Types of nuclear weapons
Two or more pieces of HEU are brought together to form a supercritical mass. The Little Boy atomic bomb dropped on Hiroshima used a 3 inch naval gun tube to fire a U-235 projectile into a U-235 hollow cylinder called the target. The cylinder was actually several rings that resembled large washers. An external neutron source was used to enhance the supercritical reaction.
A subcritical sphere of plutonium-239, called the pit, is compressed into a supercritical mass by an explosive charge. An external neutron source is used to enhance the supercritical reaction. The explosive charge is constructed of pieces of high energy explosive configured to fit together to form a sphere around the pit. The explosive components are referred to as lenses because they are machined to tolerances associated with camera lenses. Each lens must be detonated at exactly the same time. The explosive pressure waves must reach the surface of the pit at exactly the same time, compressing it into a supercritical mass, starting a chain reaction resulting in an atomic explosion.
Boosted Fission Type
A boosted nuclear warhead is a fission-fusion-fission reaction. One method of boosting employs a uranium-235 pit. A layer or shell of lithium deuteride coats the small ball of U-235 in the center. A third layer of U-235 coats the layer of lithium deuteride. This process is repeated several times. Detonation of the explosive lens compresses the layer producing a uranium supercritical fission reaction. Neutrons from the fission reaction initiate the fusion reaction, producing a limited amount of fusion—a fission reaction created by implosion, augmented by a fusion reaction—a fission-fusion-fission reaction. The U.S. uses a different method to boost its fission warheads. Boosted fission warheads can achieve yields in the hundreds of kilotons