Megaton

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A megaton or megatonne is a unit of mass equal to 1,000,000 metric tons, i.e. 109 kg or 1 teragram (Tg). The official SI symbol for the megaton is Mt, but MT is also being used; beware that the latter is the SI symbol for megatesla and also (unofficially) used for the metric ton in some contexts. See 1 E9 kg for a comparison with similar masses.

The kiloton or megaton of TNT is used as a unit of energy, approximately equivalent to the energy released in the detonation of this amount of TNT. The megaton of TNT has traditionally been used to rate the energy output, and hence destructive power, of nuclear weapons. This unit is written into various arms control treaties, and gives a sense of destructiveness as compared with ordinary explosives, like TNT. More recently, it has been used to describe the energy released in other highly destructive events, such as asteroid impacts.

  • A gram of TNT by definition for arms control purposes is 1000 thermochemical calories, which equals 4.184 kilojoules (kJ).
  • A ton of TNT, (a metric ton = 1000 kg) is therefore 4.184 × 109 J = 4.184 gigajoules (GJ).
  • A kiloton of TNT is therefore 4.184 × 1012 J = 4.184 terajoules (TJ).
  • A megaton of TNT is 4.184 × 1015 joules = 4.184 petajoules (PJ).

This definition is a conventional one. Explosives energy is normally calculated using the thermodynamic work energy of detonation, which for TNT has been accurately measured at 1120 cal/g from large numbers of air blast experiments and theoretically calculated to be 1160 cal/g, according to standard reference book Explosives Engineering (pp 406).

The measured pure heat output of a gram of TNT is only 652 thermochemical calories = 2724 J [1], but this is not the important value for explosive blast effects calculations.

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See also

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References

  • Rhodes, Richard. The Making of the Atomic Bomb, New York: Simon and Schuster, 1986.
  • Cooper, Paul. Explosives Engineering, New York: Wiley-VCH, 1996.

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