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Homework Help: Energy of a Nuclear Weapon

  1. May 29, 2009 #1
    Simple question - how is the energy release of a nuclear weapon detonation defined? We often hear numbers like 50 kilotons or 5 megatons and so on.

    Now, when a nuclear weapon explodes (lets say it's a simple fission bomb), we have U-235 or some other fissile material fissioning in a chain reaction an being converted into fission products, which are themselves more often than not radioactive. These decay and so on, right through the decay chains until eventually all nuclei present in the initial explosion reach a stable isobar, yes? OK then, so when we talk about a 5 megaton weapon, does that mean that the total energy release from the whole process, from the first instant of fission right down through the thousands of years until the last unstable nuclei decays into a stable state, is equal to 5 megatons of TNT exploding? Or does the 5 megatons come in as some sort of measure of the 'initial blast strength' - i.e. 5 million tons of TNT and the equivalent size nuclear weapon, sitting next to each other, would yield the same size explosion/impact/damage etc, with the energy release from the later radioactive decay of nuclear fission products ignored?

    It would seem to me that there would be a significant difference in initial blast size described by the label '5 megaton', depending on the definition used...

    Any knowledgeable people out there feel like chiming in?
  2. jcsd
  3. May 29, 2009 #2


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    The megaton is a unit of energy, equal to 4.184*1015 Joules. It is based on the energy released by exploding one tonne of TNT. However, that is not an exact number, and so the Megaton of energy has been standardized to be 1015 calories.

    Ref: NIST guide to SI units.
  4. May 29, 2009 #3
    Thanks sylas.

    I was more interested though in whether a given detonation energy refers to the total release of energy in a nuclear blast, including energy released in the decays of fission products all the way down through their decays chains (which could take many years to come close to completion), or whether it refers to the energy release of the initial blast /impact/explosion/destructive power itself somehow, neglecting the energy release of the (sometimes) slowly decaying radioactive fission products...

    It seems to me that a great deal of energy will be released after the initial blast in the form of radioactive decay of these product nuclei, so I would think that it is important to know whether this energy forms a part of the quoted energy designation of a particular warhead/bomb, or whether it is left out of the equation due to the energy release not really forming a part of the blast itself or it's destructive power.

    Once again, cheers for the reply.
  5. May 29, 2009 #4


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    This seems to be discussed at the Nuclear Weapons FAQ. It appears to be all the energy of the initial explosion, including heat and blast effect. I don't think subsequent decay products come into it.

    Cheers -- sylas
  6. May 29, 2009 #5
    Thanks for that - just what I was after.


  7. May 29, 2009 #6

    "Now the units of explosive energy (megatons, kilotons, or even just tons, depending on yield) are derived from attempts to compare the explosive force of a bomb to conventional explosives, the original intention was to equate it with tons of trinitrotoluene (TNT) - a workhorse military explosive." (http://nuclearweaponarchive.org/Nwfaq/Nfaq1.html#nfaq1.3)
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