The explosive yield of a nuclear weapon is the amount of energy released when that particular nuclear weapon is detonated, usually expressed as a TNT equivalent (the standardized equivalent mass of trinitrotoluene which, if detonated, would produce the same energy discharge), either in kilotonnes (kt—thousands of tonnes of TNT), in megatonnes (Mt—millions of tonnes of TNT), or sometimes in terajoules (TJ). An explosive yield of one terajoule is equal to 0.239 kilotonnes of TNT. 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. 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.
In order of increasing yield (most yield figures are approximate):
|Bomb||Yield||Notes||Weight of nuclear material|
|Davy Crockett||0.02||0.084||Variable 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 Genie||1.5||6.3||An 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 kg|
|Hiroshima's "Little Boy" gravity bomb||13–18||54–75||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 fissioned|
|Nagasaki's "Fat Man" gravity bomb||19–23||79–96||Implosion type plutonium-239 fission bomb (the second of the two nuclear weapons used in warfare).||6.2 kg of Plutonium-239, about 1 kg fissioned|
|W76 warhead||100||420||Twelve of these may be in a MIRVed Trident II missile; treaty limited to eight.|
|W87 warhead||300||1,300||Ten of these were in a MIRVed LGM-118A Peacekeeper.|
|W88 warhead||475||1,990||Twelve of these may be in a Trident II missile; treaty limited to eight.|
|Ivy King device||500||2,100||Most powerful US pure fission bomb, 60 kg uranium, implosion type. Never deployed.||60 kg of Highly enriched uranium (HEU)|
|Orange Herald Small||800||3,300||Most powerful tested UK boosted fission missile warhead.||117 kg of Uranium-235|
|B83 nuclear bomb||1,200||5,000||Variable yield weapon, most powerful US weapon in active service.|
|B53 nuclear bomb||9,000||38,000||Was 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 device||15,000||63,000||Most powerful US test. Never deployed.||400 kg of Lithium-6 deuteride|
|EC17/Mk-17, the EC24/Mk-24, and the B41 (Mk-41)||25,000||100,000||Most powerful US weapons ever: 25 megatonnes of TNT (100 PJ); the Mk-17 was also the largest by area square footage and mass cubic footage: about 20 short tons (18,000 kg). 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 produced. All were gravity bombs carried by the B-36 bomber (retired by 1957).|
|The entire Operation Castle nuclear test series||48,200||202,000||The highest-yielding test series conducted by the US.|
|Tsar Bomba device||50,000||210,000||USSR, 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 1996||510,300||2,135,000||Total 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.
The yield-to-mass ratio is the amount of weapon yield compared to the mass of the weapon. According to nuclear-weapons designer Ted Taylor, the practical maximum yield-to-mass ratio for fusion weapons is about 6 megatonnes of TNT per tonne (25 TJ/kg).[self-published source?] The "Taylor limit" is not derived from first principles, and weapons with yields as high as 9.5 megatonnes per tonne have been theorized. 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.
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.
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.
