Nuclear Weapons Glossary

50 defined terms covering nuclear weapon effects, physics, doctrine and arms control. Each term has a cited definition you can link to directly.

Nuclear weapon effects

Acute Radiation Syndrome: ARS occurs at whole-body doses above about 0.7 gray. About half of exposed people die within 60 days at roughly 2.5 to 5 gray, and survival is extremely unlikely above about 10 gray. ^[11] Symptoms progress through prodromal, latent, manifest illness, and either recovery or death.
Air Burst: In an air burst the fireball does not reach the ground, so the bomb cannot vaporize and irradiate soil. This minimizes local fallout but maximizes blast spread because the shock wave reflects off the ground and reinforces the direct wave. Hiroshima and Nagasaki were both air bursts, detonating about 600 m and 503 m above the cities. ^[7]
EMP (Electromagnetic Pulse): A high-altitude nuclear detonation (above 30 km) produces a particularly strong EMP because gamma rays from the bomb interact with the upper atmosphere to generate a downward-directed electromagnetic wave covering thousands of kilometers. A single high-altitude burst over the central US could disable power grids and unhardened electronics across most of the continent. ^[9]
Fallout: Surface bursts produce massive fallout because the fireball touches the ground, vaporizing soil and irradiating it with neutron flux. The radioactive debris is lofted into the upper atmosphere and drifts downwind, depositing as a long elliptical plume. Air bursts produce minimal fallout because the fireball does not interact with the ground. The lethal fallout zone of a large surface burst can stretch far downwind: after the Bravo test it was a belt about 270 km long. ^[6]
Fireball: The sphere of extremely hot gas created by a nuclear detonation; at the first instant it reaches tens of millions of degrees, similar to the center of the Sun. ^[1] The fireball forms within microseconds as the X-ray flux from the bomb superheats the surrounding air into plasma. Within the fireball radius, exposed material is vaporized; the plasma sphere also acts as the source of the thermal radiation pulse and the initial blast wave. Fireball radius scales roughly with yield to the 0.4 power. ^[1]
Nuclear Winter: A 2019 study found that a war in which India and Pakistan used 250 weapons on cities could release 16-36 million tonnes of soot, cut sunlight at the surface by 20-35% and cool the planet by 2-5 °C, with recovery taking more than 10 years. ^[10] The result would be widespread crop failure and famine. Even a regional nuclear war causes significant cooling on these models.
Overpressure: Nuclear blast damage is conventionally categorized by overpressure. 20 PSI levels reinforced concrete structures; 5 PSI destroys typical homes and lightly built commercial buildings; 1 PSI shatters windows and endangers people with flying glass. ^[2] Overpressure radius scales with yield to the 1/3 power. ^[3]
PSI (Pounds per Square Inch): 1 PSI is a small overpressure that breaks windows. 5 PSI collapses most residential buildings and is the standard measurement for nuclear-blast damage zones. 20 PSI destroys reinforced concrete. ^[2] By comparison, normal atmospheric pressure at sea level is about 14.7 PSI. ^[3]
Surface Burst: A nuclear detonation at ground level. Its blast reaches less far than an optimum-height air burst, and it creates heavy local fallout. ^[3] A surface burst is used when the target is hardened (silos, command bunkers) or when the attacker wants to maximize fallout. The fireball touches the ground, vaporizing soil and creating a large crater. Part of the energy goes into forming a crater and ground shock, and less thermal radiation reaches a given distance than from an air burst. ^[8] The radioactive plume can render hundreds of square kilometers uninhabitable for years.
Thermal Radiation: Thermal radiation accounts for roughly 35% of the energy released in an air burst, and less (about 18% effective) in a surface burst. ^[4] It travels at the speed of light and arrives before the blast wave. Third-degree burns can occur out to several kilometers for strategic-yield weapons. Thermal-radiation radius grows roughly as yield to the 0.4 power, faster than blast (yield to the 1/3 power). ^[5]

