CHAPTER 2 ORIGIN AND NATURE OF RADIOACTIVE DEBRIS The chemical processes that follow the explosion of a nuclear device could be of importance in determining the chemical and physical form of the radioactive cloud and hence of the particles that ultimately constitute the fallout. For this reason an attempt has been made to describe the explosion from a chemical point of view. There are obvious differences between high bursts and those in which the fireball touches the earth, so the high bursts are considered first and then an attempt is made to modify these conclusions to include various other conditions. AIR BURST In the early history of the explosion the temperatures are so high that chemistry is unimportant because all the materials exist as single ions and atoms. We will consider first the time scale for a 20-KT bomb. Upto about 0.5 millisecond (ms) the fireball may be considered to be an iso- thermal sphere, and the surface temperature corresponds to the tempera- ture of the bomb materials. After 0.5 ms the surface is cooler than the interior until after “breakaway,’ when the surface temperature again increases to the temperature of the interior (see Fig. 1 on page 12). The temperature at the second maximum is about 7000°K and this temperature is reached in approximately 0.3 sec. The fireball has in that time almost reached its maximum radius and has engulfed enough air so that oxygen may be considered to be present in large excess. The various elements, as atoms, begin to combine with oxygen atoms to form gaseous diatomic molecules at temperatures of the order of 20,000°K for the most stable oxides and at correspondingly lower temperatures for less stable oxides. Many of these gaseous oxide molecules are sufficiently stable to form before any appreciable amount of O2 11