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Nuclear weapon yield
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The explosive yield of a nuclear weapon is the amount of energy, called the yield, discharged when a nuclear weapon is detonated, expressed usually in the equivalent mass of trinitrotoluene (TNT), either in kilotons (thousands of tons of TNT) or megatons (millions of tons of TNT), but sometimes also in terajoules (1 kiloton of TNT = 4.184 TJ). Because the precise amount of energy released by TNT is and was subject to measurement uncertainties, especially at the dawn of the nuclear age, the accepted convention is that one kt of TNT is simply defined to be 1012 calories equivalent, this being very roughly equal to the energy yield of 1,000 tons of TNT.
rder of increasing yield (most yield figures are approximate):
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As a 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.
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Encyclopedia
The explosive yield of a nuclear weapon is the amount of energy, called the yield, discharged when a nuclear weapon is detonated, expressed usually in the equivalent mass of trinitrotoluene (TNT), either in kilotons (thousands of tons of TNT) or megatons (millions of tons of TNT), but sometimes also in terajoules (1 kiloton of TNT = 4.184 TJ). Because the precise amount of energy released by TNT is and was subject to measurement uncertainties, especially at the dawn of the nuclear age, the accepted convention is that one kt of TNT is simply defined to be 1012 calories equivalent, this being very roughly equal to the energy yield of 1,000 tons of TNT.
Examples of nuclear weapon yields
In order of increasing yield (most yield figures are approximate):
| Bomb | Yield | Notes |
|---|
| kt TNT | TJ |
|---|
| Davy Crockett | | 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) | | | gun type uranium-235 fission bomb (the first of the two nuclear weapons that have been used in warfare) | | | implosion type Plutonium-239 fission bomb (the second of the two nuclear weapons used in warfare) | | W76 warhead | | Twelve of these may be in a MIRVed Trident II missile; treaty limited to eight | | B61 nuclear bomb | various | Mod 7—up to Mod 10—four yield optionsMod 11—undisclosed yield | | W87 warhead | | Ten of these were in a MIRVed LG-118A Peacekeeper. | | W88 warhead | | Twelve of these may be in a Trident II missile (treaty limited to eight) | | Ivy King device | | second most powerful pure fission bomb, 60 kg uranium, implosion type | | Orange Herald | | most powerful pure fission bomb, UK | | B83 nuclear bomb | variable | | | B53 nuclear bomb | | most powerful US warhead; no longer in active service, but 50 are retained as part of the "Hedge" portion of the Enduring Stockpile; similar to the W-53 warhead that has been used in the Titan II Missile; decommissioned in 1987 | | Castle Bravo device | | most powerful US test | | EC17/Mk-17, the EC24/Mk-24, and the B41 (Mk-41) | various | | | The entire Operation Castle nuclear test series | | the highest-yielding test series conducted by the US | | Tsar Bomba device | | USSR, most powerful explosive device ever, mass of 27 short tons (24,000 kg), in its "full" form (i.e. with a depleted uranium tamper instead of one made of lead) it would have been . | | All nuclear testing | | total energy expended during all nuclear testing. |
 
Yield limits
The yield-to-weight ratio is the amount of weapon yield compared to the mass of the weapon. The theoretical maximum yield-to-weight ratio for fusion weapons is 6 megatons of TNT per metric ton (25 TJ/kg). The practical achievable limit is somewhat lower, and 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 United States once claimed they had the capability of tipping a Titan II ICBM with a fusion bomb warhead. If this were the case, the yield to weight ratio would have been about . For current smaller US weapons, yield is 600 to 2,200 kilotons of TNT per metric ton (2.5–9.2 TJ/kg). By comparison, for the very small tactical devices such as the Davy Crockett it was 0.4 to 40 kilotons of TNT per metric ton (0.002–0.167 TJ/kg).
For historical comparison, for Little Boy the yield was only , and for the largest Tsar Bomba yield was ) (deliberately reduced from the possible maximum which was twice as much). For the Mk-41, a U.S. late-generation large airplane deliverable bomb, yield was . Because of MIRV and ballistic considerations mentioned previously, efficiencies this high will not likely be seen in future weapons.
The largest pure-fission bomb ever constructed had a 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. However there is no known upper yield limit for a fusion (e.g, hydrogen) bomb. In principle a fusion bomb could be many thousand megatons. Because the maximum theoretical yield-to-weight ratio is about , and the maximum achievable ratio about , there is a practical limit on air delivery of the weapon.
For example, if the full payload of 250 metric tons of the Antonov An-225 could be used, the limit would be 250 t × 5.2 Mt/t, or . Likewise the maximum limit of a missile-delivered weapon is determined by the missile payload capacity. The large Russian SS-18 ICBM has a payload capacity of 7,200 kg, so the calculated maximum delivered yield would be . In fact, the SS-18 mod 1 yield for a single warhead is about .
Again, it is helpful for understanding to emphasize that large single warheads are seldom a part of today's arsenals, since smaller MIRV warheads are far more destructive for a given total yield or payload capacity. This effect, which results from the fact that destructive power of a single warhead scales approximately as the 2/3 power of its yield, more than makes up for the lessened yield/weight efficiency encountered if ballistic missile warheads are scaled-down from the maximal size that could be carried by a single-warhead missile.
Calculating yields and controversy
Yields of nuclear explosions can be very hard to calculate, even using numbers as rough as in the kiloton or megaton 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. Yields can be calculated in a number of ways, including calculations based on blast size, blast brightness, seismographic data, and the strength of the shock wave. 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 at how far they were moved by the shock wave of the explosion.
A good approximation of the yield of the Trinity test device was obtained from simple dimensional analysis by the British physicist G. I. Taylor. 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 ? of the air. The only number having dimensions of length that can be constructed from these quantities is:
Using the picture of the Trinity test shown here (which had been publicly released by the U.S. government and published in Life magazine), Taylor estimated that at t = 0.025 s the blast radius was 140 metres. Taking ? to be 1 kg/m³ and solving for E, he obtained that the yield was about 22 kilotons of TNT (90 TJ). This very simple argument agrees within 10% with the official value of the bomb's yield, , which at the time that Taylor published his result was considered highly-classified information. (See G. I. Taylor, Proc. Roy. Soc. London A201, pp. 159, 175 (1950).)
Where this data is 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 (a 20% margin of error), while the Nagasaki bomb, "Fat Man", is estimated to be between (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" or at a maximum of by differing political figures, either as a way for hyping the power of the bomb or as an attempt to undercut it.
See also
External links
(excerpt from official report)
, Chapter 1 in Samuel Glasstone and Phillip Dolan, eds., The Effects of Nuclear Weapons, 3rd edn. (Washington D.C.: U.S. Department of Defense/U.S. Energy Research and Development Administration, 1977); provides information about the relationship of nuclear yields to other effects (radiation, damage, etc.).
, discusses different methods used to determine the yields of the Indian 1998 tests.
from Carey Sublette's NuclearWeaponArchive.org
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