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A thermonuclear weapon, or fusion weapon, is a second-generation nuclear weapon design. Its greater sophistication over pure fission weapons may afford it vastly greater destructive power than first-generation atomic bombs, a more compact size, a lower mass or a combination of these benefits. Characteristics of nuclear fusion reactions make possible the use of non-fissile depleted uranium as the weapon's main fuel, thus allowing more efficient use of scarce fissile material (U-235 and Pu-239).

Though large quantities of vague data have been officially released, and larger quantities of vague data have been unofficially leaked by former bomb designers, most public descriptions of nuclear weapon design details rely to some degree on speculation, reverse engineering from known information, or comparison with similar fields of physics (inertial confinement fusion is the primary example). Such processes have resulted in a body of unclassified knowledge about nuclear bombs that is generally consistent with official unclassified information releases, related physics, and is thought to be internally consistent, though there are some points of interpretation that are still considered open. The state of public knowledge about the Teller¥Ulam design has been mostly shaped from a few specific incidents outlined in a section below.

Surrounding the other components is a hohlraum or radiation case, a container that traps the first stage or primary's energy inside temporarily. The outside of this radiation case, which is also normally the outside casing of the bomb, is the only direct visual evidence publicly available of any thermonuclear bomb component's configuration. Numerous photographs of various thermonuclear bomb exteriors have been declassified.

Separating the secondary from the primary is the interstage. The fissioning primary produces four types of energy: 1) expanding hot gases from high explosive charges that implode the primary; 2) superheated plasma that was originally the bomb's fissile material and its tamper; 3) the electromagnetic radiation; and 4) the neutrons from the primary's nuclear detonation. The interstage is responsible for accurately modulating the transfer of energy from the primary to the secondary. It must direct the hot gases, plasma, electromagnetic radiation and neutrons toward the right place at the right time. Less than optimal interstage designs have resulted in the secondary failing to work entirely on multiple shots, known as a "fissile fizzle". The Castle Koon shot of Operation Castle is a good example; a small flaw allowed the neutron flux from the primary to prematurely begin heating the secondary, weakening the compression enough to prevent any fusion.

Candidates for the "special material" are polystyrene and a substance called "FOGBANK", an unclassified codename. FOGBANK's composition is classified, though aerogel has been suggested as a possibility. It was first used in thermonuclear weapons with the W-76 thermonuclear warhead, and produced at a plant in the Y-12 Complex at Oak Ridge, Tennessee, for use in the W-76. Production of FOGBANK lapsed after the W-76 production run ended. The W-76 Life Extension Program required more FOGBANK to be made. This was complicated by the fact that the original FOGBANK's properties weren't fully documented, so a massive effort was mounted to re-invent the process. An impurity crucial to the properties of the old FOGBANK was omitted during the new process. Only close analysis of new and old batches revealed the nature of that impurity. The manufacturing process used acetonitrile as a solvent, which led to at least three evacuations of the FOGBANK plant in 2006. Widely used in the petroleum and pharmaceutical industries, acetonitrile is flammable and toxic. Y-12 is the sole producer of FOGBANK.

The radiation pressure exerted by the large quantity of X-ray photons inside the closed casing might be enough to compress the secondary. Electromagnetic radiation such as X-rays or light carries momentum and exerts a force on any surface it strikes. The pressure of radiation at the intensities seen in everyday life, such as sunlight striking a surface, is usually imperceptible, but at the extreme intensities found in a thermonuclear bomb the pressure is enormous.

The idea of a thermonuclear fusion bomb ignited by a smaller fission bomb was first proposed by Enrico Fermi to his colleague Edward Teller in 1941 at the start of what would become the Manhattan Project. Teller spent most of the Manhattan Project attempting to figure out how to make the design work, to some degree neglecting his assigned work on the fission bomb program. His difficult and devil's advocate attitude in discussions led Robert Oppenheimer to sidetrack him and other "problem" physicists into the super program to smooth his way.

France's journey in building nuclear weapons began prior to World War II in 1939. The development of nuclear weapons was slowed during the country's German invasion. The United States did not want France to acquire expert knowledge about nuclear weaponry, which ultimately led to the Alsos Mission. The missions followed closely behind the advancing forward-front to obtain information about how close Germany was to building an atomic weapon. Following the surrender of the Nazis, Germany was divided into "zones of occupation". The "zone" given to the French was suspected to contain several nuclear research facilities. The United States conducted Operation Harborage to seize any and all information about nuclear weaponry from the French. The Operation strategized to have American troops intercede advancing French army, allowing the Americans to seize any German scientists or records as well as destroy the remaining functional facilities.

In an interview in August 2009, the director for the 1998 test site preparations, Dr. K. Santhanam claimed that the yield of the thermonuclear explosion was lower than expected and that India should therefore not rush into signing the CTBT. Other Indian scientists involved in the test have disputed Dr. K. Santhanam's claim, arguing that Santhanam's claims are unscientific. British seismologist Roger Clarke argued that the magnitudes suggested a combined yield of up to 60 kilotonnes, consistent with the Indian announced total yield of 56 kilotonnes. U.S. seismologist Jack Evernden has argued that for correct estimation of yields, one should ‘account properly for geological and seismological differences between test sites’.

The reentry cones for the W88 and W87 are the same size, 1.75 meters (69 in) long, with a maximum diameter of 55 cm. (22 in). The higher yield of the W88 implies a larger secondary, which produces most of the yield. Putting the secondary, which is heavier than the primary, in the wider part of the cone allows it to be larger, but it also moves the center of mass aft, potentially causing aerodynamic stability problems during reentry. Dead-weight ballast must be added to the nose to move the center of mass forward.