DISTRIBUTION OF ENERGY IN NUCLEAR EXPLOSIONS.

1.22 It has been mentioned that one important difference between nuclear or conventional (chemical) explosion is the appearance of an appreciable proportion of the energy as thermal radiation in the former case. The basic reason for this difference is that, weight for weight, the energy produced by a nuclear explosive is millions of times as great as that produced by a chemical explosive. Consequently, the temperatures reached in the former case are very much higher than in the latter, namely, tens of millions of degrees in a nuclear explosion compared with a few thousands in a conventional explosion. As a result of this great difference in temperature, the distribution of the explosion energy is quite different in the two cases.

1.23 Broadly speaking, the energy may be divided into three categories: (kinetic) or external energy, i.e., energy of motion of electrons, atoms and molecules as a whole; internal energy of these particles; and thermal radiation energy. The proportion of thermal radiation increases rapidly with the increasing temperature. At the moderate temperatures obtained in a chemical explosion, the amount of thermal radiation is comparatively small, and so essentially all the energy released at the time of the explosion appears as kinetic and internal energy. This is almost entirely converted into blast and shock, in the manner described in §1.01. Because of the very much higher temperatures in a nuclear explosion, however, a considerable proportion of the energy is released as thermal radiation. The manner in which this takes place is described later (§ 1.77 et seq.).

1.24 The fraction of the explosion energy received at a distance from the burst point in each of the forms depicted in fig 1.02 depends on the nature and yield of the weapon and particularly on the environment of the explosion. For a nuclear detonation in the atmosphere below an altitude of 1000,000 feet, from 35 to 45 percent of the explosion energy is received as thermal energy in the visible and infrared portions of the spectrum (see Fig. 1.74). In addition, below an altitude od 40,000 feet, about 50 percent of the explosive energy is used in the production of air shock. At somewhat higher altitude, where there is less air with which the energy of the exploding nuclear weapon can interact, the proportion of energy converted into shock is decreased whereas that emitted as thermal radiation is correspondingly increased (§ 1.36).

1.25 The exact distribution of energy between air shock and thermal radiation is related in a complex manner to the explosive energy yield, the burst altitude, and, to some extent, to the weapon design, as will be seen in this and later chapters. However, an approximate rule of thumb for a fission weapon exploded in the air at an altitude of less that about 40,000 feet is that 35 percent of the explosion energy is in the form of thermal radiation and 50 percent produces air shock. Thus, for a burst at moderately low altitudes, the air shock from a fission weapon will be about half of that from a conventional high explosive with the same total energy release; in the latter, essentially all the explosive energy is in the form of air blast. This means that if a 20-kiloton fission weapon, for example, is exploded in the air below 40,000 thousand feet or so, the energy used in the production of blast would be roughly equivalent to that from 10 kilotons of TNT.

1.26 Regardless of the height of burst, approximately 85 percent of the energy of a nuclear fission weapon produces air blast (and shock) , thermal radiation and heat. The remaining 15 percent of the energy is released as various nuclear radiations. Of this, 5 percent constitutes the