data are omitted from this report to keep the results unclassified. This omission is not a significant loss, because the natural uranium content of the desert soil is about 10° times that received from fallout and the plutonium concentration in the soil has been measured and reported (B177, Ha76). 2. FRACTIONATION The phenomenon of fractionation is due to both chemical and physical separation of the radionuclides. in the first few minutes. the device is vaporized. Chemical separation occurs At zero time, everything in the vicinity of As the fireball cools to about 3000°C the FeO and soil components form liquid droplets in which the refractory elements dissolve (Ad60). The fireball cools to the melting point of soil and iron oxides, ~1500°C, in about 20 sec (Mi63, St58). The solidified droplets contain the refractory elements, while the volatile elements and their radioactive daughters remain in the gas phase. In 6-8 minutes, when the cloud has cooled to ambient temperature, ~~50°C, the volatile elements (except for Kr and Xe) and their daughters condense on the surface. Since 1959 our group at Lawrence Livermore National Laboratory, under P. C. Stevenson, has interpreted fractionation phenomena in terms of a two~chemical-phase model of fallout. (Mi60) have also used this model. Heft (He70) and Miller In this work I refer to the two phases as the volatile phase and the refractory phase. The volatile