fusion (as well as, usually, nuclear fission). The first stage is typically a boosted fission weapon (except for the earliest thermonuclear weapons, which used a pure fission weapon). Its detonation causes it to shine intensely with X-rays, which illuminate and implode the second stage filled with fusion fuel. This initiates a sequence of events which results in a thermonuclear, or fusion, burn. High-energy neutrons from the fusion reactions can be (and usually are) used to produce an additional large number of fission reactions, which can double the already-large yield of the weapon. This process affords potential yields hundreds or thousands of times greater than those of fission weapons, and to develop weapons that are relatively compact for their yield. Around 80% of the nuclear weapons deployed in the world today are thermonuclear weapons, as their ratio of size and weight to their energy release allows them great flexibility in deployment on bombers and in missile warheads.

The United States pioneered most major nuclear weapon design innovations, but many were either subsequently developed independently by other nuclear states, or were obtained through espionage, gift, or sale.

In early news accounts, pure fission weapons were called atomic bombs or A-bombs and weapons involving fusion were called hydrogen bombs or H-bombs. Practitioners of nuclear policy, however, have favored the terms nuclear and thermonuclear, respectively.