ON ts nn eee * Other radioactive nuclear explosives that were unconsumed in th. nuclear explosion, for example uranium-235. FalloutComposition The physical dimensions and the chemical composition of these radioactive particles from the tests (Freiling, 1965) depend upon where the nuclear explosion occurred. In particular, the composition differs if the test was over land compared to tests either over water or under water (Glasstone, 1964, and Neft, 1970). The fireballs from nuclear explosions over ground suck up vast quantities of soil and other materials. Due to the high temperature, these rise as a vapor in the fireball and cloud. The fission products are initially also vapors and these condense onto both solid and molten soil particles resulting from cooling and condensing. During the cooling, the more refractory (higher vaporiza- tion temperature) materials condense first. For example, fission products that are gaseous or have gaseous progenitors, or precursors, (parent element before beta-particle decay transmutation into a daughter element) adhere or are incorporated lastto the fallout particles during these processes. In general, this selective attachment of radioactive atoms to fallout material is called "fractionation."' The occurrence of frac- tionation is shown, for example, by the fact that in a land surface burst the larger particles, which fall out of the cloud at early times and are found near ground zero, have radiological properties different from the smaller particles that leave the radioactive cloud at later