|
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 calories equivalent, this being very roughly equal to the energy yield of one thousand tons of TNT.
Examples of nuclear weapon yields In order of increasing yield (most yield figures are approximate): As a comparison, the Oklahoma City bombing, using a truck-based fertilizer bomb, was a mere 0.002 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-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 per metric ton (6 Mt/t). * The practical achievable limit is somewhat lower. For current US weapons 600 kt/t (2.5 TJ/kg) to 2.2 Mt/t (9.2 TJ/kg). By comparison, for the Davy Crockett it was 0.4 - 40 kt/t (0.002 - 0.167 TJ/kg), for Little Boy 4 kt/t, and for the Tsar Bomba 2 Mt/t (8 TJ/kg) (deliberately reduced from the possible maximum which was twice as much), and for the Mk-41 5.2 Mt/t. The largest pure-fission bomb ever constructed had a 500 kt 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 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 of the maximum theoretical yield-to-weight ratio is about 6Mt/t, and the maximum achievable ratio about 5.2 MT/t, there is a practical limit on air delivery of the weapon. For example, if the full payload of 250 t of the Antonov An-225 could be used, the limit would be 250 t 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. Excellent approximations of yields can be approximated by the dimensionless number developed by G. I. Taylor. G. I. Taylor, Proc. Roy. Soc. London A201, 175 (1950) and G. I. Taylor, Proc. Roy. Soc. London A201, 159 (1950): Here, is the energy (in joules for SI), is the time (in seconds for SI), ρ is the density of air (kg/m³ for SI) and is the blast radius (in m for SI). With as a constant (for air ~1.033), all that is needed is a picture that shows the radius of the blast, a reference length scale, and the time since the blast to determine the energy. Using the picture of the Trinity test as an example, take the density of air to be 1 kg/m³, the radius as approximately 140 m, solving for : and substituting in, =8.889e13 J. With 1 kiloton of TNT= 4.184e12 J the result is 21.24 kilotons of TNT, which agrees nicely with commonly stated value of 20 KT. 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 12 and 18 kt (a 20% margin of error), while the Nagasaki bomb, "Fat Man", is estimated to be between 18 and 23 kt (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 Mt or at a maximum of 57 Mt by differing political figures, either as a way for hyping the power of the bomb or as an attempt to undercut it. Nuclear testing yields, as in the Tsar Bomba example, can also be used as a way of reflecting upon technical expertise, and claiming higher yields or accusations of lower yields can be used as a way of promoting or disparaging the technical abilities of a nuclear program. When India claimed to have successfully detonated a hydrogen bomb in their 1998 Operation Shakti tests, many Western observers relied on analysis of seismographic data to determine whether the Indian tests reflected a successful hydrogen bomb detonation. Some have alleged that India's reported yields have been higher than their actual test yields, a move which would apparently be for political purposes (to claim more nuclear ability than their rival Pakistan, for example, or to demonstrate their military might to other potential rivals such as nearby China) if true. See also | ||||||||
|
| |||||||||
 |
|
| |