The explosive yield of a nuclear weapon is the amount of energy released such as blast, thermal, and nuclear radiation, when that particular nuclear weapon is detonated. It is usually expressed as a TNT equivalent, the standardized equivalent mass of trinitrotoluene (TNT) which would produce the same energy discharge if detonated, either in kilotonnes (symbol kt, thousands of tonnes of TNT) or in megatonnes (Mt, millions of tonnes of TNT). It is also sometimes expressed in terajoules (TJ); an explosive yield of one terajoule is equal to . Because the accuracy of any measurement of the energy released by TNT has always been problematic, the conventional definition is that one kilotonne of TNT is held simply to be equivalent to 1012 calories.
The yield-to-weight ratio is the amount of weapon yield compared to the mass of the weapon. The practical maximum yield-to-weight ratio for fusion weapons (thermonuclear weapons) has been estimated to six megatonnes of TNT per tonne of bomb mass (25 TJ/kg). Yields of 5.2 megatonnes/tonne and higher have been reported for large weapons constructed for single-warhead use in the early 1960s.The B-41 Bomb Since then, the smaller warheads needed to achieve the increased net damage efficiency (bomb damage/bomb mass) of multiple warhead systems have resulted in increases in the yield/mass ratio for single modern warheads.
Examples of nuclear weapon yields
In order of increasing yield (most yield figures are approximate):
BombYieldNotesWeight of nuclear materialkt TNTTJDavy CrockettVariable yield tactical nuclear weapon—mass only 23 kg (51 lb), lightest ever deployed by the United States (same warhead as Special Atomic Demolition Munition and GAR-11 Nuclear Falcon missile).AIR-2 GenieAn unguided air-to-air rocket armed with a W25 nuclear warhead developed to intercept bomber squadrons.Total weight of nuclear material and bomb was 98.8 - 100.2 kgHiroshima's Gun type uranium-235 fission bomb (the first of the two nuclear weapons that have been used in warfare).64 kg of Uranium-235, about 1.38% of the uranium fissionedNagasaki's gravity bombImplosion type plutonium-239 fission bomb (the second of the two nuclear weapons used in warfare).6.2 kg of Plutonium-239, about 1 kg fissionedW76 warheadTwelve of these may be in a MIRVed Trident II missile; treaty limited to eight.W87 warheadTen of these were in a MIRVed LGM-118A Peacekeeper.W88 warheadEight of these may be in a Trident II missile.Ivy King deviceMost powerful US pure fission bomb, 60 kg uranium, implosion type. Never deployed.60 kg of Highly enriched uranium (HEU)Orange Herald SmallMost powerful tested UK boosted fission missile warhead.117 kg of Uranium-235B83 nuclear bombVariable yield weapon, most powerful US weapon in active service.B53 nuclear bombWas the most powerful US bomb in active service until 1997. 50 were retained as part of the "Hedge" portion of the Enduring Stockpile until completely dismantled in 2011. The Mod 11 variant of the B61 replaced the B53 in the bunker busting role. The W53 warhead from the weapon was used on the Titan II Missile until the system was decommissioned in 1987.Castle Bravo deviceMost powerful US test. Never deployed.400 kg of Lithium-6 deuterideEC17/Mk-17, the EC24/Mk-24, and the B41 (Mk-41)Most powerful US weapons ever: ; the Mk-17 was also the largest by area square footage and mass cubic footage: about . The Mk-41 or B41 had a mass of 4800 kg and yield of 25 Mt; this equates to being the highest yield-to-weight weapon ever mass-produced. All were gravity bombs carried by the B-36 bomber (retired by 1957).The entire Operation Castle nuclear test seriesThe highest-yielding test series conducted by the US.Tsar Bomba deviceUSSR, most powerful nuclear weapon ever detonated, yield of 50 megatonnes, (50 million tonnes of TNT). In its "final" form (i.e. with a depleted uranium tamper instead of one made of lead) it would have been 100 megatonnes.All nuclear testing as of 1996Total energy expended during all nuclear testing.
In comparison, the blast yield of the GBU-43 Massive Ordnance Air Blast bomb is 0.011 kt, and that of the Oklahoma City bombing, using a truck-based fertilizer bomb, was 0.002 kt. The estimated strength of the explosion at the Port of Beirut is 0.3-0.5 kt. Most artificial non-nuclear explosions are considerably smaller than even what are considered to be very small nuclear weapons.
Yield limits The yield-to-mass ratio is the amount of weapon yield compared to the mass of the weapon. The highest achieved values are somewhat lower, and the value tends to be lower for smaller, lighter weapons, of the sort that are emphasized in today's arsenals, designed for efficient MIRV use or delivery by cruise missile systems.
