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: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. 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. 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. The first stage is typically a boosted 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. This process affords potential yields hundreds or thousands of times greater than those of fission weapons.