|July 16, 1945||Trinity||18–20||United States||First fission-device test, first plutonium implosion detonation.|
|August 6, 1945||Little Boy||12–18||United States||Bombing of Hiroshima, Japan, first detonation of a uranium gun-type device, first use of a nuclear device in combat.|
|August 9, 1945||Fat Man||18–23||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.|
|August 29, 1949||RDS-1||22||Soviet Union||First fission-weapon test by the Soviet Union.|
|May 8, 1951||George||225||United States||First boosted nuclear weapon test, first weapon test to employ fusion in any measure.|
|October 3, 1952||Hurricane||25||United Kingdom||First fission weapon test by the United Kingdom.|
|November 1, 1952||Ivy Mike||10,400||United States||First "staged" thermonuclear weapon, with cryogenic fusion fuel, primarily a test device and not weaponized.|
|November 16, 1952||Ivy King||500||United States||Largest pure-fission weapon ever tested.|
|August 12, 1953||RDS-6s||400||Soviet Union||First fusion-weapon test by the Soviet Union (not "staged").|
|March 1, 1954||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.|
|November 22, 1955||RDS-37||1,600||Soviet Union||First "staged" thermonuclear weapon test by the Soviet Union (deployable).|
|May 31, 1957||Orange Herald||720||United Kingdom||Largest boosted fission weapon ever tested. Intended as a fallback "in megaton range" in case British thermonuclear development failed.|
|November 8, 1957||Grapple X||1,800||United Kingdom||First (successful) "staged" thermonuclear weapon test by the United Kingdom|
|February 13, 1960||Gerboise Bleue||70||France||First fission weapon test by France.|
|October 31, 1961||Tsar Bomba||50,000||Soviet Union||Largest thermonuclear weapon ever tested—scaled down from its initial 100 Mt design by 50%.|
|October 16, 1964||596||22||China||First fission-weapon test by the People's Republic of China.|
|June 17, 1967||Test No. 6||3,300||China||First "staged" thermonuclear weapon test by the People's Republic of China.|
|August 24, 1968||Canopus||2,600||France||First "staged" thermonuclear weapon test by France|
|May 18, 1974||Smiling Buddha||12||India||First fission nuclear explosive test by India.|
|May 11, 1998||Pokhran-II||45–50||India||First potential fusion-boosted weapon test by India; first deployable fission weapon test by India.|
|May 28, 1998||Chagai-I||40||Pakistan||First fission weapon (boosted) test by Pakistan|
|October 9, 2006||2006 nuclear test||under 1||North Korea||First fission-weapon test by North Korea (plutonium-based).|
|September 3, 2017||2017 nuclear test||200–300||North Korea||First "staged" thermonuclear weapon test claimed by North Korea.|
See also: Fermi problem
Yields of nuclear explosions can be very hard to calculate, even using numbers as rough as in the kilotonne or megatonne range (much less down to the resolution of individual terajoules). Even under very controlled conditions, precise yields can be very hard to determine, and for less controlled conditions the margins of error can be quite large. For fission devices, the most precise yield value is found from "radiochemical/Fallout analysis"; that is, measuring the quantity of fission products generated, in much the same way as the chemical yield in chemical reaction products can be measured after a chemical reaction. The radiochemical analysis method was pioneered by Herbert L. Anderson.
For nuclear explosive devices where the fallout is not attainable or would be misleading, neutron activation analysis is often employed as the second most accurate method, with it having been used to determine the yield of both Little Boy and thermonuclear Ivy Mike's respective yields.
Yields can also be inferred in a number of other remote sensing ways, including scaling law calculations based on blast size, infrasound, fireball brightness (Bhangmeter), seismographic data (CTBTO), and the strength of the shock wave.
|Initial ionizing radiation||5%|
|Residual fallout radiation||10%|
Enrico Fermi famously made a (very) rough calculation of the yield of the Trinity test by dropping small pieces of paper in the air and measuring how far they were moved by the blast wave of the explosion; that is, he found the blast pressure at his distance from the detonation in pounds per square inch, using the deviation of the papers' fall away from the vertical as a crude blast gauge/barograph, and then with pressure X in psi, at distance Y, in miles figures, he extrapolated backwards to estimate the yield of the Trinity device, which he found was about 10 kilotonnes of blast energy.
Fermi later recalled:
I was stationed at the Base Camp at Trinity in a position about ten miles [16 km] from the site of the explosion... About 40 seconds after the explosion the air blast reached me. I tried to estimate its strength by dropping from about six feet small pieces of paper before, during, and after the passage of the blast wave. Since, at the time, there was no wind[,] I could observe very distinctly and actually measure the displacement of the pieces of paper that were in the process of falling while the blast was passing. The shift was about 2 1/2 meters, which, at the time, I estimated to correspond to the blast that would be produced by ten thousand tonnes of TNT.
The surface area (A) and volume (V) of a sphere are and respectively.
The blast wave, however, was likely assumed to grow out as the surface area of the approximately hemispheric near surface burst blast wave of the Trinity gadget. The paper is moved 2.5 meters by the wave, so the effect of the Trinity device is to displace a hemispherical shell of air of volume 2.5 m × 2π(16 km)2. Multiply by 1 atm to get an energy of 4.1×1014 J ~ 100 kT TNT.[quantify]
A good approximation of the yield of the Trinity test device was obtained in 1950 by the British physicist G. I. Taylor from simple dimensional analysis and an estimation of the heat capacity for very hot air. Taylor had initially done this highly classified work in mid-1941 and published an article with an analysis of the Trinity data fireball when the Trinity photograph data was declassified in 1950 (after the USSR had exploded its own version of this bomb).
