Highly enriched uranium in the context of "Little Boy"

A thermonuclear weapon, fusion weapon or hydrogen bomb (H-bomb) is a second-generation nuclear weapon, utilizing nuclear fusion. The most destructive weapons ever created, their yields typically exceed first-generation nuclear weapons by twenty times, with far lower mass and volume requirements. Characteristics of fusion reactions can make possible the use of non-fissile depleted uranium as the weapon's main fuel, thus allowing more efficient use of scarce fissile material. Its multi-stage design is distinct from the usage of fusion in simpler boosted fission weapons. The first full-scale thermonuclear test (Ivy Mike) was carried out by the United States in 1952, and the concept has since been employed by at least the five NPT-recognized nuclear-weapon states: the United States, Russia, the United Kingdom, China, and France.

The design of all thermonuclear weapons is believed to be the Teller–Ulam configuration. This relies on radiation implosion, in which X-rays from detonation of the primary stage, a fission bomb, are channelled to compress a separate fusion secondary stage containing thermonuclear fuel, primarily lithium-6 deuteride. During detonation, neutrons convert lithium-6 to helium-4 plus tritium. The heavy isotopes of hydrogen, deuterium and tritium, then undergo a reaction that releases energy and neutrons. For this reason, thermonuclear weapons are often colloquially called hydrogen bombs or H-bombs.

Gun-type fission weapons are fission-based nuclear weapons whose design assembles their fissile material into a supercritical mass by the use of the "gun" method: shooting one piece of sub-critical material into another. Although this is sometimes pictured as two sub-critical hemispheres driven together to make a supercritical sphere, typically a hollow projectile is shot onto a cylindrical spike, which fills the hole in its center. Its name is a reference to the fact that it is shooting the material through an artillery barrel as if it were a projectile. Developed and deployed by the Manhattan Project, gun-type designs were quickly replaced by the more efficient implosion-type weapons.

All known gun-type fission weapons have used highly enriched uranium (HEU). The high spontaneous fission rates of plutonium isotopes make it very impractical for use in gun-type designs, as in the abandoned Thin Man design. Additionally, the efficiency is low, increasing the amount of HEU required and weapon weight. The main reason for this is the fissile material does not undergo compression (and resulting density increase) as does the implosion design. Instead, gun-type bombs assemble the supercritical mass by amassing such a large quantity of uranium that the overall distance through which daughter neutrons must travel has so many mean free paths it becomes very probable most neutrons will find uranium nuclei to collide with, before escaping the supercritical mass. HEU could be more efficiently used by the composite cores of early implosion-type weapons.

Uranium mining is the process of extraction of uranium ore from the earth. Almost 50,000 tons of uranium were produced in 2022. Kazakhstan, Canada, and Namibia were the top three uranium producers, respectively, and together account for 69% of world production. Other countries producing more than 1,000 tons per year included Australia, Niger, Russia, Uzbekistan and China. Nearly all of the world's mined uranium is used to power nuclear power plants. Historically uranium was also used in applications such as uranium glass or ferrouranium but those applications have declined due to the radioactivity and toxicity of uranium and are nowadays mostly supplied with a plentiful cheap supply of depleted uranium which is also used in uranium ammunition. In addition to being cheaper, depleted uranium is also less radioactive due to a lower content of short-lived
U and
U than natural uranium.

Uranium is mined by in-situ leaching (57% of world production) or by conventional underground or open-pit mining of ores (43% of production). During in-situ mining, a leaching solution is pumped down drill holes into the uranium ore deposit where it dissolves the ore minerals. The uranium-rich fluid is then pumped back to the surface and processed to extract the uranium compounds from solution. In conventional mining, ores are processed by grinding the ore materials to a uniform particle size and then treating the ore to extract the uranium by chemical leaching. The milling process commonly yields dry powder-form material consisting of natural uranium, "yellowcake", which is nowadays commonly sold on the uranium market as U₃O₈. While some nuclear power plants – most notably heavy water reactors like the CANDU – can operate with natural uranium (usually in the form of uranium dioxide), the vast majority of commercial nuclear power plants and many research reactors require uranium enrichment, which raises the content of
U from the natural 0.72% to 3–5% (for use in light water reactors) or even higher, depending on the application. Enrichment requires conversion of the yellowcake into uranium hexafluoride and production of the fuel (again usually uranium dioxide, but sometimes uranium carbide, uranium hydride or uranium nitride) from that feedstock.