The 25 Mt yield option reported for the B41 would give it a yield-to-mass ratio of 5.1 megatonnes of TNT per tonne. While this would require a far greater efficiency than any other current U.S. weapon (at least 40% efficiency in a fusion fuel of lithium deuteride), this was apparently attainable, probably by the use of higher than normal lithium-6 enrichment in the lithium deuteride fusion fuel. This results in the B41 still retaining the record for the highest yield-to-mass weapon ever designed.. The W56 demonstrated a yield-to-mass ratio of 4.96 kt per kilogram of device mass, and very close to the predicted 5.1 kt/kg achievable in the highest yield-to-mass weapon ever built, the 25-megatonne B41. Unlike the B41, which was never proof-tested at full yield, the W56 demonstrated its efficiency in the XW-56X2 Bluestone shot of Operation Dominic in 1962, thus, from information available in the public domain, the W56 may hold the distinction of demonstrating the highest efficiency in a nuclear weapon to date. In 1963 DOE declassified statements that the U.S. had the technological capability of deploying a 35 Mt warhead on the Titan II, or a 50–60 Mt gravity bomb on B-52s. Neither weapon was pursued, but either would require yield-to-mass ratios superior to a 25 Mt Mk-41. For current smaller US weapons, yield is 600 to 2200 kilotonnes of TNT per tonne. By comparison, for the very small tactical devices such as the Davy Crockett it was 0.4 to 40 kilotonnes of TNT per tonne. For historical comparison, for Little Boy the yield was only 4 kilotonnes of TNT per tonne, and for the largest Tsar Bomba, the yield was 2 megatonnes of TNT per tonne (deliberately reduced from about twice as much yield for the same weapon, so there is little doubt that this bomb as designed was capable of 4 megatonnes per tonne yield). The largest pure-fission bomb ever constructed, Ivy King, had a 500 kilotonne yield, which is probably in the range of the upper limit on such designs. Fusion boosting could likely raise the efficiency of such a weapon significantly, but eventually all fission-based weapons have an upper yield limit due to the difficulties of dealing with large critical masses. (The UK's Orange Herald was a very large boosted fission bomb, with a yield of 800 kilotonnes.) However, there is no known upper yield limit for a fusion bomb.
Large single warheads are seldom a part of today's arsenals, since smaller MIRV warheads, spread out over a pancake-shaped destructive area, are far more destructive for a given total yield, or unit of payload mass. This effect results from the fact that destructive power of a single warhead on land scales approximately only as the cube root of its yield, due to blast "wasted" over a roughly hemispherical blast volume, while the strategic target is distributed over a circular land area with limited height and depth. This effect more than makes up for the lessened yield/mass efficiency encountered if ballistic missile warheads are individually scaled down from the maximal size that could be carried by a single-warhead missile.
Yield efficiency The efficiency of an atomic bomb is the ratio of the actual yield to the theoretical maximum yield of the atomic bomb. Not all atomic bombs possess the same yield efficiency as each individual bombs design plays a large role in how efficient it can be. In order to maximize yield efficiency one must make sure to assemble the critical mass correctly, as well as implementing instruments such as tampers or initiators in the design. A tamper is typically made of uranium and it holds the core together using its inertia. It is used to prevent the core from separating too soon to generate maximum fission, so as not to cause a "fizzle". The initiator is a source of neutrons either inside of the core, or on the outside of the bomb, and in this case it shoots neutrons at the core at the moment of detonation. It is essentially kick starting the reaction so the maximum fission reactions can occur to maximize yield.
Milestone nuclear explosions The following list is of milestone nuclear explosions. In addition to the atomic bombings of Hiroshima and Nagasaki, the first nuclear test of a given weapon type for a country is included, as well as tests that were otherwise notable (such as the largest test ever). All yields (explosive power) are given in their estimated energy equivalents in kilotons of TNT (see TNT equivalent). Putative tests (like Vela incident) have not been included.
Date Name Country Significance Trinity United States First fission-device test, first plutonium implosion detonation. Little Boy United States Bombing of Hiroshima, Japan, first detonation of a uranium gun-type device, first use of a nuclear device in combat. Fat Man United States Bombing of Nagasaki, Japan, second detonation of a plutonium implosion device (the first being the Trinity Test), second and last use of a nuclear device in combat. RDS-1 22 Soviet Union First fission-weapon test by the Soviet Union. George 225 United States First boosted nuclear weapon test, first weapon test to employ fusion in any measure. Hurricane 25 First fission weapon test by the United Kingdom. Ivy Mike 10,400 United States First "staged" thermonuclear weapon, with cryogenic fusion fuel, primarily a test device and not weaponized. Ivy King 500 United States Largest pure-fission weapon ever tested. RDS-6s 400 Soviet Union First fusion-weapon test by the Soviet Union (not "staged"). Castle Bravo 15,000 United States First "staged" thermonuclear weapon using dry fusion fuel. A serious nuclear fallout accident occurred. Largest nuclear detonation conducted by United States. RDS-37 1,600 Soviet Union First "staged" thermonuclear weapon test by the Soviet Union (deployable). Orange Herald 720 United Kingdom Largest boosted fission weapon ever tested. Intended as a fallback "in megaton range" in case British thermonuclear development failed. Grapple X 1,800 United Kingdom First (successful) "staged" thermonuclear weapon test by the United Kingdom 70 France First fission weapon test by France. Tsar Bomba 50,000 Soviet Union Largest thermonuclear weapon ever tested—scaled down from its initial 100 Mt design by 50%. 596 22 China First fission-weapon test by the People's Republic of China. Test No. 6 3,300 China First "staged" thermonuclear weapon test by the People's Republic of China. Canopus 2,600 France First "staged" thermonuclear weapon test by France 12 India First fission nuclear explosive test by India. Pokhran-II 45–50 India First potential fusion-boosted weapon test by India; first deployable fission weapon test by India. Chagai-I 40 Pakistan First fission weapon (boosted) test by Pakistan First fission-weapon test by North Korea (plutonium-based). First "staged" thermonuclear weapon test claimed by North Korea.
Note "Staged" refers to whether it was a "true" thermonuclear weapon of the so-called Teller–Ulam configuration or simply a form of a boosted fission weapon. For a more complete list of nuclear test series, see List of nuclear tests. Some exact yield estimates, such as that of the Tsar Bomba and the tests by India and Pakistan in 1998, are somewhat contested among specialists.