Weapons & delivery systems

Atomic Bomb: Atomic bombs (also called fission bombs) split heavy nuclei to release energy. Most fission weapons yield under about 100 kilotons, though pure-fission designs have reached 500 kilotons (Ivy King, 1952). Little Boy (uranium-235, gun-type) and Fat Man (plutonium-239, implosion) were both fission weapons. ^[12]
Cruise Missile: Unlike ballistic missiles, cruise missiles fly at low altitude using GPS or terrain-contour matching for navigation. Some carry nuclear warheads (e.g., the AGM-86 ALCM with the W80 warhead). They are slower than ICBMs but harder to detect because they fly under radar.
Hydrogen Bomb: Also called thermonuclear or H-bombs, hydrogen bombs use a fission explosion to ignite fusion in a secondary fuel. Yields are scalable to tens of megatons; the largest weapon ever tested, Tsar Bomba (1961), released about 50 megatons. ^[13]
ICBM (Intercontinental Ballistic Missile): An ICBM is a long-range ballistic missile with a range greater than 5,500 km, designed to deliver nuclear warheads between continents. ^[19] ICBMs are typically launched from hardened silos or mobile launchers, follow a ballistic trajectory through space, and reenter the atmosphere to strike targets thousands of kilometers away. US Minuteman III and Russian RS-28 Sarmat are current examples. An ICBM flight between Russia and the United States takes roughly 30 minutes, about 25 of them in midcourse flight. ^[20]
MIRV: MIRV technology, deployed from the early 1970s, allows one ICBM or SLBM to deliver up to 8-10 warheads on different targets. ^{[18, 15, 16]} This was originally a counter-ABM measure but became the dominant warhead-multiplier in Cold War arsenals. New START limits MIRV deployments by warhead count, not by missile.
Neutron Bomb: Also called an "enhanced radiation weapon," a neutron bomb releases a much larger share of its energy as prompt radiation, about 40% compared with about 5% for a fission weapon. ^[22] This radiation kills biological targets without destroying buildings. Originally conceived as a battlefield anti-tank weapon, the W70-3 neutron warhead was produced from 1981 and retired in 1992. ^[22]
SLBM (Submarine-Launched Ballistic Missile): SLBMs (Trident II, Bulava, etc.) are launched from ballistic-missile submarines (SSBNs) that hide in the ocean, making them nearly impossible to target preemptively. This survivability makes SLBMs the most credible second-strike leg of the nuclear triad. The W76 and W88 warheads ride US Trident II missiles.
Strategic Nuclear Weapon: Strategic warheads typically yield about 100 to 800 kilotons; only a few still deployed exceed one megaton. ^{[15, 16, 21]} They are delivered by ICBMs, SLBMs, or strategic bombers and are governed by treaties such as New START. The US triad consists of Minuteman ICBMs, Trident SLBMs, and B-2 / B-21 bombers carrying B61 / B83 gravity bombs.
Tactical Nuclear Weapon: Tactical (non-strategic) nuclear weapons have shorter ranges; their yields run from under 1 kt to 100 kt or more (the US B61-3 bomb is rated at 0.3–170 kt). ^{[15, 16]} They include artillery shells, depth charges, short-range missiles, and gravity bombs. Modern tactical weapons like the B61 and Russian Iskander-K warhead have variable yields (sometimes called "dial-a-yield"). The line between tactical and strategic blurs at higher yields.
Teller-Ulam Design: In the Teller–Ulam configuration, X-rays from a fission primary travel through a hohlraum and ablation-compress a separate fusion secondary, raising it to fusion-ignition temperatures. This radiation-implosion principle made very large yields practical. First proven in Ivy Mike (1952), the Teller-Ulam configuration became the template for later thermonuclear weapons. ^[14]
Thermonuclear Weapon: A thermonuclear weapon uses a fission "primary" to drive radiation implosion of a fusion "secondary." This staged design (Teller–Ulam configuration) allows yields scalable from tens of kilotons to tens of megatons.
Warhead: In nuclear arsenals, "warhead" usually refers to the physics package on a delivery vehicle (ICBM reentry vehicle, SLBM RV, cruise missile, gravity bomb). Most modern warheads yield between about 100 and 800 kilotons, though some are much smaller. ^{[15, 16, 17]} The W76 and W88 are the most common US warheads; the B61 is a variable-yield gravity bomb.

