A typical thermonuclear warhead may be constructed according to a two-stage design, featuring a fission or boosted-fission primary (also called the trigger) and a physically separate component called the secondary. Both primary and secondary are contained within an outer metal case. Radiation from the fission explosion of the primary is contained and used to transfer energy to compress and ignite the secondary. Some of the initial radiation from the primary explosion is absorbed by the inner surface of the case, which is made of a high-density material such as uranium. Radiation absorption heats the inner surface of the case, turning it into an opaque boundary of hot electrons and ions. Subsequent radiation from the primary is largely confined between this boundary and the outer surface of the secondary capsule. Initial, reflected, and re-irradiated radiation trapped within this cavity is absorbed by lower-density material within the cavity, converting it into a hot plasma of electrons and ion particles that continue to absorb energy from the confined radiation. The total pressure in the cavity—the sum of the contribution from the very energetic particles and the generally smaller contribution from the radiation—is applied to the secondary capsule’s heavy metal outer shell (called a pusher), thereby compressing the secondary.

Typically, contained within the pusher is some fusion material, such as lithium-6 deuteride, surrounding a “spark plug” of explosive fissionable material (generally uranium-235) at the center. With the fission primary generating an explosive yield in the kiloton range, compression of the secondary is much greater than can be achieved using chemical high explosives. Compression of the spark plug results in a fission explosion that creates temperatures comparable to those of the Sun and a copious supply of neutrons for fusion of the surrounding, and now compressed, thermonuclear materials. Thus, the fission and fusion processes that take place in the secondary are generally much more efficient than those that take place in the primary.

In an efficient, modern two-stage device—such as a long-range ballistic missile warhead—the primary is boosted in order to conserve on volume and weight. Boosted primaries in modern thermonuclear weapons contain about 3 to 4 kg (6.6 to 8.8 pounds) of plutonium, while less-sophisticated designs may use double that amount or more. The secondary typically contains a composite of fusion and fissile materials carefully tailored to maximize the yield-to-weight or yield-to-volume ratio of the warhead, although it is possible to construct secondaries from purely fissile or fusion materials.

Enhanced designs

Historically, some very high-yield thermonuclear weapons had a third, or tertiary, stage. In theory, the radiation from the tertiary can be contained and used to transfer energy to compress and ignite a fourth stage, and so on. There is no theoretical limit to the number of stages that might be used and, consequently, no theoretical limit to the size and yield of a thermonuclear weapon. However, there is a practical limit because of size and weight limitations imposed by the requirement that the weapon be deliverable.

Uranium-238 and thorium-232 (and some other fissionable materials) cannot maintain a self-sustaining fission explosion, but these isotopes can be made to fission by an externally maintained supply of fast neutrons from fission or fusion reactions. Thus, the yield of a nuclear weapon can be increased by surrounding the device with uranium-238, in the form of either natural or depleted uranium, or with thorium-232, in the form of natural thorium. This approach is particularly advantageous in a thermonuclear weapon in which uranium-238 or thorium-232 in the outer shell of the secondary capsule is used to absorb an abundance of fast neutrons from fusion reactions produced within the secondary. The explosive yields of some weapon designs have been further increased by the substitution of highly enriched uranium-235 for uranium-238 in the secondary.

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In general, the energy released in the explosion of a high-yield thermonuclear weapon stems from the boosted-fission chain reaction in the primary stage and the fissioning and “burning” of thermonuclear fuel in the secondary (and any subsequent) stage, with roughly 50 to 75 percent of the total energy produced by fission and the remainder by fusion. However, to obtain tailored weapon effects or to meet certain weight or space constraints, different ratios of fission yield to fusion yield may be employed, ranging from nearly pure fission weapons to a weapon where a very high proportion of the yield is from fusion.

Another tailored weapon is the enhanced radiation warhead, or neutron bomb, a low-yield (on the order of one kiloton), two-stage thermonuclear device designed to intensify the production of lethal fast neutrons in order to maximize mortality rates while producing less damage to buildings. The enhanced radiation is in the form of fast neutrons produced by the fusion of deuterium and tritium. The secondary contains little or no fissionable material, since this would increase the blast effect without significantly increasing the intensity of fast neutrons. The United States produced enhanced-radiation warheads for antiballistic missiles, short-range ballistic missiles, and artillery shells.