Megaton

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Diagram of explosive yield vs mushroom cloud height, illustrating the difference between 22 kiloton Fat Man and 15 megaton Castle Bravo explosions

TNT equivalent is a method of quantifying the energy released in explosions. The tonne of TNT is used as a unit of energy, approximately equivalent to the energy released in the detonation of this amount of TNT.

The kiloton and megaton of TNT have traditionally been used to rate the energy output, and hence destructive power, of nuclear weapons (see nuclear weapon yield). This unit is written into various nuclear weapon 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.

Value

A gram of TNT releases 980–1100 calories upon explosion. To define the tonne of TNT, this was arbitrarily standardized to 1000 thermochemical calories = 1 gram TNT = 4184 J (exactly).^[1] To put this into perspective, a gram of food carbohydrate has approximately 4 kcal of energy, versus 1 kcal for a gram of TNT.

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_th/g from large numbers of air blast experiments and theoretically calculated to be 1160 cal_th/g.^[2]

The measured pure heat output of a gram of TNT is only 651 thermochemical calories ≈ 2724 J,^[3] but this is not the important value for explosive blast effect calculations.

Examples

• Conventional bunker buster bombs yield range from less than 1 ton to MOAB's 11 tonnes.

• Minor Scale, a 1985 United States conventional explosion utilizing 4,800 short tons of ANFO explosive to simulate a 4 kt nuclear explosion, is believed to be the largest planned detonation of conventional explosives in history.

• The atomic bomb dropped on Hiroshima on August 6, 1945, exploded with an energy of about 15 kilotons of TNT (≈ 6.3×10¹³ joules). The nuclear weapons currently in the arsenal of the United States range in yield from 0.3 kt to 1.2 Mt TNT equivalent, for the B83 strategic bomb.

• During the Cold War, the United States developed hydrogen bombs with a maximum theoretical yield of 25 Mt; the Soviet Union developed a prototype weapon, nick-named the Tsar Bomba, which was tested at 50 Mt, but had a maximum theoretical yield of 100 Mt.^[4] The actual destructive potential of such weapons can vary greatly depending on conditions, such as the altitude at which they are detonated, the nature of the target they are detonated against, and the physical features of the landscape where they are detonated.

• The 1908 Tunguska event, believed to have been caused by an impacting comet or meteoroid, is estimated to have had a force ranging from 10–15 Mt.

• Megathrust earthquakes record huge M_W values, or total energy released. The 2004 Indian Ocean Earthquake released 9,560 gigatons of TNT equivalent, but its M_E (surface rupture energy, or potential for damage) was far smaller at 26.3 megatons of TNT. The largest quake registered, the 1960 Chilean quake, released M_W almost 60,000 gigatons of TNT equivalent.

• On a much grander scale, supernova explosions give off about 10⁴⁴ joules of energy, which is about ten octillion (10²⁸) megatons of TNT.

By E = mc², when 1 kilogram of antimatter annihilates with 1 kilogram of matter the reaction produces 1.8×10¹⁷ J, which is equal to 42.96 Mt.^[5]

See also

• Ton
• Tonne

References

1. ^ NIST Guide for the Use of the International System of Units (SI): Appendix B8—Factors for Units Listed Alphabetically
2. ^ Cooper, Paul. Explosives Engineering, New York: Wiley-VCH, 1996, p. 406.
3. ^ "Physics for Future Presidents, a textbook", 2001–2002, Richard A. Muller, Chapter 1. Energy, Power, and Explosions
4. ^ See Currently deployed U.S. nuclear weapon yields, Complete List of All U.S. Nuclear Weapons, Tsar Bomba, all from Carey Sublette's Nuclear Weapon Archive.
5. ^ In antiproton annihilation, about 50% of this energy is carried off by effectively invisible neutrinos (see S.K. Borowski,Comparison of Fusion/Antiproton Propulsion systems); in contrast, almost 100% of electron-positron annihilation events emit their energy entirely as gamma rays.

• Guide for the Use of the International System of Units (SI)
• Nuclear Weapons FAQ Part 1.3
• Rhodes, Richard. The Making of the Atomic Bomb, New York: Simon and Schuster, 1986.

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