Nuclear physics

Boosted Fission: Boosting uses neutrons from a small amount of fusion fuel to make roughly twice as much of the fissile material fission, raising the yield; the fusion itself adds only about 1% of the energy. ^[33] The fusion neutrons released by D-T reactions cause a much higher fraction of the fissile core to fission before the bomb disassembles. Most modern fission primaries are boosted.
Critical Mass: For a bare sphere, the critical mass is about 10 kg for plutonium-239 and about 46 kg for pure uranium-235. ^[28] Critical mass is reduced by neutron reflectors (tampers) and by compression. Atomic bombs work by rapidly assembling a supercritical mass before the chain reaction blows itself apart.
Deuterium: In ocean water, about 1 hydrogen atom in 6,420 is deuterium. ^[31] In thermonuclear weapons, lithium-6 deuteride (a solid compound) is the fusion fuel: neutron irradiation of the lithium produces tritium in situ, which then fuses with deuterium to release energy.
Gamma Radiation: Gamma rays from a nuclear detonation cause acute radiation injury at close range and contribute to fallout dose at long range. Heavy shielding (lead, concrete) is required to attenuate gamma radiation. The high-altitude EMP effect is initiated by gamma-ray interaction with atmospheric atoms.
Half-Life: Different isotopes have wildly different half-lives: tritium 12.3 years, cesium-137 30 years, plutonium-239 24,100 years, uranium-235 704 million years. ^[32] Half-life determines fallout persistence: short-lived isotopes dominate early radiation, long-lived isotopes contaminate the environment for decades.
Kiloton (kt): A kiloton is the standard unit for tactical-yield nuclear weapons, equal to 1,000 tons of TNT or 4.184 × 10¹² joules. The Hiroshima bomb was about 15 kilotons; modern artillery-fired nuclear shells (now retired) were sub-kiloton. ^[24]
Megaton (Mt): A megaton equals 1,000 kilotons, or one million tons of TNT. Most modern strategic warheads are below one megaton, typically about 100–800 kt. Tsar Bomba (about 50 Mt, 1961) is the largest weapon ever detonated. ^{[25, 26, 15]}
Nuclear Fission: Fission is the energy-release mechanism in atomic bombs and most nuclear power plants. Each fission event releases roughly 200 MeV of energy and 2–3 neutrons, which can sustain a chain reaction if a critical mass of fissile material is assembled. ^[27]
Nuclear Fusion: Fusion powers stars and is the energy mechanism in thermonuclear weapons. In a hydrogen bomb, a fission primary compresses and heats lithium-deuteride fuel to fusion-ignition temperatures (tens of millions of degrees), releasing far more energy than fission alone. ^[24]
Plutonium-239: Pu-239 has a smaller critical mass than U-235 and is the preferred material for compact fission weapons. It cannot be assembled by gun-type designs (it would predetonate), so plutonium weapons use implosion. The Nagasaki bomb (Fat Man) used about 6.2 kg of plutonium. ^[30]
Second-Degree Burns: In a nuclear context, the 2nd-degree burn radius is the distance at which exposed skin receives enough thermal radiation to blister; the energy needed rises with yield. For a Hiroshima-yield bomb this radius is about 2.8 km; for a 1 Mt weapon about 15 km. ^[5]
Sievert (Sv): The sievert measures equivalent dose, weighting absorbed energy by biological effect. Acute effects are usually stated in gray: a whole-body dose above about 0.7 gray causes acute radiation syndrome; about 2.5 to 5 gray kills roughly half of exposed people within 60 days; survival is extremely unlikely above about 10 gray. ^[11] Natural background radiation averages about 2.4 millisieverts per year worldwide. Aircrew and astronauts receive notably higher doses.
Tritium: Tritium has a half-life of 12.3 years, so warhead arsenals require ongoing tritium production to replace decay. ^[32] Modern boosted-fission weapons inject tritium-deuterium gas into the fission core to enhance yield. Tritium is produced by irradiating lithium-6 in nuclear reactors.
Uranium-235: Natural uranium is about 99.3% U-238 and 0.7% U-235. Weapons-grade uranium is enriched to about 90% U-235 or more, a process requiring centrifuges or gaseous diffusion. Little Boy used about 64 kg of uranium enriched to an average of about 80% U-235. ^[29]
Yield: Nuclear yield is conventionally measured against TNT explosive energy: 1 kiloton (kt) = 1,000 tons of TNT, 1 megaton (Mt) = 1 million tons. The Hiroshima bomb was about 15 kt, Tsar Bomba 50,000 kt, and modern strategic warheads are typically 100-500 kt. ^[23]

