In order to produce a nuclear explosion, subcritical masses of fissionable material must be rapidly assembled into a supercritical configuration. The simplest weapon design is the pure fission gun-assembly device, in which an explosive propellant is used to fire one subcritical mass down a “gun barrel” into another subcritical mass. Plutonium cannot be used as the fissile material in a gun-assembly device, because the speed of assembly in this device is too slow to preclude the high probability that a chain reaction will “pre-initiate” by spontaneous neutron emission, thereby generating an explosive yield of only a few tens of tons. Therefore, gun-assembly weapons are made with highly enriched uranium, typically more than 80 percent uranium-235.

The other major assembly method is implosion, in which a subcritical mass of fissile material is compressed by a chemical high explosive into a denser critical mass. The fissile material is typically plutonium or highly enriched uranium or a composite of the two. In the simplest design, a spherical fissile core is surrounded by a reflector (also known as a tamper), which in turn is surrounded by the chemical high explosive. Other geometries are used where the diameter of the device must be kept small—to fit, for example, in an artillery shell or missile warhead—or where higher yields are desired. To obtain a given yield, considerably less fissile material is needed for an implosion weapon than for a gun-assembly device. An implosion fission weapon with an explosive yield of one kiloton can be constructed with as little as 1 to 2 kg (2.2 to 4.4 pounds) of plutonium or with about 5 to 10 kg (11 to 22 pounds) of highly enriched uranium.

Refinements to the basic implosion design came first through Operation Sandstone, an American series of tests conducted in the spring of 1948. Three tests used implosion designs of a second generation, which incorporated composite and levitated cores. The composite core consisted of concentric shells of both uranium-235 and plutonium-239, permitting more efficient use of these fissile materials. Higher compression of the fissile material was achieved by levitating the core—that is, introducing an air gap into the weapon in order to obtain a higher yield for the same amount of fissile material.

American tests during Operation Ranger in early 1951 included implosion devices with cores containing a fraction of a critical mass—a concept originated in 1944 during the Manhattan Project. Unlike the original Fat Man design, these “fractional crit” weapons relied on compressing the fissile core to a higher density in order to achieve a supercritical mass, thereby achieving appreciable yields with less material.

Another technique for enhancing the yield of a fission explosion is called boosting. Boosting refers to a process whereby fusion reactions are used as a source of neutrons for inducing fissions at a much higher rate than could be achieved with neutrons from fission chain reactions alone. American physicist Edward Teller invented the concept by the middle of 1943. By incorporating deuterium and tritium into the core of the fissile material, a higher yield is obtained from a given quantity of fissile material—or, alternatively, the same yield is achieved with a smaller amount. The fourth American test of Operation Greenhouse, on May 24, 1951, was the first proof test of a booster design. In subsequent decades approximately 90 percent of nuclear weapons in the American stockpile relied on boosting.

The fusion process

Nuclear fusion is the joining (or fusing) of the nuclei of two atoms to form a single heavier atom. At extremely high temperatures—in the range of tens of millions of degrees—the nuclei of isotopes of hydrogen (and some other light elements) can readily combine to form heavier elements and in the process release considerable energy—hence the term hydrogen bomb. At these temperatures, the kinetic energy of the nuclei (the energy of their motion) is sufficient to overcome the long-range electrostatic repulsive force between them, such that the nuclei can get close enough together for the shorter-range strong force to attract and fuse the nuclei—hence the term thermonuclear. In thermonuclear weapons, the required temperatures and density of the fusion materials are achieved with a fission explosion.

Deuterium and tritium, which are isotopes of hydrogen, provide ideal interacting nuclei for the fusion process. Two atoms of deuterium, each with one proton and one neutron, or tritium, with one proton and two neutrons, combine during the fusion process to form a heavier helium nucleus, which has two protons and either one or two neutrons. Tritium is radioactive and has a half-life of 12.32 years. The principal thermonuclear material in most thermonuclear weapons is lithium-6 deuteride, a solid chemical compound that at normal temperatures does not undergo radioactive decay. In this case, the tritium is produced in the weapon itself by neutron bombardment of the lithium-6 isotope during the course of the fusion reaction. In thermonuclear weapons, the fusion material can be incorporated directly in (or proximate to) the fissile core—for example, in the boosted fission device—or external to the fissile core, or both.