Nuclear weapon yield in the context of "Castle Bravo"

A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either nuclear fission (fission or atomic bomb) or a combination of fission and nuclear fusion reactions (thermonuclear weapon), producing a nuclear explosion. Both bomb types release large quantities of energy from relatively small amounts of matter.

Nuclear weapons have had yields between 10 tons (the W54) and 50 megatons for the Tsar Bomba (see TNT equivalent). Yields in the low kilotons can devastate cities. A thermonuclear weapon weighing as little as 600 pounds (270 kg) can release energy equal to more than 1.2 megatons of TNT (5.0 PJ). Apart from the blast, effects of nuclear weapons include extreme heat and ionizing radiation, firestorms, radioactive nuclear fallout, an electromagnetic pulse, and a radar blackout.

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.

The effects of a nuclear explosion on its immediate vicinity are typically much more destructive and multifaceted than those caused by conventional explosives. In most cases, the energy released from a nuclear weapon detonated within the lower atmosphere can be approximately divided into four basic categories:

• the blast and shock wave: 50% of total energy
• thermal radiation: 35% of total energy
• ionizing radiation: 5% of total energy (more in a neutron bomb)
• residual radiation: 5–10% of total energy with the mass of the explosion.

Depending on the design of the weapon and the location in which it is detonated, the energy distributed to any one of these categories may be significantly higher or lower. The physical blast effect is created by the coupling of immense amounts of energy, spanning the electromagnetic spectrum, with the surroundings. The environment of the explosion (e.g. submarine, ground burst, air burst, or exo-atmospheric) determines how much energy is distributed to the blast and how much to radiation. In general, surrounding a bomb with denser media, such as water, absorbs more energy and creates more powerful shock waves while at the same time limiting the area of its effect. When a nuclear weapon is surrounded only by air, lethal blast and thermal effects proportionally scale much more rapidly than lethal radiation effects as explosive yield increases. This bubble is faster than the speed of sound. The physical damage mechanisms of a nuclear weapon (blast and thermal radiation) are identical to those of conventional explosives, but the energy produced by a nuclear explosion is usually millions of times more powerful per unit mass, and temperatures may briefly reach the tens of millions of degrees.

Nuclear winter is a severe and prolonged global climatic cooling effect that is hypothesized to occur after widespread urban firestorms following a large-scale nuclear war. The hypothesis is based on the fact that such fires can inject soot into the stratosphere, where it can block some direct sunlight from reaching the surface of the Earth. It is speculated that the resulting cooling, typically lasting a decade, would lead to widespread crop failure, a global nuclear famine, and an animal mass extinction event.

Climate researchers study nuclear winter via computer models and scenarios. Results are highly dependent on nuclear yields, weather and how many cities are targeted, their flammable material content, and the firestorms' atmospheric environments, convections, and durations. Firestorm case studies include the World War II bombings of Hiroshima, Tokyo, Hamburg, Dresden, and London, and modern observations from large-area wildfires such as the 2021 British Columbia wildfires.

The Teller–Ulam design is the technical concept behind thermonuclear weapons, also known as hydrogen bombs. The design relies on the radiation implosion principle, using thermal X-rays released from a fission nuclear primary to compress and ignite nuclear fusion in a secondary. This is in contrast to the simpler design and usage of nuclear fusion in boosted fission weapons.

The design is named for scientists Edward Teller and Stanisław Ulam, who originally devised the concept in January 1951 for the United States nuclear weapons program, though their individual roles have been subsequently debated. The US Greenhouse George test in May 1951, the world's first artificial thermonuclear fusion, validated the radiation implosion principle. The US first tested the "true" Teller-Ulam design with the very high-yield Ivy Mike test in 1952. The design was independently devised and then tested by teams of nuclear weapons scientists working for at least four more governments: the Soviet Union in 1955 (RDS-37), the United Kingdom in 1957 (Operation Grapple), China in 1966 (Project 639), and France in 1968 (Canopus). There is not enough public information to determine whether India, Israel, or North Korea possess multi-stage weapons. Pakistan is not considered to have developed them. The Teller-Ulam design is the basis for all nuclear weapons tests above one megaton yield.

Operation Castle was a United States series of high-yield (high-energy) nuclear tests by Joint Task Force 7 (JTF-7) at Bikini Atoll beginning in March 1954. It followed Operation Upshot–Knothole and preceded Operation Teapot.

Conducted as a joint venture between the Atomic Energy Commission (AEC) and the Department of Defense (DoD), the ultimate objective of the operation was to test designs for an aircraft-deliverable thermonuclear weapon. All the devices tested, which ranged in weight from 6,520 to 39,600 pounds (2,960 to 17,960 kg), were built to be dropped from aircraft. However, ballistic casings, fins and fusing systems would have to be attached.

Ivy Mike was the codename given to the first full-scale test of a thermonuclear device, in which a significant fraction of the explosive yield comes from nuclear fusion.Ivy Mike was detonated on November 1, 1952, by the United States on the island of Elugelab in Enewetak Atoll, in the now independent island nation of the Marshall Islands, as part of Operation Ivy. It was the first full test of the Teller–Ulam design, a staged fusion device.

Due to its physical size and fusion fuel type (cryogenic liquid deuterium), the "Mike" device was not suitable for use as a deliverable weapon. It was intended as a "technically conservative" proof of concept experiment to validate the concepts used for multi-megaton detonations.