Doctrine & strategy

Counterforce: Counterforce attacks target missile silos, submarine bases, airfields, and command bunkers. Success requires high accuracy and prompt strike capability — historically associated with the development of MIRVs and improved guidance. Counterforce is paired with first-strike scenarios and is destabilizing because it raises pre-emption incentives.
Countervalue: Countervalue targeting is the classical approach to nuclear deterrence: hold the opponent's cities at risk so they cannot survive a war as a nation. It does not require pinpoint accuracy and was the dominant strategy during the early Cold War. MAD is fundamentally a countervalue concept.
Dead Hand: The Soviet Perimeter system, often called Dead Hand, entered service in 1985 after development that began in 1974. ^[36] If sensors detect a nuclear attack and command-and-control has been severed, the system enables surviving officers to authorize a full retaliatory launch. Russia is widely believed to maintain a modernized version.
First Strike: A counterforce first strike targets the opponent's missile silos, submarine bases, and command bunkers. To succeed it must destroy enough of the opponent's forces that the residual second strike is acceptable to the attacker. Modern MIRVed missiles and high accuracy make first-strike scenarios more plausible than during early Cold War.
MAD (Mutually Assured Destruction): MAD relies on each side maintaining a credible second-strike capability — typically through hardened ICBM silos, strategic bombers, and ballistic-missile submarines. The logic is paradoxical: weapons exist precisely so that they will not be used. MAD is the implicit framework of the post-1960s nuclear standoff.
Nuclear Deterrence: Effective deterrence requires three elements: capability (sufficient survivable forces), credibility (the adversary believes the weapons will be used), and communication (clear signaling of red lines). Deterrence is fragile: misperception, accidents, and crisis instability can break it.
Nuclear Triad: The triad provides survivability through diversity: each leg has different vulnerabilities. ICBMs are accurate but fixed; SLBMs are survivable but harder to recall; bombers are flexible but slow. The US and Russia operate full triads, and China and India field less mature ones. France relies on submarines and aircraft, and the UK only on submarine-launched Trident missiles. ^{[34, 35, 21, 17]}
Permissive Action Link (PAL): A June 1962 presidential order (NSAM 160) required PALs on US nuclear weapons in Europe, to prevent unauthorized arming or detonation. ^[37] Modern PALs disable the warhead if tampered with. The Soviet Union and Russia use a different system based on centralized release codes.
Second Strike: Second-strike survivability is achieved through redundancy and concealment: hardened ICBM silos that can withstand near-misses, ballistic-missile submarines patrolling the oceans, and dispersed strategic bombers. The submarine leg is generally considered the most survivable.

Treaties & arms control

CTBT (Comprehensive Test Ban Treaty): The CTBT cannot enter into force until nine more key states ratify it: China, Egypt, India, Iran, Israel, North Korea, Pakistan, Russia and the United States. ^[45] Despite this, all states except North Korea have observed a de-facto testing moratorium since the late 1990s. The CTBTO operates a global monitoring network.
New START: New START replaced the 1991 START I treaty. It capped each side at 1,550 deployed strategic warheads, 700 deployed missiles and bombers, and 800 total launchers. Russia suspended participation in February 2023, and the treaty expired on 5 February 2026 with no replacement. ^{[46, 47, 48, 49]} It was the last remaining bilateral US–Russia nuclear arms control agreement.
NPT (Non-Proliferation Treaty): The 1968 Treaty on the Non-Proliferation of Nuclear Weapons recognizes five nuclear-weapon states, defined as those that exploded a nuclear device before 1 January 1967. ^{[38, 39, 40]} These are the US, USSR/Russia, UK, France, and China. All other parties commit not to acquire nuclear weapons in exchange for access to peaceful nuclear technology. India, Pakistan, Israel, and North Korea (which withdrew) operate outside the NPT framework.
TPNW (Treaty on the Prohibition of Nuclear Weapons): The TPNW entered into force in January 2021. 75 states are parties to the TPNW as of September 2026. ^{[41, 42, 43]} None of the nine nuclear-armed states have signed; the treaty is intended to stigmatize possession and build pressure toward eventual abolition. ICAN, the lead civil-society coalition behind the TPNW, won the 2017 Nobel Peace Prize. ^[44]

Sources

See also: methodology · sources · weapons database.