The Energy from a Nuclear Weapon
One of the fundamental differences between a nuclear and a conventional explosion is that nuclear explosions can be many thousands (or millions) of times more powerful than the largest conventional detonations. Both types of weapons rely on the destructive force of the blast or shock wave. However, the temperatures reached in a nuclear explosion are very much higher than in a conventional explosion, and a large proportion of the energy in a nuclear explosion is emitted in the form of light and heat, generally referred to as thermal energy. This energy is capable of causing skin burns and of starting fires at considerable distances. Nuclear explosions are also accompanied by various forms of radiation, lasting a few seconds to remaining dangerous over an extended period of time.
Approximately 85 percent of the energy of a nuclear weapon produces air blast (and shock), thermal energy (heat). The remaining 15 percent of the energy is released as various type of nuclear radiation. Of this, 5 percent constitutes the initial nuclear radiation, defined as that produced within a minute or so of the explosion, are mostly gamma rays and neutrons. The final 10 percent of the total fission energy represents that of the residual (or delayed) nuclear radiation, which is emitted over a period of time. This is largely due to the radioactivity of the fission products present in the weapon residues, or debris, and fallout after the explosion.
The "yield" of a nuclear weapon is a measure of the amount of explosive energy it can produce. The yield is given in terms of the quantity of TNT that would generate the same amount of energy when it explodes. Thus, a 1 kiloton nuclear weapon is one which produces the same amount of energy in an explosion as does 1 kiloton (1,000 tons) of TNT. Similarly, a 1 megaton weapon would have the energy equivalent of 1 million tons of TNT. One megaton is equivalent to 4.18 x 1015 joules.
In evaluating the destructive power of a weapons system, it is customary to use the concept of equivalent megatons (EMT). Equivalent megatonnage is defined as the actual megatonnage raised to the two-thirds power:
EMT = Y2/3 where Y is in megatons.
This relation arises from the fact that the destructive power of a bomb does not vary linearly with the yield. The volume the weapon's energy spreads into varies as the cube of the distance, but the destroyed area varies at the square of the distance.
Thus 1 bomb with a yield of 1 megaton would destroy 80 square miles. While 8 bombs, each with a yield of 125 kilotons, would destroy 160 square miles. This relationship is one reason for the development of delivery systems that could carry multiple warheads (MIRVs).