Taylor noted that the radius R of the blast should initially depend only on the energy E of the explosion, the time t after the detonation, and the density ρ of the air. The only equation having compatible dimensions that can be constructed from these quantities is
Here S is a dimensionless constant having a value approximately equal to 1, since it is low-order function of the heat capacity ratio or adiabatic index
which is approximately 1 for all conditions.
Using the picture of the Trinity test shown here (which had been publicly released by the U.S. government and published in Life magazine), using successive frames of the explosion, Taylor found that R5/t2 is a constant in a given nuclear blast (especially between 0.38 ms, after the shock wave has formed, and 1.93 ms, before significant energy is lost by thermal radiation). Furthermore, he estimated a value for S numerically at 1.
Thus, with t = 0.025 s and the blast radius being 140 metres, and taking ρ to be 1 kg/m3 (the measured value at Trinity on the day of the test, as opposed to sea-level values of approximately 1.3 kg/m3) and solving for E, Taylor obtained that the yield was about 22 kilotonnes of TNT (90 TJ). This does not take into account the fact that the energy should only be about half this value for a hemispherical blast, but this very simple argument did agree to within 10% with the official value of the bomb's yield in 1950, which was 20 kilotons of TNT (84 TJ) (see G. I. Taylor, Proc. Roy. Soc. London A 200, pp. 235–247 (1950)).
A good approximation to Taylor's constant S for below about 2 is
The value of the heat capacity ratio here is between the 1.67 of fully dissociated air molecules and the lower value for very hot diatomic air (1.2), and under conditions of an atomic fireball is (coincidentally) close to the STP (standard) gamma for room-temperature air, which is 1.4. This gives the value of Taylor's S constant to be 1.036 for the adiabatic hypershock region where the constant R5/t2 condition holds.
As it relates to fundamental dimensional analysis, if one expresses all the variables in terms of mass M, length L, and time T:
(think of the expression for kinetic energy, ),
and then derive an expression for, say, E, in terms of the other variables, by finding values of , , and in the general relation
such that the left and right sides are dimensionally balanced in terms of M, L, and T (i.e., each dimension has the same exponent on both sides).
Where these data are not available, as in a number of cases, precise yields have been in dispute, especially when they are tied to questions of politics. The weapons used in the atomic bombings of Hiroshima and Nagasaki, for example, were highly individual and very idiosyncratic designs, and gauging their yield retrospectively has been quite difficult. The Hiroshima bomb, "Little Boy", is estimated to have been between 12 and 18 kilotonnes of TNT (50 and 75 TJ) (a 20% margin of error), while the Nagasaki bomb, "Fat Man", is estimated to be between 18 and 23 kilotonnes of TNT (75 and 96 TJ) (a 10% margin of error).
Such apparently small changes in values can be important when trying to use the data from these bombings as reflective of how other bombs would behave in combat, and also result in differing assessments of how many "Hiroshima bombs" other weapons are equivalent to (for example, the Ivy Mike hydrogen bomb was equivalent to either 867 or 578 Hiroshima weapons — a rhetorically quite substantial difference — depending on whether one uses the high or low figure for the calculation).
Other disputed yields have included the massive Tsar Bomba, whose yield was claimed between being "only" 50 megatonnes of TNT (210 PJ) or at a maximum of 57 megatonnes of TNT (240 PJ) by differing political figures, either as a way for hyping the power of the bomb or as an attempt to undercut it.
The maximum fireball radius presented on the computer is an average between that for air and surface bursts. Thus, the fireball radius for a surface burst is 13 percent larger than that indicated and for an air burst, 13 percent smaller.
approximately 82% of the fission energy is released as kinetic energy of the two large fission fragments. These fragments, being massive and highly charged particles, interact readily with matter. They transfer their energy quickly to the surrounding weapon materials, which rapidly become heated.